FO-03875     Reviewed 1991

To Order

Irrigation Water Management Considerations for Sandy Soils in Minnesota

Available Water in Root Zone

The frequency and application depth of irrigations depend heavily on the amount of soil water available to the crop's roots.

In most fields there are several soil types and the irrigation manager should review the available water-holding capacity of each type. The irrigation system should be managed to meet the crop water needs from the soil type covering at least 30 percent of the field and having the lowest available water-holding capacity.

County soil surveys identity soil types and available water-holding capacity for most fields. County personnel from the Soil and Water Conservation District (SWCD), Soil Conservation Service (SCS), or the University of Minnesota Extension Service can help determine the soil water storage characteristics.

Table 1. Crop rooting depths for irrigation water management
Crop	Depth
Alfalfa (established)	48
Corn, sugar beet	36
Potato, small grain	24
Soybean, field bean	24
Tomato, muskmelon	12-24
Broccoli, cauliflower	12-18
Blueberry, strawberry	12-18

Available soil water capacity is that portion of the total soil water available for plant use. The maximum amount of available water stored in the soil, soil texture, soil's available water-holding capacity, and the crop's rooting depth are all related. Table 1 shows typical irrigation management rooting depths for several common crops.

Tables 2 and 3 show examples of the available water-holding capacity of two typical irrigated soils. In table 2 note that at a depth of 18 inches there is a root restricting layer of gravel and sand which limits the available water capacity at 3.5 inches for any crop having rooting potential of 18 inches or greater. In table 3 the soil profile allows plant roots to go much deeper. However, for most irrigated crops only the top 2-3 feet would be managed and this would yield an available water capacity of 1.8-2.5 inches for this soil.

Table 2. Available Water Capacity (AWC) of a Renshaw Soil Series
Profile Depth inches	Texture class	AWC-Inches
		per inch	per zone	cumm.
0-12	Loam	.21	2.52*	2.52
12-18	Sandy Loam	.16	.96	3.48
18-60	Sand and Gravel	.02	.84	4.32
* calculated by multiplying 12" x .21 inches per inch = 2.52"

Table 3. Available Water Capacity (AWC) of a Hubbard Soil Series
Profile Depth inches	Texture class	AWC-Inches
		per inch	per zone	cumm.
0-12	Sand	.09	1.08	1.08
12-24	Sand	.06	.72	1.80
24-36	Sand	.06	.72	2.52
36-60	Sand	.06	1.44	3.96

The second purpose for determining available water capacity within a field is to establish maximum allowable soil water depletion limits for managing the soil moisture and irrigation system. This is described later.

This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9180.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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