Abstract: External crop production inputs such as mineral fertilizers, organic amendments, microbial inoculants and pesticides are applied with the ultimate goal of maximizing productivity and economic returns, while side effects on soil health are often neglected. This study summarized the current understanding of how crop production inputs affect soil health (soil physical, chemical and biological properties). Mineral fertilizers have limited direct (such as soil physical property) effects but their application can enhance soil biological activity via increases in system productivity, crop residue return and soil organic matter. Another important indirect effect such as N fertilization is soil acidification, with considerable negative effects on soil health such as on amount, activity and diversity of organisms. Organic amendments such as manure, compost, biosolids and humic substances provide a direct source of C for soil organisms as well as an indirect C source via increased plant growth and plant residue returns. Non-target effects of microbial inoculants appear to be small and transient. Among the pesticides, herbicides have few significant effects on soil health, whereas negative effects of insecticides and fungicides are more common and their application warrants strict regulation. The sound management of crop production inputs must attempt to ensure both an enhanced and safeguarded environment; therefore, an Integrated Pest and Nutrient Management (IPNM) strategy that combines the use of chemical, organic crop production inputs together with cultural practices must be developed and evaluated as suggested by Integrated Pest and Nutrient Management Protocols (IPNMP).
INTRODUCTION
Currently, increase in food production is the one of the primary objective of all countries as world population is expected to grow to nearly 9-10 billion by 20501. In developing countries the population growth rate is 3% year–1. Demand for food increases1 by 3.8% year–1. However, food production is increasing by only 1.2% year–1. World food production needs to increase by 70%, in order to keep pace with the demand of growing population2. However, increase in food production is faced with the ever-growing challenges especially the new area that can be increased for cultivation purposes is very limited3. Moreover, the soil fertility of developing countries has become deteriorated4 and occurrence of various pest infestations increased5. The increasing world population coupled with other challenges has therefore put a tremendous amount of pressure on the existing agricultural system so that food needs can be met from the same current resources like land, water etc.6. In the process of increasing crop production, crop production inputs (fertilizers and pesticides) are now being used in higher quantities than in the past with proper or improper application system4,7.
Crop production inputs to crop production systems include mineral fertilizers such as urea, ammonium nitrate, sulfates and phosphates8; organic fertilizers such as animal manures, composts and biosolids; various other organic products such as humic acids and microbial inoculants6,9 and pesticides including herbicides, insecticides, nematicides, fungicides and soil fumigants8. All these products are applied with the ultimate goal of maximizing crop productivity and economic returns9.
Mineral fertilizers are a major physical input into world agricultural production. The global total nutrient capacity (N+P2O5+K2O) was 284 million tons in 2014, out of which the total supply was 240 million tons10. During 2015, the total capacity was increase by 2.9% and supplies were grown by 1.6%. Based on the production process, it can be roughly categorized into three types: chemical, organic and bio fertilizer. Each type of fertilizer has its advantages and disadvantages. Common types of mineral fertilizers as used in this review are ammonium nitrate, ammonium sulfate, calcium nitrate, diammonium phosphate, elemental sulfur, phosphate rock, sodium dihydrogen phosphate, superphosphate, triple superphosphate and urea. Though, there is lack of information on organic fertilizer consumption globally, Organic agriculture using organic fertilizer is now practiced in more than 130 countries with a total area of 30.4 million ha in 0.7 million number of organic farms11,12. It is clear that early many long-term studies have shown that combinations of both organic and inorganic nutrient sources lead to enhanced nutrient availability and synchronization of nutrient release and uptake by crops and positive effects on soil properties6.
Pesticides are a diverse group of inorganic and organic chemicals13. Now it become an integral part of our modern life and are used to protect agricultural land, stored grain, flower gardens as well as to eradicate the pests transmitting dangerous infectious diseases mainly in developing countries14. It has been estimated that globally nearly $38 billion are spent on pesticides each year15. Manufacturers and researchers are designing new formulations of pesticides to meet the global demand. Ideally, the applied pesticides should only be toxic to the target organisms and unwanted plants, should be biodegradable and eco-friendly to some extent to the soil health. Unfortunately, this is rarely the case as most of the pesticides are non-specific and may kill the organisms that are harmless or useful to the ecosystem. In general, it has been estimated that only about 0.1% of the pesticides reach the target organisms and the remaining contaminates the surrounding environment16. On account of this behavior then, they can best be described as biocides (capable of killing all forms of life)17. Researches showed that pesticides have not only entered into the food chain and have bio-accumulated in the higher tropic level but also more recently causing several human acute and chronic illnesses7,18. The repeated uses of persistent and non-biodegradable pesticides and herbicides have polluted various components of water, air and soil ecosystem17. Pesticides and herbicides affect various properties of soil such as, soil physical and chemical properties and mainly soil microorganisms7,19.
This study summarized the current understanding of the effects of crop production inputs on soil health. The underlying concept is that these inputs can affect soil health through direct or indirect effects. Schreck et al.6 and Bunemann et al.8 summarized this effect in to two categories: (1) Direct effects (example: Increased amount and/or activity after removal of nutrient limitations and decreased activity due to high nutrient availability and decreased amount and/or activity due to toxicity) and (2) Indirect effects [Example: change in pH, change in soil physical properties (aggregation, porosity), change in productivity, residue inputs and soil organic matter levels]. Due to difficulty to distinguished the above classification, existing data are presented for the different amendments separately but the effects are discussed together. Note that, this study did not focused on the direct uptake of nutrients and its effect on crop productivity rather mainly focused on what happen on some soil chemical properties and the amount, activity and diversity of soil microorganisms when the crop production inputs are applied for crop production systems. The evidence from Ethiopia is rather almost none and therefore this study includes literature from overseas, in an attempt to establish the main effects and to draw some conclusions applicable to all users in the world.
