Plant Available Water & Irrigation Efficiency

Increase the Water Stored in Your Soil 

Conventional sprinkler irrigation at Leafy Greens, operated by farmer Tom Heess, in the Salinas Valley of California on Thursday, June 16, 2011. Leafy Greens grows row crops of lettuce, broccoli, cauliflower sweet peas and seed beans. He uses rotational crop plantings to control weeds and plant disease. When a plot of land is at rest, he plants a cover crop of barley and rye grass because the roots hold the topsoil reducing erosion of the soil. He is converting his irrigation system from conventional sprinklers (seen) to micro irrigation. Where one system produces runoff and erosion of the soil (seen in foreground); the other has little or no erosion, less maintenance, easy harvest and less water is needed. When seasonal rains produce runoff, the silt that flows with it is caught in sediment ponds (left and right of pathway). The ponds have grass, bushes and trees to hold the structure and allow the silt to settle. Spillways lead to holding ponds and eventually the Salinas River, a tributary of the Monterey Bay National Marine Sanctuary. So far, because of its design and efficiency, no water has made it to the river. USDA Photo by Lance Cheung.
USDA Photo by Lance Cheung.

California’s drought conditions have improved, yet irrigation allocations will remain tightly regulated due to continued groundwater depletion and erratic weather. In order to cope with limited water rations, growers can benefit from testing the soil to measure water holding capacity, texture, organic matter, and other factors affecting plant available water.

 Knowing the soil’s water holding capacity and plant available water allows growers to saturate the soil to a level optimal for their goal. Plant available water is the moisture held in the soil between field capacity and permanent wilting point. The soil is said to be at “field capacity” once a saturated soil profile has fully drained. At field capacity the water left in the soil is loosely bound to pore walls by capillary action. The amount of water held at field capacity is the soil’s maximum water holding capacity. Soils reach permanent wilting point when the profile becomes so dry that plants wilt beyond recovery. Between field capacity and permanent wilting point, root suction can overcome capillary forces to extract and absorb moisture. Most of the time, growers want to irrigate just enough to facilitate plant nutrient uptake and evapotranspiration while maintaining sufficient soil aeration to support healthy roots. Growers also need to periodically flush the soil with excess water to reduce salt buildup and lower electrical conductivity in the root zone. No matter the goal, growers can achieve better irrigation accuracy by including values for water holding capacity and plant available water in their irrigation management calculations.

What determines the soil’s Plant Available Water?

Plant available water is influenced by soil texture, structure, and percent organic matter. Together, these three components determine the number, size, and connectivity of soil pores where water and air are stored. Soil texture is the ratio of sand, silt, and clay sized particles in the soil’s mineral fraction. Texture falls on a continuum from very course (sandy soil) to very fine (clayey soil). Large particles in sandy soils create large pore spaces that allow water to drain quickly down the soil profile and out of the root zone. Fine particles in clay produce very small pores that hold water at a very high suction pressure, requiring plants to exert more energy to extract moisture from soil. As seen in the diagram, clay soils hold more water than loamy soils, but less of that water is plant available. Clays hold more water because the fine particles have very large surface areas capable of attracting and holding more water than a similar volume of silt or sand particles. Plant available water is maximized in medium textured loamy soils with enough surface area to retain plenty of water with pore sizes ranging from small to large. Small and medium sized pores store water and facilitate plant uptake, while larger pores allow drainage and soil aeration.


Plant Available Water Diagram from P. Kramer, 1983


Regardless of texture, soil organic matter (SOM) affects plant available water in several ways. Compared with a similar volume of mineral particles, SOM adsorbs much more water on its surfaces, increasing the soil’s water holding capacity. Soil organic matter also helps buffer soil temperature, keeping soil cool in the summer and preventing water loss through evaporation. Soil organic matter indirectly affects plant available water by enabling soil structure development. Without adequate organic matter, the soil will not be able to form the structure that provides even water and nutrient distribution to plant roots. Structureless soils often either drain quickly, causing crop dehydration, or don’t drain enough, causing soil saturation and oxygen depletion. Given enough organic matter, soil particles will clump together into stable soil aggregates with lots of pore spaces providing pathways and storage for air, water, and nutrients.


Soil Health
Soils high in organic matter retain more moisture than low organic matter soils with similar texture and mineral composition. The dark color and granular structure in this soil indicates high organic matter and good soil health. Courtesy of NRSC


How Can Growers Increase the Soil’s Plant Available Water?

While growers cannot change their soil’s texture, they can adjust management practices to increase or preserve soil organic matter. Leaving crop residue on the field, incorporating cover crops, reducing tillage, or adding compost, green waste, or manure, are all great ways to increase soil organic matter and maximize plant available water. In addition, growers can protect existing soil organic matter by using fertilizers high in carbon, in addition to N, P, and K. Many fertilizers including AgroThrive, Wiserganic, and O-MEGA contain about 16% carbon, feeding not only the crop, but also the soil microorganisms. Without the extra carbon input the microbes will consume the existing plant material and quickly deplete the soil’s organic matter, lowering water holding capacity and plant available water. By protecting and building soil organic matter, plant available water will increase so that more of the water applied at irrigation will be stored in soil and used to grow the crop.