Frequently Asked Question
By Jon Frank, International Ag Labs
For years International Ag Labs has been preaching the merits of potassium sulfate in place of potassium chloride i.e. potash. Here are 7 quick reasons I came up with to support that claim.
1) More Energy for the Money
Commercial fertilizers are concentrated packages of energy. This energy is looked as either producing growth/vegetation or as reproductive/producing seed. Let’s never forget that energy is what grows a crop. Energy is associated with the elements in the fertilizers. Here is the bottom line: One dollar spent on potassium sulfate (K2SO4) will purchase less fertilizer by weight than will one dollar spent on potassium chloride (KCl) but the one dollar spent on potassium sulfate will buy significantly more crop growth energy. Here's why: 47% of potassium chloride is chloride and useless as an element for crop production. In contrast 100% of potassium sulfate is useable by the plant.
2) Lower Salt Index
The salt index is an easy way to judge how damaging a fertilizer is to plant roots and its impact on germination. The higher the salt index the more it can damage roots and kill emerging seedlings resulting in a lower plant population. Potassium sulfate at 46 is less than half of potassium chloride at 116.
3) Better Uptake of Potassium
Proper uptake of potassium requires it to be in the phosphate of potassium form. When there is an excess of chlorides the bonding of potassium with phosphate is blocked. The end result is less potassium uptake into the plant in the preferred form. The high level of chlorides in the soil solution following an application of potassium chloride undermines the very reason it was applied for. The sulfate form does not overwhelm the soil solution with chloride ions and consequently more potassium is taken up by the plant in the phosphate of potassium form.
4) Microbial Stimulation vs. Microbial Suppression
Sulfates have a stimulating effect on the microbial system in the soil, whereas chlorides at high levels are very hard on soil biology and is never recommended by International Ag Labs. In defense of chlorides I must say that a very small amount is actually beneficial for soil microbes. This modest requirement is easily met by the small amount of chlorides present in potassium sulfate. Chlorides usually run 1-2% in potassium sulfate. High rates of chlorides destroy soil carbons.
5) Plants and Soils Need the Sulfur
Most intensely-farmed soils are sulfur deficient. In the past rainfall picked up sulfur out of the air and continuously supplied sulfur with every precipitation. Today with stringent environmental codes and cleaned up smoke stacks the free sulfur is a thing of the past. In order for plants to make oils and sulfur bearing amino acids such as cysteine and methionine the plants need an adequate supply of sulfur in the sulfate form. This is exactly what potassium sulfate supplies.
6) Better Palatability
Forages, vegetables, and fruit taste poorly when the potassium comes from potassium chloride. This happens because, like it or not, chlorides are also taken up by the plants. Apples regularly sprayed with calcium chloride taste like ____. Instead use Amaze for calcium.
7) Less is More
The application of 100 lbs. of potassium sulfate will give a greater plant response than 200 lbs. of potassium chloride. We have found that 100 lbs. of potassium sulfate is quite sufficient for most crops. The only exception to this is high-value, potassium-loving crops such as tomatoes, potatoes, pumpkins, and melons where 200-300 lbs. are used per acre.