Changes in Soil Organic Matter and Soil Aggregation of a Zambian Oxisol after Applying Lime
J. Manjoka ,
O.A. Yerokun ,
In this study we investigated factors that contribute to poor crop response to lime. Soil samples were collected from three depth increments (0-15, 15-30 and 30-45 cm) in trial plots with the following treatments: lime only, urea only, urea + lime, no lime no urea-cultivated and fallow no lime (control). The samples were analyzed for Soil Organic Matter (SOM), exchangeable cations (Ca, Mg and K) and soil aggregate stability. The highest SOM content (36.8 g kg-1) was measured in the fallow plot and the least in the plot fertilized with urea. While exchangeable Ca increased in limed plots the level of exchangeable K decreased. The low SOM in fertilized plots was corroborated by the measured decrease in soil aggregate stability. Lime had a positive effect on soil aggregate size, but the best effect was obtained from the fallow plot. This study has shown that the effect of lime on acid soils is variable and probably soil specific. Therefore, in practice, soil amendment with lime has to be considered on a case by case basis.
Soil acidity poses a serious challenge to the crop production capacity of many soils in the tropics and subtropics (Giller, 2001). Experience in Zambia has shown that crop yields can be drastically depressed due to this phenomenon (Lungu and Chinene, 1993). In order to reduce the problems caused by soil acidity, there is convincing evidence that ground agricultural limestone (lime) is effective hence it is used widely and successfully (Yerokun, 2006; Bolan et al., 2003). Lime works by neutralizing acidity and complexing exchangeable aluminum which can be toxic to and limit the functioning and growth of plant roots (Ritchie, 1989). It raises soil pH to levels that are desirable for crop production such that the availability of most of the elements that are essential to plants is improved. In addition, lime promotes an optimum population of useful soil microorganisms that are essential to decompose soil organic matter (Haynes and Naidu, 1988). Lime has also been shown to be effective in attenuating heavy metals in contaminated soils (Adriano, 2001) and reducing their uptake by plants.
While agricultural lime is widely recognized for its many positive attributes when applied to soil, there are also some negative effects that may arise from the application of lime to soil (Bolan et al., 2003). For instance, because lime promotes soil organic matter decomposition, the increased mineralization of nitrogen, phosphorus and sulphur may lead to a higher potential for the loss of these elements through leaching and erosion (Borah et al., 2000; Neale et al., 1997; Marchner and Wilczynski, 1991). There is conflicting information about the effect of lime on soil structure. Chan and Heenan (1999) observed that lime increased the aggregate stability of soil while Roth (1991) and Roth and Pavan (1991) reported that lime negatively affected soil structural stability. In their study, Chan and Heenan (1999) also observed that lime induced soil organic carbon leaching. When considered therefore, while application of lime may improve soil structural stability from Ca-bridging, leaching losses of nutrients may also be enhanced.
It has been suggested that several of the negative effects to soil as well as negative plant response associated with the use of lime are more likely to occur on tropical and subtropical soils (McLean, 1971). This is probably because when compared with temperate soils, they have a higher rate of leaching and weather faster from high soil temperatures and precipitation (Wilcke et al., 1999). Therefore, application of high rates of lime in the tropics, based upon information derived from temperate regions should be done with caution as it can be detrimental (McLean, 1971). Due consideration must be given to the differences in soils and the processes they undergo.
In recent times, there has been resurgence in the promotion and use of lime especially in the small-holder farm sector in Zambia. Aside from the government extension system, non-governmental organizations have been instrumental in setting up field demonstration plots, providing training, as well as input and logistics to support use of lime. These activities are based upon the positive results from earlier research work on the use of lime in Zambia. However, most of these studies which have been reviewed by Goma and Singh (1993) focused upon determining the response of various crops, particularly maize, groundnuts, soybeans and wheat to lime. There was little attention paid to the effects of lime on soil properties other than its effectiveness in reducing exchangeable aluminum concentrations and acidity. Given the potential for negative consequences, it is necessary to gain better understanding about the changes that occur in soils when lime is applied. The objective of this study was to determine the distribution and redistribution of soil organic matter as well as basic cations following cultivation of land and the application of lime and urea fertilizer. We hypothesized that lime would not effect downward leaching of organic matter and cations in this soil because of the constant nature of soil charge.
