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No abstract

Ryegrass (Lolium perenne L.) was grown in a pot trial in soil of high phosphate adsorption capacity at 10 levels of P-supply and at two water regimes, one regularly adjusted to the water-holding capacity of the pots corresponding to a water potential of -2 to -3 kPa, and the other at 20% less water volume corresponding to - 20 kPa. P uptake was lower in the lower water treatment at all P-supply levels, even when the P-supply was severely limiting P uptake. This reduction in P uptake at all P levels was equivalent to the reduction in water volume between the two water treatments, and is apparently a result of a lower supply of solution phosphate in the lower water treatment in the highly buffered soil.

Plants respond to lowering Nand P nutntlOn by increasing their relative root growth (lowering S:R ratio) and decreasing their root cation exchange capacity (CEC); the 2 responses were correlated. The smaller the normal CEC of the plant the greater these changes were. Part of the competitive advantage of low CEC plants may lie in their greater increase in relative root growth in response to limiting concentrations of phosphate.

Fifteen rates of superphosphate (0-897 kg/ha ) were applied to white clover grown alone or with browntop on soil deficient in Nand P. Plots were mown and all herbage removed.Browntop reduced clover dry matter and N yields, especially when swards were younger or when fertiliser N was applied when swards were older.Competition for P was important when the supply of P was suboptimal and when swards were younger, but with time, other factors, which probably included shading of clover stolons by the browntop mat, became more important. Increases in superphosphate, often substantial, only partially overcame the reduction in yield. These competitive factors affected clover herbage N yields but did not alter increases in soil N.It is suggested that competition will be stronger under grazing than under mowing, and it limits the realisation of the N-fixing potential of white clover.