Superphosphate: This is the basic phosphorus fertilizer. It is a mixture of monocalcium phosphate and calcium sulfate, produced by treating the raw rock with sulfuric acid. The regular grade contains about 20 per cent phosphoric acid, while triple superphosphate may go as high as 48 per cent. Someone has said that without this source of phosphorus, American agriculture would grind to a halt. While this is a bit extravagant, the statement does point up the vital role played by this one material.
Muriate of Potash: It's odd how this old-fashioned name remains in use! Muriate comes from Muria, the Latin for brine. Muriate of potash is potassium chloride containing between SO and 60 per cent potash. It was deposited eons ago by ancient seas and should be considered a natural product, blessed by organocultists, but it is not. Its chlorine content passes off rapidly when applied to soil. As explained under soil organisms, however, muriate of potash is harmful to certain beneficial bacteria. Some authorities think sulfate of potash is better.
Sulfate of Potash: This contains 48 per cent potash. It is more expensive than muriate of potash but is considered less harmful to bacteria and plant roots.
Wood Ashes: About the only generally-available organic source of potash, this material is treasured by organic gardeners. Wood ashes contain about 6 per cent potash, plus considerable lime. Before corn cobs were used industrially, the cobs were burned in huge piles. The
resultant ashes were peculiarly rich in potash-up to 35 per cent. Almost any ash resulting from burning organic materials that contain some fiber should be a fair source of potash. Wood ashes are particularly good to use for adding potash to a compost heap.
Most home gardeners prefer to buy their plant nutrients as pre-mixed products-the so-called complete fertilizers (often erroneously called balanced fertilizers). Many years ago, before we knew as much as we do today about plant nutrition, three elements were said to be essential. Although we still know too little about nutrition, we do at least realize that these "Big Three"-nitrogen, phosphorus and potash-by no means supply all elements vital to growth. Nevertheless, any mixed fertilizer containing these three elements may legally be labeled "complete." According to law, at least in most states, such complete fertilizers must contain at least twenty units of plant food. Figures that state how many units are contained in a product must appear on the bag or package, with nitrogen, phosphorus and potash appearing in that order. In one or two southern states, however, the figures for the last two elements are sometimes listed in reverse order.