Sections: Main Page White Mold and Dry Beans Irrigation Scheduling Strategy Soil Water Monitoring Methods Soil Moisture Sensor Installation

Soil Water Monitoring Methods

Figure 8. Regular readings of in-field moisture sensors is one method of estimating soil water depletion.

Regular monitoring of the soil water status within the field is very necessary to assist an operator in deciding when to irrigate (Figure 8). The most common tools for estimating soil water depletion are (1) soil water tension sensors; (2) feel and appearance of the soil; and (3) soil water accounting by the "checkbook" method, where soil water deficit (SWD) is estimated by recording daily crop ET (evapotranspiration) and the amount of rainfall and irrigation.

Soil water tension can be monitored at given locations in the active root zone by electrical resistance soil water sensors or tensiometers (Figure 9). Soil tension or suction describes how tightly water is held to the soil particles and how much force plant roots need to remove water from the soil. Tension is usually expressed in centibars.

Figure 9. Positioning for use of two types of soil moisture monitors.

Table 2 shows estimated soil water deficit for several soil water tension levels and soil textures. Tensiometers directly measure the soil water tension between 0 and 80 cbs. and work best in sandy loam or lighter textured soil.

Resistance sensors work in a wider range of soil textures and soil water tensions. Some types, like the Watermark granular matrix sensor, operate as well as a tensiometer in sandy textured soils.

Reading the installed sensors every 1 to 2 days is the best way to keep track of the soil water status. Figure 6 shows a good way to summarize the daily readings.

For additional information on sensors or their availability, contact your local irrigation supplier or consult an appropriate mail order product catalog.

	Soil water tension in centibars, cbs
Soil Texture	10 30 50 70 100 200 1500^
	Soil water deficit - inches per foot of soil
Coarse sand	0 0.1 0.2 0.3 0.4 0.6 0.7
Fine sand	0 0.3 0.4 0.6 0.7 0.9 1.1
Loamy sand	0 0.4 0.5 0.8 0.9 1.1 1.4
Sandy loam	0 0.5 0.7 0.9 1.0 1.3 1.7
Loam	0 0.2 0.5 0.8 1.0 1.6 2.4
^ 1500 cbs refers to the permanent wilting point, soil deficit equals the soil's available water capacity.
Table 2. Soil water deficit estimates for several soil textures and tensions.

The feel/appearance method involves collecting soil samples in the root zone with a soil probe or a spade. The soil water depletion of each sample can be estimated by feeling the soil and comparing its appearance to developed tables (such as that of Table 3).

Soil sampling should be taken from several depths in the root zone and at several locations across the field. Summing up the SWD estimations for a location from each depth will predict the total soil water depletion in the root zone at that site. This method requires frequent use by an operator to develop consistency in the art of estimating soil water status by feel. For assistance in determining total soil water capacity for the different soils in a field review a local soil survey atlas or contact the local Soil and Water Conservation District or Extension office.

Soil Texture Classification
Moisture deficiency	Coarse (loamy sand)	Sandy (loamy sand)	Medium (loam)	Fine (clay loam)	Moisture deficiency
in./ft. .0 .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0	(field capacity) Leaves wet outline on hand when squeezed. Appears moist, makes a weak ball. Appears slightly moist, sticks together slightly. Appears to be dry, will not form a ball under pressure. Dry, loose, single-grained flows through fingers. (wilting point)	(field capacity) Appears very dark, leaves wet outline on hand, makes a short ribbon. Quite dark color, makes a hard ball. Fairly dark color, makes a good ball. Slightly dark color, makes a weak ball. Lightly colored by moisture, will not ball. Very slight color due to moisture, loose, flows through fingers. (wilting point).	(field capacity) Appears very dark, leaves wet outline on hand, will ribbon out about one inch. Dark color, forms a plastic ball, sticks when rubbed. Quite dark, forms a hard ball. Fairly dark, forms a good ball. Slightly dark, forms weak ball. Lightly colored, small clods crumble fairly easily. Slight color due to moisture, powdery, dry, sometimes slightly crusted but easily broken down in powdery condition. (wilting point)	(field capacity) Appears very dark, leaves slight moisture on hands when squeezed, will ribbon out about two inches. Dark color, will slick and ribbons easily. Quite dark, will make thick ribbon, may slick when rubbed. Fairly dark, makes a good ball. Will ball, small clods will flatten out rather than crumble. Slightly dark, clods crumble. Some darkness due to unavailable moisture, hard, baked, cracked sometimes has loose crumbs on surface. (wilting point)	in./ft. .0 .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0
Table 3. Guide for judging soil water deficit based on soil feel and appearance for several soil textures.

The irrigation checkbook accounting method involves keeping track of the estimated daily crop ET and measured rainfall and irrigation amounts within the field. These values are placed into a balance sheet (manual or computerized) to predict the soil water deficit (SWD) at the end of each day. Daily crop water use (ET) estimations for dry beans can be obtained by applying the values in Table 4 when the crop achieves full canopy closure shortly after first bloom. If the canopy shortly after bloom does not close completely, the estimated ET from Table 4 should be reduced by 10 to 15 percent depending on closure. Daily ET rate may also be available from a regional crop ET hotline service that is estimated from local weather station measurements. Crop ET may also be estimated from a daily ET potential map available on the internet at http://bob.soils.wisc.edu/wimnext/ that is developed from local airport weather measurements and satellite imagery by the University of Wisconsin and supported by the University of Minnesota.

Week After Emergence
Temp F 50-59 60-69 70-79 80-89 90-99	1 .02 .04 .05 .06 .07	2 .03 .04 .06 .07 .09	3 .05 .07 .09 .11 .13	4 .06 .08 .11 .13 .16	5 .07 .11 .14 .18 .21	6 .08 .12 .06 .20 .24	7 .09 .13 .17 .22 .25	8 .10 .15 .19 .24 .27	9 .09 .14 .19 .23 .26	10 .09 .13 .17 .22 .25	11 .08 .12 .15 .19 .23	12 .06 .09 .11 .14 .17	13 .04 .06 .07 .09 .11	14 .02 .03 .04 .04 .05
Field Beans Growth Stage				2nd trifoliate		1st flower			seed filling			leaves yellowing
Table 4. Estimated daily crop water use (ET) in inches/day for field beans in central Minnesota with normal sunshine.

Predicted daily soil water deficit (SWD) from checkbook method should be in-field verified at least once every 7 to 10 days with a soil probe or soil water sensors and then the SWD on respective checkbook balance sheet adjusted accordingly (Figure 10).

Figure 10. A soil water balance sheet is used to track moisture levels using the irrigation checkbook accounting method. A full-size copy of the form can be found in University of Minnesota Extension Service publication F0-1332. The form is used in conjunction with an "average water use table."

Figure 6 shows how well in-field daily soil sensor readings trace the predicted daily soil water deficit (SWD) as estimated by the Minnesota Checkbook method (daily SWD needed correcting five times during this period based on the soil water sensors readings) for a 1998 dry bean field in Wadena county.

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