R.J Carsky

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 21 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 21 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) 21 and from 7 to 30 (3 plants) 21 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 21 ), 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. q 2000 Elsevier Science Ltd. All rights reserved.

A land management based approach to integrated Striga hermonthica control in sub-Saharan Africa

Striga hermonthica, an obligate root parasite of grasses, Is one of the most severe constraints to cereal production in sub-Saharan Africa. In the recent past, prior to increased production pressure on land, S. hermonthica was controlled in African farming systems by prolonged crop rotations with bush fallow. Because of increasing need for food and concomitant changes in land management practices, however, these fallow rotations are no longer extensively used. Shorter crop rotations and fallow periods have also led to declines in soil fertility which present a very serious threat to African food production. A sustainable solution will be an integrated approach that simultaneously addresses both of these major problems. An integrated programme that replaces traditional bush fallow rotation with non-host nitrogen-fixing legume rotations, using cultivars selected for efficacy in germinating S. hermonthica seeds, is outlined. The programme includes use of S. hermonthlca-free planting material, biological control, cultural control to enhance biological suppressiveness, host-plant resistance, and host-seed treatments.

Differential N uptake by maize cultivars and soil nitrate dynamics under N fertilization in West Africa

Nitrate is prone to leaching in the sandy soils of the West African moist savannas. Better management of nitrogen (N) resources and maize cultivars with enhanced genetic capacity to capture and utilize soil and fertilizer N are strategies that could improve N-use efficiency. In two field experiments conducted at Zaria, northern Nigeria, five maize ( Zea mays L.) cultivars planted early in the season were assessed under various N levels for differences in N uptake, soil N dynamics, and related N losses. Cultivar TZB-SR accumulated more N in the aboveground plant parts in both years than the other cultivars. All, except the semi-prolific late (SPL) variety, met about 50–60% of their N demand by the time of silking (64–69 DAP). In both years, SPL had the greatest capacity to take up N during the grain filling period, and it had the highest grain-N concentration and the least apparent N loss through leaching in the second year. There were no significant differences in soil N dynamics among cultivars in both years. At harvest, the residual N in the upper 90 cm of the profile under all the cultivars ranged from 56 to 72 kg ha −1 in the first year and from 73 to 83 kg ha −1 in the second year. Apparent N loss from 0 to 90 cm soil profile through leaching ranged from 35 to 122 kg ha −1 in both years. N application significantly increased N uptake by more than 30% at all sampling dates in the second year of the experiment, but had no effect on apparent N loss. Results indicate that the use of maize cultivars with high N uptake capacity during the grain filling period when maximum leaching losses occur could enhance N recovery and may be effective in reducing leaching losses of mineral N in the moist savanna soils.

Soybean Maturity and Environmental Effects in Savanna Systems: I. Dry Matter Accumulation

ABSTRACT Growing soybean varieties with high residue yield may help to sustain the soil organic matter (SOM) content when recycled. Replicated field trials were conducted in four Guinea savanna sites in Nigeria to study the dry matter partitioning in six soybean genotypes and evaluate them for biomass production. The varieties were early TGx1485-1D and late TGx1670-1F in Trial 1 and early TGx1485-1D, TGx1805-2E and TGx1681-3F, medium TGx1809-12E and late TGx923-2E and TGx1670-1F in Trial 2. On average, the proportion of total dry matter accumulated in soybean plant parts was 42% in grain, 36% in stover, 12% in leaf litter and 11% in roots and nodules. While maturity class had no significant effect on the grain yield of soybean, significantly higher dry matter accumulation of roots and nodules, leaf litter, and stover was observed in medium and late varieties compared with early varieties (P < 0.05). Thus, medium and late varieties would be better able to sustain the SOM content than early varieties when the residues are recycled. Strongly acid soils in a high rainfall environment limited soybean biomass production and potential to maintain SOM.

Response of selected crop associations to groundwater table depth in an inland valley

Abstract The success of inland valley crop production in humid and subhumid areas of subSaharan Africa depends on the availability of appropriate crop varieties with some tolerance to excess moisture. Performance of upland crop associations [cassava (Manihot esculenta) + sweet potato (Ipomoea batatas) and cassava + soybean (Glycine max)] was observed in a trial conducted along a small valley toposequence. Groundwater table depth (GWT) was a significant covariate for cassava yield and regression analysis showed that cassava clones of similar yield potential responded similarly and positively to increasing GWT depth (r2=0.77) regardless of associated crop. Sweet potato tuber production was related to GWT depth for variety TIS 9465 (r2=0.47) but not for TIS 8441. Sweet potato was more tolerant of shallow GWT than cassava. Combined cassava and sweet potato tuber production was also positively correlated to GWT depth (r2=0.83). The range of GWT depth to be considered by plant breeders screening cassava and sweet potato varieties for excess moisture in inland valleys should include depths 0.2 to 0.4 m, because those depths were shown to be stressful to these crops.