SOIL HEALTH
There is increasing recognition of soil as an important non-renewable asset that needs to be managed well and looked after19. Soil health is defined as the capacity of a soil to function, within ecosystem and land use boundaries, to sustain biological productivity, maintain environmental quality and promote plant and animal health20. It considers the chemical, physical, biological and ecological properties of soils and the disturbance and ameliorative responses by land managers. Soil health also describes the capacity of soil to meet performance standards relating to nutrient and water storage and supply, biological diversity and function, structural integrity and resistance to degradation6. Nonetheless, it should be emphasized that these functions operate by complex interaction with the abiotic physical and chemical environment of soil. Both natural and agricultural soils are the habitat for many different organisms which collectively contribute to a variety of soil-based goods and services21. However, these soil-based biological processes may become disturbed or improved by different factors such as addition of agricultural inputs, improper land cultivation, climatic conditions etc.22. More specifically, Under-use, over-use and adequate use of crop production inputs determine the soil health of an environment as discussed previously6. For instance Fig. 1 shows differences in nutrient inputs and outputs at three locations representing under-use, over-use and adequate use of fertilizers. Western Kenya is characterized by low inputs of N and P in marked contrast to the situation in China and the USA. The N outputs at the Kenyan site are much larger than the inputs, leading to substantial nutrient depletion or "soil mining" and consequent long-term degradation of soil health. On the other hand, high fertilizer nutrient inputs in China greatly exceed nutrient outputs and point towards substantial risks of nutrient losses to the environment. With almost similar inputs and outputs of both N and P, soil health in the Midwest USA is better than in either the Kenyan or Chinese sites23.
In Ethiopia and worldwide productions and consumption of crop production inputs: Pesticides include chemically synthesized compounds, devices or organisms that are regularly utilized in agriculture to manage, destroy, attack or repel pests, pathogens and parasites. Pesticides include both organic and inorganic moieties and may be classified into different groups based on their chemical composition24. Over 1990s, the global pesticide sale remained relatively constant, between 270 to 300 billion dollars, of which 47% were herbicides, the remaining were insecticides, fungicides /bactericides and the others. Over the period 2007-2008 herbicides ranked the first in three major categories of pesticides (insecticides, fungicides/bactericides, herbicides). Fungicides/bactericides increased rapidly and ranked the second7,17 . Europe is now the largest pesticide consumer in the world followed by Asia. Countries like China, United States, France, Brazil and Japan are the largest pesticide producers, consumers or traders in the world. Although, most of the pesticides worldwide are used to fruit and vegetable crops, developing countries like Ethiopia used to mainly cereal crops25. In the developed countries pesticides, mainly herbicides are mostly used7.
| Fig. 1: | Total agronomic inputs and outputs of N and P in agricultural soils at Western Kenya, North China and Midwest USA23 |
Although chemical pesticide usage in Ethiopia was historically low, recent developments in increased food production and expansion in floriculture industry have resulted in higher consumption of chemical pesticides. Recently, Ethiopia has been considered as the largest accumulator of obsolete pesticides in the whole of Africa25. According to the Ethiopian Federal Environmental Protection Authority (EFEPA)26 pesticides are mainly imported for agricultural purposes while some amounts of pesticides are imported for health care and industrial purposes. Both public and private enterprises are engaged in pesticide importation business. In contrary, there is one factory that formulates pesticides within the country25,26. Large quantities of pesticides are imported annually to Ethiopia. In this regard, currently, about 20 organizations are actively involved in importation and sale of pesticides and over 3000 tons of various types of pesticides that are worth more than USD 20 million are imported annually26.
In the medium term, global fertilizer consumption would grow moderately at an annual rate of 1.7%, to reach 199.4 Mt nutrients in 201927. Increases are projected for all three major nutrients, with average annual growth rates of 1.3% for N, 2.1% for P and 2.4% for K. Total sales in the fertilizer and industrial sectors in 2019 are forecast at 264 Mt nutrients, representing a 10% increase over 2014. In Ethiopia, over the last 10 years, total fertilizer imports have increased by more than 50%, from less than 370,000 Mt in 2002 to almost 570,000 Mt in 2011, with a spike of 627,000 Mt in 2009. Fertilizer carryover stocks averaged 33% of imports between 2002 and 2011, with a high of 61% in 2002 and a low of 12% in 2007. Fertilizer sales and consumption have increased by more than 100% between 2002 and 2011, with an average rate of 6% year–1, more so for urea than for diammonium phosphate28. The information on world consumption of organic fertilizer is very fragmented and difficult to present a comprehensive review in this study. However, organic agriculture using organic fertilizer is now practiced in more than 130 countries with a total area of 30.4 million ha in 0.7 million number of organic farm11,12. This constitutes about 0.65% of the total agriculture land of the world12.
IMPACT OF FERTILIZERS ON SOIL HEALTH
For optimum plant growth, nutrients must be available in sufficient and balanced quantities. Soils contain natural reserves of plant nutrients but these reserves are largely in forms unavailable to plants and only a minor portion is released each year through biological activity or chemical processes. This release is too slow to compensate for the removal of nutrients by agricultural production and to meet crop requirements29. Therefore, fertilizers are designed to supplement the nutrients already present in the soil. The use of chemical fertilizer, organic fertilizer or biofertilizer has its advantages and disadvantages in the context of nutrient supply, crop growth and environmental quality. The advantages need to be integrated in order to make optimum use of each type of fertilizer and achieve balanced nutrient management for crop growth8,30,31.
