Utilisation of soil organic P by agroforestry and crop species in the field, western Kenya

Abstract

A field experiment in western Kenya assessed whether the agroforestry species Tithonia diversifolia (Hemsley) A. Gray, Tephrosia vogelii Hook f., Crotalaria grahamiana Wight & Arn. and Sesbania sesban (L) Merill. had access to forms of soil P unavailable to maize, and the consequences of this for sustainable management of biomass transfer. The species were grown in rows at high planting density to ensure the soil under rows was thoroughly permeated by roots. Soil samples taken from beneath rows were compared to controls, which included a bulk soil monolith enclosed by iron sheets within the tithonia plot, continuous maize, and bare fallow plots. Three separate plant biomass samples and soil samples were taken at 6-month intervals, over a period of 18 months. The agroforestry species produced mainly leaf biomass in the first 6 months but stem growth dominated thereafter. Consequently, litterfall was greatest early in the experiment (0–6 months) and declined with continued growth. Soil pH increased by up to 1 unit (from pH 4.85) and available P increased by up to 38% (1 μg P g−1) in agroforestry plots where biomass was conserved on the field. In contrast, in plots where biomass was removed, P availability decreased by up to 15%. Coincident with the declines in litterfall, pH decreased by up to 0.26 pH units, plant available P decreased by between 0.27 and 0.72 μg g−1 and Po concentration decreased by between 8 and 35 μg g−1 in the agroforestry plots. Declines in Po were related to phosphatase activity (R2=0.65, P<0.05), which was greater under agroforestry species (0.40–0.50 nmol MUB s−1 g−1) than maize (0.28 nmol MUB s−1 g−1) or the bare fallow (0.25 nmol MUB s−1 g−1). Management of tithonia for biomass transfer, decreased available soil P by 0.70 μg g−1 and Po by 22.82 μg g−1. In this study, tithonia acquired Po that was unavailable to maize. However, it is apparent that continuous cutting and removal of biomass would lead to rapid depletion of P stored in organic forms.