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SOIL FERTILITY

Soil and Plant Influences on Crop Response to Two African Phosphate Rocks

Department of Natural Resource Sci. and Landscape Arch., Univ. of Maryland, College Park, MD 20742-4452 USA

rw17{at}umail.umd.edu

Affordable technologies are needed to allow smallholder farmers to effectively use the phosphate rocks (PRs) found in many African countries. A pot study was conducted in Tanzania using two PRs (Panda and Minjingu) and two soils (an Alfisol and an Andisol) to assess responses of several types of crops to these PRs and to determine whether changes in crop responses to PR with time are due to crop sequence or merely contact time with soil. The Panda PR had no effect on growth or tissue P content in maize (Zea mays L.), bean (Phaseolus vulgaris L.), and pigeon pea [Cajanus cajan (L.) Millsp.], but it nearly tripled these parameters for cabbage (Brassica oleracea L.) on the Alfisol. Freshly applied Minjingu PR only slightly stimulated maize and pigeon pea, but nearly tripled cabbage yield in both soils. Previous crop had a greater effect than previously applied PR on second crop maize. Yields and P content of maize were always lowest following cabbage and highest following pigeon pea. Minjingu PR, but not Panda PR, had residual benefits on maize. Severe Mn toxicity occurred in all crops on the unamended Andisol. The calcareous Minjingu PR, but not the Panda PR, increased yields dramatically on the Andisol, partly by raising the soil pH in water enough (from 4.6 to 5.6) to alleviate Mn toxicity. Future P fertility work in Africa should pay adequate attention to the effects of crop sequences and soil biological properties.

Abbreviations: PR, phosphate rock • TSP, triple superphosphate • AM, arbuscular mycorrhizae