MATERIALS AND METHODS
The study was carried out at the Golden Valley Agricultural Research Trust
Station in Chisamba, about 65 km north of Lusaka, Zambia. The soil is classified
as a Mutwale series, fine mixed Isohyperthermic Humic Eustrostox (Soil Survey
Staff, 1999). It is a red sandy clay with 43% sand, 16% silt and 41% clay. In
March 2003, soil samples were collected from a field study where phosphorus
trial was initiated in November 2000. The original experiment was an evaluation
of the agronomic effectiveness of Partially Acidulated Phosphate Rock (PAPR)
and it is fully described by Lungu and Munyinda (2002). Briefly, 12 treatments
that included applications of 60 or 120 kg PAPR ha-1, 60 or 120 kg
MAP ha-1, 0 or 1.5 tons lime ha-1 and 60 or 120 kg N ha-1
(supplied as 130 or 260 kg urea: 46% N) were arranged in a randomized
complete block design with three replications. The lime was broadcast on the
soil surface one week before the first planting and urea was always applied
as a top-dressing to the soil surface. The following treatments were selected
for the current study: (I) Fallow (FL), (ii) No lime no urea (NT), (iii) 260
kg Urea ha-1 (UR), (iv) 1.5 ton Lime ha-1 (LM) and (v)
1.5 ton Lime + 260 kg Urea ha-1 (LU).
Soil Chemical Analysis
Soil samples were collected from the 0-15, 15-30 and 30-45 cm depths of
each of the three replications of the selected treatments. The samples were
air-dried and crushed to pass through a 2 mm sieve. Soil
was < 4.6 for plots that had not received lime and > 5.9 for the limed
plots. Soil organic matter was determined by the standard Walkley-Black potassium
dichromate oxidation method (Nelson and Sommers, 1982). Exchangeable calcium,
magnesium and potassium were extracted in neutral 1 N NH4OAc and
their concentrations were determined on an atomic absorption spectrophotometer
(Lanyon and Heald, 1982). All samples were analyzed in duplicates.
Soil Physical Analysis
Undisturbed clods of soil were collected from the 0-20 cm layer and carefully
packed to minimize the disturbance to soil aggregates. In order to dry sieve,
500 g of soil aggregates > 9.5 mm in diameter were weighed and placed on
a nest of sieves with 4.75, 1.00, 0.50 and 0.30 mm openings. The nest was shaken
for five minutes and the soil retained on each sieve was weighed and expressed
as a percentage of the total soil weight. This procedure was repeated for wet
sieving, except that the soil was moist. After shaking, the soil retained on
each sieve was dried at 105°C, weighed and expressed as a percentage of
total soil weight. The Mean Weight Diameter for the soil was then calculated
using the equation:
Analysis of variance to determine the effect of treatment and soil depth
on soil organic carbon, extractable base cations and mean weight diameter of
soils was carried out on the data using MSTAT computer program (MSTATC, 1993).
Treatment means were separated using Duncans Multiple Range Test at the
five percent level of significance.
RESULTS AND DISCUSSION
The Soil Organic Matter (SOM) contents detected in the plots were significantly
different (p<0.05) among the five treatments, the three soil depths and their
interaction (Table 1). The fallow plot (FL) had the highest
mean SOM content of 36.8 g kg-1 (Table 2). This
was probably as a result of accumulation of litter from continuous residue addition
and an absence of soil disturbance for an extended period. Without activities
that invert the soil and expose SOM in this plot, it is very likely that there
would be an appreciable degree of physical and chemical protection of SOM by
clays. When lime or urea was applied to the soil, SOM content was observed to
decrease (Table 2).
