Pieter Pypers

Integrated Soil Fertility Management: Operational Definition and Consequences for Implementation and Dissemination

Traditional farming systems in Sub-Saharan Africa depend primarily on mining soil nutrients. The African green revolution aims to intensify agriculture through the dissemination of integrated soil fertility management (ISFM). This paper develops a robust and operational definition of ISFM based on detailed knowledge of African farming systems and their inherent variability and of the optimal use of nutrients. The authors define ISFM as a set of soil fertility management practices that necessarily include the use of fertilizer, organic inputs and improved germplasm, combined with the knowledge on how to adapt these practices to local conditions, aimed at maximizing agronomic use efficiency of the applied nutrients and improving crop productivity. All inputs need to be managed in accordance with sound agronomic principles. The integration of ISFM practices into farming systems is illustrated with the dual-purpose grain legume–maize rotations in the savannas and fertilizer micro-dosing in the Sahel. Finally, the dissemination of ISFM practices is discussed.

The performance of DGT versus conventional soil phosphorus tests in tropical soils - an isotope dilution study

Background and aimsA soil test that samples nutrients only from fractions that are accessible to plants will predict availability and uptake more robustly than empirical tests. This can be tested by comparison of the isotope ratios (specific activity, SA) of the nutrient between plant and the soil extract. This study was set up to assess this requirement for the diffusive gradients in thin films technique (DGT), recently proposed as a soil P test, in comparison with conventional soil P tests viz. Olsen, Colwell, Bray-1, Mehlich-3, ammonium oxalate, anion exchange membranes (AEM) and 0.01 M CaCl2 solution.MethodsMaize (Zea mays L.) was grown in two P-deficient soils from western Kenya with contrasting P sorption characteristics, amended with a low and a high P rate and labelled with 33P.ResultsThe SA in the plant shoot corresponded with that of the extracts of the different soil tests, except for CaCl2 and ammonium oxalate extracts, at the low P rate in the soil with low P sorption capacity, Teso soil. For the high P rate on this soil, differences in SA between maize shoot and soil test were small for all established soil tests, but significant for the Colwell, Bray-1, Mehlich-3 and AEM tests. The SA in the soil extracts was significantly smaller than that in the maize shoot for Sega the strongly P-sorbing soil at both P rates for all conventional tests, including AEM. This indicates that these tests extracted P from a pool that is not accessible to the plant. For the DGT test, however, there was no difference in SA between the maize shoot and the soil test, for any of the treatments.ConclusionsMost conventional soil tests can extract a fraction of P which is not available to maize. The DGT technique, however, only samples P from the plant-accessible pool.

Management Impacts on Biological Phosphorus Cycling in Cropped Soils

Phosphorus (P) is a limited resource and P deficiency limits crop production on large areas worldwide. Future food security, therefore, will largely depend on efficient P use in cropping systems. In this review, we present the impact of farmers’ interventions on biological P cycling in cropped soils of temperate and tropical regions, with emphasis on microbial functions in soil P dynamics. We exemplify the effects of (1) soil tillage, with a focus on the comparison of conventional tillage versus direct seeding systems; (2) fertilizer input, using organic and/or mineral nutrient sources; and (3) integration of legumes into cropping systems. We analyze whether and how biological processes can be influenced to increase the use efficiency of soil and fertilizer P. Finally, we formulate recommendations for an integrated P management. Future research should target improved biological access to recalcitrant inorganic and organic P forms.

Root hairs explain P uptake efficiency of soybean genotypes grown in a P-deficient Ferralsol

Background and aimsIncorporating soybean (Glycine max) genotypes with a high nitrogen fixation potential into cropping systems can sustainably improve the livelihoods of smallholder farmers in Western Kenya. Nitrogen fixation is, however, often constrained by low phosphorus (P) availability. The selection of soybean genotypes for increased P efficiency could help to overcome this problem. This study investigated the contribution of different root traits to variation in P efficiency among soybean genotypes.MethodsEight genotypes were grown in a Ferralsol amended with suboptimal (low P) and optimal (high P) amounts of soluble P. Root hair growth was visualized by growing plants in a novel agar system where P intensity was buffered by Al2O3 nanoparticles.ResultsIn the pot trial, P uptake was unaffected among the genotypes at high P but differed about 2-fold at low P. The genotypes differed in P uptake efficiency but not in P utilization efficiency. Regression analysis and mechanistic modeling indicated that P uptake efficiencies were to a large extent related to root hair development (length and density) and, to a lower extent, to colonization by mycorrhizal fungi.ConclusionBreeding for improved root hair development is a promising way to increase P uptake efficiency in soybean.

ENHANCING MAIZE PRODUCTIVITY AND PROFITABILITY USING ORGANIC INPUTS AND MINERAL FERTILIZER IN CENTRAL KENYA SMALL-HOLD FARMS

