This Tip sheet will discuss soil type and how it affects irrigation depth and frequency
Once you know the precipitation rate of your irrigation system and have an estimate of the crop’s daily water-use or evapotranspiration (ET), the question remains: How often and for how long should I run my system for optimum efficiency? To answer this question, you must know the soil’s water holding capacity and water intake rate. These factors can be estimated from the soil texture (proportions of sand, silt and clay particles) which can be determined by a soils laboratory or estimated from a NRCS soil survey. A sandy soil, for instance, will have a relatively low water-holding capacity but a high intake rate (i.e. it will quickly absorb applied water). A clay or loam soil on the other hand, will hold more water than a sandy soil but will not absorb water as quickly (i.e. the intake rate will be low). Table 1 lists the approximate water holding capabilities and basic intake rates of various soil types or textures.
Table 1. Available water holding capacities and water intake rates of various soil textures.
Sandy loam soils, which are the most prevalent type found on the Hammond Conservancy District (HCD), have a water holding capacity ranging between about 1 and 1.5 inches of water per foot, and a basic intake rate of about 0.70 inch per hour. To satisfy the crop’s water requirements while preventing deep drainage of water below the crop root zone on these soils, irrigations must be applied frequently and at relatively low volumes. Water runoff is usually not a problem with these soils if irrigating with a sprinkler system because the system’s precipitation rate will usually not exceed the soil’s intake rate. On loam and clay loam soils, a higher volume of water may be applied if the precipitation rate is low enough (i.e. 0.30 inch or less) to prevent runoff and the interval between irrigations can be lengthened because the soil can hold more plant-available water. In heavy clay soils (i.e. ‘slick spots’), total water holding capacity may be high but plant available water decreases because much of the water is held so tightly that plant roots cannot easily extract it (Table 1). Water will readily pool or run off these soils.
In addition to knowing water characteristics of the soil, the irrigator should also have estimates of the crop’s effective root depth so he/she knows how much total water is available. Because alfalfa and cool season pastures are the predominant plants grown on the HCD, this discussion will be restricted to those two crops. For root depths for some other crops click HERE. In a coarse sandy soil, alfalfa tap roots can extend to a depth of 10 feet or more and pasture grass roots can grow to depth of 5 feet. For irrigation management purposes, however, depths of 4 feet and 2.5 feet are usually considered for established alfalfa and pasture, respectively, as this is where most of the water-uptake occurs. In new plantings, of course, rooting depths are much shallower than this.
Another consideration is related to the concept of maximum allowable depletion (MAD). When the soil is at field capacity (i.e. it is holding all the water that it possibly can) plant roots can easily extract the moisture in the soil. As the soil begins to dry down, it becomes more difficult to extract the moisture and plant growth is restricted. This plant ‘water stress’ as it’s often called results in decreased crop yield. For most plants, including alfalfa and pasture grass, it is recommended that no more than 50% (i.e. MAD = 0.5) of available water be used before refilling the soil (root-zone) profile.
Putting it all together:
The use of the information discussed in these three tip sheets (irrigation system output and efficiency, estimates of crop water-use or ET and soil water holding capacity and intake rates, and crop rooting depth) to manage irrigations can best be explained by presenting an example:
Crop = established alfalfa. Root depth = 4 feet. Maximum allowable depletion = 50%
Irrigation system sideroll sprinkler
Determined precipitation rate = 0.50 inches per hour (evenly between two sets)
Efficiency = 80% (0.80)
Soil Type: Fruitland sandy loam
Available water capacity = 1.2 inches per foot
Intake rate = 0.65 inches per hour (precipitation rate is less than soil intake rate)
The date is June 1.
An irrigation has just been applied and the soil is at field capacity.
Average daily water-use (ET) of alfalfa from Tip sheet 2, Figure 1 = 0.30 inches.
Question: When should the next irrigation be applied and for how long should the system be run?
- Total available soil water = 4.8 inches (1.2 inches per foot x 4 feet)
- Total allowable water extraction before next irrigation = 2.4 inches (MAD of 0.5 x 4.8)
- Total days it will take to extract this 2.4 inches = 8 (2.4/0.30 daily ET)
- Total water to apply corrected for efficiency = 3.0 inches (2.4/0.8 system efficiency)
- System run-time to apply 3.0 inches = 6.0 hours (3.0/0.5 inch per hour)
Answer: The next irrigation should be applied on June 9 (8 days after the last irrigation) and it should be run for 6 hours.
- If using a sideroll system, the set should be in the same location as the June 1 set.
- The above scenario does not include a ‘leaching fraction’. This is additional water that must be applied to leach excess salts out of the root zone. The volume of additional water will depend upon the soil’s (and irrigation water’s) salt concentration. This can be determined by a reputable soils lab. The normal recommendation for the HCD is about 10%.
- Run-time adjustments may be required as irrigation efficiency can vary with wind conditions, time of day, etc.
- It’s a good practice to apply a heavy irrigation at the beginning of the season to insure the soil profile is filled before beginning irrigation scheduling. Particularly if the winter has been dry. Also, keep in mind that even with a wet winter, much of the available soil water will have been extracted from the soil by the alfalfa, which may break dormancy and begin growing in February, long before irrigation water is available on April 15.