Impact of Missing Elements on Nutrient Use Efficiency of Sweet Corn (Zea mays L. Saccharum) on Five Tropical Soils

INTRODUCTION

Sustainable farming is often limited on tropical soils by inherently low fertility. The situation is further worsened by continuous cropping, coupled with poor soil management practices (Agboola and Unamma, 1991). In most developing countries, including Nigeria, food production has had serious setbacks due to general shortage and unaffordable cost of chemical fertilizers. Most farmers do not adopt fertilizer recommendations. Instead, they indulge in its abuse (Chude, 1999). The unsatisfactory use efficiencies of essential nutrient elements by crops have also resulted in increased environmental problems. The ability to improve the effectiveness of applied nutrients (and hence reduce pollution) depends on matching supply of the nutrients with plant demand and maintaining nutrient availability (Shaviv et al., 1985). Yet, the response of crops to fertilizers is often site specific (Balasubramanian et al., 1978; Mustapha et al., 1997), varying as soils’ native fertility statuses vary.

Sweet corn (Zea mays L. Saccharum) has become a popular and an economically important vegetable in the world. Reversing soil fertility depletion for sustainable production of the crop in sub-Saharan Africa demands improved nutrient use efficiency (NUE) in the face of fertilizer procurement difficulties (price, accessibility and availability) in the region (Lafitte, 1998). In the determination of NUE, native soil nutrient is as important as the applied one. Akinrinde et al. (2000) observed marked differences between the use efficiency of P by cowpea plants fertilized with organic and inorganic fertilizers. Akintoye et al. (1999) also noted that NUE of single, double and synthetic maize lines grown in three ecological zones of West Africa decreased significantly with increasing N level.

This study investigated the efficiencies of use of inorganic N, P, K, Ca and Mg fertilizers by sweet corn (variety Shrunken-2) in five soils located within Nigeria. This was with a view to ensure optimal utilization of fertilizers, thereby saving crop production costs and reducing environmental pollution.

MATERIALS AND METHODS

The reserach involved a soil culture study conducted (between April and May 2003) in the greenhouse of the Department of Agronomy, University of Ibadan, Nigeria.

Table 1:	Locations, ecological zones, parent materials, classification and (B) physico-chemical properties of the soils used

A set of 21 plastic pots (17 cm diameter, 29 cm height) were filled with 4.0 kg surface soil samples collected from each of five locations-Enugu, Rockefellar, Alabata, Barthroad and NIFOR (Benin-city), giving a total of 105 pots for all the soils. The pots were perforated at the bottom to facilitate drainage. They were arranged on a raised platform but in a Completely Randomised Design (CRD) in the greenhouse. Ten sweet corn (Var. shrunken-2) seeds were sown in each pot and later thinned to 4 plants one Week after Sowing (WAS).

The experimental treatment involved seven N, P, K, Ca and Mg fertilizer treatment combinations (control, Complete/adequate nutrient supply, minus N, -P, -K, -Ca and -Mg) repeated three times in each of the five location soils, giving a total of 7x5x3 (= 105) experimental units. Application rates were 120 kg N (Urea, 45%N), 45 kg P (single super phosphate, SSP), 60 kg K₂O (muriate of potash, KCl, 60% K₂O), 20 kg Ca (Calcium carbonate, CaCO₃, 40% Ca) and 20 kg Mg (Magnesium sulphate, MgSO₄, 20% Mg). The fertilizer materials were mixed with the soils according to treatment before sowing and watering was maintained at 60% Field Capacity (FC). Heights of plants were measured at weekly intervals for six weeks. Fresh and dry forage (roots and shoot) yields, macronutrient (N, P, K, Ca and Mg) as well as micronutrient (Mn, Zn and Fe) contents and uptake values were also determined after 6 weeks of growth.

The pre-cropping soil samples (taken 0-15 cm depth) were analysed for the parameters given in Table 1 by the standard procedures described by Page et al. (1982). Nutrient Use Ffficiency (NUE) was estimated as yield produced (Y_g) per unit of Nutrient supplied N_f) and expressed as Y_g/N_f (Moll et al., 1982).

All statistical analyses were performed using the Statistical Analysis System (SAS, 1985).

RESULTS AND DISCUSSION

Experimental soils: Details of the locations, ecological zones, parent materials, physico-chemical properties and classification of the surface soils used are presented in Table 1. The quality and magnitude of the various parameters varied widely among the soils although they are all located in the rainforest ecological zone. Soils from three of the locations (Rockefellar, Alabata and Barthroad) are Alfisols derived from basement complex rocks while the Enugu and NIFOR location soils are Ultisols and Oxisols formed on Imo shale and Coastal plain sands, respectively.

