Elke Vandamme

Phosphorus in smallholder farming systems of sub-Saharan Africa: implications for agricultural intensification

Current efforts in combating food insecurity in sub-Saharan Africa (SSA) focus on agricultural intensification. Given the high soil nutrient depletions, replenishing soil fertility is a major component of such efforts. One of the key nutrients limiting crop production is phosphorus (P). Overcoming P deficiency in smallholder farming in SSA faces many challenges, mainly because the causes of P deficiencies vary, and viable options to replenish soil P have limitations. In some areas, P deficiency is associated with a low P reserve, while in others, it results from a high soil P-adsorption capacity. Numerous studies have focused on developing approaches and strategies with potential to replenish soil P or improve its availability to crops. This paper highlights approaches and strategies that have been studied, including the use of soluble P fertilizers, phosphate rocks and organic resources. The contribution of soluble P fertilizers is mainly limited by their high cost, while most phosphate rocks are not mined. Replenishing P through organic resources is constrained by their often low P content and inadequate availability in smallholder farms. Optimizing the P use efficiency appears as the most plausible target pending an increased accessibility of P sources to farmers. Practices towards this optimization include strategic crop sequences and P allocation to crops, use of P-efficient genotypes, and targeting the residual P. Research is needed towards gaps in understanding processes governing benefits associated with these practices. Since P replenishment is fertilizer dependent, greater attention is required in enhancing the accessibility of P fertilizers to smallholder farmers.

The future of phosphorus in our hands

AbstractWe live in a global phosphorus (P) system paradox. P access is becoming increasingly limiting, leading to food insecurity but at the same time an over-application or abundance of P in many agricultural and urban settings is causing environmental degradation. This has been recognised in the academic literature and at regulatory levels, but swift action and multi-level cooperation of all stakeholders is required to ensure the economically, environmentally and socially responsible use of P. To provide foundations for future cooperation, a conceptual model describing the elements of P need, P availability and P use in different systems and at different scales was developed during the Young Scientists Workshop in P Week 2014 in Montpellier, France. Here we describe our extended conceptual model and a theoretical P balance calculation tool for describing multi-scale P balances and imbalances to impartially advise all stakeholders on more sustainable P use across the world.

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.

Genotypic Variation in Grain P Loading across Diverse Rice Growing Environments and Implications for Field P Balances

More than 60% of phosphorus (P) taken up by rice (Oryza spp.) is accumulated in the grains at harvest and hence exported from fields, leading to a continuous removal of P. If P removed from fields is not replaced by P inputs then soil P stocks decline, with consequences for subsequent crops. Breeding rice genotypes with a low concentration of P in the grains could be a strategy to reduce maintenance fertilizer needs and slow soil P depletion in low input systems. This study aimed to assess variation in grain P concentrations among rice genotypes across diverse environments and evaluate the implications for field P balances at various grain yield levels. Multi-location screening experiments were conducted at different sites across Africa and Asia and yield components and grain P concentrations were determined at harvest. Genotypic variation in grain P concentration was evaluated while considering differences in P supply and grain yield using cluster analysis to group environments and boundary line analysis to determine minimum grain P concentrations at various yield levels. Average grain P concentrations across genotypes varied almost 3-fold among environments, from 1.4 to 3.9 mg g−1. Minimum grain P concentrations associated with grain yields of 150, 300, and 500 g m−2 varied between 1.2 and 1.7, 1.3 and 1.8, and 1.7 and 2.2 mg g−1 among genotypes respectively. Two genotypes, Santhi Sufaid and DJ123, were identified as potential donors for breeding for low grain P concentration. Improvements in P balances that could be achieved by exploiting this genotypic variation are in the range of less than 0.10 g P m−2 (1 kg P ha−1) in low yielding systems, and 0.15–0.50 g P m−2 (1.5–5.0 kg P ha−1) in higher yielding systems. Improved crop management and alternative breeding approaches may be required to achieve larger reductions in grain P concentrations in rice.