Soil physical conditions of winter and the growth of ryegrass plants

Three pot trials were conducted to compare the growth of ryegrass plants in soil having free access of air with growth in similar soil from which the air had been displaced by nitrogen gas. In the nitrogen‐flushed soils levels of soil oxygen, as indicated by the current flowing to a platinum electrode at a depth of 2.5 em, were mostly below the critical level of 2 micro‐amps previously found just adequate for seedling ryegrass plants. In the soils permeated with air, soil oxygen was above this critical level. In two trials on seedling plants with initially small root systems, ryegrass roots grew freely through surface soil in which the mean flux of oxygen to a platinum electrode was 7.5 × 10 ‐8 g cm> min ‐1 , corresponding to a current of 1.5 micro‐amps. This is considerably less than the oxygen fluxes found by overseas workers to be prohibitive to root growth of Newport bluegrass. As in previous experiments the growth of the seedling plants was reduced in the treatments with low soil oxygen. Lower levels of carbon dioxide in the atmosphere about the leaves in these treatments may have contributed to this result in the present trials. In a third trial the gas flushing treatments were applied to plants already possessing well‐developed root systems. Although further root growth was reduced in the ill‐aerated soils, top growth was not affected.