AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 2003, No. 1
Fertilizer salts form soluble ions in soil water. Increased concentration of ions increases osmotic pressure and decreases water potential, making it harder for plants to take up water. This is why plants affected by “fertilizer burn” look about the same as if they had been stricken by drought. They can’t get the water, because there’s too much salt in it.
Fertilizer doesn’t have to burn, though. It’s all a matter of dosage. Plants can’t grow without salt, either. The nutrients they need are salts. The dissolved ions are exactly the form they take up. As long as the dosage is controlled, there is no harm applying a salt to the soil.
The kind of salt is important. Specific salt ions have greater effects than others. The ammonium ion in particular can release free ammonia, especially at higher soil pH. Ammonia moves directly into plant cells. High concentrations can prevent root growth and even kill seedlings. On the other hand, phosphate ions hardly pose a salt hazard at all, since they never get to high concentrations in soil water.
Salt is often associated with sodium, because common table salt is sodium chloride. Sodium ions can destroy soil structure and clog the flow of soil water. But there’s hardly any sodium in most commercial fertilizers.
The chloride ion is one of the most soluble. Grapevines, woody trees, and many legumes are sensitive to it. Research in the southern states showed some soybean varieties to be sensitive to chloride. But research in many other places has found muriate of potash (potassium chloride) to be an effective source of potassium for soybeans grown in deficient soils. And crops like wheat and corn show great benefits from fertilizing with chloride.
Several fertilizers aren’t truly salts. Urea, for example, is a soluble substance that isn’t a salt. Nevertheless, its solubility means it can have an osmotic effect, just like any other fertilizer. It also quickly decomposes to form a salt–the ammonium ion. Elemental sulfur is neither salt nor soluble–but it oxides into sulfate, a salt.
Manures and composts contain inorganic salts, organic salts, and insoluble organic forms of nutrients. But their salt content per unit of nutrient may not differ much from fertilizer, since their nutrient content is a lot lower, and they contain salts not necessarily needed for the crop. Also, as they decompose, the nutrients turn into salts.
How to avoid salt injury? Guidelines for safe rates are specific for each crop and are based on distance from the seedling, soil texture, soil moisture content, and the specific ions in the nutrient source. For ammonium, soil pH is an additional consideration.
A high salt content is a consequence of a unique advantage commercial fertilizers possess. Using sources with concentrated soluble nutrients cuts transportation costs–less fuel is wasted in transporting non-nutritive material. In order to utilize that advantage, commercial fertilizers need to be applied at judicious rates.
Why use salt for fertilizer? Because plants use salt for food.
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