Soils from Enugu and Barthroad were the poorest in fertility. They had very low Organic Matter (OM) contents, total N and available-P. Only the NIFOR (Agbaro) location soil had 48.5 g OM kg^-1 soil that is higher than the 30 g kg^-1 which Agboola (1973) had suggested as the level at which response to N fertilization is not expected. The soil also had 2.20 g N kg^-1, which is higher than 1.5 g kg^-1 believed to be optimal for maize production (Singh and Uriyo, 1980).

Table 2:	Influence of fertilizer treatment and soil type on height and biomass yield of maize

On the other hand, only Alabata and Barthroad location soils had available P values (5.6 and 4.1 mg kg^-1 soil, respectively) that are lower than the critical level of 10 mg kg^-1 (Adeoye and Agboola, 1985). Considering the 0.16-0.20 cmol kg^-1 critical range for K (Ayodele, 1984), Enugu and Rockefellar location soils were sufficient in the element while Alabata, Barthroad and NIFOR location soils were marginal. Based on 2.0 cmol kg^-1 critical Ca level (Akinrinde and Obigbesan, 2000), only Alabata location soil should respond to Ca fertilizer application. As regards Mg, Barthroad, Agbaro and Enugu had less than 0.40 cmol Mg kg^-1 soil given as the critical level (Lombin, 1974). It could be expected from these analyses that plants growing on these soils would perform and respond to fertilizer application differently. The magnitude of crop response to fertilizer input is abundantly dependent on soil native (inherent) fertility level (Singh and Uriyo, 1980; Akinrinde and Obigbesan, 2000). As such, for optimum crop utilization of soil and fertilizer nutrients on the soils, recommendation of fertilizer use should be specific to each soil type and location.

Height growth and yield of sweet corn: A summary of height growth and yield of sweet corn both across the various soil types and fertiliser treatments is given in Table 2. At the end of first week, Enugu location soil supported crop growth to an average height of 22.2 cm. This was significantly (p<0.05) higher than the means of 19.9 and 16.7 cm obtained in respect of Alabata and Barthroad location soils. During this initial growth period, plants grown in Rockefellar or NIFOR location soils had least heights (15.4 and 15.0 cm, respectively). Between the 2nd and the 6th weeks, however, the NIFOR location soil consistently produced the tallest plants (35.4-55.1 cm) compared with those produced on Barthroad (31.9-50.0 cm), Rockefellar (31.0-50.5 cm) and Enugu (33.3-50.0 cm) location soils. Alabata location soil gave the shortest plants (29.4-48.0 cm) at the successive growth periods. The summarised fresh and dry yield data in respect of roots; shoots and total biomass (Table 2) also confirm this order of performance among the soils. Thus, at the end of 6 weeks, NIFOR location soil (an Oxisol) gave the highest yield of 41.0 g pot^-1 of fresh roots (5.1 g dry matter, DM) and 25.9 g pot^-1 of fresh shoot (6.6 g DM) compared with the least (4.5 g dry shoot pot^-1) on Alabata location alfisol. It is worthy to note that variations in growth and yield of the crop are closely associated with differences in fertility status of soil types and locations as given in Table 1.

Table 2 also shows that sweet corn plants significantly responded to the missing fertiliser nutrient application schedule (across the soils), particularly after 2 weeks of growth when native fertility started to be insufficient for crop growth. The untreated (control) plants were consistently the shortest (16.1-36.2 cm), indicating that the growth medium (soil) is deficient in one or more nutrient elements and the plants are in dire need of such nutrients for their growth. Minus Ca treated plants were the most vigorous in growth (17.7-60.1 cm tall), implying the irrelevance of including the element in crop fertilization schedule on majority of the soils. Continuous application of nutrient elements that occur in sufficient amounts in soil might lead to nutrient imbalance after some time (Adeoye and Agboola, 1985) and antagonises the absorption of other essential nutrients by the crop (Mengel and Kirkby, 1987).

Table 3:	Influence of fertilizer treatment and soil type on content and uptake of nutrients by maize

Minus N treated plants performed less (39.7 cm in height) than minus P treated ones (52.6 cm), suggesting that N is the “key” nutrient element for sweet corn production in the experimental soils. This is in consonance with the general reports on tropical soils (Sanchez, 1976; Sanchez and Uehara, 1980; Singh and Uriyo, 1980). Minus Mg and K¯ treated plants were as tall or even taller (56.5 cm and 55.0 cm, respectively) than those that had complete nutrient fertilisation (54.0 cm). However, in most cases, complete nutrient supply and -Ca treatment gave the highest total biomass (13.9 and 13.8 g pot^-1, respectively) while the control again produced the least (4.9 g g^-1) followed by -N (5.3 g g^-1) and -P (10.7 g g^-1). These results further confirm that the differences in crop performance are in consonance with the initial variation in the fertility levels of the experimental soils.

