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Fertilizer sulfur and food production: research and policy implications for tropical countries
Unfortunately there has been a problem with your order. Moreover, increasingly shorter fallow periods are inadequate for soil fertility regeneration.
Poor productivity also decreases soil vegetative cover and water transpiration, which in turn results in increased erosion and nutrient leaching through the soil - thus perpetuating a downward spiral of soil fertility. Map 1. Major soil types of Africa. Map 2.
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Fertilizer use kg ha -1 for various regions and markets, Cereal yields in various parts of the world, Monitoring annual nutrient losses on a scale as large as Africa is a complex task that must take into account soil type Map 1 , agro-ecological zone Map 2 , topography and land use.
Building on the nutrient depletion work by Stoorvogel and Smaling , , , Henao and Baanante estimated soil nutrient balances for Africa. Inputs include fertilizers, organic residues and manures, nitrogen fixation and sedimentation, while outflows include losses from soil erosion, leaching, volatilization and crop removal.
Table 2 derived from Henao and Baanante, shows current major sources of nutrient losses and inflows in various agro-ecological zones of Africa. Under the low yield scenario prevalent throughout most of Africa, crop removal harvest plus crop residues accounts for some 43 percent of N, 87 percent of P and 73 percent of K losses. This calculation excludes the Mediterranean and arid North Africa, which are not part of SSA and receive high fertilizer inputs. Erosion accounts for an additional 43 percent of N loss.
As yields increase due to appropriate fertilization, one would expect reductions in the proportion of nutrients lost by means other than crop and residue removal. Increased fertilizer use increases crop production grain and biomass , thereby providing additional benefits beyond the yield increases.
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Because biomass in SSA is primarily used for livestock fodder or for cooking fuel, additional production increases the possibilities for a larger portion remaining on or being returned to the field. Crop residues left on the field are important in preventing the loss of topsoil, maintaining soil organic matter and recycling crop nutrients, particularly K. Given the diversity of environments in Africa, one cannot refer to K in African soils in general, but must highlight the diversity of K availability in African soils.
No simple soil test exists to evaluate K availability for all soils, but ammonium acetate-exchangeable K which correlates closely with other methods such as the Mehlich-3 or dilute acid solutions is a commonly used method.
The Booker Tropical Soils Manual highlights the various interpretations of the exchangeable K value. For sandy, sandy loam and red-brown clays in Zimbabwe the exchangeable K values are 0. Maria and Yost report critical K norms of 0. However, potassium availability is not solely a function of exchangeable K.
An exchangeable K value of 0. Soils have non-exchangeable and labile gradually available K reserves, held primarily in clay minerals such as smectites and illites, which are gradually released and as such buffer K supply. These minerals are generally associated with less weathered soils. Because of this buffering capacity, some soils can be cropped for many decades without depleting K reserves, particularly if plant residues, which contain substantial quantities of K, are recycled. In contrast, N and P reserves are generally depleted more rapidly, particularly with cereal crops, and crop response to N and P can often be observed without K application, whereas the reverse is less commonly true.
Under conditions of low fertilizer use that prevail throughout SSA, low yields limit K removal, though notable exceptions exist, and are discussed below. Exchangeable K levels vary widely in Africa. Kanyanjua et al. In Nigeria, Ataga found very low K levels in sandy acid soils under palm oil production 0. Acquaye found exchangeable K levels in Ghanaian soils ranged from 0. In Tanzania, Hartemink et al. Decline in soil fertility indices is commonly observed under continuous cropping with no or limited inputs.
In most cases, declines in soil organic matter, soil pH and exchangeable bases Ca, Mg, K occur together. Hartemink found that, in general, fertility decline in Cambisols and Luvisols was substantially less, though K decline was still considerable in these soils. Mouttapa noted that the most severe K deficiencies in Africa are in the savanna belt on sandy soils of the humid inter-tropical zone, and stated further that K deficiencies generally occur in the humid forest zone only after many years of continuous cropping, and are seldom found on Cambisols or Luvisols in semi-humid regions.
Mukashema found that soil fertility indices in the Gishwati watershed in the Rwandan highlands declined over a period of 25 years under various land uses, with agricultural land degrading more rapidly than forests or pastures, and sandy acid soils deteriorating more rapidly than soils of volcanic origin. Haefele et al. They concluded that the K buffering capacity in these soils was such that yields could be sustained for decades without K addition. A seemingly obvious but often neglected fact is that farmers will adopt fertilizers only when fertilizer use is profitable.
Most farmers in SSA have limited means, and are more inclined to invest in opportunities that balance the twin objectives of return and risk. Those promoting nutrient inputs often have a tendency to direct their use to the most broadly cultivated crops - commonly cereal crops such as sorghum or maize. But these crops do not always represent the best return on fertilizer investment. In years of good production, large-scale fertilizer use can result in supply gluts at harvest, particularly in countries with low capacity to store harvest, depressing prices. If farmers are not linked to a specific market opportunity such as a milling operation or a processor or do not have access to warehouses to store produce until prices improve, low or even negative returns can result.
In many cases it is the less cultivated crops that can provide the best returns on fertilizer investment.
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As an example, leguminous crops such as groundnuts and soybeans require a relatively low fertilizer investment often only P and K as they fix their own nitrogen, but can result in strong returns as their value is high. Farmers throughout Africa invest in NPK blends for vegetables, which in general offer good returns on fertilizer investments. An additional key to accelerating fertilizer consumption is having targeted fertilizer blends.
The most available fertilizers in Africa are urea, di-ammonium phosphate and NPK blends, commonly or Formulae best suited to specific crops are commonly not available. However, African-based fertilizer blenders are increasingly creating blends for specific vegetables, legumes, cereals and root crops.
Some blends now contain secondary and micronutrients to overcome deficiencies that are limiting response to N, P and K in many environments.