Using a Soil Sample to Choose the Right Calcium Amendment

Introduction

Irrigated agricultural soils can show symptoms of excessive sodium accumulation, which is marked by poor water infiltration, crusted soils, low oxygen in the root zone, decline in crop performance and yield, and a measurable loss in soil structure.  Soils that have excessive sodium can also have elevated chloride and sometimes boron, which impacts crop yields, generally due to poor drainage and soil structure. In order to correct these symptoms, amendments containing calcium are applied to help restore soil structure, reduce root zone salts, and help improve the salinity status of the field.

The question that I will help answer in this article is: “Which calcium amendment do I use?”

Taking a soil sample will help you figure out the following important parameters critical to your reclamation program:

-          Scope and severity of salinity issue

-          Which amendment to use (e.g., gypsum, lime, sulfur product)

-          Dosage of correct amendment to apply

Critical parameters on the soil report worth looking at are: soil pH, fizz test results, EC (dS/m), SAR, sodium results, and chloride results.  The fizz test measures the amount of free lime in your soil (CaCO3), and this is either expressed qualitatively (low, high, etc.) or quantitatively (%). In fact, the fizz test results are so important that it is a great place to start when deciding which calcium amendment is the right one for your field (Figure 1).

Figure 1 - A soil sample can help direct you to the most optimal use of a calcium amendment on your ranch. Lime can be used in a manner similar to gypsum and is most soluble in acidic soils.

Gypsum and Lime

Gypsum (CaSO4) is a great calcium amendment when one has low or very low fizz test readings (Figure 2). When gypsum dissolves, it is able to supply calcium directly to the soil, which improves soil structure and allows excess salts (e.g., sodium) to drain. The reaction sequence is how gypsum contributes to the salinity reclamation program. In soils that are acidic with structural issues, lime (CaCO3) can be applied to fields to both provide a calcium source, similar to gypsum, while also improving soil pH (acidic à neutral).  Gypsum will have little impact on soil pH.

Figure 2 - Gypsum supplies calcium directly to the soil, which improves soil structure and helps excess sodium and chloride move out of the rooting zone. Lime will also provide a direct calcium source and can help raise the pH of acidic soils.

Sulfur Products

Sulfur products, such as sulfuric acid and elemental sulfur, require the presence of free lime in the soil (CaCO3). As such, soils that have medium to very high fizz test ratings are a great candidate for the use of sulfur products in your reclamation program.  Briefly, sulfur products react with the free lime in the soil to create gypsum as a by-product. The calcium in the gypsum is then able to go to work to improve soil structure and reduce accumulated salt as discussed previously (Figure 3). One advantage of using sulfur products is that you generally do not need as much material to get the job done as you do gypsum (1 ton gypsum = 0.57 ton sulfuric acid = 0.19 ton elemental sulfur) which can greatly impact field logistics and application costs. Also, repeated applications of sulfur products can reduce problematic soil pH areas (alkaline à neutral). One disadvantage to using sulfur products is that they have several reaction steps that can slow the reclamation speed of the field relative to lime and gypsum. Also, sulfuric acid products have safety concerns that must be considered.

Figure 3 - Sulfur products must be reacted in the soil with free lime (CaCO3) so that it can turn into gypsum. Sulfuric acid has a two-step reaction process (top), whereas elemental sulfur has a three step reaction process (bottom). Once gypsum is formed, it supplies calcium directly to the soil, which improves soil structure and helps excess sodium and chloride move out of the rooting zone.

Leaching Fractions

Now that you have matched your field conditions (e.g., fizz test, required amendment dose, soil pH) to the right calcium amendment, you are ready for the next step (Figure 4).  All amendments, similar to fertilizers, must be solubilized in water for them to go to work in the soil.  A reclamation program will not work if the soil remains dry.  Additionally, extra irrigation water is required to physically carry the excess salinity down out of the rooting zone in order to improve the salinity status of the field.  This extra water is called the leaching fraction and is essential for restoring your fields to optimum performance.  While there are leaching fraction equations that have been published, Figure 4 shows a general rule of thumb on how to reduce your soil salinity (EC = dS/m) by X% by applying Y inches of extra water.

- Dr. Karl Wyant, Lead Agronomist, Western Division

Figure 4 - A general rule of thumb for calculating leaching fractions is shown above. From your soil report, look at your EC (dS/m) and make a feasible salt reduction goal by applying 6-24 inches of additional water in a future irrigation after amendments are applied.