This paper discusses the merits of an alternative food crop, cassava Adanihot esculenta, as a means to alleviate the effects of climate-induced drought on human and livestock food security in southern Africa. Although both tubers and leaves of cassava can be processed for consumption, the crop is grown to a very limited extent in this region. However, local awareness of cassava's potential has increased over the last decade.
Abstract. Nitrate leaching in lysimeters containing a tropical sandy agricultural soil was studied over two summers with maize (Zea mays L.) and one winter season with wheat (Triticum aestivum L.). The treatments included two moisture regimes and two nitrogen sources, cattle manure and inorganic fertilizer‐N (either ammonium nitrate or ammonium sulphate) applied at 100 kg N/ha in the summers. Neither manure nor fertilizer‐N was applied in the intervening winter. Leachate volume from the manured lysimeters was mostly larger than from fertilized ones because of poor growth and less evapotranspiration. The largest seasonal nitrate loads (17–39 kg N/ha) were obtained in the wet summer immediately after installation of the lysimeters. Nitrate loads in winter (3.7–18.6 kg N/ha) were larger than those obtained in fertilized (0.6 and 9.3 kg N/ha) and manured (0.3 and 3.0 kg N/ha) lysimeters for the two moisture regimes in the second summer. The drier conditions in the second summer decreased N‐mineralization and leaching of manure.
The fate of 15N-ammonium sulphate fertilizer that was applied to four lysimeters in the 1990/91 summer was studied over three consecutive growing seasons during which either maize or wheat was grown. Aboveground portions of tSN-labelled maize plants from the first harvest were applied to four other lysimeters at 5 t ha -1. Two lysimeters in each of the sets of four were assigned a low and a high moisture treatment using irrigation. In both moisture treatments, plant recovery of fertilizer-15N in the first season was 27 % and a further 2% was recovered by plants during the next two seasons. During the second and third seasons, total recovery of 15N by aboveground plant portions from lysimeters that received t5N-labelled maize material was equivalent to 2.5% of applied fertilizer-15N. This corresponded to ca. 18% recovery of the 15N added in maize material. Leaching of fertilizer-N over the three growing seasons did not exceed 0.3% in total. During the first season, a maximum of 0.25 kg N ha -I , equivalent to 0.25% of the applied fertilizer-N, was leached in the high moisture treatment. This represented 1.8% of the nitrate load in leachates. Less than 0.002% of the applied fertilizer-N was leached in the low moisture treatment during the first season.
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