Soil health is important for the water cycle

Soil is a critical component of the water cycle. Soil captures rainwater and holds it for use by plants. Soil health is important to improve the water cycle. Below, I will describe soil water-holding properties and why soil health is important for the water cycle.

Soil’s physical characteristics determine, in part, how much water the soil can hold. Soil is made up of very fine mineral particles made from the parent material, commonly rock. The rock breaks down into smaller and smaller particles, creating what we know as soil. Water fills the spaces, or pores, between the particles and is held in the pores by the surface of the mineral particles. The amount of water that can be held by the soil depends partially on the size of the mineral particles. If you poured water over large rocks, not much water would be held. The rocks are so large, and the surface area of the soil “particles” is so small relative to the size of the rocks, that the large pores cannot hold the water.

Water is held within the pore spaces and on the surface of the particles through two forces. One of the forces is the attractive force between water molecules. The other force is the attractive force between the mineral particle and the water. Smaller mineral particles have greater surface area, smaller pores and stronger force to hold the water. The larger the individual pores, the lower the energy of attraction that holds the water within the pore. As the pores become smaller, the attractive forces between the soil mineral particles and the water increase, holding the water in the pore. Large soil pores require more energy to hold water within the pore; instead, the water drains out of the soil quickly. More water can be held in smaller pores. For this reason, clay soils, with very small particles and small pores, hold more water than sandy soils.

The mineral structure of the soil also determines how much water is available to plants. As the mineral particles of soil decrease in size, the pore size becomes very small, increasing how tightly water is bound within the pores. However, while there may be more water in the soil, it is no longer available to plants. The water is held so tightly in the pore spaces between the soil particles that plant roots are not able to remove it. Even though there is water in the soil, plants will wilt because of lack of water. The “plant-available water” is an important characteristic of soils that indicates how much water can be stored in the soil and how much water is available for the plant roots to take up. For a sandy soil, the large pores in the soil matrix do not hold water easily, limiting the amount of water that can be stored within the soil, but making the water that is there readily available to plants. In contrast, a clay soil, with very small pores, can hold more water than sandy soils but the water is not readily available to plants as it is bound tightly within the soil matrix.

Soil health is critical for determining both the amount of water that can be stored in the soil, and the amount of stored water that is available to plants. Healthy soil has good soil structure. The soil structure depends partially on the mineral elements of the soil, but also on the other components of soil—especially the biological components. The plant roots, soil microbes (bacteria and fungi), and decaying vegetation make up an important part of the soil and help give the soil good structure. A soil with good structure will form stable aggregates that allow easy infiltration of rainwater. The soil aggregates will hold water, yet release the water for greater availability for plants. While tillage initially increases the pore space in the soil, it destroys the soil structure by breaking apart soil aggregates, disrupting plant root and fungal hyphae networks, and reducing organic matter. Tilled soils eventually become more compacted because of this loss in soil structure. In contrast, no-till management preserves the plant and fungal networks, increases the organic matter in the soil, and creates soils with stable aggregates. Organic matter readily absorbs water and holds it until needed by plant roots. More soil organic matter increases the ability of the soil to absorb rain water, rather than having it run off. As the organic matter in the soil increases, the plant-available water also increases. It has been estimated that for every 1% increase in soil organic matter, the plant-available water in the soil increases by 25,000 gallons per acre. During the rapid growing phase, corn in southeast Kansas uses about ¼” of water per day. Every 4 days, a corn crop needs an additional 1” of soil water. Soils with greater amounts of organic matter increase both the amount of water held in the soil and the water available to that growing corn crop.

We cannot change the mineral composition of our soils, but we can improve the soil biological component through better management practices. Reducing tillage operations will improve soil health by preserving root and fungal networks and increasing the organic matter, which in turn contribute to better aggregate stability of the soil. Planting cover crops can also increase the organic matter in the soil. By improving the soil health, we can improve the resiliency of our soils—increasing the amount of water stored in the soil, reducing runoff and increasing the water available for crop production.