Impact of inorganic fertilizers on soil health: Most inorganic fertilizers in the world are applied to systems with regular and significant nutrient exports in harvested products, such as lands under arable cropping8,30. Various studies conducted at laboratory, pot and filed level revealed that mineral fertilizers had different effects on soil health. The application of chemical fertilizers causing accumulation of acid soils such as hydrochloric and sulfuric acids creates a damaging effect on soil referred to as soil friability. The different acids in the soil dissolve the soil crumbs which help to hold together the rock particles. Soil crumbs result from the combination of humus or decomposed natural material such as dead leaves, with clay. These mineral rich soil crumbs are essential to soil drainage and greatly improve air circulation in the soil. As the chemicals in the chemical fertilizers destroy soil crumbs, the result is a highly compacted soil with reduced drainage and air circulation32,33.
Addition of Organic fertilizer increased Effective Cation Exchange Capacity (ECEC) by 16% while it reduced in inorganic fertilizer plots34. Bulk Density (BD) and soil pH decreased while total porosity, Water Holding Capacity (WHC) improved with the application of increased level of N fertilizers34,35. Gypsum amendment was superior in reducing the chemically available heavy metals (Fe, Mn, Zn, Cu, Ni and Cd) in the heavy clay salt-affected soil35. The experiment at laboratory inoculation pointed out that the addition of 200 mg N kg–1 soil as ammonium sulfate to 2 pasture soils of varying P status resulted in a decrease in microbial P, no change in the turnover of added C and an increase in N mineralization during 168 days of incubation36. Long term experiment on chemical fertilizers usage showed that soil physical characteristics such as bulk density were changed in long-term and it was increased compared to control soil. The heavy metals accumulations in soil were highly affected and the concentration of some metals such as cadmium has reached a limit beyond the standard for agricultural purposes. The results also showed that fortunately the concentration of other metals is not beyond the standard36.
In contrary, an increase in soil respiration and microbial P but no effect on microbial N and a decrease in various enzyme activities upon addition of 500 mg P kg–1 soil as calcium diphosphate were reported by Bunemann et al.8. The addition of N, P, K and S at 100, 20, 100 and 20 mg kg–1 soil, respectively, to a range of soils followed by incubation for 20 days, resulted in minor changes (increase or decrease) of soil respiration and microbial C, N and P that remained within 20% difference from the non-amended controls8. Ammonium addition did not change the composition of the microbial community during 28 days of incubation but led to community shifts after 16 weeks of incubation37. Using molecular techniques in a range of pot experiments, soil pH and N and P fertilization can affect the microbial community composition but that substrate availability, e.g. in the form of root exudates in the rhizosphere, appears to be the main factor determining the community composition in the rhizosphere38. It is thus, important to consider the potential feedback from improved plant nutrition when examining fertilizer effects on soil health8. Moreover, the addition of mineral N did not affect Arbuscular Mycorrhizal Fungi (AMF) while increasing additions of inorganic P decreased the rate of root length colonization39. A decrease in AMF root colonization was also observed in pastures after 15-17 years of mineral P and N fertilization40 (Table 1).
Many field experiments have shown a lack of response of the microbial biomass and earthworms to inorganic fertilizers (Table 2, 3). Where a decrease in microbial C was observed, it was usually accompanied by a decrease in soil pH after application of N or S fertilizers41. Other methods such as microbial enumeration by plate counts, enzyme activities42 and nematode counts43, which are possibly more sensitive than measurements of microbial biomass, show variable changes due to mineral fertilization (Table 1). For instance, although the total number of nematodes was not affected by N fertilization and a concomitant decrease in pH, some nematode species increased, whereas others were decreased41.
There was absence of changes in microbial C in response to N fertilization and a related decrease in pH in the 2 long-term field experiments44. This study is interesting, because in the same study microbial C was found to be correlated to levels of organic C as induced by different crop rotations. Several long-term field experiments in which inorganic and organic fertilizer inputs have been compared are indicated in Table 3 and 4. Several studies showed that there was a good correlations between the microbial biomass and soil organic C. Although soil organic C levels are often increased by inorganic fertilization compared with the non-fertilized control, even greater increases in soil organic C are usually achieved in treatments receiving organic amendments8. This is also reflected in the fact that whereas inorganic fertilizers show variable effects on soil organisms and soil pH, organic amendments have only been reported to have insignificant or positive long-term effects (Table 2).
The amounts of microbial C and N under sugarcane after 59 years of differential crop residue management and NPK fertilization and showed that the microbial biomass was directly influenced by residue management and indirectly by NPK fertilization through increased residue inputs42. Another study in the same trial revealed the interaction of soil acidification with negative effects and organic matter accumulation with positive effects on soil organisms and enzyme activities42. The long term field experiment exemplifies that agro-ecosystems can be relatively slow to respond to changes in management and thus illustrates the value of long-term field experiments8. The excellent correlation between microbial C and soil organic C found after 100 years of constant management practices remained disturbed 2 years after a change in crop rotation and crop residue management. The time required to reach a new equilibrium is a factor that may confound the results from many short-term studies.
The comparison of various long term experiment on N, P and K fertilizers, liming and manure treatments revealed that ammonium fertilizers decreased pH and CEC, causing a degradation of hydraulic properties, whereas basic amendments increased pH and CEC45,46. Aggregate stability was lowest in acid plots, intermediate in basic plots and highest in plots treated with manure. A short term study suggested that ammonium nitrate enhanced soil porosity by 18%, compared with 46% increase in a manure treatment30. Since soil respiration almost doubled in the mineral fertilizer treatment compared with the unfertilized control, the authors discussed a potential priming effect of N addition on the decomposition of soil organic matter (Table 1). A decreased amount or activity of soil organisms after mineral fertilization could be due to the toxicity of metal contaminants contained in mineral fertilizers8,36,41. In general, N and K fertilizers contain very low levels of contaminants, whereas P fertilizers often contain significant amounts of cadmium, mercury and lead8. Metal contaminants are, however, most prevalent in waste products from urban and industrial areas8,47. Long term chronic toxicity due to gradually accumulating metals appears to be far more common than immediate, acute toxicity. Quality control of fertilizer products is therefore required. This applies in particular to any new products. For example, the application of rare earth elements such as lanthanum, which is increasing in China, was shown to decrease soil respiration and dehydrogenase activity at high application rates8,48. Such observations warrant more detailed investigation into processes of accumulation, bioavailability and threshold levels of elements contained in fertilizers that can be toxic to soil health.
