Subsoil phosphorus concentration and tolerance of heavily grazed legume-based pastures to dry soil conditions

Abstract

Recent research has highlighted how higher rates of phosphorus (P) fertiliser, above those that were considered to provide adequate maintenance P, have enabled higher than normal stocking rates and increases in wool production. However, rationale and physiological understanding of increased P supply on heavily stocked legume-based pastures is not well understood, particularly in terms of ability of the legume to acquire P when subjected to increased grazing pressure and soil drying. This paper explores whether the stressed plant has an increased demand for P or, alternatively, whether these stresses reduce the ability of the legume to take up water and P from the soil matrix. The total water and P uptake increased with increasing P supply for the frequently defoliated clover plants under dry conditions. This was possible due to a positive carbon balance and maintained root growth with increased P supply, which enabled plants to maintain a reduced but continued growth and prevented carbohydrate reserves from becoming completely depleted under frequent defoliation. The positive carbon balance with increased P supply also enabled the frequently defoliated plants to respond positively to the additional stress of dry soil conditions. Findings in this study suggested that a higher concentration of plant-available P in the soil matrix was required by the stressed plants for two reasons. Firstly, frequent defoliation reduced root growth. Reduced root growth in turn reduced the capacity of the roots to take up P, due to reduced surface area. Secondly, dry soil conditions increased the tortuosity in the diffusion pathway and reduced the rate of diffusion of P to the roots. Therefore, to enable the stressed plants to take up adequate P, the supply of P in the soil matrix had to be increased.