Problems with Excessive Sodium
Sodium is found in the soil both naturally and as a result of the products applied by man. Sodium affects both the structure of the soil and the plants growing in it. Too much will permanently alter growth and the microbial activity in the soil.
Plants are affected by the movement of water in the soil. This water movement, which carried the sodium, is mainly up or down. If the soil is wet the movement will be largely downward. As the soil dries, the water moves upward by capillary action and carries with it any dissolved salts. Without adequate internal drainage LEACHING OUT OF SODIUM BY IRRIGATION NEVER OCCURS because capillary action keeps the salts where they affect the plant roots.
OSMOSIS is a physical process by which the plant absorbs water and dissolved nutrients from the soil through the root hairs. This movement takes place only when the osmotic pressure within cells of the root hairs is GREATER than that of the soil solution. This pressure reverses with an increase in the soluble salt content of the soil and the result is EXOSMOSIS. Large amounts of irrigation WITH DRAINAGE will generally offset this problem, however it only masks it so that it is not recognizable as a problem. When this concentration of dissolved salts is high, water will be pulled out of the plant cells making them very susceptible to dessication, stress and various other growth and health problems.
Sodium also affects the PHYSICAL condition of the soil, which affects the internal drainage and causes compaction. As the salinity increases so do the amount of sodium ions which are measured as the exchangeable sodium percentage -%Na- on the soil test. Excessive sodium ions break apart the soil structure, causing large, porous particles to become many very small particles. These particles then COMPACT and further restrict the flow of water through the soil. As a result, microbial activity and root growth (among other factors) are unfavorably affected due to the lack of oxygen. THE WATER JUST CAN’T GET OUT OF THE WAY TO LET THE AIR IN !! This problem of salinity compaction also results in other negatives: development of fungus, thatch and a MUCH lessened effectiveness of added granular nutrients and those already in the soil both because the minerals need microbial activity to break them down and the microbes need air as they are living organisms. Thus as the sodium toxicity gets worse, the soil situation breaks down even further.
When considering the fact that in non-saline (normal soil), only 50% of the water is available for the plant to use, it becomes obviously important that the grower need to be aware of the sodium in his soil and to understand the effect of that sodium on his crop. Base Saturation Soil Testing is an excellent method of monitoring exchangeable sodium (a % of 3 or less is considered normal). Excessive sodium is THE major LIMITING FACTOR in soil management today. All others are secondary when this happens to you.
Reference Material:
1) Our Soils and Their Management, 5th Edition, 1983 Donahue, Folett, Tulleh, Page 143, Chapter 10
2) Turf Maintenance Guide, Duble (Texas A & M), pages 636-638
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