Associated with many salt problems is a problem of sodium. The sodium tends to displace the other cations on the exchange complex, to accumulate In the soil solution, and to interfere internally with the plant physiology. The sodium problem is more complex. Sodium may exist in soil either as free salt or as part of the exchange complex. Free sodium will leach readily or wash from the soil. Exchange absorbed sodium must be displaced by another cation, such as calcium. Gypsum (calcium sulfate) is commonly used for this purpose. Prerequisite for gypsum applications to reclaim an alkali soil is good drainage, followed by leaching with excess water by flooding or sprinkling.

Sodium content of soils should not exceed 2-3 meq/7100 g or 15% of the cation concentration depending upon the particular soil and situation.

Soils are classified by salt content as follows

When requesting soluble salt and sodium the laboratory normally determines a total sodium. Be sure to designate your request as exchangeable sodium if desired for reclamation. Be sure to keep the type of analysis in mind when interpreting the test results or making reclamation recommendations.

Toxicity

Several elements and many commercially prepared compounds are very toxic to plants, animals , or both. The best advice is to follow manufacturers directions carefully on commercial compounds and to check to find out how much if any has previously been applied to the soil in question.

The major naturally toxic elements are arsenic, boron, manganese, selenium, and molybdenum.

Arsenic in soil is usually non-toxic to plants at 5 ppm or less, becomes toxic to sensitive seedlings at about 20 ppm, and is lethal to replants of perennial material at about 80 ppm, although established, well-rooted crops may continue to survive and not show damage.

Selenium may be required in small amounts in animal nutrition, but is very toxic in larger amounts. One part per million or more, of soluble selenium in soil can be considered to be dangerous to animals being fed the forage grown on the soil.

Boron should always be checked as part of a salinity diagnosis, especially an arid soils or any soil receiving supplemental irrigation.

Manganese is most generally toxic on soils of pH less than 5, and is not generally a serious problem on soils of near neutral (6.5), or more basic pH. Manganese becomes much less soluble as the pH increases from the strongly acid side, and this solubility decrease is often the major dividend from line application.

Molybdenum exhibits a reverse action; as pH rises the molybdenum becomes soluble. Where mild molybdenum deficiencies exist, lime applications often overcome the problem. Other acidic soils often contain sufficient molybdenum that toxic forage results when liming is done. Some alkaline soils also contain sufficient molybdenum to produce toxic forage.

When potentially toxic trace elements are being considered for application, the soil in question should be thoroughly analyzed and the side effects for future crops as well as the current or next crop should be carefully considered. It cannot be emphasized too strongly that over-application of these elements can be very difficult to rectify, but when used wisely on deficient soils they are a tremendous crop production assist.

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