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