Abstract
With the wide adoption of conservation agriculture (minimal soil disturbance, stubble retention, crop rotation), soil nutrient stratification is becoming more prevalent especially for poorly mobile phosphorus (P), potassium (K), copper (Cu), zinc (Zn), and manganese (Mn) that concentrate in the fertilized topsoil (0–10 cm). In water-limited environments, surface soil drying limits root access to the topsoil nutrients, but the nutrients in moist subsoil may play a substantial role in crop nutrition and growth. Although the subsoil is generally lower in available nutrients and organic matter than the topsoil, there is strong evidence that subsoil can contribute significant amounts of nitrogen (N), P, and K taken up by crops. Placing fertilizers deeper in soil profiles increases plant nutrient efficiency in low rainfall regions, because deep fertilizing can induce deeper root growth and leave fertilizer-supplied nutrients in moist subsoil for longer periods during the growing season. However, the contribution of subsoil nutrition to crop growth is limited by subsoil constraints that restrict deeper rooting, including physical constraints, e.g. gravel layers and soil compaction, and chemical constraints, e.g. acidity, alkalinity, salinity, sodicity, nutrient deficiency, and element toxicity. On the other hand, crops and genotypes efficient in nutrient uptake under drought are likely to have an extensive, deep root system and thus a large surface area of contact between roots and soil. The uptake of soil water from moist subsoil and its release into dry topsoil by roots – hydraulic redistribution – may maintain the growth of fine roots and thus prolong nutrient uptake from drying surface soils. A good understanding of subsoil nutrient acquisition by crop species and their response to subsoil constraints is required for designing crop rotations and nutrient management programmes that allow for effective use of subsoil water and nutrients, especially in rainfed agriculture.