| Table 1: | Summery of effects of inorganic fertilizers on soil health |
| #mg kg–1 soil | |
| Table 2: | Comparative effects of inorganic and organic fertilizers on soil organisms as concluded from field experiments (FYM, farmyard manure) |
| Table 3: | Comparative effects of inorganic and organic fertilizers on soil health as concluded from field experiments (FYM, farmyard manure) |
| Table 4: | Effects of animal manures, biosolids and composts on soil health |
Soil health is also influenced by increased rate of decomposition of low quality or high C:N ratio organic inputs and SOM when fertilizers are applied to the soil. Fertilizer application leads to enhancement of microbial decomposer activity, which has been previously limited by low nutrient concentrations in the organic materials, although in a few studies added inorganic N has had either a neutral or even an inhibitory effect on the decomposition of low-N plant materials49. Long-term use of fertilizers in crop production, however, leads to soil organic matter accumulation6 and soil health improvement through addition of increasing amount of litter and root biomass to the soil. It suggests that the application of N fertilizer can have complex interactive effects on C transformations in the soil.
Some of the commonly used chemical fertilizers such as ammonium sulfate [(NH4)2SO4], ammonium nitrate (NH4NO3), urea (NH2CONH2), triple super phosphate [Ca (H2PO4)2] and potassium chloride (KCl) affects soil health (Table 3-4). Ammonium sulfate (NH4)2SO4) is one of the synthetic N fertilizer and contains 21N-0P-0K + 24% S. In the soil, it reacts with water to produce sulfuric acid (H2SO4). Sulfuric acid has a pH of less than 1. It is extremely toxic and kills organisms. Hydrogen ions released from the acid replace alkaline elements on the cation exchange sites, depleting the soil of nutrients. The free oxygen created in this reaction oxidizes the organic matter of the soil causes a low level "combustion" (burning) of the organic matter. This is a purely chemical reaction which depletes the organic matter. In calcareous soils (soil with excess calcium) the sulfuric acid reacts with calcium carbonate (CaCO3) to form gypsum (CaSO4 = Calcium sulfate)50. Gypsum is a salt and attracts water to it and away from soil organisms and plant roots. In anaerobic conditions gypsum and water form hydrogen sulfide (H2S), which is a toxic gas. Gypsum is banned from landfills. Sulfuric acid is a major component of acid rain50.
Ammonium nitrate (NH4NO3) contains 34N-0P-0K. In the soil, it breaks down into ammonium (NH4+) and nitrate (NO3‾). The ammonium is consumed by plants and fungi or by denitrifying bacteria which eventually convert it to nitrate51. The nitrates are consumed by soil organisms, leached or converted to nitrogen gas and volatilized. The free oxygen created through these processes oxidizes the organic matter of the soil and causes a low level "combustion" (burning) of the organic matter. This is a chemical reaction which depletes the organic matter. Some biological soil scientists advocate the use of small amounts of ammonium nitrate under specific circumstances even though it is prohibited for use under organic standards51,52. Urea (NH2CONH2) contains 46N-0P-0K. The urea is consumed by bacteria which convert it to (excrete) anhydrous ammonia (which is a gas) and carbon dioxide (= 2(NH3)+CO2)52. Anhydrous ammonia is highly toxic and kills soil organisms. If urea is applied to the soil surface, the gases quickly dissipate. However, in the presence of high air humidity anhydrous ammonia gas vapours form. These are heavier than air and can accumulate in low lying areas. If urea is incorporated into the soil, the ammonia gas reacts with water (H2O) to produce ammonium hydroxide (NH4OH), which has a pH of 11.653. It is highly caustic and causes severe burns. This creates a toxic zone in the immediate vicinity of the applied urea that kills seeds, seedlings and soil dwelling organisms. Within a few days further chemical reactions in the soil release the ammonium ion NH4+, which then follows the same path as naturally occurring ammonium, with any excess nitrate created in this way leached into the environment, etc.
Triple super phosphate [Ca (H2PO4)2] contains 0N-46P-0K. This is produced by treating phosphate rock (apatite) with either sulfuric acid or phosphoric acid, making it extremely acidifying52. When applied to the soil it reacts with calcium to form tri-calcium phosphate, which is water insoluble, i.e., requiring microbial action for breakdown. Even in a soil with healthy microbial activity only about 15-20% of this phosphorous is easily available to plants, considerably less in soil which does not have good microbial diversity53. The production of each ton of phosphoric acid is accompanied by the production of 4 ½ tons of calcium sulfate, also known as phosphogypsum. This is a highly radioactive product and also contains heavy metals and other impurities. Depending on the production process, radioactive substances and heavy metals can be extracted into the fertilizer. The high concentration of radioactive polonium-210 in tobacco is thought to be associated with the use of acid-extracted phosphate fertilizers54.
Potassium chloride (KCl) contains 0N-0P-60K.This product contains about 50% potassium and 50% chloride. In the soil the chloride combines with nitrates to form chlorine gas. This kills microbes. Applying 1 pound of potassium chloride to the soil is equivalent to applying 1 gallon of clorox bleach. Or in other words: 2 ppm chlorine are generally thought to be sufficient to sterilize drinking water potassium chloride application typically results in chloride levels as high as 50-200 ppm53. Potassium chloride contains very high amounts of potassium, which can result in an unbalanced phosphate:potash ratio. This ratio ideally ranges from 2:1 (most soils) to 4:1 (grasses). Excess potassium in the soil can lead to a calcium deficiency in plants, since plants absorb calcium, magnesium and potassium largely in the ratio in which they are present in the soil. In the soil excess potassium causes a loss of soil structure53. Reduced soil air levels result in reduced root respiration and the production of toxic compounds in plants. Reduced soil air and insufficient calcium each also result in the reduction of soil microbes and the corresponding reduced breakdown of organic matter/nutrient availability to plants. In drilling potassium is used to "close" the soil, because it disintegrates the clay particles ("ages" the clay) and effectively seals the soil55.
