Phosphorus Uptake and Balance in a Soyabean-Maize Rotation in the Moist Savanna of West Africa

This study investigated the effect of maturity class on phosphorus (P) balance in a soyabean-maize rotation in the West African moist savanna.F our soyabean varieties of different maturity classes were grown with or without P fertilizer followed by a maize crop. Soyabean stover P content averaged 1.77 kg ha−1 compared with 5.13 kg ha−1 in the grain. The late soyabean variety TGx1670-1F accumulated a significantly higher P in the grain (6.56 kg ha−1), and stover (2.57 kg ha−1) than the others. While P harvest index averaged 79%, P application increased grain P by 63-81% and stover P by 100% or more. When either soyabean grain or grain+stover was exported, P balance was negative and was not statistically different for varieties when no P was applied. At 30 or 60 kg P ha−1, P balance was negative but significantly lower in TGx1670-1F compared with other varieties. In creasing P rate applied to soyabean significantly (p 0.01) increased maize grain P by 35-66% in the second year. When P was exported only in soyabean grain, cumulative P balances after maize grain harvest (with no P or 30 kg ha−1 applied previous year) were not significantly different for previous soyabean crops. At 60 kg ha−1, however, P balance in previous TGx1670-1F plot was significantly lower than for other varieties. A further export of soyabean stover reduced P balance. Significant residual P effect was observed emphasizing the need to focus P fertilizer application in the cropping system rather than on the single crop. Also with more P in soyabean grain, a reduction in the extent of P depletion will be achieved by returning soyabean stover to the field after threshing

ESTABLISHMENT OF OPTIMUM PLANT DENSITIES FOR DRY SEASON SORGHUM GROWN ON VERTISOLS IN THE SEMI-ARID ZONE OF CAMEROON

Dry season transplanted sorghum is grown on Vertisols in the Lake Chad Basin at approximately 10,000 plants ha-1. Increasing plant density was hypothesised to be one way of increasing yields in this cropping system. To test this hypothesis, a trial was conducted in four environments near Maroua in northern Cameroon (one year at Yoldeo and three years at Salak) examining densities ranging from 10,000 to 50,000 plants ha-1. Grain yields were not significantly increased by increasing planting density in any of the environments because of reduced panicle size. For example, as planting density was doubled from 10,000 to 20,000 ha-1, the mean density of panicles harvested was increased by 85% but mean grain weight per panicle was decreased by 45%. Thus, in all environments, mean grain yields increased by 100 kg ha-1 (9%) at the transplant density of 20,000 ha-1 and 150 kg ha-1 at 26,667 plants ha-1. A comparison of results from three years at Salak suggests that the fraction of plants bearing panicles is influenced by the annual rainfall and, especially, the amount of rain during August and September. However, even after a season of adequate rainfall, panicle grain weight decreased with increasing panicle density, suggesting that there is little scope for increasing dry season sorghum transplant density without supplemental irrigation. Considering increases in labour input for nursery establishment, transplanting and harvest, the increased revenue from increasing planting density does not compensate for increased costs. The economic optimum is around 10,000 ha-1, which is similar to the current farmers' practice

Effect of soybean on subsequent maize grain yield in the Guinea savanna zone of West Africa

(African Crop Science Journal, 1997 5(1): 31-38

Utilization of rock phosphate by crops on a representative toposequence in the northern Guinea savanna zone of Nigeria: response by maize to previous herbaceous legume cropping and rock phospate treatments

