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Irrigation Soil Types: Part II

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Written by Valley Irrigation |

In "Irrigation Soil Types: Part I," we discussed how to manage water applications on variable soil types in the same field. We concluded that soils with lower available water holding capacity (AWC) didn’t necessarily need more water, but needed water more frequently than soils with higher AWC. 

f you have heard news about how Variable Rate Irrigation (VRI) technology can improve irrigation efficiency and increase crop productivity by applying water site-specifically to different parts of the field based on varying soil types, you may be wondering how that is possible. If, in general, crops use water at the same rate regardless of the soil type they are grown in, how does VRI justify that soil types need watered differently? The answer to this question deals with how irrigation scheduling is managed. 

Using VRI, the general approach is to supply necessary amounts of water to the sandy soils because, as discussed earlier, these soils will show first signs of crop stress; while at the same time, applying less water to heavier soils having higher AWC because of their greater ability to store and supply plant available water. This approach can be effective because heavier soils will not reach their minimal allowable depletion, or MAD (typically around 50% of field capacity), as soon as the soils with lower AWC. 

Table 1 (below) summarizes a VRI scenario with two soils (clay and sand) at different AWCs. At field capacity, a clay soils can hold 1.90 inches of water/foot of soil, whereas sandy soils can hold 0.80 inches of water/foot of soil. Therefore, if we have a corn (maize) crop with a root zone of 3 feet, the available water at field capacity (as shown on Day 1 in the table) in clay soils will be 5.70 inches and 2.40 inches in sandy soils per root zone. Corn, at full coverage, may consume water (also known as evapotranspiration [ET], or the combination of evaporation and transpiration) at a rate of 0.32 inches per day in a moderately hot climate. By Day 5, sandy soils will have reached the MAD, which requires irrigation in order to reduce the risk of crop-water stress. Therefore, water was applied to the sandy soil at a depth of 1.25 inches, which increased the available water to the plant (column 6) up to 85% (note: it is not watered up to 100% of field capacity in order to allow room for precipitation). Conversely, only 0.63 inches of water, or half of what was applied to the sandy soil, was applied to the clay soil, which brought its available water up to 83% of field capacity. This irrigation scheduling was then repeated on Day 9 and again on Day 13. Finally, on Day 16, over two weeks later, the clay soil reaches its MAD when it is at 49% of field capacity. 

Irrigation schedule

Table 1. Irrigation schedule of a sandy soil and a clayey soil using VRI.

In the irrigation scheduling example described above, both soils would have the same amount of water applied to them if they had been filled back up to field capacity. The only thing that changed is the irrigation rate: sandy soils were watered at twice the irrigation depth of the clay soils. So how does VRI help improve water efficiency and increase crop productivity? 

Water efficiency can be improved by two ways. First, it is possible to reduce runoff and deep percolation caused by heavy rainfall during this two week period. Under uniform irrigation management (Table 2, below), the clay soils remains between 75-100% of field capacity and never reaches the MAD. The example in Table 2 shows what would happen if a rainfall event of 1 inch occurred on Day 11 of the irrigation schedule. If this happened, both the clay and the sand soil would become saturated, which would cause runoff and deep percolation, resulting in a loss of 0.6 inches of water. However, with the VRI system, a rainfall event of over 1.95 inches would be needed before the clay soils become fully saturated on Day 11. 

Irrigation schedule
Table 2. Irrigation schedule of a sandy soil and a clayey soil using uniform irrigation management. On Day 11, a precipitation event added 1 inch of water to the field, causing saturation in both soil types

Secondly, water efficiency can be improved by reducing runoff caused by limited infiltration rates in heavier or compacted soils. By applying less water periodically to clay soils, we can be assured that our infiltration rate will not be exceeded. This decreases the amount of ponding occurring on heavier soil types, which can then help increase crop productivity in these areas. 

There are a multitude of other benefits resulting from using VRI to target variable soil types. Because we are actually applying less water to our clay soils with every pass, the pivot will speed-up over sectors of the field with less than our base application rate (which, as explained earlier, is targeted toward the soils with the lower AWC). This will then result in faster pivot revolutions, which will enable the pivot to make it around more quickly in order to water those soils with lower AWC more frequently. 

There are many factors that influence the amount of water taken from a unit of soil by a plant. If we look into each one of these factors, we will realize that water is not actually taken out equally in all soils. For instance, different soil types have different cation exchange capacities, which regulate the amount of readily available nutrients to plants. Therefore, plants may be growing more vigorously in heavy soils because there are more nutrients readily available. This will cause differences in plant growth, maturity, and leaf area, which in turn affects the amount of water crops transpire. So, VRI can actually water site-specifically toward these areas with different crop-water use requirements. 

Variable Rate Irrigation isn’t just a package of hardware and/or software incorporated with a center pivot; it requires accurate irrigation scheduling, soil mapping, and decision support.

For more information on VRI, visit valleyirrigation.com.

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