Nitrogen, phosphorus, and potassium are all essential to normal plant growth. They serve different purposes in building the plant structure.Nitrogen
Nitrogen is vital to the formation of all proteins. It is an essential element of chlorophyll, the green chemical that permits plants (and plants alone) to manufacture starches and sugars. Many of the compounds in plants - amino acids, aromatic compounds, etc. - must have nitrogen. It is the "grow" element, forcing soft, lush growth when used in excess. Because of the importance of nitrogen in plant growth, it is essential to have an available supply. Since nature cannot provide nitrogen in sufficient quantities, other sources are essential, namely, commercial fertilizers and manures. Nitrate of soda, sulfate of ammonia, dried blood, cottonseed meal, and the various manures are some of the materials used for supplying extra nitrogen when it is needed in greater amounts than the soil seems able to provide.Phosphorus
Phosphorus also enters into the composition of proteins and amino acids. It is associated with cell division. It contributes to stiff stems that hold foliage to the sunlight. Flowering and seed formation must have this element. It is a difficult element to manage, sine it locks up rapidly as soon as it touches soil and so must be applied in excess of actual use by plants. Many gardens that have been fertilized for years are practically low-grade phosphate mines, but this phosphorus is an insoluble form that plants cannot use.
Superphosphate, mistakenly called acid phosphate - it is alkaline in reaction - is processed to delay this "fixing" process as long as possible. Phosphate rock, often recommended by organic garden enthusiasts, becomes slowly available as it is dissolved by soil acids. It is seldom of much use the first year it is applied and, weight for weight, is far less effective than super phosphate.
Perhaps one of the most sacred cows in gardening is bone meal. True, in Victorian England, bone meal was one of the few good fertilizers available, but it was a far different product than the dehydrated, devitalized product of today. Every dedicated gardener of that day had a bone grinder in his potting shed or garden house. Fresh, raw bone, with scraps of meat clinging to it, was ground and used immediately as a fertilizer. It included the marrow, blood, meat scraps, and valuable minor elements in addition to uncooked phosphorus. Today's bone is all but worthless.Potassium
Potassium - in the form of potassium hydroxide (potash) - has been neglected in pant nutrition. This is due to the fact that it is abundant in most American soils and when analysis is made for total nutrients, seems to be in adequate supply. Unfortunately, soil-bound potash is in a form that is difficult for plants to use. As a result, annual applications of potash in soluble form are essential, particularly if root crops are being grown. Potassium is an important catalyst in photosynthesis, is essential for starch formation and the movement of sugars in the plant, and is important to seed formation. It helps form stiff stems and is essential in formation of tubers and roots. It is a vital element in protein synthesis and in the utilization of nitrogen by plants.
Wood ashes, which also supply lime, are a convenient source of potassium. One point often overlooked in using wood ashes is that until the potash they contain has been dissolved and absorbed by the soil, a free lye solution is present. This can cause injury to roots for two or three days following application.
Muriate of potash and sulfate of potash are chemical salts that also need to be used carefully to avoid injury to plants.