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Soil Water Basics

Soil moisture is difficult to define because it means different things in different disciplines. For example, a farmer's concept of soil moisture is different from that of a water resource manager or a weather forecaster. Generally, however, soil moisture is the water that is held in the spaces between soil particles. Surface soil moisture is the water that is in the upper 4 inches of soil, whereas root zone soil moisture is the water that is available to plants, which is generally considered to be in the upper 5 feet of soil.

Available soil moisture is the difference between the amount of water in the soil at field capacity and the amount at the permanent wilting point often referred to as the available water storage capacity in Table 1.

Field capacity is the water content of the soil where all free water has been drained from the soil through gravity. Sandy soils may drain within a few hours but fine textured soils such as clay may take a few days to drain. Proper irrigation brings soil moisture up to filed capacity.

Permanent wilting point is the soil moisture content at which the plant will wilt and die. While there still may be water in the soil, the plant is not able to extract sufficient water from the soil to meet its needs.

Figure 1. Soil Water Moisture Terms

Soil Moisture Example: NW Minnesota

Table 1. Soil Water Moisture Terms

The Northwest Research and Outreach Center measures available soil moisture on a monthly basis for three soil series at Crookston (Table 2). This spring, the Bearden soil series had 4.07 inches of available stored soil moisture in the top five feet of the soil profile, the Hegne series had 5.30 inches and the Wheatville series had 6.61 inches. Following wheat harvest in 2006 the Wheatville series had a deficit of -0.72 inches of available soil moisture in the upper five feet. The stored soil moisture brought the 2006 crops through a major drought will minimal loss in production. The precipitation amounts for the growing season (April 1 – July 31) was 5.84 inches which is 52% of normal. With virtually all the soil moisture depleted this fall (Figure 2) this will have significant consequences for the 2007 growing season if fall recharge of the soil profile is not achieved as there will not be a buffer of soil moisture in the soil profile to supply water to the crops between rain events next year.

Table 2. Soil moisture measurements at the NW Research and Outreach Center, Crookston, MN

Figure 2. National Weather Service Climate Prediction Center soil moisture map located at www.cpc.ncep.noaa.gov

Management Practices for Conserving Soil Water

Management practices to conserve soil moisture will be paramount this fall and going into next spring if major recharge of the profile is not achieved. Fall tillage influences the amount of snow catch, which is important in many years to help recharge soil moisture after a previously dry summer and fall. Snow catch is influenced by surface roughness, but mostly by stubble height. Conservation tillage, and especially no-till increases the amount of stubble left on the surface (Table 3).

Table 3. Influence of tillage on residue cover

A dry spring means that conditions are right for moisture loss from normal spring tillage operations. Without sufficient topsoil moisture, seed germination could be impaired, resulting in poor crop development and low yield. Other drought-related risks include increased insect infestation, low resistance to plant disease, and the possibility of wind erosion on highly erodible soils where crop residue is minimal. The good news is that soil moisture can be partially managed with tillage, even in a dry year. If you've experienced low levels of precipitation since harvest you should consider modifying your spring tillage management plans. You could limit your crop's susceptibility to drought (from emergence to maturity) by minimizing the number of tillage or cultivation passes. Growers also may remove or control weeds to save moisture for the crop and reduce weed competition for moisture. To conserve moisture, the kind of tillage and the number of tillage operations affects the amount of moisture remaining following tillage and the ability of the soil surface to attract and store additional moisture (Table 4).

Table 4. Moisture loss with tillage over four days

Conservation practices can play a major role in managing soil moisture conditions. The absence or reduction of soil disturbance in no-till both minimizes soil moisture loss from the soil's surface and maximizes soil moisture storage. They also enhance beneficial soil physical properties such as increased infiltration rate, maintenance of soil macropores, and reduction of surface runoff during rainfall, thus increasing soil moisture storage.

Generally, every soil disturbance of a tillage pass can cause the loss of 1/4 inch of soil moisture. However, this number varies based on soil texture, percentage of soil organic matter, and the amount of residue on the soil surface. Getting the crop off to a good start next year may be critical, especially if subsoil moisture is limiting.

If tillage is needed for herbicide incorporation, check the label requirements and tillage timing accordingly. Do not till deeper than needed for incorporation. If your soil is very dry, consider post-emergence weed management that limits or avoids incorporating herbicide.

Soils that received primary tillage this fall may need to be leveled with a secondary tillage operation before planting. Two strategies can be used to conserve soil moisture: early spring or just before planting. With an early spring strategy, the soil is opened up for some drying, but any spring rain that does occur can cause soil erosion. With a late spring strategy, the tillage and leveling operation is delayed until as close to planting time as possible. Using tillage just before planting preserves soil moisture present in the tilled depth and reduces soil moisture before planting.

It also helps to know your soil. If you have soils with poor internal drainage, consider light tillage passes with a field cultivator or disk just before planting. This strategy dries the surface enough to plant without causing undue moisture loss. For soils with adequate internal drainage, no-till may offer the best management choice.

Additional drought information can be found at the NOAA Drought Information Center web site www.drought.noaa.gov such as the Palmer Drought Index shown in figure 3. Also Extension has a drought link on out home page at: www.extension.umn.edu/administrative/disasterresponse/wi_drought.htm

Figure 3.

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