Soil-water basics for irrigation scheduling

By Vasudha Sharma- Extension Irrigation Specialist

During the past few years, irrigated agriculture has increased significantly in Minnesota. Most of the irrigation in the state happens in the glacial outwash region where irrigation makes this region highly productive because of low water holding capacities and rapid drainage, however this is where most people depend on groundwater for their drinking water supply. Contamination of groundwater due to agricultural nitrate leaching and decreased recharge to lakes and streams because of high groundwater withdrawals for irrigation are two critical environmental problems in the central sands region of Minnesota.

Strategic irrigation management can address the complex challenges we face in the central sands region. Irrigation management enables the irrigator to apply the right amount of water at the right time, which increases irrigation efficiency and reduces nitrate-N leaching. However, proper irrigation management is a difficult task. Over-irrigation wastes water, causes nutrients to leach from the rooting zone, contaminates ground water, increases energy and labor costs, and reduces soil aeration. On the other hand, under irrigation creates plant water stress and reduces yield.

Understanding terms and principles

Irrigation scheduling is a practical tool for preventing the over-application of water while optimizing crop growth. Scheduling can be done by timely monitoring of the moisture content of the soil directly using soil moisture sensors, and indirectly by estimating crop water use of different crops (evapotranspiration) to model soil water depletion. A better understanding of the basic principles, definitions, and terms behind soil-water-plant relationship is fundamental and can aid communication between agricultural producers, extension educators, crop consultants, agency personnel and researchers.

Figure 1. Soil water reservoir components.

Soil is a plant’s water reservoir. Understanding basic terms associated with the soil water reservoir (Figure 1) is vital for determining the amount and timing of irrigation.

Saturation – The water which readily percolates or drains out from the root zone by gravitational force. Also called gravitational water.

Field capacity – It is the amount of water that remains in the soil after all the excess water at saturation has been drained out. When the soil is allowed to drain for approximately 24 hours after saturation, field capacity is reached. 

Permanent wilting point – When plants uptake all the available water for a given soil, soil dries to a point that it cannot supply any water to keep plants from dying.

Available water holding capacity (AWC) –The amount of water that soil can store to be extracted by the plant. It is the water held between field capacity and permanent wilting point.

Management allowable depletion (MAD) –The soil water content where crops begin to experience water stress. Usually, most of the crop do not experience water stress before 40-60% of AWC has removed.

Soil water deficit –The amount of water removed by the crop from active rooting depth.

Soil type differences

Figure 2. Soil moisture conditions for various soil textures.
Ref: The COMET program, University Corporation of
atmospheric research

Different soil types have different AWC. For example, as shown in Figure 2, coarse soils, such as sands and sandy loam, have relatively large pores when compared to a finer textured soil such as clay. Fine soils, like clays or clay loams, have small mineral particles and very small pores. Having a larger number of small pores means that a fine textured soil can hold more water than a coarse textured soil. For irrigation scheduling, whenever the soil water deficit is equal to or higher than MAD, irrigation should be triggered. Irrigation amounts should be refilling the rooting zone to approximately 80% AWC, leaving some room for possible precipitation.

The information about the AWC for your field can be obtained from the NRCS web soil survey (https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm) or testing in the field can also be done. In general, coarse textured soils can have roughly 0.5 inches of available water per foot of soil depth and silt loam soils have roughly 2 inches of available water per foot of depth.

Soil water deficit can be monitored directly using soil moisture sensors or indirectly by estimating crop water use (evapotranspiration) and using water-balance equation.

Field trials for 2019

Dr. Vasudha Sharma will be working with the Pope SWCD on the establishment of field trials to compare various soil and climate based as well as crop growth models based irrigation management strategies and to test the performance of various soil moisture sensors in coarse textured soils. This research will answer as to which irrigation method is best suited for coarse textured soils of this region that reduces nitrogen leaching while maintaining the high yields.

The other field research trials will be focused on developing and evaluating deficit irrigation management strategies and developing crop coefficients for various cropping systems in Minnesota that will aid in better irrigation management. The upgraded variable rate irrigation system at the Rosholt farm provides tremendous opportunities for irrigation research in this region. In addition to Rosholt Farm, eight new sprinkler irrigation systems at the U of M’s Sand Plain Research Farm in Becker provides a tremendous opportunity for irrigation related research in the Sand Plain area.

For more information, please feel free to contact Dr. Vasudha Sharma at vasudha@umn.edu.

This article was first published in January 2019.