There is an additional dimension to plant-available water capacity of soils: The water in the soil may be available, but roots also need to be able to access it, along with the nutrients contained in the water. Consider the soil from the compacted surface horizon in figure 5.6 (left), which was penetrated only by a single corn root with few fine lateral rootlets. The soil volume held sufficient water, which was in principle available to the corn plant, but the roots were unable to penetrate most of the hard soil. The corn plant, therefore, could not obtain the moisture it needed. The corn roots on the right (figure 5.6) were able to fully explore the soil volume with many roots, fine laterals, and root hairs, allowing for better water and nutrient uptake.
Similarly, the depth of rooting can be limited by compaction. Figure 5.7 shows, on the right, corn roots from moldboard-plowed soil with a severe plow pan. The roots could not penetrate into the subsoil and were therefore limited to water and nutrients in the plow layer. The corn on the left was grown in soil that had been subsoiled, and the roots were able to reach about twice the depth. Subsoiling opened up more soil for root growth and, therefore, more usable water and nutrients. Thus, plant water availability is a result of both the soil’s water retention capacity (related to texture, aggregation, and organic matter) and potential rooting volume, which is influenced by compaction