Chloride is taken up by plants as the Cl- ion. A major function of chloride in plants is as a counter ion for cation (Ca+2, K+, Mg+2, N4H+) transport and as an osmotic solute. In addition, chloride serves an essential role in maintaining cell hydration and turgor. A critical role of chloride is as a cofactor in the oxidation of water in photosynthesis and as an activator of several enzymes.
Physical symptoms of chloride deficiency in plants vary and are not always consistent. In wheat, some varieties show a characteristic leaf spotting, best described as random chlorotic spots on the leaves (Figure 1). The spots resemble tan spot lesions, but are smaller and do not have the characteristic “halo” at the edge of the spot. On low-chloride soils in Kansas, some varieties consistently show the leaf spotting, while other varieties never spot. Other research indicates no obvious visual deficiency symptoms occurred on corn or grain sorghum, even where chloride fertilization increased yields.
Figure 1. Chloride deficiency symptoms in wheat resemble tan spot lesions, some varieties might not show a visual deficiency symptom. Photo by Dorivar Ruiz Diaz, K-State Research and Extension.
Chloride is normally present in the soil in sizeable quantities, particularly in U.S. coastal areas where chloride deposition is high. Evaluations in Kansas indicates fairly low soil chloride levels. This could be due to low chloride atmospheric deposition due to the distance from oceans and the relatively high indigenous potassium levels of the majority of Kansas soils, which means little potassium chloride (KCl) fertilizer has been applied. Summaries of soil test data in Kansas show a majority of the samples had chloride levels below 40 pounds per acre, with a significant number of samples less than 10 pounds per acre (on 0- to 24-inch samples).
As an anion, a negatively charged ion, chloride is not readily adsorbed on the soils exchange complex and is subsequently not attached. Because of this, chloride moves readily with soil water. Chloride is quite leachable, even more so than nitrate.
Soil testing for chloride
The Kansas State University Soil Testing Laboratory and most commercial labs offer a chloride soil test. Because of the leaching potential of chloride, we recommend sampling to a depth of 24 inches to best assess soil chloride status (just like nitrogen and sulfur). When testing for pH, phosphorus (P), potassium (K), organic matter, and zinc, a 0- to 6-inch sample is recommended. When testing for the mobile nutrients (nitrogen, sulfur, or chloride) a 0- to 24- inch sample is recommended.
Potassium chloride (KCl) is the most common and readily available chloride-containing fertilizer in Kansas. On an elemental basis, KCl fertilizer is 53 percent potassium and 47 percent chloride. For ease of calculating, assume a ratio of roughly 50 to 50 potassium to chloride. For example, if 50 pounds of KCl fertilizer is applied, about 25 pounds of chloride would be furnished. Since P and K in fertilizer are reported on an oxide basis (P2O5 and K2O), it can be confusing because many fertilizer dealers know potassium chloride as 0-0-60 or 0-0-62. For ease of calculating chloride application, just remember the product is about 50 percent chloride.
Other chloride-containing fertilizers include: ammonium chloride (NH4Cl), calcium chloride (CaCl2), magnesium chloride (Mg2Cl), and sodium chloride (NaCl). These fertilizers contain 66 percent, 65 percent, 74 percent, and 60 percent chloride, respectively. Calcium chloride, ammonium chloride, and magnesium chloride are sometimes available as liquid fertilizer. Research in Kansas has evaluated all of these sources of chloride. Results show each of these fertilizers to be equally effective in supplying chloride.
Chloride research in Kansas
Considerable research with chloride fertilization has been conducted in Kansas on wheat, corn, and grain sorghum. Positive yield responses have been noted on these crops. To date, response to chloride fertilization on other crops such as soybean has been limited.
Soil test recommendations and fertilizer recommendations
Research indicates the likelihood of a response to chloride fertilizer is directly related to soil chloride levels. Chloride levels in Kansas soils vary, but levels below 25 pounds per acre are not uncommon, particularly where potassium chloride fertilizer is not normally used. Since most central and western Kansas soils are high in potassium, use of potassium chloride fertilizer has been limited, and low soil-chloride levels are often found. In eastern Kansas, however, where potassium chloride is routinely applied, low soil-chloride levels are not widespread. On soils low in chloride, optimum yields of crops may require addition of chloride fertilizer. The information in Table 1 summarizes our interpretation of soil test chloride information.
Table 1. Soil test chloride interpretation and fertilizer recommendation.
(pounds per acre)
(pounds per acre)
*Interpretations valid for 0-24 inch samples on wheat, corn, and grain sorghum
Plant tissue analysis also has proven valuable in assessing a potential need for chloride. Research shows that whenever leaf chloride concentrations are in the 0.10 to 0.12 percent range or less, this is a good indicator of low soil chloride levels. Again, research has been limited to wheat, corn, and grain sorghum. Research with wheat used leaf samples taken at boot stage, while corn and grain sorghum leaf samples were taken at the 6- to 8-leaf stage.
When soil tests indicate a need for chloride, the recommendation is to apply 10 to 20 pounds of actual chloride per acre, depending on soil test chloride level. For example, if potassium chloride is being used, application of 30 pounds per acre of potassium chloride would supply about 15 pounds per acre of actual chloride. Research shows equal performance of chloride applied either pre-plant or topdress (November through early March) for wheat. On corn and grain sorghum, pre-plant or planting time applications are preferred. With the good solubility of all chloride fertilizers, surface broadcast applications work well with sufficient rainfall or irrigation after application.
Remember, response at any given soil chloride level in a specific year may vary with several factors, including variety, disease pressure, timing of moisture or temperature stress relative to the effect of chloride on plant development, and soil chloride distribution relative to crop root distribution.
Chloride, an often-overlooked nutrient, is essential for plant growth. Deficiencies of this nutrient have been verified in Kansas. Chloride is essential for photosynthesis and serves other critical roles in plants. Plants take up chloride as the Cl- ion. This ion is very mobile in the soil and is subject to leaching.
Soil testing and plant analyses have proven useful in identifying potential deficiencies of chloride. Recent Kansas research has verified a need for chloride fertilization on some soils. Chloride recommendations are based on soil test chloride levels. If supplemental chloride is needed, several sources of soluble chloride fertilizers are available and agronomically effective.