Using SoyWater to Schedule Irrigations
April 30, 2010
Irrigation Strategy 1. Using Only the Soil Water Balance Method
When using SoyWater as a decision aid for the Soil Water Balance Method, the user chooses an irrigation “trigger,” a percentage depletion of the maximum plant available water (field capacity) in the crop root zone. (The default is set at 35%.) This depletion is caused by crop withdrawal of soil water. For example, in a silty clay loam soil that has a field capacity water content of 2 inches per foot, there will be 6 inches of plant available water in the 3-foot root zone.
The furthest right column in the SoyWater table shows the estimated amount of depletion on each successive calendar date of the season, based on the depletion trigger selected by the user (in this example, 35%). SoyWater will yellow highlight all calendar dates when 2.1 inches of water will have been depleted from the 6 inches of water that a silty clay soil can hold at field capacity (35% of 6 inches = 2.1 inches). Figure 1 shows inches of water for a silty clay loam soil – other soil texture types would have different numbers.
Based on the producer-chosen depletion percentage, the first yellow highlight would be the suggested date to schedule the first irrigation. Assuming the irrigation is applied on that date, the next yellow highlighted date would be the suggested date for scheduling the next irrigation.
Note that the producer always determines how much irrigation water to apply. For example, for the same 35% depletion scenario described above, Producer A might apply only 0.75 inch of irrigation while Producer B might apply 1.50 inches. Given those choices, Producer A must irrigate more frequently (perhaps every three days, if there is no intervening rain), and with less application efficiency (due to canopy interception of applied water). However, Producer A could capture a larger amount of possibly unexpected (but free) rainfall that might occur the day after an irrigation, given that there is “room” in the soil to store up to 1.35 inches of rain (2.1 – 0.75 inches). Conversely, Producer B irrigates less frequently (perhaps every five days) and applies water more efficiently (assuming a rate of application that ensures no irrigation water run-off), yet can capture and store only 0.6 inch of an unexpectedly large rainfall event the day after an irrigation.
Irrigation Strategy 2. Defer all Irrigation to the Soybean R3-stage (Beginning Pod Elongation)
This method has been documented in research conducted by Dr. Specht as an alternative to the Water Balance Method (Strategy 1) for fields whose soils have a high water-holding capacity (e.g., silty clay loam soils) and in years when the field water content is at full capacity at planting or emergence. In this strategy, the producer allows the soybean crop to extract the water it needs for vegetative growth and early reproductive development from the six inches of stored soil water present at emergence and any postemergence rainfall stored in the root zone prior to the R3 stage.
Figure 2. A field is considered to be at the R3 stage (beginning of pod elongation) when at lesat 50% of the soybean plants in the field have at least one pod 3/16 inch long at the four uppermost stem nodes. Producers using an irrigation strategy that defers all irrigation until this stage or afterward can see the projected date of R3 in the SoyWater table.
Irrigation need not be applied between emergence and R3 (beginning pod stage, Figure 2) for two reasons. Dr. Specht’s research has shown that irrigation during the vegetative stage provides little or no yield benefit on the heavier soil textures (even when rainfall is sparse from emergence to R3). But more importantly, with this method the producer avoids putting on an irrigation that ultimately produces little or no yield benefit and can make more effective use of the “free” rainfall that occurs in May and June – the period with the highest probability of rain. That rainfall can be stored in the soil profile so the crop can use it rather than the more costly irrigation water.
Dr. Jessica Torrion, Dr. Specht’s collaborator in the development of the SoyWater website, has shown that, for seedling emergence (VE) dates of about May 10, the soybean taproot, which is already six inches long at emergence (VE), will extend downward at the rate of 0.8 inch per day. Within seven to eight days of emergence, the taproot tip will be 12 inches deep (i.e., 6 inches + 0.8 x 8 = 6 + 6.4 = 12.4 inches). It will be 24 inches deep within 22-23 days after VE, and 36 inches deep within 36-37 days after VE.
SoyWater projects the soybean R-stage dates and soil water depletion amounts on those dates, so that the producer can determine how much cumulative depletion has occurred by the date of R3, and thus how much irrigation will be needed to bring the soil water depletion back to 35% within a 7-10 day period after R3 occurs.
This irrigation deferral strategy may require more frequent irrigation and/or greater irrigation application amounts after R3 in those few years when rainfall is sparse prior to R3. In most years, however, it will allow the producer to likely forego one unneeded irrigation event during the vegetative stage.
Dr. Specht's research also has shown that a sprinkler irrigation applied during the R2 stage (full flowering, about 5-10 days before R3) is risky because of the potential for
- white mold infection (its spores can only germinate on “wet” senesced flower petals)
- greater plant lodging because of an irrigation-induced elongation of upper internode lengths (internode lengthening does not slow until R3).
Also, there is documented evidence that little yield benefit accrues from an irrigation applied at the R2 stage of full flowering.
UNL Research Associate, Lincoln
Charles E. Bessey Professor of Agronomy and Horticulture, Lincoln