Impact of long-term pig slurry application on phosphorus fractions in profiles of contrasting soils in KwaZulu-Natal, South Africa

Abstract

Long-term applications of pig slurry (PS) can lead to phosphorus (P) accumulation in soil, but its distribution across soil profiles remains poorly studied, particularly after 50 years of PS application on pasture soils, compared to undisturbed and cropped Luvisols, and across different durations on Ferralsols under maize-soybean rotations in KwaZulu-Natal, South Africa. Estimated annual P additions through PS were 147  kg P ha⁻¹ in Luvisol and 58.8  kg P ha⁻¹ in Ferralsol, plus 30  kg ha⁻¹ from mineral fertiliser. Soil samples from multiple depths were analysed for total P, available P, and other P fractions. Total P increased to 2 824 and 3 415  mg kg⁻¹ in Luvisol and Ferralsol, respectively. Available P reached 101 (Luvisol) and 112  mg kg⁻¹ (Ferralsol) in the top 400  mm of soil. Organic P also increased, reaching 2 597  mg kg⁻¹ (Ferralsol) and 1 891  mg kg⁻¹ (Luvisol). In both soils, Fe-P (355 and 491  mg kg⁻¹) and Al-P (255 and 239  mg kg⁻¹) increased after over 20 years of PS application. In Luvisol, Al-P accumulation (255  mg kg⁻¹) was confined to the top 100  mm, while in Ferralsol it extended to 400  mm. This indicates a low risk of leaching but a high risk of surface water pollution through runoff and erosion. The study underscores the importance of monitoring the effects of PS application to mitigation environmental risks while maintaining soil fertility.