||Analysis of variance of the effect of applying lime and urea
on soil organic matter (SOM), exchangeable potassium, calcium and magnesium
concentrations and mean weight diameter of a Mutwale series soil
||Bulk soil average soil organic matter content, cations concentrations
and mean weight diameters following cultivation and application of lime
and urea to a Mutwale series soil
|SOM = Soil organic matter, MWD
= Mean Weight Diameter, §FL = Fallow, NT = No lime no urea,
UR = Urea only, LM = Lime only, LU = Lime plus, urea, ¶All
means with the same letter within a column are not significantly different
at the 5% probability level using the Duncan Multiple Range Test
Consequently, SOM in the lime only (LM) plot was 17% less than in the FL plot
while that in the urea only (UR) plot was 21% less than what was found in the
FL plot. The decline in SOM was more when urea alone was applied than when lime
alone was applied. However, it is clear that the effect of combining lime and
urea in a plot was not additive since SOM content in the lime plus urea (LU)
plot was only 23% less than in the FL plot.
Soil organic matter content in the untreated-no lime no urea plot (NT) which had only been subjected to physical disturbance from cultivation was 32.3 g kg-1 (Table 2). This value exceeded SOM content in the LM plot by 5%, the UR plot by 11% and the LU plot by 14%. Conversely, SOM content in the NT plot was 12% less than in the FL plot. It would, therefore, appear that physical disturbance of the soil by cultivation induced a slower rate of SOM decomposition than found in the plots that received chemical amendments. The reductions in SOM that were observed with the chemical treatments may be explained by the accompanying rise in soil pH and increase in soil microbial activity which occurs following liming (Haynes and Swift, 1988). This will cause SOM to decompose at a faster rate. Reports of loss of SOM content following application of lime has also been reported by Chan and Heenan (1999) in New Zealand and Baldock et al. (1994) in Australia. Variable charge or more weathered soils appear to be more susceptible to this phenomenon. Additions of nitrogen to the soil, such as in urea result in a narrowing of the soil C:N ratio which similarly increases microbial population and activity, creating a more conducive environment for faster decomposition of SOM (Giller, 2001).
As the depth of soil sampling increased, a significant (p<0.05) decrease
in SOM contents was observed for almost all the treatments (Table
1 and 3). Generally, this would be expected since biomass
that is returned to the soil as residue is basically mixed into the surface
soil. Present observation is in contrast to that by Chan and Heenan (1999) where
the lime induced soil organic carbon leaching in a variable charge soil. Temminghoff
(1998) suggested that while the rise in pH from liming may enhance solubilization
of SOM, the Ca in lime is also effective in reducing the amount of negative
charges on Dissolved Organic Carbon (DOC). This will lead to coagulation of
DOC, which conceivably may account for the absence of noticeable leaching of
SOM following liming in our study.
The soil cation concentrations were generally low. In the case of potassium
(K), McLean (1971) suggested that tropical soils have a low K release tendency.
Bulk soil exchangeable K concentrations were more than 6% higher in the LM and
UR plots than in the other treatment plots (Table 2). It is
likely that lime-derived calcium (Ca) entered into competition with K for cation
exchange sites, causing K to be displaced into the soil solution more easily.
Galindo and Bingham (1977) demonstrated this replacement effect in a batch experiment
where they observed a decrease in K sorption by soil hence more K displaced
into soil solution following addition of increasing Ca concentration to the
soil. In our case, lime would cause this increase in soil solution Ca concentration.
The increase in extractable K observed in the UR plot was also probably as a
result of urea derived NH4+ ions entering into competition
with K. Whereas K and NH4+ ions have similar non-hydrated
and hydrated ion diameters, a higher concentration of NH4+
ions would displace K from the clay into solution (Tan, 1998). When the soil
depths are considered, K concentrations increased with depth in the plots that
received lime, LM and LU (Table 3). The reverse was true for
the plots that did not receive lime application. Potassium concentration either
decreased with depth or remained stable. This observation for the limed plots
strengthens earlier suggestion that K was being displaced from exchange sites
by the lime derived Ca and probably also leached downwards in the soil profile.
