Impact of a Decade of Tillage, Crop Rotation and Residue Management on Subsoil P and N Dynamics in Haplic Plinthosols Data

Conventional tillage (CT) has shaped agricultural landscapes for decades and has long-standing use in South Africa. However, recently, it has been criticised for being unsustainable, prompting a re-evaluation of CT practices in the context of South Africa’s agricultural sustainability goals. This study investigated the effect of conservation agriculture (CA) on soil profile distribution of P and N stocks in the Eastern Cape, South Africa. Conservation agriculture is the farming practice that helps protect arable land from degradation. The trial was laid out in a randomised complete block design with strip-split-split plot treatment structure. The main plots, sub plots, sub-sub plots, and sub-sub-sub plots were allocated to no-tillage (NT) and conventional tillage (CT); crop rotations maize-fallow-maize (MFM), maize-soybean-maize (MFS), maize-wheat-maize (MWM), maize-wheat-soybean (MWS), and crop residue management practices; residue retention (R+) and residue removal (R-), and soil depths 0 – 20 cm, 20 – 40 cm, and 40 – 60 cm. Analysed soil samples were collected in the 2023/24 cropping cycle. Soil pH was significantly higher (P<0.05) under CT with R⁺, especially in legume-based rotations like MFS (7.12) and MWS (7.10), likely due to the less acidic nature of legume residues. Electrical conductivity peaked in NTR⁺ × MFM (68.70 mS/m) compared to CTR⁻ × MWS (27.73 mS/m) at 0–20 cm, attributed to improved moisture retention. NT increased phosphorus by 67.62 mg/kg which was 40.49% more than CT at 0–20 cm, with both NT and CT showing nutrient decline with depth. Total nitrogen was concentrated in surface layers under NT, which also enhanced ammonium (NH₄⁺) retention and reduced nitrate (NO₃⁻) leaching. Importantly, the study revealed novel insights into nutrient dynamics at deeper soil layers (40–60 cm), a zone often overlooked in soil fertility research. Calcium and magnesium were more abundant at depth, while potassium accumulated near the surface and sodium leached downward, likely due to ionic charge properties. These findings underscore the importance of integrated soil management practices and highlight the value of examining subsurface layers to better understand long-term soil health and nutrient distribution.