5.7: Soil-Water Relations

Soil water is of great importance because of the many biological and chemical reactions occurring due to the presence of moisture in the soil. In a completely dry soil, very few physical or chemical reactions occur. In a moist soil, many reactions occur, while in a very wet soil, a completely different set of reactions occur. Additionally, the behavior of a soil is completely altered once the moisture/saturated conditions are altered, and these dynamics are specifically dependent upon soil texture and structure.

Plants use several mechanisms to obtain water from soil. Passive absorption is the most important process and accounts for more than 90% of the water absorbed by plants. Passive absorption is the movement of water into a plant root resulting from the pulling force (suction) on soil water caused by the continuous water column moving upward through the plant as water is lost at the leaf surface by transpiration. The transpiration process is the actual cause of the water loss due to the air being much drier than the water in the plant. The more water a plant removes from a soil, the more difficult it becomes for the plant roots to continue to obtain the remaining water from the soil. The plant roots must exert a greater suction on the water in the soil pores to enable the roots to extract more water for continued crop growth.

Plants are able to use only a portion of the total water held in the soil pore space. It is convenient to think of soil water as a continuum from completely wet (saturated) to completely dry (oven dry). This continuous spectrum of soil water is conveniently separated into crop and soil response regions (Figure 3). These divisions are based mainly on the soil water potential (or suction) designated in negative kilopascals (kPa) of pressure or positive kilopascals (kPa) of suction.

Figure 3. Detailed description of the association between crop and soil response regions and soilmoisture availability. Source: Rhoads and Yonts, 1984.

Water moves through a soil in the large pores and drains away due to the force of gravity. This water is called gravitational water. Gravitational water represents the difference in soil moisture content between the saturation condition and field capacity.

Gravitational water = soil moisture content at saturation - soil moisture content at field capacity.

Water held within small soil pores by adhesion and cohesion is termed capillary water. Only a portion of the total capillary water in the soil is available to plants. Plant available water can be obtained from the soil. Plant available water is the difference between the water content of a soil at field capacity and the soil water content at the permanent wilting point.

Plant available water = water content at field capacity - water content at permanent wilting point

Available water-holding capacity (plant available water) is an estimate of the water held between field capacity and permanent wilting point within the rooting zone or the top 150 cm of the soil if there is no root-limiting layer. The total water is calculated by summing the amount of water held in each horizon or portion of horizon, if the horizon extends beyond 150 cm. If a horizon or layer is unfavorable for roots, this and all horizons below should be excluded in calculating the available moisture.¹

The relationship between available water retained per centimeter of soil and the textures is given in Table 2 below. If a soil contains many large pebbles and/or rock fragments, the volume occupied by the rock fragments must be estimated and subtracted from the total soil volume.

For example, if a silt loam A horizon is 25 cm thick and contains rock fragments which occupy 10% of its volume, the available water of the horizon would be 25 cm x 0.20 cm/cm x [(100-10)/100] = 4.50 cm of water.

To calculate the available water for a soil profile, first calculate the available water for each horizon to the nearest hundredth, sum all horizons, then round the grand total to the nearest tenth. For example, 14.92 would round to 14.9 in the low class; 15.15 would round to 15.2 in the moderate class.

Table 2. Calculation factors for water-holding capacity (Adapted from: 2018 Region VII Official Handbook)

Available Water Calculation Factors
Available water (cm of water / cm of soil)	Texture classes
0.05	sands, loamy sands
0.15	textures not included in the other classes
0.20	silt loam, silt, silty clay loam, peat, muck, mucky peat
The available water classes are:
Very low. Up to and including 7.5 cm of water
Low. Greater than 7.5 cm of water but less than or equal to 15.0 cm of water
Moderate. Greater than 15 cm of water but less than or equal to 22.5 cm of water
High. Greater than 22.5 cm of water

For most field crops, the permanent wilting point is equal to -1500 kPa of water potential or 1500 kPa of water suction. When a soil is air dry, the water present is held at water potentials ranging from -3000 to –10,000 kPa. The water in air dry soil is in equilibrium with the soil pore atmosphere which has about 98% relative humidity. The Hygroscopic Coefficient is the condition when the last micro pore is drained of water and only films of water exist surrounding the soil particles. Soil dried at 105°C to a constant weight is considered oven dry. The oven dry weight of soil is used as the reference weight to quantify the amount of water in mineral soils for all moisture conditions. Hygroscopic water is the water held between the Hygroscopic Coefficient and Oven dry.

Hygroscopic water content = Water held below the Hygroscopic Coefficient - Oven dry mass

¹ For available water calculations, the properties of the lowest horizon designated for description can be assumed to extend to 150 cm, if the presence of a restrictive layer is not evident. If a restrictive layer is present between the lowest described horizon and the 150 cm depth, the depth to the restrictive layer should be considered for available water estimations. Materials not suited for plant root growth include: (i) horizons with coarse sand textures and some unfilled voids located directly underneath a horizon of finer-textured soil materials (i.e., textures of very fine sand, loamy very fine sand or finer), (ii) bedrock, (iii) fragipans, (iv) densic materials, (v), horizons cemented across 90% or more of the soil profile, and (vi) SiC, C, or SC textures that are very firm or firmer and have structureless grade and massive shape of structure.