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Abstract: It is generally accepted that phosphorus is taken up by plants and fungi as inorganic phosphate. In addition to utilising soluble, inorganic phosphate, recent work suggests some microorganisms can mineralise a number of organic phosphorus compounds extracellularly and subsequently absorb the inorganic phosphate (Casida, 1959; Cosgrove, Irving and Bromfeld, 1970; Ko and Hora, 1970; Bartlett and Lewis, 1973). Also, the production of organic acids by some fungi allows them to solubilise insoluble inorganic phosphates such as tricalcium phosphate (Johnston and Miller, 1959). However, much of the soil phosphorus is bound in complex organic compounds, and is unavailable to plants and micro-organisms. Thus for the purpose of crop production many field soils are supplemented with phosphate fertilisers. In studies on the nutrition of mushroom mycelium in liquid culture by Styer (1928), Treschow (1944) and Bohus (1959), phosphorus was supplied as a soluble inorganic salt. Styer and Bohus both found that the omission of phosphate was more inhibitory to growth than omission of other minerals. Treschow determined the optimum concentration of phosphate in a synthetic medium to be 0.005 to 0.008 M. Below 0.0017 M. phosphate, mycelial growth was greatly reduced. He also found that concentrations above the optimal range were inhibitory, this effect being reduced in the presence of calcium ions. There have been a few attempts to correlate the phosphorus content of horse manure/straw composts with yield of mushrooms. Stoller (1943), Solari (1956) and Lemer (1959) gave P2O5 levels of 1%, near to 1% and 0.8 to 1.3% respectively for pasteurised compost. Bretzloff and Fluege11 (1963) determined total and available phosphorus contents of composts from stacking through to the end of the crop. They recorded a rise in the total level from 0.32% to 0.062% on a dry weight basis by the time of spawning. This value declined to 0.55% by the end of the crop. Available phosphorus increased from 0.1% to 0.22% at spawning amd rose a little higher by the end of the crop. However, the relative availability of phosphorus from stacking to spawning declined and they suggested low phosphorus availability could reduce yield. In support of their suggestion they recorded low yielding composts were low in phosphorus.
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