Effects of Organic fertilizers on soil health: Since most organic fertilizers are waste products; their application rate is often determined by availability rather than demand7,8. Most amendments are applied primarily to benefit plant growth. In contrast to inorganic fertilizers, however, effects on the soil’s physical, chemical and biological properties are intended as well in the following section. Besides, in the following sections, we try to establish some links between the properties of various organic inputs and their effects on soil health (Table 2, 3).
Compostable organics: Compostable and composted materials vary widely in characteristics such as dry matter content, pH, salinity, carbon content, plant nutrient concentrations, non-nutrient elements and microbial types, numbers and activity6. Onwudiwe et al.56 reported that application of municipal solid waste did not cause significant improvement in some soil physical properties such as soil aggregate stability, saturated hydraulic conductivity, bulk density and porosity. Although studies of amendments vary widely in nature of materials, application rates and experimental conditions, amendment with raw and composted organics generally results in increased microbial proliferation in the soil (Table 3). The duration of observed increases in soil organisms depends on the amount and proportions of readily decomposable carbon substrates added and the availability of nutrients, particularly nitrogen57. However, soil chemical, physical and microbial characteristics of amended soils often return to their baseline within a few years58. Sustained changes in microbial biomass, diversity and function are more likely where organic amendments are ongoing as is the case in organic and biodynamic farms59. However, an increase in microbial populations may not be seen when system productivity is limited by nutrient input or water supply60.
Manures and sewage sludge generally have higher salinity than municipal garden wastes and salts can build up in soil with repeated heavy applications61,62. Sewage sludge (biosolids) often contains heavy metals such as copper, zinc or cadmium, especially where industries contribute to the waste stream. Heavy metals can affect microbial processes more than they affect soil animals or plants growing on the same soils. For example, nitrogen-fixing rhizobia were far more sensitive to metal toxicity than their host plant clover. This resulted in N deficiency of clover due to ineffective rhizobia in sludge-amended soils. Sewage sludge and livestock manure may also contain active residues of therapeutic agents used to treat or cure diseases in humans and animals63. Green wastes from farms and gardens are typically lower in nutrient concentrations than manures or sewage sludge’s but may contain residues of synthetic compounds such as herbicides, insecticides, fungicides and plant growth regulators. Composting degrades some but not all such compounds, depending on the nature of the pesticide and the specific composting conditions.
In general terms compost will modify Soil Organic Matter (SOM) levels depending on compost quality and when/where applied. This often leads to increases in organic carbon and total nitrogen in topsoil. Equilibrium is achieved after long periods of time and this is affected by soil type, climate, by the means of exploitation and the quality/quantity of the compost. Soil pH is generally increased or stabilized this can save lime inputs in some circumstances. The Cation Exchange Capacity (CEC) of SOM is higher than that of clay minerals so raising SOM will lift overall soil CEC. In terms of the effects on physical properties compost use can lead to larger and more stable aggregates. Mature compost is better than young compost in this respect. Over the longer term (>3 years) soil density decreases-increased aeration has been seen but there has only been a small number of studies. The majority of studies showed increased water-holding capacity and infiltration though this was in the longer term studies. It was noted that many field studies did not provide a consistent picture of the details of the composts used e.g. ingredients, management of the composting process and quality parameters. If mineral fertilizers were added this was often not stated (Table 1).
Humic substances: Humus in soil has traditionally been separated into humin, humic acid and fulvic acid based on extraction with an alkaline solution and subsequent precipitation after addition of an acid8. The fractions typically rank in their resistance to microbial decomposition in the order humic acid> fulvic acid>humin70. Concentrated sources of organic material such as peat, composts and brown coal (oxidized coal, lignite, leonardite) also contain humic substances and are often marketed on the basis of their humic and fulvic acid contents as determined by similar procedures. Contents of humic acids vary, however. Some of the chemically extracted humic and fulvic acid separates are themselves sold as soil amendments. In discussion of organic amendments, a clear distinction must be made between products containing humic substances and those products that are humic (or fulvic) acids extracted from the primary sources listed above. Humic substances can stimulate microbial activity directly through provision of carbon substrate, supplementation of nutrients and enhanced nutrient uptake across cell walls. Valdrighi et al.71 reported that increasing amounts of compost or brown coal-derived humic acid stimulated aerobic bacterial growth but had only slight effects on actinomycetes and no effect on filamentous fungi.
Differences in microbial response were related to the molecular weight of the humic acids, with the lower weight fractions, typical of composts, causing greater microbial stimulation than the higher molecular weight fractions extracted from brown coal71. Application of humic substances may induce changes in metabolism, allowing organisms to proliferate on substrates which they could not previously use. Both heterotrophic and autotrophic bacteria can be stimulated by humic acid addition, mostly through the enhanced surfactant-like absorption of mineral nutrients, although heterotrophs also benefit from the direct uptake of organic compounds70,71. Nitrifiers (chemotrophs) cannot use humic acids as an alternative carbon and energy source71.
