inkcube

this is from a nutrition lecture i give to my intro to hort class:

to understand salts, one must understand what a salt is. salts are the products, other than water, of the reaction of an acid with a base. the classical concept of acids and bases is the neutralization process of combining a hydrogen ion and a hydroxyl ion to form water, (H+) + (OH-) = H2O.

fertilizer salts are compounds made up of positive and negative ions that may be related only indirectly to acid-base reactions. the positive ions most frequently found in fertilizer salts are the simple metal ions such as K+, Ca++, Mn++ , Mg++, Fe++, Zn++ and the solvated hydrogen ion NH4+. the negative ions can include polyatomic ions such as NO3-, SO4- and PO-4.

when a fertilizer salt such as K2SO4 (potassium sulfate) is applied to the soil, the compound divides into K+ and SO4-. the potassium ion can then be used by plant materials or be attached to clay colloids (particles0, while the sulfate ion stays in the soil solution and moves with water to the roots to be used by plants, or can be leached below the root zone and away from the plant materials. all fertilizer salts react the same way, whether they're potassium chloride (KCl), ammonium nitrate (NH4NO3 or ferrous oxide (FeO). negative ions, such as NO3-, SO4-, and Cl-, all stay in the soil solution and don't attach to the soil colloids.

Salt Buildup

this brings to light the chloride ion (Cl-), which needs some discussion. some feel that chloride ions might add to "salt" buildup in soil. this is false, as the chloride ion doesn't attach to the soil. sulfate of potash adds just as much "salt" to the soil as muriate of potash (potassium chloride).

about 95 percent of all potash used is muriate of potash. all potash fertilizer salts (KCl, KSO4 and KNO3) are soluble in water and are considered readily available. in general, it can be said that there's little difference in their effects on crop production, except in tobacco, where sulfate of potash produces a superior burning-quality leaf.

chlorides are readily leached from soils, thus toxicity is rarely seen. although little is known about chlorine as a plant nutrient, it has been demonstrated that plants grown on soils that contained only 3-5 milliequivalent Cl/100 grams showed severe chlorine deficiency. the symptoms of chlorine deficiency aren't easily identified, but affected plants are said to wilt, become chlorotic and necrotic in some areas, and exhibit leaf bronzing. potassium chloride also has shown fungicidal properties in turf, wheat and other crops.

Salt Index

some horticulturists (those who grow field crops) and turf managers are concerned about the "salt index". the salt index compares the solubility of chemical compounds. most nitrogen and potassium compounds have higher salt indexes, while phosphorus compounds tend to have a lower salt index. salt indexes are the measurement of burn potential and are based on sodium nitrate (an index of 100). the following are some common indexes of standard fertilizer products:

ammonium sulfate 69.0
potassium sulfate 46.1
potassium chloride 116.3
ammonium nitrate 104.7
urea 75.4
calcium nitrate 52.5
manure 115

all of these fertilizers have a burn potential, but none of them will burn if they're applied at the right amount and irrigated in. also, none of these fertilizers add any harmful "salts" to the soil, and they all add good salts that will use to facilitate proper growth.

Adding Nutrients

although fertilizer salts add necessary nutrients to the soil, that doesn't mean that the elements will be available to the plant materials. iron is a good example. iron is taken up by plants as ferrous ions (Fe++). most of the soil iron is unavailable to plants, and deficiencies are common in soils with a high pH. iron deficiency, can be caused by an imbalance of metallic ions such as copper and manganese; excessive amounts of phosphorus; a combination of high pH, high lime, high soil moisture and cool temperatures; and high levels of HCO3- (bicarbonate) in the rooting medium and occasionally manures.

there's a lot to understand about fertilizer salts, but the more you understand, the better you will be able to choose fertilizers for optimum performance.