Sulfuric acid has played a major role in the progress of fertilizer applications, both organic and chemical. Bone meal, various phosphorus fertilizers and nitrogen fertilizers all make use of sulfuric acid treatment to alter the chemical composition so that more nutrients are available to roots when applied.
Bones have been used for hundreds of years to fertilize the soil for crops. But it wasn’t until 1808 that Sir James Murray of Ireland discovered that treating the bones with sulfuric acid converted the phosphorus in bones to a form that plants could readily absorb. But this fertilizer was primarily a solid form. The liquid form of phosphates came onto the market after it was discovered that liquid fertilizer could be added to irrigation lines to simultaneously water and fertilize plants.
The fertilizer industry consumes most of the more than 40 million tons of sulfuric acid produced in the United States each year. A large quantity is used to produce superphosphate. Pulverized rock phosphorus is treated with sulfuric acid and water, then allowed to react and dry. For liquid superphosphate fertilizer, the miniscule superphosphate granules are remixed with water and a stabilizer.
Phosphorus is a tricky nutrient to provide to plants. Once added to the soil, phosphate readily binds to the soil, becoming ‘fixed.’ Once fixed, it won’t leech away with rain or irrigation, but it’s also no longer in a form that plants can absorb. What little phosphorus doesn’t become fixed is available for plants, but this means that phosphorus must be reapplied, and that much of it is wasted in each application.
Ammonium sulfate is a fertilizer product that mixes sulfuric acid with urea. While sulfur is a necessary nutrient for plant growth, it is far more available in soil than nitrogen, phosphorus or potassium. Ammonium sulfate is used primarily for the nitrogen in the ammonium, which breaks away from the sulfur for plant uptake. This process can also slightly lower soil pH.
As a relatively quick-release nitrogen fertilizer, ammonium sulfate should be used with caution in liquid form. Many plants have leaves sensitive to over applications of nitrogen, and will burn when ammonium sulfate is applied. To be safe, use an ammonium sulfate liquid fertilizer with low amounts of nitrogen (less than 10 on the N-P-K label), avoid spraying on the leaves altogether, or dilute according to foliar application rates on the label.