The Voice of Reason
Join Date: May 2006
Understanding Inorganic Salts and Their Relationship to Plant Nutrition
Understanding Chemical Salts and Their Relationship to Plant Nutrition
The simplest definition of salt is this. A salt is chemical compound (other than water) formed by a chemical reaction between an acid and a base.
For the purpose of explaining how salts are formed and named and classified, and because of everyone’s familiarity with it, I will be using Table Salt (NaCl) as an example. Please note that table salt only relates to these examples, and has no place in any nutrient and is generally harmful to freshwater microbial and plant life.
The most familiar salt is sodium chloride, the principal component of common table salt. Sodium chloride, NaCl, and water, H2O, are formed by neutralization of sodium hydroxide, NaOH, a base, with hydrogen chloride, HCl, an acid. The reaction is written like this: HCl+NaOH→NaCl+H2O.
Most salts are ionic compounds; they are made up of ions rather than molecules.
The chemical formula for an ionic salt is an empirical formula; it does not represent a molecule but shows the proportion of atoms of the elements that make up the salt. The formula for sodium chloride, NaCl, indicates that equal numbers of sodium and chlorine atoms combine to form the salt. In the reaction of sodium with chlorine, each sodium atom loses an electron, becoming positively charged, and each chlorine atom gains an electron, becoming negatively charged; there are equal numbers of positively charged sodium ions and negatively charged chloride ions in sodium chloride. The ions in a solid salt are usually arranged in a definite crystalline structure, each positive ion being associated with a fixed number of negative ions, and vice versa.
A salt that has neither hydrogen (H) nor hydroxyl (OH) in its formula, e.g., sodium chloride (NaCl), is called a normal salt.
A salt that has hydrogen in its formula, like sodium bicarbonate (NaHCO3), is called an acid salt. A salt that has hydroxyl in its formula, like basic lead nitrate (Pb[OH]NO3), is called a basic salt. Since a salt may react with a solvent to yield different ions than were present in the salt (hydrolysis), a solution of a normal salt may be acidic or basic; e.g., trisodium phosphate, Na3PO4, dissolves in and reacts with water to form a basic solution.
In addition to being classified as normal, acid, or basic, salts are categorized as simple salts, double salts, or complex salts. Simple salts, e.g., sodium chloride, contain only one kind of positive ion (other than the hydrogen ion in acid salts). Double salts contain two different positive ions, like the mineral dolomite, also known as calcium magnesium carbonate, CaMg(CO3)2.
A hydrate is a salt that includes water in its solid crystalline form; Glauber's salt and Epsom salts are hydrates.
Salts are often grouped according to the negative ion they contain, for example, bicarbonate or carbonate, chlorate, chloride, cyanide, fulminate, nitrate, phosphate, silicate, sulfate, or sulfide.
A salt may react with an acid to form a different salt and acid.
Two salts may react with one another (in solution) to form two new salts.
A salt undergoes dissociation when it dissolves in a polar solvent, like water, the extent of dissociation depending both on the salt and the solvent.
The pH of the water influences the potential dissociation.
This is the point where the rubber meets the road, so to speak, and our discussion can move on into plant nutrition. Dissociation is the separation of a substance into atoms or ions. Electrolytic dissociation occurs when an electrolyte is dissolved in a polar solvent. For example, when hydrogen chloride, HCl, is dissolved in water to form hydrochloric acid, most of its molecules dissociate into hydrogen ions (H+) and chloride ions (Cl-). Some pure substances spontaneously dissociate. For example, in pure water some of the molecules dissociate to form hydrogen ions and hydroxyl ions. Dissociation is generally reversible; when the atoms or ions of the dissociated substance are returned to the original conditions, they recombine in the original form of the substance.
Plants can only use elements in their ionic state.
So, since we now know that Common useful natural plant nutrient components, like dolomite lime(calcium magnesium carbonate) and potash (potassium carbonate), are inorganic salts, and since we now know that salts separate into their ionic components in water; nutrients made from inorganic salts don’t have to be such a mystery.
Properly armed with all of the facts, we can eliminate the need for the grower to worry about the salt in their GH flora, or their Pure Blend pro.
There are only two ways to harm your plants, or the microbiology in your soil, using salts. Introducing salts containing toxic elements, and over concentrating salts.
Obviously, no successful nutrient formula uses salts containing elements which are toxic to plants, So I’m just going to address the concentration issue.
How can more of what a plant needs be harmful?
As the concentration of a salt in a solution of water rises, the dissociation which frees up the ion to be used by the plant as nutrient, begins to reverse. This not only strengthens the ionic bonds within the salts, making less of each ion available for uptake, but it also disrupts the plants osmotic potential (ability to intake water). Just as salty food can make a person thirsty, salty soil can dehydrate a plant.
This is the main reason that the ‘less is more’ type of GenHydro approach is so effective, and the reason that the solution to a nutrient deficiency is not always to add more of said nutrient.
Salts in my organics?
The Salt Index (SI) is a measure of the salt concentration that a given fertilizer induces in the soil solution.
The salt index is estimated by measuring the amount of electric current that a 1% solution will conduct. The higher the soluble salt content, the more current the product will conduct.
The SI of a material is expressed as the ratio of the increase in osmotic pressure of the salt solution produced by a specific fertilizer to the osmotic pressure of the same weight of Sodium Nitrate (NaNO3), which is based on a relative value of 100. Sodium nitrate was chosen as the standard because it was completely water soluble and it was a commonly used nitrogen fertilizer when the SI concept was first proposed.
Knowing the salt index of a given fertilizer gives us a tool to gauge the potential harm to the plant, and the potential harm to the soil microbiology, and gives us an idea of how dilute given fertilizers need to be.
The higher the salt index, the higher the tendency of the product to cause injury to seed germination. The salt index supplies no information as to the quality of the product or quantity or quality of plant food. Most starter fertilizer products on the market that are being recommended for placement on the seed have a salt index in the range of 40-50. A product with a salt index over this range will not necessarily result in seed damage, but the tendency of the product for this problem is higher if conditions are not ideal.
Double nutrient salts, such as ammonium phosphate and monopotassium
phosphate, help keep the salt index down.
Here is a list of some common Organic and Inorganic fertilizers' SI:
Sodium nitrate - 100
Potassium Sulfate- 43 (potash)
Calcium Sulfate - 8 (gypsum)
Manure salts - 92
Seabird guano - 43
Feather meal - 1.4
Bone meal - 1.8
Blood meal - 2.8
Meat and bone meal - 3.9
Ammonia - 47
Ammonium sulfate - 68
Mono-potassium phosphate - 9
Potassium chloride - 120
Potassium sulfate - 43
Calcium nitrate - 55
Super phosphate - 10
Ammonium phosphate - 32
Note: Manure salts are the water soluble salts present in manure. Manures commonly contain 4 to 5% soluble salts (dry weight basis) and may run as high as 10%. You can't measure the EC of the manure itself, so the salts have to be dissolved in water at the appropriate concentration for comparison.
A common misconception is that organic fertilizers are safer for plants and the environment than inorganic (chemical) products. Another common misconception is that organic fertilizers contain no salts. Many organic materials contain high levels of salts. These salts will burn plants if organic materials are over-applied. Improper organic fertilizer application may induce a plant nutrient deficiency or toxicity, or cause salt burn. Properly used, both organic and inorganic fertilizers are safe for plants and the environment.
Understanding Inorganic Salts
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Last edited by Grat3fulh3ad; 11-26-2008 at 11:44 PM..
Reason: Bolding of some text.