THE RELATION BETWEEN BULK DENSITY, AVAILABLE WATER CAPACITY, AND AIR CAPACITY OF SOILS

Summary The effect of bulk density on moisture content at 50 mb tension in four soils of different textures was studied. The volumetric water content increased linearly with bulk density over a wide range of densities. Depending on texture, a maximum bulk density was reached above which continued compaction decreased the water content. This is shown to be the point at which the air capacity of the soil at this tension approaches zero. Accepting that the gravimetric wilting point depends mainly on texture, the available water capacity varies in a manner similar to the 50 mb water content. If the relationships described are valid in the field, the available water capacity and air capacity may be optimized using cultivation techniques to adjust the bulk density. The available water capacity of coarse‐textured droughty soils may be increased by increasing the bulk density provided that the air capacity remains above acceptable lower limits (10–15 per cent). Conversely, the air capacity of compacted soils with large available‐water capacities could be increased by reducing the bulk density to a value corresponding to an acceptable available‐water capacity. In very compacted soils a decrease in bulk density will benefit both available‐water capacity and air capacity.