Sustainable long-term intensive application of manure to sandy soils without phosphorus leaching: a case study from Ghana

摘要

Long-term application of manure to sandy soils to ensure high crop productivity may lead to phosphorus (P) leaching, which, in turn, may deteriorate the quality of recipient waters because of eutrophication. The risk of P leaching depends on contents of aluminum (Al) and iron (Fe) oxides that are strong P sorbents and of calcium (Ca) that can form sparingly soluble Ca-P compounds. While P sorption by Al and Fe oxides is limited, the formation of sparingly soluble Ca-P may theoretically continue as long as enough Ca is available, i.e. under such conditions long-term application of manure may be environmentally sustainable. This hypothesis has been tested on soils formed on beach sand at Keta, Ghana, which have been used since the 1930s for intensive cultivation based on manure application. Soil samples were collected from different depths down to 80 cm in profile pits on cultivated soils that received manure over 70 years and on uncultivated plots (controls). The samples were analyzed for texture, pH, total C and P and poorly ordered Al and Fe oxides as well as different P forms as assessed by the Hedley fractionation method. The results showed a fourfold P increase in the top 40 cm of the cultivated soils, whereas in the deepest soil layers the P contents in the cultivated and uncultivated soils were almost the same indicating very limited downward P transport despite long-term manure application. This was supported by comparable P concentrations in groundwater taken under cultivated and uncultivated sites. P fractionation showed that the cultivated top soils were dominated by sparingly soluble Ca-P compounds that accounted for about 70% of the P gain. Thus, the results suggest that intensive crop production under tropical semi-arid conditions may be environmentally sustainable when based on manure fertilization ensuring enough Ca supply and neutral to alkaline pH.