Early Season Soil Moisture Status in Central Nebraska - UNL CropWatch, May 10, 2013

Early Season Soil Moisture Status in Central Nebraska - UNL CropWatch, May 10, 2013

Soil moisture chart

Figure 1. Soil moisture deficits in the top 1, 2, 3, and 4 feet of soil in samples taken in central Nebraska in early April, near the normal start to the growing season. At the time these samples were taken Field 1 would have required only 1 inch of water to reach field capacity in the top 2 feet, but would have needed 4 inches of moisture to reach field capacity in the top 4 feet. Field 16, on the other hand, would have needed almost 4 inches of moisture to reach field capacity in the top two feet and 7.5 inches to reach it in the top 4 feet. Early season soil moisture measurements can be used as a baseline on which to add precipitation and irrigation to estimate current soil moisture.

May 10, 2013

The effects of the 2012 drought, which affected more than half of the U.S., continue into this planting season. Usually we enter spring with a near-full to full soil profile; however, that is not the norm this spring. I have written articles about the importance of soil sampling for residual nitrate-nitrogen and surface samples for other nutrients needed to raise a crop, but I have never sampled for soil moisture.

In early April I did just that, randomly sampling 18 fields in several counties of central Nebraska. Most of the fields were center-pivot irrigated and had been previously planted to corn or soybeans. Fields No. 3, 6, and 18 were dryland. Moisture status was detected by calculations based on the difference in wet and dry weight of the samples collected by the use of a Giddings soil probe. At each site four replicated samples were taken to a four-foot depth and divided into one-foot increments, from which the moisture status was determined. Soil moisture deficits — the amount of water needed to reach field capcity — from each of the fields are shown in Figure 1.

Cumulative deficits varied from field to field, but had one thing in common, they increased the deeper the sample. Deficit values are affected by several factors, including previous irrigation practices, tillage, and off-season precipitation. Table 1 represents the averages and ranges of each sampling depth calculated from all the fields sampled.

Table 1. Average soil moisture deficits and minimum and maximum deficits in the top 4 feet of soil from 15 irrigated fields sampled in central Nebraska.
   Average Deficit (in.) Min. Deficit (in.)  Max. Deficit (in.)

 1st Foot  0.84  0.19  1.91
 2nd Foot  1.25  0.53  2.86
 3rd Foot  1.59  0.56  3.36
 4th Foot  1.18  0.26  1.86
 Total  4.78  2.79  6.88

These samples were taken only to a depth of 4 feet, and deficits would likely have been higher below that depth. Crops such as alfalfa and wheat have longer roots and often can access water below 4 feet. 

Deficit reductions can only come from adding water to the soil profile, either through irrigation or rain. April rains in central Nebraska ranged from less than 1 inch to more than 6 inches and would be a factor in current soil moisture levels.

We have no control over Mother Nature but we do have control over how we choose to irrigate to help replenish moisture status of the profile. Research on preseason irrigation has shown that it is an inefficient practice. “So should we irrigate or shouldn’t we?” See the March 22, 2013 CropWatch article, Evaluating the Use of Preseason Irrigation, for recent research addressing this question.

Dean Krull
UNL Project Coordinator/Central Platte NRD