The fact that K concentrations decreased with depth in the FL plot (Table
3) which was only subject to growth of plants on it, suggests that removal
of K by plant roots from lower soil profile may play a considerable role in
K distribution and re-distribution in this undisturbed soil.
||Soil organic matter, potassium, calcium and magnesium distribution
in the soil profile following application of lime and urea to a Mutwale
|SOC = Soil organic matter, FL
= Fallow, NT = No lime no urea, UR = Urea only, LM = Lime only, LU = Lime
A uniform K concentration in the NT plot may have been the result of mixing
of layers from cultivation, without chemical amendments that compete with K.
Calcium and Magnesium
Calcium concentration in bulk soil was only affected by the application
of lime and this resulted in a 25% higher exchangeable Ca concentration in the
LM plot than the other treatment plots (Table 2). Generally,
Ca concentrations in the 15-30 cm layer of treatments were lower than in the
0-15 cm layer and then increased again in the 30-45 cm layer (Table
2). This may suggest some degree of natural leaching of Ca, probably with
water flux. Whereas magnesium (Mg) was supplied in the dolomite lime, the pattern
of influence of treatment on Mg concentrations was not obvious (Table
2 and 3). When lime is added to soils, the soil solution
concentrations of Ca and Mg change from the input of these ions. However, a
concurrent change in pH of the soil also has effect on the extent and nature
of the cation exchange complex. All these contribute to the possibility of shifting
the equilibrium between solution and exchangeable cations (Bolan et al.,
2003). Additionally, the interactions which take place between organic matter
and Ca and Mg can be rather complex and situation specific and can confound
explanation of some of the observations made in this study.
Mean Weight Diameter
Soil mean weight diameter ranged from 1.6 to 2.6 mm (dry) and 1.4 to 2.5
mm (wet) and the patterns observed for dry and wet sieved MWD of soil aggregates
were similar (Table 2). The highest values were observed in
the fallow plot. As seen from the lower MWD values for the NT treatment (1.9
and 1.8 mm) without addition of chemical amendments, cultivation alone was adequate
to induce aggregate disintegration. Soil aggregate sizes in the plots that received
lime, LM and urea, UR were significantly (p<0.05) lower than in the FL plot.
However, the positive effect of lime on soil structure is seen in the 21% (dry)
and 17% (wet) larger aggregate sizes in the LM plot than in the NT plot. It
would appear that Ca contributed by lime acted as a bridge to bind the soil
particles together. Chan and Heenan (1998) observed that soil aggregate formation
and saturated hydraulic conductivity were improved by Ca in the liming material.
This effect was achieved by the influence of Ca on dispersion and flocculation
of the soil. Calcium reduces the thickness of the diffuse-double layer, causing
soil particles to become attracted to one another with increase in flocculation
Additionally, when lime promotes an increase in microbial biomass and enzyme activity (Haynes and Swift, 1988), this leads to production of polysaccharides that can act as binding agents with positive influence on soil aggregation.
The plot treated with urea, UR consistently had the lowest MWD of 1.6 (dry) and 1.4 mm (wet) (Table 2). As was observed with SOM levels, it is likely that the N supplied to soil in urea contributed to a lower C:N ratio, increased microbial population and activity and accelerated the loss of SOM and organic binding agents. Wagner et al. (2007) demonstrated that incorporating barley straw to build up soil organic matter in Australian soil enhanced soil aggregation. Since both lime and urea increase microbial population, the better MWD with lime suggests a stronger contribution of Ca to aggregate stability in this instance. The process of urea transformation leads to the release of protons into and acidification of the soil. This can cause acid decomposition of aggregates hence poorer soil structure. The similarities in the general pattern of MWD between dry and wet sieved soil suggests that the Ca or urea were the principal agents that influenced differences in soil aggregation among the treatments.
This study has shown that while lime may raise the pH of soil and neutralize exchangeable aluminum, it may also have some negative effects on soil properties. Lime did not induce leaching of soil organic matter but accelerated the rate of decomposition. This may be good for release of nutrients into the soil but detrimental in acid soils where farmers are encouraged to raise soil organic matter levels. Lime displaced K from the soil exchange complex and this led to its leaching into lower horizons. Calcium was apparently effective in the bridging of soil aggregates and this role could be important in the structural stability of many soils. These conflicting results from application of lime may in part explain why high lime rates do not work in many tropical region soils.
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