Orchard Irrigation:  Soil Water Content

Water Holding Capacity

• Field Capacity is the amount of water that can be held in the soil after excess water has percolated out due to gravity.  
• Permanent Wilting Point is the point at which the water remaining in the soil is not available for uptake by plant roots. When the soil water content reaches this point, plants die.
• Available Water is the amount of water held in the soil between field capacity and permanent wilting point. 
• Allowable Depletion (readily available) is the point where plants begin to experience drought stress. For most fruit trees, the amount of allowable depletion, or the readily available water represents about 50% of the total available water in the soil.
• Rooting Zone is the soil volume that roots can access based on rootstock, cultivar, soil depth, texture, restrictive layers, and management practices.

The goal of a well-managed irrigation program is to maintain soil moisture between field capacity and the allowable depletion, or in other words, to make sure that there is always readily available water.

The amount of readily available water is related to the effective rooting depth of the plant, and the water holding capacity of the soil. The effective rooting depth depends on soil conditions, variety and rootstock (rooting zone). Although tree roots can grow to several yards deep, nearly all of the roots of a mature tree are typically in the top 2 to 3 feet (Atkinson, 1980). The water holding capacity within that rooting depth is related to soil texture, with coarser soils (sands) holding less water than fine textured soils such as silts and clays (see below). A deep sandy loam soil at field capacity at an effective rooting depth of three feet, for example, would contain 3.6-4.5 inches of available water, half of which (1.8 to 2.25 inches) is readily available water. Assuming that the tree roots extend 3 feet out from the trunk, the rooting zone could hold 64-79 gallons of available water at field capacity.

Available water holding capacity for different soil textures, in inches of water per foot of soil. Available water is the amount of water in the soil between field capacity and permanent wilting point. Readily available water is approximately 50% of available.*

Soil Texture	Available (inch/foot)	Readily Available (inches)* 2 ft root depth	Readily Available (inches)* 3 ft root depth
Sands and fine sands	0.5-0.75	0.5-0.75	0.75-1.13
Loamy sand	0.8-1.0	0.8-1.0	1.2-1.5
Sandy loam	1.2-1.5	1.2-1.5	1.8-2.25
Loam	1.9-2.0	1.9-2.0	2.85-3.0
Silt loam, silt	2.0	2.0	3.0
Silty clay loam	1.9-2.0	1.9-2.0	2.85-3.0
Sandy clay loam, clay loam	1.7-2.0	1.7-2.0	2.6-3.0

Measuring Soil Moisture

In order to assess soil water content, one needs to monitor soil moisture at several depths, from just below the sod layer or cultivation depth (4 to 6 inches), to about 70 percent of effective rooting depth (2 feet). One of the more cost effective and reliable methods for measuring soil moisture is by electrical resistance block, such as the Watermark sensor (Irrometer Co., Riverside CA). These blocks are permanently installed in the soil, and wires from the sensors are attached to a handheld unit that measures electrical resistance. Resistance measurements are then related to soil water potential, which is an indicator of how hard the plant roots have to “pull” to obtain water from the soil. The handheld unit reports soil moisture content in centibars, where values close to zero indicate a wet soil and high values represent dry soil. The relationship between soil water potential and available water differs by soil type. The maximum range of the sensor is 200 centibars, which covers the range of allowable depletion in most soils. The sensors are less effective in coarse sandy soils, and will overestimate soil water potential in saline soils. Remember that allowable depletion is 50% of available water, which roughly corresponds to soil water potentials of 50 centibars for a loamy sand soil, and 90 centibars for a loam (see below, 50% depletion threshold values for each soil texture). 

Recommended Watermark sensor values at which to irrigate.

Soil Type	Sensor Reading at 50% depletion of soil water (centibars)
Loamy sand	40-50
Sandly loam	50-70
Loam	60-70
Silt loam, silt	70-90
Clay loam or clay	90-120

Growers may also estimate soil moisture by feel. This is a skill that takes some practice, but can be used to estimate soil moisture with in about 5 percent. Soil samples must be taken at various intervals in the full rooting zone of the crop based on soil texture, depth and stratification. Each sample is examined based on a set of criteria including can it form a ball, how strong is that ball, does the soil stain hands and fingers, and is water visible. For more information on using this method to estimate soil moisture visit the NRCS’s Guidelines for Estimating Soil Moisture.