The effectiveness of deep placement of fertilisers is determined by crop species and edaphic conditions in Mediterranean-type environments: a review

Much of our knowledge of plant growth in response to soil nutrient supply comes from studies under homogeneous soil conditions. However, the adoption of reduced or nil tillage and shallow banding of fertilisers at the time of seeding causes spatially variable distribution and availability of soil nutrients in agricultural lands. Soil available nutrients, particularly the poorly mobile ones such as phosphorus (P), potassium (K), zinc (Zn), manganese (Mn), and copper (Cu), stratify within the fertilised topsoil. In water-limited environments where the topsoil is prone to drying, soil nutrient stratification may influence nutrient availability and plant uptake because of impeded root growth or reduced diffusion of immobile nutrients to the root surface, or more likely a combination of both factors. Placing fertilisers deeper in the soil profile could increase nutrient acquisition and utilisation by plants as fertiliser nutrients are in the moist soil for a longer part of the growing season. However, the effectiveness of deep placement of fertilisers may also be determined by soil texture, tillage, fertilising history, nutrient mobility, and crop species. In Mediterranean-type climates of southern Australia, a yield response of winter crops to deep fertiliser mostly occurs on infertile sandy soils in low rainfall regions. This contrasts with the responses of winter and summer crops in northern Australia on soils with optimum-to-high nutrients but subjected to rapid and frequent drying of topsoil because of high temperatures and high evaporation demand during the growing season. The pattern of nutrient accumulation by crop species (indeterminate v. determinate) and the mobility of mineral nutrients in the phloem would also modify the effectiveness of deep-placed nutrients under drought. The complexity of plant responses to subsoil nutrition may suggest that before adopting deep fertiliser practice in a paddock it is essential to understand the effects of edaphic and climatic conditions, soil management, and plant–soil interactions in order to achieve maximum yield benefit.