Mucuna pruriens and Lablab purpureus can immediately benefit from rock phosphate (RP), but as both herbaceous legumes have only limited economic value, these benefits are only meaningful if yields of a subsequent maize crop are improved. The impact of RP application to Mucuna and Lablab on grain yield, total N, and total P uptake of a subsequent maize crop was evaluated for a set of non-acidic soils on a representative toposequence (‘plateau', ‘slope', and ‘valley' field) in the Northern Guinea savanna (NGS) zone. Urea-N (45 kg N ha−1) was applied to all treatments on the ‘slope' and ‘valley' fields, while on the ‘plateau' field, plots were split into sub-plots with and without fertilizer N addition. Application of RP to Mucuna and Lablab led to site- and species-specific increases in grain yield, total N, and total P uptake of a subsequent maize crop. On the ‘plateau' and ‘valley' fields, maize grain yields, and total aboveground N and P uptake were significantly higher (1741 kg grains ha−1 on the ‘plateau' field and 910 kg grains ha−1 on the ‘valley' field) in the treatments following legumes treated with RP, compared to the treatments following legumes without RP addition. These increases were highest for the Mucuna treatment in both fields. On the ‘slope' field with high initial Olsen-P content application of RP to preceding legumes did not influence maize yield or N and P uptake. Improvements of maize yields on the ‘plateau' field were the results of an improvement in the soil P status after growing legumes supplied with RP, as maize did not respond to the addition of urea in the absence of RP addition to the preceding legumes. Recovery of RP-P by the maize varied between 3.1 and 5.8%. On all fields, legumes also improved the soil N status, giving average apparent legume N recoveries of 17% for Mucuna and 32% for Lablab. The results clearly show that soil fertility management in the NGS needs to take into consideration both N and P. The studied legume–maize rotations supplied with RP during the legume phase and minimal amounts of inorganic N during the maize phase are good examples of promising soil fertility management technologies alleviating N and P deficiencies. Recommendations need to be diversified following the position of the field on the toposequence and previous P application history

Utilization of rock phosphate by crops on a representative toposequence in the northern Guinea savanna zone of Nigeria: response by Mucuna pruriens, Lablab purpureus, and maize

The availability of P from rock phosphate (RP) is often too low to demonstrate an immediate impact on cereal production. Legumes may improve the immediate availability of P from RP and eventually benefit subsequent maize crops. The ability of Mucuna pruriens (L.) var utilis (Wright) Burck and Lablab purpureus L. to use P from RP and the changes in selected plant and symbiotic properties and in the soil available P and particulate organic matter (POM) pool as affected by the addition of RP were measured for a set of soils on a representative toposequence (‘plateau', ‘slope' and ‘valley' field) in the Northern Guinea savanna zone of Nigeria. At 18 weeks after planting (WAP), Mucuna accumulated significantly more N and P in the total biomass in the plots treated with RP compared to the plots without RP addition on all fields. Nitrogen accumulation of Mucuna reached 175, 177 and 164 kg N ha−1 in the treatments with RP on the ‘plateau', ‘slope' and ‘valley' fields, respectively. Phosphorus accumulation of Mucuna was highest at 18 WAP in all sites and reached 10, 14 and 10 kg P ha−1 in the treatments with RP on the ‘plateau', ‘slope' and ‘valley' fields, respectively. Lablab accumulated significantly more N and P at 18 WAP only on the ‘plateau' field, but some of the potential differences in N or P accumulation may have been masked by various pests especially affecting Lablab. A highly significant negative correlation was observed between the aboveground biomass at 16 WAP and the nematode population. The addition of RP significantly increased arbuscular mycorrhizal fungi (AMF) infection of the Mucuna (from 24 to 33%) and Lablab roots (from 15 to 28%) to a similar extent in all fields. This increased AMF infection was most likely caused by specific processes in the rhizosphere of the legumes as AMF infection of the maize roots (8%) was not affected by RP addition. Increases in nodule numbers and fresh weight were site- and species-specific and highest for the ‘plateau' and ‘slope' fields. The number of nodules increased on average from 8 to 19 (3 plants)−1 and from 7 to 30 (3 plants)−1 for Mucuna and Lablab, respectively, after RP addition. Although nearly all the aboveground legume biomass had disappeared from the soil surface at 51 WAP, both the Olsen-P status and POM N concentration were increased by the presence of legumes. Mucuna significantly enhanced the Olsen-P content of the soil after RP addition compared to the Lablab or maize treatments on the ‘plateau' and ‘valley' fields. Due to the relatively high initial Olsen-P content of the ‘slope' field (14 mg kg−1), differences between treatments were not significant. The N concentration of the POM pool was significantly higher under legumes than under maize on the ‘slope' and ‘valley' fields, and indicates incorporation of part of the legume biomass in the POM pool. The addition of RP to herbaceous legumes was observed to lead to site- and species-specific changes in the tripartite legume–rhizobium–mycorrhizal fungus, driven by processes taking place in the rhizosphere of the legumes, and in the soil available P pool. A cereal following these herbaceous legumes could benefit from this improvement in soil fertility status