Soil-fertility depletion in smallholder farms is the fundamental biophysical root cause for declining per capita food production in sub-Saharan Africa. An average of 660 kg N ha-1, 75 kg P ha-1, and 450 kg K ha-1 has been lost during the last 30 yr from about 200 million ha of cultivated land in 37 African countries. We propose an alternative approach, the replenishment of soil fertility as an investment in natural resource capital. This approach combines basic principles of soil science with environmental economics. Combinations of P fertilizers and organic inputs can replenish soil N and P nutrient stocks in Africa and restore service flows to near original levels. Phosphorus replenishment strategies are mainly mineral-fertilizer based, with biological supplementation. Nitrogen replenishment strategies are mainly biologically based with mineral-fertilizer supplemen-tation. Africa has ample phosphate rock (PR) deposits that can be either used directly or processed to reverse P depletion. Decomposing organic inputs may facilitate the use of PR in P-depleted soils. Leguminous tree fallows and herbaceous cover crops grown in situ play a major role in N capture and internal cycling in ways compatible with farmer con-straints. Soil-fertility replenishment was found profitable in three case studies, but small-holder farmers lack the capital and access to credit to make the initial investment. A cost-shared initial capital investment to purchase P fertilizer and germplasm for growth of organic inputs combined with effective microcredit for recurring costs such as fertilizers and hybrid seed is seen as the way forward.
Tag: inorganic fertilizers
Interactions of organic materials with phosphate rocks and triple superphosphate
Phosphorus uptake and maize response to organic and inorganic fertilizer inputs in Rubona, Southern Province of Rwanda
Green manure of multipurpose trees is known to be a good source of nutrients to cropHowever, most agroforestry species do not have adequate phosphorus (P) in their leaves. Supplementing green manure with moderate dose of P is a beneficial strategy to improve food security in Rwanda. This study examines the effects of Calliandra calothyrsus Meissner, Tithonia diversifolia Hensley A.Gray and Tephrosia vogelii Hook.f. green manure applied independently or in combination with triple super phosphate (TSP) and lime on maize yield and P uptake in the Oxic Tropudalf of Rubona, Rwanda. The treatmentswere the control, lime at 2.5 t ha-1,TSP at 25 and 50 kg P ha-1, leafofC. calothyrsus, T. diversifolia, and T. vogelii each at 25 and 50 kg P ha-1, respectively. Leaf shrubs biomass,TSPand limewere applied for four consecutive seasons (2001-2004). The results showed that the combination of greenmanure with TSP at a rate of 50 kg P ha-1 significantly increased maize yield from 24 to 508% when compared to the control and T. divesifolia combined with TSP was leading (508%). Equally, the same treatments as indicated above showed higher P uptake (15.6-18. 6 kg P ha-1) than the control (5 kg P ha-1) and 65% of maize yields variation was explained by total P uptake. The plant residues quality such as C:N ratio, total plant N, and P significantly Calliandra calothyrsusinfluenced the variability of maize grain yields.
Agronomic and economic evaluation of organic and inorganic sources of phosphorus in Western Kenya
Manys oils in the highlands of East and Central Africa are depleted of soil nutrients, particularly P. Ouro bjective was to comparec attle manure, Calliandra calothyrsus Meissner leaf biomass, and triple superphosphule (TSP) as sources of P for maize (Zea mays L.), both individually and as mixtures of organic (manure or calliandra) and inorganic (TSP + urea) sources. Field experiments were conducted on a Kandiudalf at two sites in western Kenya. Net benefits were computed as the difference between the value of additional maize yield accruing from nutrient inputs and the associated additional costs. Maize grain yield was 0.6 Mg ha1 for application of urea without P. Application of 10 kg P ha1 as organic, inorganic, and mixtures of organic and inorganic sources significantly increased maize yield. Grain yield for manure at least equaled and sometimes exceeded (P 0.05) yield for calliandra and TSP + urea. Net benefits in U.S. dollars (USD) for two seasons were highest for manure spot placed in the planting bole (293 USD ha1), broadcast manure (255 USD ha1), and broadcast TSP + 44 kg urea-N ha1 (313 USD ha1 at P = 30 kg ha1 and 98 USD ha1 at P = 10 kg ha1). Net benefits for calliandra leaf biomass were highest (136 USD ha1) when biomass was valued at cost of production and integrated with TSP, such that it provided all the N for maize and TSP provided the additional P not supplied by calliandra. Cailiandra valued at its opportunity cost as a protein supplement for dairy cattle was not an economic source of P. Sensitivity analyses suggest that organic materials most suitable for use as P sources have high P content and low cost of production.