Nutrient content and uptake by sweet-corn: Nutrient concentration in sweet-corn plant tissue also varied exceedingly under the influence of the various soils and the fertiliser treatments (Table 3). The alfisol from Rockefellar location enhanced the greatest accumulation of P, K, Fe, N and hence crude protein while the alfisol from Alabata enhanced the accumulation of P, Ca and Mn and the oxisol from NIFOR permitted plants’ accumulation of P and Mn. Removal of N from the fertilisation schedule caused plants to accumulate the greatest amounts of P, (0.27%) and K (2.74%) while the non application of P enabled plants to accumulate the greatest amounts of N (0.70%). The control plants had the highest concentrations of Ca (1.02%), Mg (0.48%) and Mn (329 mg kg^-1). It is worthy to note that complete (balanced) nutrient supply did not permit exceptional accumulation of any of the nutrients. According to Janssen (1998) balanced nutrition is the best guarantee for the simultaneous optimum use of all nutrients.

The influence of soil type and fertiliser treatment on nutrient uptake by sweet corn plants were of quite similar trends to those obtained in respect of the forage yield and almost like the exact opposite of the response of the crop in terms of the nutrient content. Thus, the NIFOR location soil produced plants with highest uptake of N, P, Ca, Mg, Mn and Zn while Alabata location soil produced the least. Table 3 also shows clearly that -Ca fertiliser treatment favoured the uptake of N, P, K, Ca, Mg and Fe while the control led to the least uptake values.

Nutrient use efficiency by sweet corn: The effects of soil type and fertiliser treatments on N-, P-, K-, Ca-and Mg-use efficiency by sweet-corn are indicated in Tables 4 and 5. It is evident from these data that N is the least efficiently used nutrient by the crop. The ranges are 2.43 (in NIFOR soil) to 7.0 g g^-1 (in Barthroad location soil) across fertiliser treatments and 2.4 g g^-1 (with -N) to 6.8 g g^-1 (with complete nutrient application) across soil types.

Table 4:	Influence of fertilizer treatment and soil type on nutrient use efficiency by maize

These values are low compared with a mean of about 30.0 g g^-1 obtained for single, double and synthetic lines by Akintoye et al. (1999). The disparity is, however, most probably due to differences in soil nutrient status and the restriction of root growth in plastic pots used in this work unlike the situation on the field.

An intensive study of the data summarised in Tables 4 and 5, however, made the categorisation of the various nutrients’ use efficiency values (into low, medium and high levels) possible. The corresponding soil location and fertiliser treatment were also considered and indicated in Table 6. It is evident from these evaluations that values of less than 2.6, 2.6-6.8 and above 6.8 g g^-1 are indicative of the capacity (low, medium and high) to efficiently use N, respectively. The respective P, K, Ca and Mg levels are also given in Table 6. The use efficiency of each of the nutrients was highest when that nutrient was omitted in the fertiliser application schedule and/or when its initial content in the soil was low.

Table 6 also highlights the facts that the use efficiency of a particular nutrient can best be achieved by improving the supply of the other nutrients. This is not unexpected since nutrient use efficiency is the yield per nutrient applied. The more the fertiliser nutrient quantity applied, the less the nutrient use efficiency value (Akinrinde et al., 2000). When other nutrient(s) is/are applied the increase in use efficiency of the original nutrient by the crop is at the expense of the new one (s). This is in line with the report by Janssen (1998) that efficient use of nutrients is an art of balancing, which enhances nutrient synergism (Mengel and Kirkby, 1987).

The soil as a medium for plant growth, also determines the capacity of crops to efficiently use nutrients in as much as they contain native nutrients.

Table 5:	Influence of the interaction between fertilizer treatment and soil type on nutrient use efficiency by maize

As in the case of fertilizer nutrient additions, use-efficiency of native nutrient elements by the crop decreases with increase in the available quantities in soil. Environmental contamination (particularly via seepage of excess fertilizer nutrients into underground water) is more certain with low nutrient use efficiency level than with medium and high levels.

Nitrogen (followed by P) is the “key” nutrient element for sweet corn production in the soils. Yet, N was the least efficiently used nutrient element, suggesting that soil N removal (or losses) other than by crop uptake is more than for any of the other elements. Sound soil management practices (including complete nutrient application) that guarantee gradual release of fertilizer nutrients should be adopted.

Table 6:	Calibration of sweet-corn nutrient (N, P, K, Ca and Mg) use efficiency values

To ensure that the crop efficiently uses a nutrient, fertilizer application should not be based on “blanket recommendation”.