Abstract
We tested the effects of triple superphosphate (TSP) and Minjingu phosphate rock (MPR), when applied at phosphorus (P) rates of 50 or 250 kg P ha−1 in a factorial combination with urea or Tithonia diversifolia green manure as nitrogen sources, on P availability and maize yields for two seasons at Nyabeda and Khwisero in Kenya. Phosphorus availability was determined by the Olsen method or sequential fractionation. There was no significant difference in Olsen P as influenced by TSP and MPR at 50 kg P ha−1 irrespective of the N source at both sites in both seasons. However, at 250 kg P ha−1, TSP gave significantly higher Olsen P than MPR. The labile P fractions generally followed the same trend as the Olsen P. Maize yields increased with increasing amount of P applied. Generally, there was no significant difference between TSP and MPR on maize yields irrespective of the N source. The Olsen-P, Resin-P, and sodium bicarbonate inorganic P correlated well with maize yields when TSP was used but the correlations between these P tests and maize yields for MPR were not consistent and therefore their use on soils treated with MPR should be exercised with caution.
Highlights
More than 80% of the soils in the densely populated highlands of western Kenya are inherently low in P and this seriously limits the productivity of maize which is the staple food crop in the area [1]
To increase the range of organic materials (OMs) that smallholder farmers can use in integrated soil fertility management (ISFM), some nontraditional sources of nutrients such as agroforestry shrubs are being tested with Tithonia diversifolia green manure showing great promise [7]
The nutrient inputs were evenly spread within the appropriate experimental plots and incorporated to a depth of 0–0.15 m at the time of planting in each of the seasons, apart from Minjingu phosphate rock (MPR) or triple superphosphate (TSP) at the P rate of 250 kg ha−1, which were applied in the first season only
Summary
More than 80% of the soils in the densely populated highlands of western Kenya are inherently low in P and this seriously limits the productivity of maize which is the staple food crop in the area [1]. The use of organic materials (OMs) and phosphate rocks (PR) has in particular received considerable research attention in recent years [3,4,5]. This has been given impetus by a paradigm shift in soil fertility management towards the integrated soil fertility management (ISFM). In this strategy, the use of organic and inorganic nutrient sources in combination is advocated on smallholder farms [6] with the OMs being utilized mainly as sources of N, because of their low P content, while inorganic fertilizers are used to supply P. The combination of OMs such as tithonia with inorganic P fertilizers is attractive since it has been demonstrated to reduce P sorption and increase P availability in these P fixing soils [8]
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