Abstract

Media summary

Keywords

Crop sequences, residual phosphorus effects, legumes, maize-cowpea rotations, sandy soils

Introduction

Crop sequences are utilized by the majority of farmers in rural Zimbabwe. Cowpea is a widely grown legume in both the high and low rainfall smallholder farming areas of Zimbabwe (Shumba et al. 1990). Although maize is the staple crop in Zimbabwe, the popularity of cowpea with farmers can be attributed to its multiple uses and its adaptability to different environments. It is often used for relish so it an important source of proteins for humans, as livestock feed, and for enhancing soil fertility through biological N fixation (Giller 2001; Johnson 1970). However, the lack of appropriate fertilizer recommendations, poor seed availability, and a limited product market were identified as major constraints to the expansion of cowpea production in Zimbabwe (Nhamo et al. 2003). The aim of this study was to determine an agronomic rationale for P application rates and strategies in maize-cowpea sequences.

Materials and methods

The experiments were conducted on-farm on sandy soils in two smallholder farming areas of Zimbabwe, Shurugwi and Zimuto, where the average annual rainfall ranges from 450 to 600 mm. Phosphorus was broadcast at rates of 0, 20, 40 and 60 kg/ha of P₂O₅ on all maize and cowpea plots in the first season. In the second season, the crops were rotated and grown on residual P from the previous year’s application. Nitrogen (60 kg N/ha) was applied to the maize, whilst K was applied in both years to cowpea and maize at 45 kg K/ha. Cowpea residues from the first season were incorporated into the soil, but the maize residues were fed to livestock. Grain yields were measured at the end of each season. The relative P effect was calculated as the ratio of the yield benefit of each P treatment to the control (0P).

Results

Selected properties of the soils at the trial sites are given in Table 1. Most of the soils had a low pH (less than 6.2), low clay and organic matter contents, and hence low total exchangeable bases and CEC. Soils from homestead fields (e.g. at the Maganyani site in Zimuto) had higher nutrient contents than the soils from topland fields in their respective areas.

Table 1. Selected properties of the sandy soils (0-20cm) at the trial sites in Zimbabwe.

Abbreviations: locality; Shg, Shurugwi; Zmt, Zimuto; field type; H, homestead; T, topland.

Cowpea yields from the sites in Zimuto were significantly (P<0.05) higher than those in Shurugwi. However, average yields at each of the regions relative to the country average of 200 kg/ha were high, 816 kg/ha and 1212 kg/ha for Shurugwi and Zimuto, respectively (Tables 2 and 3). Maize yields at Shurugwi (1556 kg/ha) were higher than those at Zimuto (885 kg/ha) in the first season. In the second season, yields of cowpea and maize from the 0P control plots were significantly lower than from the plots that previously received the higher rates of P fertilizer (Tables 4 and 5).

Table 2. Cowpea grain yields (kg/ha) in the first season for application of different P rates in the Shurugwi and Zimuto smallholder farming areas of Zimbabwe.

See Table 1 for locality and field type of each site

Table 3. Maize grain yields (kg/ha) in the first season of application of different P levels in the Shurugwi and Zimuto smallholder farming areas of Zimbabwe.

See Table 1 for locality and field type of each site

Discussion

There was an increase in the total grain yield of both crops over the two years at each site due to the P applied in the first year. In maize, the 0P control received the blanket N and K application and this resulted in yields that were usually not significantly different from the treatments with applied P. However, the rate of N may have been inadequate, and the low yield responses of maize to P at some sites could be a result of N deficiency masking the effects of the applications of P since N is the main limiting nutrient on sandy soils in the Shurugwi and Zimu regions.

Table 4. Cowpea grain yields (kg/ha) in the second season (residual P effects) to the rates of P fertilizer applied in the previous season in the Shurugwi and Zimuto smallholder farming areas of Zimbabwe.

See Table 1 for locality and field type of each site

Table 5. Maize grain yields (kg/ha) in the second season (residual P effects) to the rates of P fertilizer applied in the previous season in the Shurugwi and Zimuto smallholder farming areas of Zimbabwe.

See Table 1 for locality and field type of each site

Fertility of different field types

The higher yields of cowpea and maize at Mapfidza, Shava, Chibango and Maganyani reflect the higher fertility of the soils at these homestead sites (Table 1). Homestead fields (those close to the home) are better managed and more fertile than topland fields as they receive additional inputs such as kitchen refuse, composts and ash. However, the higher fertility levels of the homestead fields had the effect of reducing the yield per unit P applied in all the treatments relative to the control.

Nutrient interactions

In crop sequences in Zimbabwe, the major nutrient inflows are a result of nutrient cycling and biological N fixation by legumes. Maize following cowpea benefited from residual fertilizer P, and P and other nutrients from the decomposing above- and below-ground cowpea residues. However, cowpea following maize only benefited from the residual fertilizer P, since most of the maize residues were removed to feed animals and maize does not improve the N budget through N fixation. This effect shows the importance of small applications of P on the crop sequences, as there was a positive correlation between P application rate and the yield responses in both the first and the second year. Hardter et al. (1991) reported higher yields and land-use efficiencies from maize-cowpea sequences following application of both N and P fertilizers. Hence, in these systems management of crop residues can play a significant role in the cycling of nutrients and improvement of the soil fertility (Giller 2001; Kouyate et al. 2000).

Targeting phosphorus application

The major benefit of applying P to cowpea is the increased leaf and root biomass that can be incorporated into the soil. The increased maize yields obtained when planted after cowpea show this effect (Table 5). Eagleton et al. (1991) also found that P application increased both cowpea and maize yields. With the application of N, maize uses more P in the first year, leaving much less residual P in the soil for the subsequent crop, in contrast to cowpea which recycles both P and N. The higher relative P effect obtained with maize following cowpea is a result of the recycling of nutrients from the decomposition of the cowpea biomass.

Conclusions

The application of P to cowpea increases biomass production, and thus the benefit to the following maize crop through the efficient recycling of both N and P. The unit grain to unit P ratio favoured the application of P to cowpea, with the following maize crop benefiting from the residual effects. In maize-cowpea sequences in the Shurugwi and Zimu regions, adding a small amount of P is important, and it is better applied to cowpea than to maize.

Acknowledgements

This work was supported in part by CIMMYT’s Risk Management Project and TSBF-CIAT’s IFAD-funded project.

References

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