Retention and release of applied phosphorus by the benchmark soils of Lesotho

English: A lack of information on the behaviour of applied phosphorus (P) in and on the P requirements of the benchmark soils of Lesotho precludes the establishment of proper application recommendations of P fertilizer for crop production. Therefore, the objectives of this thesis were to determine the P retention capacities and P requirement factors of these soils and identify soil properties implicated therein. Eleven benchmark soil senes, each replicated at least five times were sampled in the lowlands and foothills for P retention studies. These soil samples were prepared as usual for laboratory analyses and characterized especially for the soil properties implicated in P retention. The soil series varied with regard to those properties, providing a good indication of their variability in Pretention. A pilot experiment was conducted on some of the soils to investigate an optimum incubation period for P retention and release studies. The soils were incubated with P levels equivalent to 0, 50, 100, 200 and 400 kg P ha-1, respectively at 85% water filled porosity, which was checked fortnightly. The experiment was laid out in a randomized complete block design at room temperature. Phosphorus retention was then determined 7, 14, 21, 28, 42 and 63 days after P application using the Olsen extraction procedure. The results of this experiment indicated an incubation period of 42 days as practically suitable for Pretention and release studies on the benchmark soils. Thereafter a P retention experiment with Il soil series, five P levels and five soil phases, all replicated three times, was conducted. The soils were incubated with the same levels of P as in the pilot experiment, for 42 days at 85% water filled porosity that was again checked fortnightly. The experiment was set-up in a split-split plot design, at room temperature. After 42 days P was extracted with Bray and Olsen extractants, respectively and retained P was calculated as the difference between applied and extracted P. Percentage of applied P retained against the Bray and Olsen extract ants varied from 6 to 97 and 21 to 91, respectively. The slopes of retained P against applied P, viz. P retention indices (PRI's) varied from -0.04 to 0.28 for Bray and 0.07 and 0.27 for Olsen while the respective slopes of retained P against extracted P, viz. P buffering indices (PBI's) varied from 2 to 55 and 12 to 103. The PRI's and PBI's were closely correlated, r = 0.94 and 0.81 (P < 0.0001) for Bray and Olsen, respectively. The series Fusi, Thabana, Machache and Sefikeng had higher PRI's and similarly higher PBI's than the rest of the series, viz. Khabos, Leribe, Rama, Sephula, Tumo, Matela and Berea, particularly according to the Bray method. In the case of the Olsen method however, the series Khabos and Tumo had comparable PBI's with the series Fusi, Thabana, Machache and Sefikeng though their PRI's were significantly lower. The series Fusi, Thabana, Machache and Sefikeng also had the highest P retention capacity at an application of 400 kg P ha-1 (PRC at P400) of about 69 to 83% for Bray and 75 to 81% for Olsen. The respective PRC at P400 for the rest of the series were 0 to 39 and 47 to 66%. For all the soils the slopes of applied P against extracted P, viz. P requirement factors (PRF's) ranged from 0.85 to 1,1.40 (Bray) and 1.45 to 9.07 (Olsen). The high Pretaining series (Fusi, Thabana, Machache and Sefikeng) had high mean PRF's of 3.36 to 7.13 for Bray and 3.85 to 5.47 for Olsen. For both the Bray and Olsen procedures the parameters PRI, PRC at P400, PBI and PRF were with a few exceptions highly correlated (r2> 0.60) with sample density, sand, clay, organic carbon, cation exchange capacity, acid ammonium oxalate and citrate bicarbonate dithionite extractable iron and aluminium. Multiple linear regression models were also obtained for each of the parameters with some of the soil properties. Recommendations were made with regard to reducing P retention and increasing P availability in the high P sorbing soils and hence to improve crop production.