5.4: Available Water and Rooting
- Page ID
- 25129
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There is an additional dimension to plant-available water capacity of soils: The water and nutrients not only need to be stored and available in the soil pores, but roots also need to be able to access them. This may be a problem if the soil is compacted. 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 holds sufficient water, which in principle would be available to the corn plant, but the roots are unable to penetrate most of the hard soil volume. The corn plant, therefore, could not obtain the moisture and nutrients it needed. Conversely, the corn roots on the right (Figure 5.6) are able to fully explore the soil volume with many roots, fine laterals, root hairs and mycorrhizal fungi (not shown) 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 (a hard layer right below the depth of tillage). The roots cannot penetrate into the subsoil and are therefore limited to water and nutrients in the plow layer near the surface. The corn on the left is grown in soil that was subsoiled, and the roots are able to reach about twice the depth. Subsoiling opens 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 strongly influenced by compaction.
Different crops need different amounts of water, supplied by precipitation or by irrigation. For example, crops like alfalfa require a lot of water for maximum yields and the plant’s long taproot helps it access water deep in the soil. On the other hand, vineyards and crops such as wheat need much smaller amounts of water. And many crops such as corn and potatoes are in between in their water needs. This may influence farmers’ choice of crops to grow as some regions of the United States and other parts of the world are projected to become drier and warmer as the climate changes and water for irrigation becomes harder to obtain.