The principal indirect effects of humic substances on soil organisms are through increased plant productivity by mechanisms as listed below but excessive applications can negatively affect plant growth71,72, possibly through reduced availability of chelated nutrients73. Field studies vary widely in the applied amounts of humic substances and in outcomes8,73 found no effect of commercial humate applied at 8.2 t ha–1 on microbial activity or microbial functional groups (total fungi, actinomycetes, total Gram-negative bacteria, fluorescent pseudomonas and P. capsici) in a sandy soil used to grow bell peppers. Similarly, after 5 years of annual applications of 100 L ha–1 liquid humic acid to a horticultural soil74, found no effect on microbial biomass or enzyme activity. They ascribed the lack of effect to the low rates recommended by the manufacturer because of high product costs. Municipal solid waste compost and sewage sludge were more affordable and led to significant increases in microbial biomass in the same study73. Calculated from laboratory studies that 67.5 kg ha–1 of humic substances were needed for effective application to a sandy soil but thought beneficial effects to plants may only occur in semi-arid or arid areas when applied in combination with irrigation and mineral nutrients.
Combined use of chemical and organic fertilizers: There is increased emphasis on the impact on environmental quality due to continuous use of chemical fertilizers75. The integrated nutrient management system is an alternative and is characterized by reduced input of chemical fertilizers and combined use of chemical fertilizers with organic materials such as animal manures, crop residues, green manure and composts. Management systems that rely on organic inputs as plant nutrient sources have different dynamics of nutrient availability from those involving the use of chemical fertilizers. For sustainable crop production, integrated use of chemical and organic fertilizer has proved to be highly beneficial. Several researchers have demonstrated the beneficial effect of combined use of chemical and organic fertilizers to mitigate the deficiency of many secondary and micronutrients in fields that continuously received only N, P and K fertilizers for a few years, without any micronutrient or organic fertilizer29,31,75. A field experiment was conducted for 7 years continuously to evaluate the influence of combined applications and organic and chemical fertility buildup and nutrient uptake in a mint (Mentha arvensis) and mustard (Brassica juncea) cropping sequence76. Results indicated that integrated supply of plant nutrients through FYM (farmyard manure) and fertilizer NPK, along with Sesbania green manuring, played a significant role in sustaining soil fertility and crop productivity. Based on the evaluation of soil quality indicators, the use of organic fertilizers together with chemical fertilizers, compared to the addition of organic fertilizers alone, had a higher positive effect on microbial biomass and hence soil health77. Application of organic manure in combination with chemical fertilizer has been reported to increase absorption of N, P and K in sugarcane leaf tissue in the plant and ratoon crop, compared to chemical fertilizer alone (Table 2-5)78.
The comparison of the change of chemical and biological properties in soils receiving FYM, poultry manure and sugarcane filter cake alone or in combination with chemical fertilizers for 7 years under a cropping sequence of pearl millet and wheat results showed that all treatments except chemical fertilizer application improved the soil organic C, total N, P and K status. Increase in microbial biomass C and N was observed in soils receiving organic manures only or with the combined application of organic manures and chemical fertilizers compared to soils receiving chemical fertilizers79. This study showed that balanced fertilization using both organic and chemical fertilizers is important for maintenance of soil organic matter content and long-term soil productivity in the tropics where soil organic matter content is low. The effects of organic fertilization and combined use of chemical and organic fertilizer on crop growth and soil fertility depend on the application rates and the nature of fertilizers used. Application of 15 tons FYM ha–1 significantly increased soil organic matter and available water holding capacity but decreased the soil bulk density, creating a good soil condition for enhanced growth of the rice crop80. Positive balances of soil N and P resulted from combined application of FYM and inorganic N and P sources. Application of 15 tons ha–1 FYM and 120 kg N ha–1 resulted in 214.8 kg ha–1 N positive balance while application of 15 tons ha–1 FYM and 100 kg P2O5 ha–1 resulted in a positive balance of 69.3 kg P2O5 ha–1 available P. The compost manure and compost manure+NPK showed a greater potential for increasing plant macronutrients (N, P, K, Ca and Mg) contents81. The result showed that combined application of Municipal solid waste with NPK performed better than sole application of either Municipal solid waste or NPK fertilizer82 (Table 2, 5).
Microbial inoculants: Inoculation with natural or genetically engineered microbial formulations can be broadly categorized according to whether they are intended to (a) exist on their own in the bulk soil, (b) populate the rhizosphere, (c) form symbiotic associations with plants or (d) promote microbial activity on leaf or straw surfaces. To achieve the desired effect in the field, the inoculant organism must not only survive but establish itself and dominate in the soil or rhizosphere8. Survival depends firstly on the quality of the inoculant itself, i.e., purity, strain trueness, viable numbers, the degree of infectivity and level of contaminants89. Secondly, the establishment and proliferation of inoculant in the soil environment are determined by many edaphic and climatic factors, the presence of host organisms (for symbionts and endophytes) and, most importantly, by competitive interactions with other microorganisms and soil fauna90. Effects of inoculation on indigenous soil organisms can therefore either result from direct addition effects and interactions with indigenous soil organisms or from indirect effects via increases in plant growth by one or several of the mechanisms.
Positive effects of inoculants on the soil microbial biomass may be short-lived and increases in biomass or activity can even be due to the indigenous population feeding on the newly added microorganism89,90. The most successful and widely studied inoculants are the diazotroph bacteria (Rhizobium, Bradyrhizobium, Sinorhizobium and Frankia) used for symbiotic fixation of N2 from air. Provided soil conditions are favorable for rhizobia survival90,91 inoculation can increase microbial C and N in the rhizosphere compared with uninoculated soils91. Population changes can be limited to the season of inoculation if the newly added organism is not as well adapted to the soil conditions as the indigenous population90. Inoculant application research is increasingly focusing on co-inoculation with several strains or mixed cultures enabling combined niche exploitation, cross-feeding, complementary effects and enhancement of one organism’s colonization ability when co-inoculated with a rhizosphere-competent strain.
