Some more information, which will end up edited into the original article when I have time...
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
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.
From Ohio State University Extension
Department of Horticulture and Crop Science:
High levels of organic materials applied to soil may increase salts in soil, impair seed germination and reduce plant stands. Most organic materials also contain much more potassium than magnesium or calcium. Long term application of this material to soils may raise the ratio of potassium to magnesium and calcium sufficiently to retard crop growth. To adjust the ratio, additional magnesium and/or calcium may have to be added as dolomitic or calcitic limestone