The limitations to plant growth caused by compaction and water extremes can be combined into the concept of the optimum water range for plant growth—the range of water contents under which plant growth is not reduced by drought, mechanical stress, or lack of aeration (figure 6.12). This range, referred to by scientists as the least-limiting water range, is bounded on two sides—when the soil is too wet and when it’s too dry.
The optimum water range in a well-structured soil has its field capacity on the wet end, as water above that moisture content is quickly drained out by gravity. On the dry end is the wilting point—beyond which the soil holds water too tightly to be used by plants. However, the soil water range for best growth in a compacted soil is much narrower. Even after a severely compacted soil drains to field capacity, it is still too wet because it lacks large pores and is poorly aerated. Good aeration requires at least 20% of the pore space (about 10% of the volume of the whole soil) to be air filled. On the dry end, plant growth in a compacted soil is commonly limited by soil hardness rather than by lack of available water. Plants in compacted soils therefore experience more stress during both wet and dry periods than plants in soils with good tilth. The effects of compaction on crop yields usually depend on the length and severity of excessive wet or dry periods and when those periods occur relative to critical times for plant growth.