Retention of applied phosphorus by the benchmark soils of Lesotho and quantification of their phosphorus retention indices

Abstract

This study was conducted with the objective of determining and quantifying the capacity of Lesotho's benchmark soils to retain applied P. The soils were incubated with a range of P levels for a period of 42 days, whereafter P retention was determined. During incubation the soils were kept at room temperature and at a water content of 85% of porosity. Indices used to describe the P retention characteristics of the soils were: (i) percent Pretention across all the P application levels; (ii) phosphorus retention index (PRI), which is the slope of retained P versus applied P; and (iii) phosphorus retention capacity at an application level of 400 kg P ha-1 (P400). Two P extraction procedures, namely the Bray-1 and Olsen methods, were used. According to the Bray method, Pretention ranged from 7 to 87% across all the P application levels, and from 1 to 83% at P400. Values of PRI varied from -0.063 to 0.237. The soil series were grouped in the following descending order of the P retention indices: Fusi, Machache, Sefikeng and Thabana > Khabos, Sephula and Tumo > Berea, Leribe, Matela and Rama. In the case of the Olsen method, P retention ranged from 53 to 83% across all P application levels, and from 47 to 81% at P400. Values of PRI varied from 0.108 to 0.232. The grouping of the soil series according to these P retention indices was slightly different from the Bray method, viz. Fusi, Machache, Sefikeng and Thabana > Khabos, Matela and Tumo > Berea, Leribe, Rama and Sephula. The conclusion, therefore, is that the Fusi, Machache, Sefikeng and Thabana series exhibit high P retention characteristics and the Berea, Leribe and Rama series exhibit low P retention characteristics, while the remaining soil series, viz. Khabos, Matela, Sephula and Tumo, had moderate P retention characteristics. This grouping of the benchmark soil series can facilitate P fertilization recommendations in Lesotho.