An example is the use of phosphorus-solubilizing bacteria to increase available phosphorus along with mycorrhizae that enhance phosphorus uptake into the plant. Saini et al.92 achieved maximum yields of sorghum and chickpea at half the recommended rates of inorganic fertilizer when a combination of mycorrhizae, N2-fixing bacteria and phosphorus-solubilizing bacteria was added. Increases in microbial biomass C, N and P in soils of inoculated treatments were strongly correlated with N and P uptake of the plants. Specific ‘helper’ bacteria may improve the receptivity of the root to the fungus to enhance mycorrhizal colonization and symbiotic development with plant roots. Similarly, legume root nodulation can be enhanced by co-inoculation with Azospirillum, which increases root production and susceptibility for rhizobium infection and may also increase secretion of flavonoids from roots that activate nodulation genes in Rhizobium. A significant reduction in indigenous actinobacterial endophytes upon inoculation of soil with a commercial multi-organism product, compared with no change in diversity after inoculation with a single species93. Trial with ‘Effective Microorganisms’ (EM), a proprietary combination of photosynthetic bacteria, lactic acid bacteria and yeasts used as a soil and compost inoculant, showed enhanced soil microbial biomass, plant growth and produce quality94. The interactions of microbial inoculants with indigenous soil organisms are likely to be complex and a better mechanistic understanding is necessary to predict short- and long-term effects.
Combined use of bio fertilizers with chemical or organic fertilizers: The activity of soil organisms is very important for ensuring sufficient nutrient supply to the plant. If the microorganisms find suitable conditions for their growth, they can be very efficient in dissolving nutrients and making them available to plants. The application of PSB, Bacillus megatherium var. phosphaticum, increased the PSB population in the rhizosphere and P availability in the soil. It also enhanced sugarcane growth, yield and quality95. When used in conjunction with P fertilizers, PSB reduced the required P dosage by 25%. In addition, 50% of costly superphosphate could be replaced by a cheap rock phosphate, when applied in combination with PSB. The effects of a combined treatment of multifunctional biofertilizer (mixture of Bacillus sp. B. subtilis, B. erythropolis, B. pumilus and P. rubiacearum) plus 50% chemical fertilizer (½ CF+biofertilizer) on a treatment of with chemical fertilizer (CF) and biofertilizer on the growth of lettuce were compared by Young et al.96. Results showed there was a 25% increase of lettuce yield for the treatment of ½ CF+biofertilizer compared to that of the CF treatment, indicating that at least 50% of chemical fertilizer can be saved as multifunctional biofertilizer was used along with chemical fertilizer. The effects of multifunctional biofertilizer (mixture of Bacillus sp. B. subtilis, B. erythropolis, B. pumilus and P. rubiacearum) on rhizosphere microbial activity and the growth of water celery in a field experiment96. Results showed that the dry weight of water celery in the treatment with 50% organic compound fertilizer with multifunctional biofertilizer (MC+½ OCF) was increased by 34% compared to the treatment with 100% Organic Compound Fertilizer (OCF) added. In addition, the beneficial bacterial counts including mineral PSB, cellulolytic bacteria and N2-fixing bacteria in the rhizosphere of water celery in the MC +½ (OCF) treatments were increased 102-104 CFU/gin (Table 4-5).
Integrated use of chemical, organic and bio fertilizers: Increased attention is now being paid to developing an Integrated Plant Nutrition System (IPNS) that maintains or enhances soil productivity through balanced use of all sources of nutrients, including chemical fertilizers, organic fertilizers and bio fertilizers. The basic concept underlying the IPNS is the adjustment of soil fertility and plant nutrient supply to an optimum level for sustaining desired crop productivity through optimization of the benefits from all possible sources of plant nutrients in an integrated manner75. The experiment conducted in a field to evaluate the effects of Chemical Fertilization (CF), organic fertilization (compost-n and compost-p), combined use of chemical and organic fertilizer (compost-p + urea) and integrated use of chemical and organic fertilizer along with biofertilizer [½ (compost-p+urea) +biofertilizer] on the growth of cabbage and maize (residual effect) and soil fertility. The contents of mineral N (NH4+ +NO3‾) and Bray-1 P in all treatments, except CK, were higher than those of the CF treatment, showing the benefits of biofertilizer and compost by supplying and enhancing the release of N and P. Excess inputs and excess accumulation of P in soil usually increase their potential to contribute soluble and particulate P to surface waters and result in P-driven eutrophication. The content of Bray-1 P in the test soil was 69 mg kg–1, rated as "high" in terms of P availability. There are many suggestions in the literature that significant rising of Bray-1 P content is not good for a soil with a high level of P availability, from the economic and environmental standpoint. The Bray-1 P content of the soil was approximately twice as high in the Compost-N treatment compared to the CF treatment, showing significant P accumulation in the soil. This indicated that reducing half the amount of compost and urea combined with inoculants of mixed strains of beneficial microorganisms has considerable potential to lessen P accumulation in the soil and saves the input of chemical and organic fertilizers75.
