Effect of P fertilizer application on N balance of soybean crop in the guinea savanna of Nigeria

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Abstract

Soybean (Glycine max (L.) Merr.) is becoming increasingly important in the cereal-based cropping system of the Nigerian Guinea savanna zone and this justifies research on its effects on soil N. Although soybean can obtain 50% or more of its N requirement from the atmosphere, the N contribution of the crop to the system depends on the amount of N contained in roots, haulms, and fallen leaves after grain harvest. At four sites in the northern Guinea savanna, the effects on N balance of P fertilizer and soybean varieties of different duration were tested. The varieties received P fertilizer at the rates of 0, 30, and 60 kg P ha−1. The total N accumulated aboveground at harvest averaged 104 kg N ha−1 in the early and medium varieties, and 135 kg N ha−1 in the late varieties. Across all varieties and sites, total N content was increased by 40–47% when P was applied. Apparent N harvest index averaged 85% but was not significantly affected by variety or P rate. When only grain was exported, the calculated N balance of the early and the medium varieties was −2.6 to −12.2 kg N ha−1 while the longer duration varieties had positive N balances ranging from 2 to 10.9 kg N ha−1. The N accrual was negative when P was not applied and ranged from 2.4 to 5.2 kg N ha−1 with P application. The interaction of variety and site on the N balance was significant at P<0.05. N balance at the southernmost site was −14.2 kg N ha−1 compared with 2.6–10 kg N ha−1 at the northern sites where N2 fixation was higher. The estimate of N balance is reduced when soybean haulms are exported. A positive N contribution by soybean is, therefore, possible in a soybean–cereal rotation when: (i) P is applied, (ii) the soybean variety is late maturing, and (iii) only grain is exported.

Introduction

The harvest of a good crop results in the removal of ample amounts of crop residues from the field and consequently large amounts of nutrients. As a result, nutrient balances in crop fields in the savanna and other ecological zones are generally negative. Thus, soils of low nutrient status are further depleted (Rhodes et al., 1996). Positive nutrient balances occur mainly in home gardens and concentric rings close to settlements where soil fertility is maintained by the incorporation of plant and animal wastes (Prudencio, 1993, Rhodes et al., 1996). In other field types far from the household, inorganic fertilizers are appropriate because they require minimal transport labor per kilogram of nutrient. In the absence of fertilizer, however, the most promising way to maintain soil N status is by growing legumes. In legume–cereal rotation systems, the availability of the N fixed by rhizobia has been shown to benefit the subsequent cereal. This should, therefore, reduce the amount of fertilizer N required to achieve similar cereal yields.

A high N harvest index (NHI) is characteristic of soybean (Eaglesham et al., 1982, Piha and Munns, 1987) so much of the N in the crop is exported in the grain. The application of P to soybean increases the amount of N derived from the atmosphere (Ndfa) by the soybean–rhizobium symbiotic system (Chien et al., 1993, Sanginga et al., 1996). Therefore, the application of P should increase the N content of roots, haulms and fallen leaves. When these residues are not exported from the field, their mineralization should add to the soil part of the N derived from the atmosphere. Farmers may, therefore, be able to reduce the amount of N fertilizer applied to cereals when grown after soybean. The objective of this study was to determine the effect of P on the N content of soybean plant parts and to estimate the amount of N that could be contributed to cropping systems involving soybean.

Section snippets

Materials and methods

Field studies were carried out in 1996 at Mokwa (9°18′N, 5°04′E), Fashola (7°56′N, 3°45′E), Gidan Waya (9°28′N, 8°22′E), and Kasuwan Magani (10°24′N, 7°42′E) in the Nigerian Guinea savanna. Two late varieties (TGx1670-1F and TGx923-2E), maturing in about 115–120 days; a medium variety (TGx536-02D) maturing in about 100 days; and early TGx1485-1D maturing in about 95 days were sown at all sites. For the physical and chemical characteristics of the soils at the various sites see Table 1.

Aboveground N content

The amount of N contained in the total aboveground dry matter (grain N, residue N, and litter N) at the final harvest of soybean was significantly (P<0.01) affected by variety and P application as well as by the interactions of site with P, and variety with P. Except at Mokwa, the amount of N contained in the varieties at final harvest and at all sites increased with increasing duration to maturity (Table 2). The N content of the late TGx1670-1F, which averaged 140.9 kg ha−1, was highest among

Effect of P fertilizer on soybean N balance

The response of soybean N content to P shows that the application of fertilizer P was justified at all sites where P levels were below 7.0 mg kg−1. These P levels are characteristic of savanna soils (Jones and Wild, 1975). The effect of P application was observable in the significant increases in total aboveground, grain, residue, and litter N contents. At Mokwa, where soil available P was high, the application of 30 and 60 kg P ha−1 did not increase soybean N content and slightly depressed grain N

Conclusions

By increasing N2 fixation, P application to soybean was able to increase N balance. Positive N contributions are attainable in a soybean–cereal cropping system when the soybean variety is late maturing. This is because long duration allowed the late varieties to fix more N2 than the early and medium varieties. To maximize the benefits of N accumulated through biological N2 fixation and avoid the depletion of soil N, the return of soybean residue is important and this will improve the

Acknowledgements

The authors are grateful to Messrs R. Oyom, L. Ushie, S. Bako, L. Ajuka and A. Azeez for assisting with fieldwork, and to Drs. G. Tian and N. Sanginga for providing laboratory facilities. This is IITA manuscript No. IITA/01/CP/01.

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