SUMMARY Declining land productivity is a major problem facing smallholder farmers today in Sub-Saharan Africa, and as a result increase in maize grain yield has historically staggered behind yield gains that have been achieved elsewhere in the world. This decline primarily results from reduction in soil fertility caused by continuous cultivation without adequate addition of external nutrient inputs. Improved soil fertility management practices, which combine organic and mineral fertilizer inputs, can enable efficient use of inputs applied, and can increase overall system’s productivity. The trials were established at two sites with different soil fertility status to determine the effects of various organic sources (Tithonia diversifolia, Mucuna pruriens, Calliandra calothyrsus and cattle manure) and their combinations with mineral fertilizer on maize grain yield, economic return and soil chemical properties. Drought spells were common during the peak water requirement periods, and during all the seasons most (90%) of the rainfall was received before 50% flowering. In good and poor sites, there was a significant (p < 0.001) effect of season on maize grain yield. Tithonia diversifolia recorded the highest (4.2 t ha −1 ) average maize grain yield in the poor site, while Calliandra calothyrsus gave the highest (4.8 t ha −1 ) average maize grain yield in the good site. Maize grain yields were lower in treatments with sole fertilizer compared with treatments that included organic fertilizers. The maize grain yields were higher with sole organics compared with treatments integrating organic and inorganic fertilizers. Soil pH increment was statistically significant in the sole manure treatment in good and poor sites (t-test, p = 0.036 and 0.013), respectively. In the poor site, magnesium increased significantly in the sole manure and manure + 30 kg N ha−1 treatments with t-test p = 0.006 and 0.027, respectively. Soil potassium was significant in the sole manure treatment (t-test, p = 0.03). Generally the economic returns were low, with negative net benefits and benefit cost ratio of less than 1. Inorganic fertilizer recorded the highest net benefit and return to labour (p < 0.001 and <0.01, respectively) in the good site. The treatments that had very high maize grain yields did not lead to improved soil fertility, thus there is need for tradeoffs between yield gains and soil fertility management when selecting agricultural production technologies.

Occurrence and genetic diversity of phosphate-solubilizing bacteria in soils of differing chemical characteristics in Kenya

This study focused on the isolation, identification (sequencing of 16S rDNA gene) and determination of the phosphorus (P)-solubilizing efficiency of native populations of phosphate-solubilizing bacteria (PSB) in 13 Kenyan soils with differing chemical characteristics. Air-dried soil samples were serially diluted and plated on NBRIP media and enumerated. Their phosphate-solubilizing efficiency was assessed on Frioni’s agar. Pearson correlation coefficients were determined between PSB populations and soil properties. The PSB populations varied among the sites tested and had a positive and significant correlation (p ≤ 0.05) with organic carbon (r = 0.76), exchangeable calcium (r = 0.93) and exchangeable magnesium (r = 0.92). A total of 150 isolates were identified to the genus and species level. Among the isolates, Bacillus megaterium, Bacillus sp. and Arthrobacter sp. were the most abundant and well-distributed strains. However, only 5% of the total isolates were efficient in terms of phosphate-solubilizing efficiency. The results indicate that although there were many PSB strains in the soils tested, only a few (5%) were effective in terms of their phosphate-solubilizing ability. It is therefore unlikely that native PSB contribute significantly to solubilizing phosphate in the soils tested, which would ultimately benefit plant growth. Therefore, inoculation with effective strains with a high P solubilization potential is necessary.

Seed weight affects shoot and root growth among and within soybean genotypes beyond the seedling stage: implications for low P tolerance screening

AimsSoybean is a relatively large-seeded species and variation in seed size can affect plant establishment especially under phosphorus-deficient conditions. We evaluated to what extent differences in seed weight and seed phosphorus (P) content among genotypes and within genotypes affect plant growth at low and high P supply.MethodsSeedling growth of 42 soybean genotypes was evaluated in a pot trial. In addition, plant growth of selected genotypes was evaluated at different growth stages in two pot trials using various seed lots differing in seed weight per genotype.ResultsCorrelations between seed weight and plant biomass, shoot P content and root length among 42 genotypes were stronger at low than at high P supply. Within-genotype variation in seed weight affected shoot and root growth at different plant growth stages. At low P supply, plant growth was correlated with seed weight up to the flowering stage.ConclusionsIt is concluded that seed weight largely affects growth especially at low P supply. Confounding effects due to variation in seed weight must be taken into account when screening soybean genotypes for low P tolerance.

Do Commercial Biological and Chemical Products Increase Crop Yields and Economic Returns Under Smallholder Farmer Conditions

During recent decennia, new commercial products have appeared on the market as alternatives to common fertilizers. While some of these products are based on well-established technologies, such as rhizobium inoculation, others have not been subjected to scientific scrutiny. During 3 years, we evaluated over 80 of these new products, including microbial inoculants and chemical products on major legume, cereal and banana crops across diverse agro-ecological conditions in Ethiopia, Nigeria and Kenya in the laboratory, greenhouse and field conditions. Amongst the rhizobial inoculants, several products from different companies were found very effective in increasing nodule biomass on soybean and increasing grain yield by up to 30 %, and benefit-cost ratio of up to 5.0 realized. Except for tissue cultured bananas, the effect of arbuscular mycorrhizal fungal (AMF) inoculants was less evident with optimal yield of legumes realized with co-inoculation with P solubilizing bacteria and when supplemented with inorganic fertilizer such as DAP. Other products containing Trichoderma or Bacillus spp. improved growth under field conditions, soil-dependent growth improvements of over 40 % in tissue culture bananas. The potential of products to reduce soil pathogenic rhizosphere organisms, particularly Fusarium, was also observed. Chemical products evaluated, special attention was given to alternative P fertilizers such as leaf sprays, seed coatings and conditioners with humic acids. The effect on cereals depended on the crop, the soil and accompanying agronomic measures. Benefit-cost ratios were favorable for seed P coating Teprosyn, because this is a fairly inexpensive treatment (US

Effect of Organic Inputs and Mineral Fertilizer on Maize Yield in a Ferralsol and a Nitisol Soil in Central Kenya

3 ha−1). Results demonstrate economic returns of US

Increasing Productivity Through Maize–Legume Intercropping in Central Kenya

4 for every dollar invested for soybean production and US