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Evaluation of Phosphorus Fixation Capacity of Tropical Savannah Soils of Western Nigeria¹

ABSTRACT

It is generally believed that tropical soils fix most Phosphorus (P) applied as fertilizers into forms not available to plants. To study fixation characteristics of savannah soils of western Nigeria, 22 surface soil samples varying in cropping histories, physical and chemical properties, and derived from igneous and metamorphic rocks were evaluated for capacities to fix soluble-P with time.

The fractional recovery (FR) which is the proportion of added P extracted by Bray's P₁ (0.03N NH₄F + 0.025N HCl) decreased with time following a pattern of rapid decline during the first few days of incubation and a slower decline thereafter. The decrease suggests an exponential relationship, and fits into the equation

FR = -K ln t + C.

The equations were used in calculating probable residual values of applied-P at times within or outside the range of the experimental period.

The high FR and residual values indicate that these soils fix little P in contrast to earlier-held views about the P-fixation capacity of tropical soils. The FR was significantly correlated with organic matter, clay, citrate-dithionite-extractable oxides of Fe and Al while there was no correlation with soil pH.

¹ Contribution from the Dep. of Agronomy, Univ. of Ibadan, Nigeria.

² Graduate Student, Dep. of Agron., Univ. of Ibadan; and Research Officer, Nat. Inst. for Horticultural Res., Ibadan, respectively.

Received for publication December 26, 1979. Accepted for publication January 19, 1981.