Soil physical conditions of winter and the growth of ryegrass plants

Six pot trials with seedling plants of perennial ryegrass ( Latium perenne L.) have been carried out to investigate the effects, on growth, of compaction and saturation of the topsoil. In each trial identical soil was packed, in 1 em layers, into a number of 17 em‐diameter pots, treatments comprising different degrees of compaction or puddling in the surface 5 em of the soil. Seedling plants at the two‐leaf stage were planted in holes 1 em in diameter and 1.2 em deep, their root systems being previously cut down to one root 1.2 em long. The pot soils were kept at high and constant moisture contents by sub‐irrigation from shallow water tables, rain being excluded, and the trials were run at winter temperatures. Growth was related to measurements of physical properties of the surface 5 em of the soil. These comprised dry density, moisture content, volume of air pores, the diffusivity of oxygen gas through an extracted core of soil, and the current flowing to a platinum micro‐electrode at a depth of 2.5 em. Taking the trials as a whole, growth did not appear to be influenced by variations in soil moisture, within the range found in the pots. In these soft wet soils it fell with increasing soil density only when higher density was accompanied by low soil aeration. Of the three measures of soil aeration used, currents to the platinum cathode gave the most consistent relationship with plant growth, growth being invariably reduced when currents averaged below 2 micro‐amps. Above 2 microamps any increased growth accompanying higher electrode currents was smaller or undetectable. This critical level is considerably lower than the optimum levels of electrode current found for crop plants by overseas investigators. Reduced growth was associated with intrinsic gaseous diffusivities of. 005 or less, a very low value and barely measurable with the apparatus used. The lowest content of air pores associated with optimum growth varied from 7 or 8 per cent of the soil volume in some trials down to about 2 per cent in one trial. As determined on core samples, it appeared to be higher for structureless soils previously puddled and partly dried than for granular flooded soils.