An evaluation of the causes of soil infertility in Niger

John William Scott-Wendt, Purdue University

Abstract

Poor soil fertility is a major constraint to agricultural production in Niger and throughout West African Sahel. Crop stands can diminish from highly productive to completely barren regions over distances as short as 2 meters. The Barber-Cushman model for nutrient uptake was used to evaluate the supply of nutrients in adjacent productive and unproductive soil pairs from 19 sites in the Niamey and Maradi regions of Niger. Phosphorus was severely deficient in all productive and unproductive soils. K was also deficient, but modelling of K uptake did not account for K supply from soil micas. Unproductive soils were deficient in Mg. Productive soils mineralized an average of 260% more N than did unproductive soils. Since soils were sampled at the end of the growing season after nutrients had been depleted, a further study to examine P supply after P fertilization was undertaken. Productive and unproductive soils were fertilized with 0, 5.5, 10.9, 21.8. and 43.7 $\mu$g P $\cdot$ g$\sp{-1}$ soil. Most productive soils supplied adequate P at the lowest fertilization rate, while unproductive soils supplied almost no P. Predicted P uptake correlated best with soil solution P (r$\sp2$ = 0.99), and correlated poorly with labile P (r$\sp2$ = 0.40). Soil solution P correlated strongly with exchangeable Al and oxalate-extractable Fe (R = 0.93 at the highest P fertilization rate). Mixing the soil in a 0-15 cm deep soil sample had the effect of reducing P availability when compared to the P available in separate 0-5, 5-10, and 10-15 cm depth increments. Mixing soils from various depths tends to reduce P availability by permitting the deeper soil material, which has a relatively high P adsorption capacity, to adsorb P from surface horizons. Soil clays were dominated by kaolinite, and had small amounts of goethite, mica, and hydroxy-interlayered smectite. The study shows that (1) unproductive soils are deficient in P, K, and Mg, have a slower rate of N mineralization, and adsorb large amounts of applied P relative to productive soils; (2) P adsorption is strongly related to exchangeable Al and oxalate-extractable Fe; and (3) P availability can best be evaluated by soil solution P analysis rather than by standard "labile P" tests. It is recommended that soil solution P analysis be adopted as the standard procedure for P evaluation in Niger.

Degree

Ph.D.

Advisors

Ahlrichs, Purdue University.

Subject Area

Agronomy

COinS