Effect of soil physical condition, and phosphorus and nitrogen availability on pasture persistence

Soils are the substrate at the base of any grazing or cropping system providing plants with nutrients, water and physical support, a habitat for beneficial organisms and a physical base on which animals stand. A change or a loss of any one of these soil services has the potential to impact on plant growth and persistence. The findings of two ongoing field-based studies are presented and discussed as they relate to the current debate on pasture persistence. One study is investigating the influence of the physical condition of the soil on a ryegrassbased pasture response to added phosphorus (P). The second study is investigating the P requirements of high producing perennial ryegrass and tall fescuebased pastures, where the constraints to pasture growth and associated P uptake imposed by low nitrogen availability and low soil moisture over summer-autumn have been removed. The critical Olsen P value for maximum pasture production appears to increase as the constraints to pasture growth are removed. This challenges the continued use of a single relative yield Olsen P function and one critical Olsen P value for near maximum yield regardless of the absolute pasture yield. Loss of soil pore function also appears to increase the critical Olsen P value necessary for optimum pasture growth. Additional P can compensate for a lack of pore function to a degree, but there appears to be an upper limit to this effect. Given that the physical condition of many of our pasture soils in intensive lowland situations is below optimum, nutrient stress may be more of a factor in poor pasture-persistence, than previously thought. Inclusion of a measure of soil physical condition would appear to warrant further study in defining the conditions for optimum plant growth and persistence. Keywords: fertiliser nitrogen, Festuca arundinacea, Lolium perenne, Olsen P, pasture persistence, soil pore function, soil services