PESTICIDES
The results from literature on the effects of selected pesticides on soil health are shown in Table 4 (herbicides), Table 5 (insecticides and nematicides) and Table 6 (fungicides). There is clearly a paucity of data (particularly on physical and chemical property of the soil) in international literature on the effects of a large number of pesticides on soil health. Most of this review has focused on soil microorganisms. Pesticides that reach the soil can alter the soil microbial diversity and microbial biomass. Any alteration in the activities of soil microorganisms due to applied pesticides eventually leads to the disturbance in soil ecosystem and loss of soil fertility120. Numerous studies have been undertaken which highlight these adverse impacts of pesticides on soil microorganisms and soil respiration77,121. In addition to this, exogenous applications of pesticides could also influence the function of beneficial root-colonizing microbes such as bacteria and arbuscular mycorrhiza, fungi and algae in soil by influencing their growth, colonization and metabolic activities122. The pesticides that reach the soil can interact with soil microorganisms in several ways:
| • | It can adversely affect the growth, microbial diversity or microbial biomass of the soil microflora. For example, sulfonylurea herbicides (metsulfuron methyl, chlorsulfuron and thifensulfuron methyl) were reported to reduce the growth of the fluorescent bacteria Pseudomonas strains that were isolated from an agricultural soil123. The Pseudomonas spp. is known to play an important ecological role in the soil habitat and hence its reduction can adversely affect soil fertility |
| • | Pesticide application may also inhibit or kill certain group of microorganisms and outnumber other groups by releasing them from the competition124. For example, increase in bacterial biomass by 76% was reported in response to endosulfan application and that reduced the fungal biomass by 47%125 |
| • | Applied pesticide may also act as a source of energy to some of the microbial group which may lead to increase in their growth and disturbances in the soil ecosystem. For example, bacterial isolates collected from wastewater irrigated agricultural soil showed the capability to utilize chlorpyrifos as a carbon source for their growth126 |
| • | Pesticides can alter and/or reduce the functional structure and functional diversity of microorganisms but increase the microbial biomass127. In contrast, application of pesticides can also reduce the microbial biomass while increasing the functional diversity of microbial community. For example, methamidophos and urea decreased the microbial biomass and increased the functional diversity of soil as determined by microbial biomass and community level physiological profiles128 |
| Table 5: | Effect of animal manures, biosolids and composts on soil organisms |
| Table 6: | Impact of herbicides on non-target soil organisms |
Herbicides: The herbicides (Table 6) generally had no major effects on soil health, with the exception of butachlor, which was shown to be very toxic to earthworms at agricultural rates. The authors showed, however, that butachlor had little effect on acetylcholinesterase activity. Phenmedipham induced avoidance behavior in earthworms and collembola. These effects are expected to be relatively short lived as phenmedipham is broken down moderately rapidly (25-day half-life) in soil. Other effects of herbicides on soil organisms were mainly isolated changes in enzyme activities. Glyphosate, for example, was shown to suppress the phosphatase activity by up to 98% in a laboratory study; however, urease activity was stimulated by glyphosate as well as atrazine.
Insecticides: Insecticides (Table 7) were generally shown to have a greater direct effect on soil health than herbicides8. Organophosphate insecticides (chlorpyrifos, quinalphos, dimethoate, diazinon and malathion ) had a range of effects including changes in bacterial and fungal numbers in soil, varied effects on soil enzymes as well as reductions in collembolan density and earthworm reproduction. Carbamate insecticides (carbaryl, carbofuran and methiocarb) had a range of effects on soil organisms, including a significant reduction of acetylcholinesterase activity in earthworms, mixed effects on soil enzymes and inhibition of nitrogenase in Azospirillum species.
Persistent compounds including arsenic and lindane caused long-term effects, including reduced microbial activity129, reduced microbial biomass and significant decreases in soil enzyme activities. Azoxystrobin, have recently been shown to effect on a biocontrol agent used for the control of Fusarium wilt130, illustrating potential incompatibilities of chemical and biological pesticides. The insecticides methyl parathion and especially pentachlorophenol have been shown to interfere with legume-rhizobium chemical signaling. Reduction of these symbiotic chemical signaling results in reduced nitrogen fixation and thus reduced crop yields131. Root nodule formation in these plants saves the world economy $10 billion in synthetic nitrogen fertilizer every year132. When the natural nutrient cycling in the ecosystem is interfered in any way by pesticides or other sources of pollution, it will lead to decline in soil fertility and soil productivity. Long term experiment on insecticides usage showed that soil physical characteristics such as bulk density were changed in long-term and it was increased compared to control soil133. The heavy metals accumulations in soil were highly affected and the concentration of some metals such as cadmium has reached a limit beyond the standard for agricultural purposes. The results also showed that fortunately the concentration of other metals is not beyond the standard36.
Soil fumigants: Soil fumigants are designed to eliminate harmful soil organisms and any competition for soil resources between soil organisms and the crop8. In spite of this, soil fumigants have not always been found to have significant effects on soil health (Table 8). Soil fumigants had long-term effects on various soil functions141. The long-term effects of fumigants were shown to be reduced by the addition of composted steer manure, with normal biological activity being observed 8-12 weeks following high application rates of the fumigant142. In the absence of the organic amendment, little recuperation (resilience) of soil function was detected even after 12 weeks.
Pesticide formulation: The formulation is the chemical and physical form in which the pesticide is sold for use. The active ingredient (a.i.) is the chemical in the formulation that has the specific effect on the target organism. In addition to the active ingredient, the formulation of a pesticide may also influence soil health. This is, however, an aspect that is rarely investigated. Little is known about the environmental fate of adjuvants after application on agricultural land. Adjuvants constitute a broad range of substances, of which solvents and surfactants are the major types8. Non-ionic surfactants such as Alcohol Ethoxylates (AEOs) and alkylamine ethoxylates (ANEOs) are typical examples of pesticide adjuvants8. The surfactant in the Roundup formulation polyoxyethylene amine (POEA) was significantly more toxic to Microtox bacterium than glyphosate acid or the IPA salt of glyphosate. Even Roundup was found to be less toxic143. The toxicity of glyphosate acid was concluded to be a result of its inherent acidity.
| Table 7: | Impact of insecticides and nematicides on soil health |
| Table 8: | Impact of fungicides on soil health |
In another study, demonstrated that the presence of ethylamine in a glyphosate formulation had major effects on Bradyrhizobium144, 145, whereas the active ingredient (glyphosate) had little if any effect. In formulation, effects included reduced nodulation in a soybean crop8.
CONCLUSION
Organic amendments such as manure, compost, biosolids and humic substances provide a direct source of C for soil organisms as well as an indirect C source via increased plant growth and plant residue returns. Herbicides showed less significant effects on soil health, whereas negative effects of insecticides and fungicides were more common. The sound management of crop production inputs must attempt to ensure both an enhanced and safeguarded environment.