Understanding the Basics of Water in Soils


In much of the world, water is the most limiting factor that affects crop plant growth. In Minnesota, the challenge is to manage crop production where relatively small amounts are held by the sandy soils. Conversely, it’s necessary to remove excess water when the fine-textured heavy soils are used for crop production.

Recognizing, that it’s important to manage water in order to achieve optimum crop yields, a basic understanding of water in soils is important.

There are two attractive forces that work together and affect the availability of water in soils. Water molecules are attracted to each other via cohesive chemical bonds. Although water (H2O) is a molecule, there is a positively charged end as well as a negatively charged end. These positive and charges are like the positive and negative terminals in a battery but current does not flow. So, water molecules are attracted to each other. This attraction is called cohesion. The water molecule is also attracted to soil particles. This is an adhesive force which is much stronger than the cohesive forces.

Any volume of soil contains solid materials (about 50%) and pore spaces (space between soil particles). This pore space is about 50% of any volume of soil.

Consider what happens to water that enters soil after a heavy rain. At first, the pores are filled with water. This is called saturation – a common term. For some soils (sandy, silt loams) this water does not normally remain in the soil pores. It moves downward through the soil profile in response to gravity. By contrast, for fine textured soils (clay loams, silty clay loams) this extra water can remain in the pores unless removed with tile. Most of the extra water, called GRAVITATIONAL WATER, drains through the root zone of a well-drained soil in about 48 hours. This movement takes longer in the fine textured soils.

When drainage stops, the soil moisture level is at FIELD CAPACITY. At this point, air fills the larger pores and thick films of water (cohesive water) surrounds each soil particle. This is ideal moisture and plants grow best at this moisture level. There are some who believe that tiling soils can lead to drought. This is not the case. Tile lines simply remove gravitational water. Drainage ceases when field capacity is reached.

When drainage stops, water use by plants and evapotranspiration continue to deplete the cohesion water. The size of the water films around the soil particles decreases and the remaining water is held more tightly. Plants start to wilt at this point. Without any water input, it is increasingly difficult for plants to absorb water and they wilt permanently. The cohesion water is gone and the adhesion water remains adhered to the soil particles. Water remaining in the soil is unavailable for plant growth. The water content of the soil at this point is called the PERMANENT WILTING POINT.

The water that can be absorbed by plants is referred to as AVAILABLE WATER. This varies with soil texture (see below). The textures listed are the most common in Minnesota.

Texture       Available Water
(inches per foot of soil)
loamy fine sand                  1.10
sandy loam                  1.40
loam                  1.95
silt loam                  2.03
clay loam                  1.95

A knowledge of available water in soils is important for those who irrigate. This is a major component of the scheduling process. For fine-textured soils where excess water must be removed, this information is important is the design of drainage systems.

The actual study of water in soils is much more complicated than discussed in the above paragraphs. Some excellent researchers have spent their careers studying the various aspects of water in soils. The above is intended to be a condensation of that research.

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