Soil Moisture and Irrigation SchedulingBy

The ability to judge soil moisture content is an important skill for all irrigators to learn. Knowing the soil moisture content allows you to accurately decide when to irrigate and how much to apply. By examining the soil’s appearance and feel you can determine the existing soil moisture and estimate the timing of an irrigation.

The process involved in this determination is relatively simple. You will need a soil probe or suitable tool to properly sample the soil profile. Since the effective root zone for most crops is approximately 3 feet, take a soil sample which represents this entire depth. Once the soil sample has been removed from the profile, the procedure to follow in assessing your soil moisture situation is:

Sand/Sandy Loam	Soil will not form a ball and crumbles easily.	Soil tends to ball under pressure, but seldom holds together.	No free water appears on soil when it is squeezed tightly, but wet outline of ball is left on the hand.
Loam/Silt Loam	Soil is crumbly but holds together under pressure.	Soil will form a ball and sometimes stick slightly or form small ribbons under pressure.	Soil will form a ball which will not produce free water when squeezed, but make a wet outline on the hand.
Silty Clay Loam/ Clay Loam	Soil is usually somewhat crumbly, but will hold together when squeezed.	Soil forms a ball that will ribbon out between the thumb and forefingers.	No free water appears after squeezing, but wet outline of the soil ball will appear on the hand. Soil also easily ribbons between fingers and has slick feeling. The higher the clay content, the more the sample will ribbon.

After determining soil type and estimating percent available water remaining, calculate the amount of water in the soil profile. Next, determine the amount of water needed to bring the soil to field capacity. Field capacity is the maximum amount of water that a soil will hold without leaching or runoff. This would be equivalent to 100% available water remaining. The following table shows water holding capacity for various soil types in inches of water per foot of soil depth. Also, it shows the approximate inches of available water in the top three feet of soil depth for each soil type at 50% and 100% capacity.

Fine Sand/Loam Sand	1.0 - 1.1	1.5 - 1.65	3 - 3.3
Sandy Loam	1.4	2.1	4.2
Loam/Silt Loam	2.0 - 2.5	3 - 3.75	6 - 7.5
Silty Clay Loam/ Clay Loam	1.8	2.7	5.4

Example: Using Chart A, you determine that the moisture level of your silt-loam soil is approximately 50% of capacity. By referring to Chart B, you can see that this soil has a water-holding capacity of 2 - 2.5 inches per foot of soil. At 50% moisture, this means that approximately 3 - 3.75 inches of water is available for plant use in the top three feet of the soil profile (or 1 -1.25 inches per foot of soil depth). Since the available water is 50%, the amount of unused water storage in the soil is equal to the amount of water available. It will require 3 - 3.75 inches of either rainfall or irrigation to refill the top 3 feet of soil to field capacity.

Saturated or droughty soil conditions are detrimental to crop growth. It is best to start the growing season with the whole root zone at field capacity. Don’t over or under irrigate. Irrigation scheduling can be used to maintain acceptable soil moisture.

Over irrigation is when more water is applied than the soil will hold. This will result in excessive runoff, leaching, or waterlogged soils. If a crop is under irrigated, portions the field may reach the crop wilting point. This is a moisture stress to the crop from which it will never fully recover. All of these situations result in an economic loss to the producer either through unnecessary pumping costs or reduced crop yield.

There are several specific irrigation scheduling methods that can be used to avoid the above conditions. Often your county extension office or local irrigation equipment dealer can supply a worksheet for a scheduling method. Regardless of the method used, irrigation scheduling is dependent on a soil’s water holding capacity and evapotranspiration. Evapotranspiration, ET, is the water removed from the soil due to crop use and evaporation. Weekly and daily ET reports for field crops are available from various agricultural news services, extension offices, and radio stations.

If you know the soil’s water holding capacity and the percent available water remaining, you can determine how much storage is left in the soil to avoid over irrigation. By keeping track of what the current crop water use is in the form of ET, it is possible to estimate how long the current soil moisture will last. Combining these two pieces of information will tell the irrigator how much water can be applied and how soon it will be needed.

An important consideration to keep in mind is how long it will take to irrigate the entire field. If irrigation is delayed too long, the last portion of the field to be irrigated may have already reached a point of moisture stress. The same may happen if an unforeseen disruption in irrigation service occurs. For these reasons, the wise irrigator will build a safety net into the irrigation schedule to allow for such delays without causing crop stress.