Soils hold water in them, and they provide that water to plants. When there isn’t enough water in the soil, plants suffer drought stress. Drought stress can also lead to nutrient deficiencies, because roots can only take up nutrients that are dissolved and moving with water in the soil. Furthermore, when soil moisture declines, so does microbial activity, and nutrient cycling slows. To avoid drought stress, we usually manage soils for more water holding capacity and water infiltration, taking into account the constraints of certain soil types.
The exception to trying to increase water infiltration is the case of an excessively drained (i.e., sandy) soil. With coarse and sandy soils, increasing infiltration rates does not improve soil quality, because the rate of infiltration is already plenty high. The solution for coarse soils, as it is in all cases, is the addition of organic material. While it is easy to reduce soil organic matter, it is extremely difficult to make significant long-term improvements in soil organic matter levels in sandy soils. When there is more oxygen available, aerobic organisms in the soil will break down (i.e. consume) more organic matter.
In coarse and sandy soils, as in almost every situation, for healthy soils, it is a good idea to consider keeping the soils covered. If they’re being cropped, reduce and no-till planting methods and creating and maintaining a surface layer of plant debris will help maintain soil structure, and reduce evaporative loss. Coarse-textured soils inherently have a lot of oxygen available, and tillage adds more. Tilling burns up soil organic matter like blowing on a fire; you’ve just added more oxygen which fans the flames. The more we can keep soil covered, the more we can build organic matter, increasing the those two key components: water holding capacity and water infiltration.
Healthy Soils Don’t Hold Too Much Water
We know that plants need enough water to grow, but plant roots also need oxygen (this is why prolonged flooding and ice-sheets can kill plants). Too much water is also a problem. Beyond the lack of oxygen, water-saturated soils also create conditions favorable to crop pathogens.
Fine-textured soils tend to restrict infiltration, which keeps water on the surface or in the upper levels of the soil profile. With too much water, we get flooded and ponded conditions.
There are three ways to help move excessive water through the soil profile, 1) construct appropriate drainage systems; 2) break up the restrictive layer (e.g., sub-soiling); and 3) add organic matter within and throughout the restrictive layer. The first idea usually involves tile drainage, which is a topic for another time. The other two methods, which often work together, are worth discussion here.
Subsoiling creates paths for water and roots to move through the restrictive layer. This reduces saturated conditions, and allows roots to access a greater volume of soil than would otherwise be available, including during dry periods of the summer. This and a few other types of careful soil disturbance can facilitate the deposition of organic material (plant roots) throughout the soil profile. Subsoiling isn’t always the answer, though. You may have really stony fields and pastures, you may have steep hills, or you may just not want to put your resources in that direction.
If you do subsoil, unfortunately, it often does not take long for the restrictive layer to re-form in the zones through which the subsoiler shanks passed. If you find this happening, one option is to wait until a field is rotated back into a grass crop and then to subsoil after a hay cutting when conditions are as dry as they typically get in our area.
Other tools break up the soil and help get organic matter deeper in the profile. Manure application spreads nutrients and organic matter on the surface, but now there are lots of equipment options for manure injection. “Bio-drilling” plants with deep taproots can also break up subsurface compaction.
Overall, if you’re trying to get more water in your soil, but not too much, you can keep planting deep-rooted crops, and avoid too much disruption. And you need to use one other tool: patience.
Thanks to Rachel Gilker for helping with this article.