Phosphorus Status, Inorganic Phosphorus Forms, and Other Physicochemical Properties of Acid Soils of Farta District, Northwestern Highlands of Ethiopia

Soil acidity and low availability of P limit crop production in the highlands of Ethiopia. The objective of this study was to determine the P status, distribution and forms of inorganic P and relate them to selected chemical properties of eight representative acidic surface soil samples from Farta District. Soil pH (H 2 O) varied between 4.74 and 5.50. The moderate to high CEC suggests that besides kaolinite, the soils also contain expandable 2 : 1 clay minerals. Though the total P content was high, the available Olsen P content was very low or low in all soils except one. In most soils, the abundance of inorganic P fractions was as follows: P bound by oxalate extractable iron (Feox-P)>reductant soluble Fe-P>occluded Al-Fe-P>P bound by oxalate extractable aluminum (Alox-P)>calcium bound P (Ca-P). Olsen P had a very strong positive correlation (P≤0.01) withAlox-P (r=0.98),Feox-P (r=0.96), and oxalate extractable P (r=0.94). Though Fe bound P reserves were quite abundant and the degree of P saturation ofAlox+Feox(median 3.3%) was moderate, the extremely low P saturation ofAlox(median 0.5%) explains the P deficiency of the soils.