FO-03875 Reviewed 1991
Table 4 lists recommended allowable soil water depletion limits and management strategies for several irrigated crops grown in Minnesota. These recommendations result from several research projects in the North Central states and published guidelines from other states.
|Table 4. Recommended allowable soil moisture depletion limits by crop growth stage|
|Crop stage of growth|
|allowable soil moisture depletion percentages|
|Corn||Emerg||12 leaf||Pollination||E. Dent||Maturity|
|10* -->||70 -->||50 -->||40 -->||50 -->||60 -->||70 -->|
|Potatoes||Emerg||Tuber Yield formation|
|10* -->||60 -->||40 -->||30 -->||40 -->||65 -->|
|60 -->||Full Pod
50 --> 70 -->
50 --> 70 -->
|Small grain||1st Node
50 --> 70 -->
* 10% depletion at this period refers to only the seeed germination zone.
Source: adapted from the results of several research projuects in the north central states and published guidelines from other states (Dorn et al., 1989-- Nebraska; Stegman, 1988-- North Dakota; Fishbach et al., 1988-- Nebraska; Curwen et al., 1985-- Wisconsin)
Here are some guidelines to consider in developing a management plan and setting allowable depletion limits. In the spring, always make sure the soil in the germinating seed zone and deeper is uniformly moist when crop planting begins. If necessary, irrigate to wet this zone. A dry soil layer below the seed will restrict root development and result in a shallow rooting depth. For corn, the soil water deficit can be allowed to reach 70 percent depletion during the vegetative growth stage up to the 10th-12th leaf under average weather conditions without affecting plant development. If irrigation is needed then, apply a lighter than normal application (0.5-0.7 inches) to partially refill the soil water deficit.
This strategy maximizes the use of rainfall while minimizing the leaching potential of agrichemicals in the soil profile. If irrigation keeps the soil moisture near field capacity, normal rainfall could result in significant leaching of some chemicals. May and June generally produce more rainfall than evapotranspiration for most crops and this also coincides with most agrichemical application events. Figure 2 compares the normal monthly crop water use pattern for corn with the respective monthly precipitation amount for west central Minnesota.
As a crop nears mid-season, typical period for most crop's critical growth stages and the peak crop water use, reduce the allowable water depletion limit to minimize the risk of plant moisture stress and subsequent economic yield losses. For most crops, this may mean setting a 30-40 percent depletion limit. Reduction in allowable depletion should start ahead of the crop's critical growth stage. For corn this is pollination and the recommended period of transition for reducing the depletion limit begins at the 12th leaf stage (table 4).
During these critical periods of high crop water demands, project the next three to four days' water needs regularly to avoid stressing any part of the field before irrigating. For example, if a center pivot takes three days to travel the field, project what the soil water deficit will be in three days and use this to determine when to start irrigating. To reduce the leaching potential of rainfall, always consider the weather forecast for the next couple of days in scheduling the next irrigation.
As most crops near maturity, the soil water depletion may be allowed to increase to greater limits without causing stress. For example, after corn kernels have begun to dent, research shows that allowing the soil water depletion to increase to 70 percent does not reduce yield. This approach allows greater storage of the late rains in the soil profile and reduces possible leaching.
For irrigation systems with limited pumping capacities on sandy soils (less than 5 gallons per minute per acre) water management alternatives become more restrictive over the season. For example, research with irrigated corn in west central Minnesota shows that to reduce the risk of stress, set the allowable depletion to no more than .75 inch starting in mid-vegetative stage (Bergsrud et al., 1982). For corn this is from the 8th-10th leaf stage and continuing until late dent. This approach increases the potential for leaching due to normal rainfall but is necessary to avoid stress during peak use periods. To minimize leaching, follow the latest weather forecasts closely before irrigating.
This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9180.
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