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Orchard Irrigation:  Soil Water Content

 

Water Holding Capacity

soil water content
Soil water content from saturated to dry.  Optimal levels for plant growth are between field capacity and allowable depletion.

allowable depletion
The amount of allowable depletion, or the readily available water, represents about 50% of the total available water.

  • 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.

The goal of a well-managed irrigation program is to maintain soil moisture between field capacity and the point of 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.  Although tree roots can grow to several yards depth, 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.  A deep sandy loam soil at field capacity, for example, would contain 1.8 to 2.25 inches of readily available water in an effective rooting depth of 3 feet.

Measuring Soil Moisture

Recommended Watermark sensor values at which to irrigate.

Soil Type
Irrigation Needed
(centibars)
Loamy sand 40-50
Sandy loam 50-70
Loam 60-70
Silt loam, silt 70-90
Clay loam or clay 90-120

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 WatermarkTM 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.