Why is My Soil Report Missing the Soil Texture?
Sending soil samples to the lab on an annual basis gives us great insight on soil nutrient status. If the reports are compiled in a spreadsheet where we can track soil status year over year, a quick determination can be made on whether our fertilizer program is keeping up with crop demand.
Soil levels change according to crop removal and application. Similarly, pH may increase or decrease according to our irrigation practices and amendment program; sodium and salinity are in a constant state of flux.
One soil property that doesn’t change is texture; it’s a fixed characteristic unless a significant volume of soil is moved. Slip plowing could pull up subsoil clay and deposit it on the sandy surface, but most often, the texture we have today is here to stay. Soil texture is the fraction of sand, silt and clay of the soil we work with. Soil labs can determine texture with a device called a hydrometer. A dry volume of soil is treated with a dispersant, dropped in a graduated cylinder of DI water and agitated. After some period of time, sand drops out first, followed by silt, and lastly, the clay particles settle. The fraction of each can then be calculated and texture determined. Labs charge an additional fee for this service, so we typically do not order a textural analysis. To get to the texture answer, we look at two other calculations on the lab report – Saturation Percentage and Cation Exchange Capacity.
Saturation percentage is a measurement of how much water a specific volume of soil will hold at saturation. De-ionized water is added to dry, ground soil until it makes a paste and then SP is calculated by dividing the weight of the water by the weight of the soil. Clays and silts hold more water than sands, so higher SPs indicate clay soils.
Cation Exchange Capacity is a measurement of the soil’s ability to hold cations, which are positively charged nutrients like potassium K+, calcium Ca++, magnesium Mg++ and sodium Na+. Since clay soils hold more cations, higher CECs indicate the soil to be a clay texture. In contrast, if the report lists a CEC of <7 it indicates the soil texture to be sandy.
Above is a table from the UC Davis Walnut Production Manual that shows the relationship between soil texture, SP and CEC.
Why is texture important to a grower? If the soil has a low pH, ag lime could be applied, and the calculation for the tons required is based on texture. To correct a high pH soil by banding elemental sulfur or sulfuric acid, the pounds required are determined by texture. In building soil levels of potassium with fertilizer, texture guides us in the quantity of fertilizer needed. Texture also influences our choice of nitrogen fertilizer and how it’s applied.
Perhaps the greatest impact of soil texture is in irrigation management. As the percentage of clay and silt increases in texture, so too does the soil’s capacity to hold water. A clay loam soil can hold 2” of water per foot of depth. In contrast, a sandy loam soil holds half that, or 1” of water per foot of soil depth. It means the period between applications of irrigation water on clay loam could be twice that of sandy loam. For example, an irrigation schedule for a clay loam might apply 3” of water every 14 days, but a schedule for a sandy loam soil would apply 1.5” of water every 7 days. At the end of the 28-day period, each soil type would receive the same 6” of water, but the frequency of application is quite different.
Texture is an extremely important soil physical property, yet seldom reported on a soil report; using SP and CEC as substitutes, we are able to gain valuable insight into soil texture. For more information, contact your local Helena representative.
- Don Wolf, Agronomist