In our last post, we discussed the influence of soil infiltration rate, application rate, and water holding capacity on soil moisture content. In this week’s post, we delve into how water distributes itself throughout the soil, discuss the factors that affect soil infiltration rate, and outline significant benefits to improving your infiltration rate.

FIRST THINGS FIRST: SOME IMPORTANT TERMS

Saturated soil is the state of the soil following irrigation or precipitation. Gravity automatically draws some amount of the water downward (called gravitational water); this water is not available for plant usage.

Field capacity is the amount of water remaining in the soil after 24-48 hours of drainage. It includes the soil’s plant available water (which has two equivalent subsets relating to plant stress) and unavailable water.

Plant available water (AW) is the difference in water content between field capacity (FC) and permanent wilting point (PW).

At the permanent wilting point, the plant can no longer extract water from the soil; the only water remaining in the soil is unavailable water. This water cannot be absorbed by plants but is tightly bonded to soil aggregates.

Dry soil is completely devoid of moisture.

WHERE DOES THE WATER GO?

Water is a hot commodity in soil. Once water in the form of irrigation or rainfall hits the ground, gravity wants its share, as does the crop and the soil itself. As shown in Figure 2, in soils with balanced soil texture (e.g. loam), the soil’s water resources are essentially split into thirds:

• ⅓ of the water is removed by gravity or is considered excess water
• ⅓ of the water is considered “available water.” Within this group, only about half of the water is available for easy-access plant use. The other half of the available water is difficult for the plant to absorb and often induces plant stress.
• ⅓ of the water is considered “unavailable water.” Unavailable water is strongly attached to soil particles and aggregates and cannot be extracted by plants.

Ratios for how water distributes itself within the soil vary for each soil type. For example, in sandy soil, which is very porous and has large particles, over half of all water that hits the soil is excess or is lost to gravity. This means that less than half of the water being applied to the soil through irrigation or precipitation actually goes to the plant itself. 

On the other end of the spectrum, soils that have small particles and are less porous (e.g. silty clay loam), lose less water to gravity, but almost half of all water that hits the soil becomes unavailable to the plant as it attaches itself to soil particles. 

WHAT FACTORS AFFECT SOIL INFILTRATION RATE OTHER THAN SOIL TEXTURE?

How the water distributes itself following irrigation or precipitation depends on:

• soil texture,
• soil evapotranspiration rate, and
• plant root depth

Water located in the first 6 inches of soil can be lost to evaporation. So, in terms of soil moisture management, in extreme heat, shallow sensors might not even register a change in moisture after large rain or irrigation events.

In terms of the effect of plant root depth on water distribution, plants with an extensive root system (e.g. cotton) can hold greater quantities of available water than those of their shallow-root counterparts, as shown in Figure 3. 

HOW CAN I IMPROVE MY SOIL INFILTRATION RATE?

Soil infiltration rate is driven mostly by soil texture, which can’t be changed. The more porous the soil, the higher the infiltration rate.  In our last post, we talked about how wide the infiltration rate varies based on the soil texture (see Table 1, below).

Sandy or gravelly soil has large pores, meaning it’s able to absorb more water per hour. By contrast, clay soil has smaller pores and a very low infiltration rate. Clay soils can have high infiltration rates in cases of extreme drought; when clay is dry and cracked, it is easily penetrated by water.

Generally speaking, growers can improve soil infiltration rates by increasing the organic matter content in their soils and limiting soil compaction. More specifically, growers should:

• Use designated field roads or rows for equipment traffic  
• Reduce number of trips across field  
• Use continuous no-till  
• Use rotations with high-residue crops, such as corn and small grain, & perennial crops, such as grass or alfalfa  
• Plant cover crops & green manure crops  
• Establish terraces or other runoff- and erosion-control structures