Forms of phosphorus and the fate of applied water soluble phosphorus in some Kenya soils

Abstract

Soils from Gitunmba, Kabete, Katumani and Lanet areas of Kenya were sampled for analysis. The major objectives of the investigation were to determine the quantities of the inorganic and organic forms of phosphorus and to determine the fate of applied water soluble phosphorus in these soils and their phosphorus adsorption properties using Langmuir adsorption isotherms. Gituamha soil had the highest amount of iron and aluminium phosphates, followed by Kabete, Lanet and Katumani soils in that order. Calcium phosphate was found to be low in most of the soil samples analyzed. Gituamba soil had higher levels of organic phosphorus than the other three soils. OrgRnic phosphorus in Kahete soil decreased sharply with depth. In Lanet and Katumani soi Is organic phosphorus decreased gradually with depth. The distribution of organic phosphorus was closely related to the distribution of organic matter. High organic phosphorus in Gituamba soil was associated with high soil acidity which most likely decreased microbial mineralization of orga~ic phosphorus. (viii) The amount of phosphorus fixed as aluminium and iron phosphates increased between 3 to 30 days, while calcium phosphate decreased in all the soils. Phosphorus fixed was correlated with oxalate-extractable Al203 and Fe203. Gituamba soil sorbed the highest amount of phosphorus and its phosphorus adsorption max~mum was 99 ~g pig and the constant related to bonding energy (k) was 4.4 x 10-3 M-1. Phosphorus adsorption maximum for Kabete soil was 49 ~g Pig and the k value was 6.3 x 10-3 M-1. For Katumani soil phosphorus adsorption maximum was 14 ~g Pig and the k value was 34 x 10-3 M-1. Phosphorus adsorption maximum for Lanet soil was 33 ~g Pig and the k value was 10 x 10-3 M-1. Phosphorus sorption was strongly influenced by oxalate-extractable Fe203' AI203 and organic carbon (their correlation copfficients being 0.96, 0.79 and 0.93 respectively, thtlS being highly significant for Fe203 and organic carbon). The T,angmuir constant, k was not corrected for the inita1 surface phosphate.