A salt is formed when an acid reacts with a base. Almost everyone knows about mixing baking soda (bicarbonate of soda), and vinegar (acetic acid). The reaction has two stages, the first being the dramatic release of carbon dioxide bubbles. Once the bubbles and pressure release, the reaction produces a salt, sodium acetate in water.
An important acid-base reaction for many Colorado farmers is the reaction between rock phosphate (calcium phosphate) and sulfuric acid. Because Colorado soils are high in calcium, and thus relatively high in pH, rock phosphate is not readily available in most soils. This is true in much of the Western United States due to the calcium-based parent materials of the soils. Eastern soils, on the other hand, are acidic and can use the calcium to lower the pH, so rock phosphate is a good fertilizer material east of the hundreth meridian.
Another reason rock phosphate is not popular in the Western States is that it is only about 10 percent phosphorus, and an even lesser percent phosphate, which is what the plants utilize, so shipping it any distance to a farmer who does not need the calcium, is not cost effective. In response to this situation, fertilizer miners and producers take rock phosphate (a base) and spread it out in settling ponds and mix it with sulfuric acid, which etches away the calcium leaving behind a more dense form of phosphate, resulting in 11-52-0 and 18-46-0, variously known as “superphosphate,” or “triple super phosphate” which is denser for shipping, and more concentrated for Colorado soils, in that the unneeded calcium is reduced, or non-existent in the shipped product.
Super-phosphate does, however, come in a salt form, because it is produced from the reaction of an acid and a base. Some farmers with soils that are already salty with calcium and magnesium, or sodium salts, have some negative effects from the added salt, and, of course, super-phosphate does not qualify for organic farming, even though all the processes involve natural ingredients, though some would call sulfuric acid a “chemical.”
Because super-phosphate is a salt and highly soluable in water, it has a natural and strong tendency to unite with the calcium in Western soils, so broadcasting it over a field—particularly on alfalfa and hay field — results in much of the phosphate salt being tied up with the calcium, and essentially turning the super-phosphate back into rock phosphate, calcium phosphate. After many years of broadcasting super-phosphate on a field, the phosphorus does slowly become available and is often liberated by soil amendments that stimulate carbonic acid formation, which releases the phosphate to plants.
It is interesting to note that phosphate, being P2O5 (P being phosphorus and O being oxygen) is, by atomic weight only about one-third actual phosphorus, but the plant uptakes phosphorus with oxygen. When buying phosphate, the farmer is actually buying quite a bit of oxygen.
I discovered that if I could concentrate super-phosphate by drilling it in bands in the soil using an old double-disk grain drill, I could use about one-third the product and get the same production results. This was true because the super-phosphate, in a band, was not in contact with as much calcium in the soil, and thus did not bind up immediately and become calcium phosphate, or rock phosphate, again. This also reduced the influence of the salt on the soil.
Drilling the super-phosphate in an alfalfa field was, however, a major pain in the rear with timing issues in the spring, so it became more common to use liquid phosphate products such as foliar fertilizer, or use other more organic forms of phosphate available from fish-based fertilizers.
While salt, and salts, are abundant on earth, phosphorus is scarce, and in the time I have farmed (since 1969) the price of phosphorus fertilizers has increased dramatically, so being wise with phosphorus fertilizer is a key to farming success in the West. ❖