The Soil Triangle

All soils are made up of three basic soil components; sand, silt and clay. These components are defined by the size of the particles in each class: sand defined as being smaller than 2 mm and larger than 0.05 mm, silt is between 0.05 to 0.002 mm, and clay less than 0.0002 mm in diameter. The relative amount of each one of these determines which soil type you have.

This information is plotted on an equilateral triangle where the legs of the triangle are the percentage of each soil component from 0 to 100. This is known as the soil triangle. The triangle is subdivided into 11 regions based on the percentages of the various components. Each of these regions have different infiltration rates (the rate at which water can enter the soil) and soil moisture holding capacity. This information can then be used to design irrigation systems and set preliminary managed allowable depletions (MADs) for different crops on the different soil types.

Determining the various components can be fairly easy and inexpensive to time consuming and pricey based on the level of accuracy required for your application. The easiest way to determine the components is to take a soil sample, remove the twigs and large gravel, and place the sample in a glass mason jar. Add a dash of liquid detergent and water to cover the soil by 1” or more then screw on a lid tightly. Shake the jar for about 4 minutes then place on a flat surface and leave for two days. At that time the water above the soil should be clear and you should discern three soil layers in the jar. The bottom layer is the sand component since it has the largest particles and settles the fastest. The silt is the middle layer and the clay is the top. To determine the percentages of the components you only need to measure the height of each individual layer and sum them together. Dividing the height of each layer by the total height will give you the percentage of each component. The other way to determine this is to collect a soil sample and send it to a testing laboratory in your area. Normally this testing will cost $25-30 per sample.

Once you know which region of the soil triangle you can use the following graph and tables to estimate the properties of the soil. Just remember that this information was generated in a laboratory setting for bare soils on a flat surface. The first table shows the basic intake rates for seven regions of the soil triangle. This information is useful when designing irrigation systems and determining the effectiveness of rain fall events. Any water applied to these soils at rates above those listed in the table has a strong potential to leave the site as runoff. This could result in erosion, down stream water quality problems, wasted water, and/or wasted energy.

The next table shows the water holding capacity for the various regions of the soil triangle. The values present in this table are percent of the soil volume. (Permanent Wilt Point is defined as the point at which the plant can no longer extract water from the soil.) Let’s take a peach tree in sandy loam as an example. We will use the parameter that the tree has a 20” root zone. As a result the soil has 20% by volume total water or 20” times 0.2 for 4” of water in the soil at field capacity. The tree can use 25% of this water without experiencing any stress or 4” multiplied by 0.25 for 1” of usable water. If this same tree is planted in silty clay with the same considerations then the tree has 20” times 0.4 times 0.25 or 2” of usable water before plant stress. Therefore a peach tree planted in sandy loam will be irrigated twice as often for half the irrigation duration as the same tree planted in silty clay under the same environmental conditions. This example demonstrates that it is extremely important to know the soil type(s) being cultivated to define some of your starting irrigation parameters. The information present in this table is shown in the following graph.

There is one important caveat to remember when doing this analysis: the soils are highly stratified in both verticle and horizontal directions within the District.  What this means is that one irrigation block can be grown on more than one soil type so the area prone to be most sensitive to water stress should be used as the monitoring area.  Further, the root zone can be composed of more than one soil type so you need to be aware of these soil transitions when applying water or monitoring for plant stress.  Within EID there are some heavy clay soils where it takes 2-3 weeks for the irrigation water to reach the 3'-soil level as monitored by the neutron probe.