Putting the P into agricultural productivity in SSA
Phosphorus (P) is essential for life. So it is no surprise that phosphorus deficiencies in acid weathered soils across sub-Saharan Africa is a common constraint for agricultural productivity.
P deficiency can be overcome by adding fertilizers, but, while recent work by CIAT highlights the importance of P fertilization (Kihara & Njoroge, 2013) soil acidity limits the efficiency of costly P inputs, making pH management a promising, if underexplored, strategy to improve P availability.
For many small holder farmers, the high cost of P inputs, particularly those that are imported, is prohibitive, especially if they result in limited impact. If farmers can manage the pH of their soil, they might get more bang for their buck on P inputs, and may therefore be more likely to use them.
CIAT is exploring different options for managing soil phosphorus in weathered soils, with the aim of finding new and cheaper ways for farmers to improve agricultural productivity.
Working in western Kenya, scientists are examining the amount, availability and movement of P in the soil across several field trials. The research is examining the effect of different types of P fertilizers (mineral and organic), and tillage and residue retention components of conservation agriculture, which are also a source of P.
In November 2014, Andrew Margenot, a PhD candidate in Soil Science & Biogeochemistry at the University of California-Davis, presented the results from one of these field trials at the annual Soil Science Society of America Meeting in Long Beach, CA, USA:
The trials included tests with triple super phosphate (TSP) and Minjingu phosphate rock (MPR), two phosphorus inputs available for farmers to buy in East Africa. The effects of TSP and MPR on the amount and type of P were measured over the course of a decade.
Since the acidity of soils in western Kenya drives ‘fixation,’ or locking up, of native and added P, the potential of pH management to improve P availability was also evaluated by simulated liming in the laboratory. Finally, because different P fertilizers (and lime) offer trade-offs in quality and cost to farmers, local prices of inputs, such as P and liming agents, were used to estimate economic feasibility and cost efficiency of P management strategies.
This research can be used to improve efficiency of limited inputs in these regions (Margenot and Sommer, 2014). For example, evaluation of the addition of P fertilizers of different cost and quality over ten years found that low-cost alternatives provide the same beneficial increases in crop available P as higher quality expensive inputs.
This is significant because the majority of studies in the region recommend high application rates, which may be useful during the limited window of most studies in the region (<2-3 yrs), but are likely unnecessary after longer-term application due to the strong residual effect of P fertilizers.
This work confirmed that the cheaper, locally available alternative phosphate rock provides increases in crop available phosphorus at no difference in cost to more expensive, higher quality (and typically imported) inputs like TSP.
A CIAT study on the potential of liming to remove soil acidity constraints to further improve the cost-effectiveness of these inputs is currently underway in field trails on a related long-term field trial in Maseno, Kenya.
Written by Andrew Margenot
Andrew is a PhD candidate in Soil Science & Biogeochemistry at the University of California-Davis. With the support of a Graduate Research Grant from Purdue University’s Borlaug Center for Global Food Security, Andrew is working with Dr. Rolf Sommer (Soil Scientist, CIAT) to understand current and potential management strategies for increasing soil phosphorus availability in western Kenya.