Fertilizer is any substance added to soil in order to enrich it with nutrients needed by plants in manufacturing their foods. Soil for farming is not fertile unless it contains all nutrients necessary for plants and contains them in the proper proportion and the right form. Every crop and every animal takes from the soil a part of these nutrients. A portion of these used nutrients is replaced from the atmosphere, but another part is forever lost unless it is replaced by man. For instance, not less than 50 pounds of phosphate of lime are carried from the soil in beef, butter, and cheese for every cow kept in a pasture through a summer. To know how to supply what is lost, the farmer must know something about the chemical nature of plants and of soils.
Plants need oxygen and hydrogen, which they get from water; they need carbon, which they get from the carbon dioxide in the atmosphere; they need nitrogen, which comes from soluble compounds in the soil. Barnyard manure, dried blood, and any animal refuse contain ammonia or other nitrogen compounds, from which the plant can extract nitrogen. Nitrates are brought from Chile to make fertilizer for our soil, and chemical nitrates are made in huge quantities by extracting nitrogen from air.
Plants also need phosphorus, potassium, and calcium. Most soils, before crops have been grown on them, contain phosphorus compounds, but these are soon depleted. Rock phosphate and fertilizers manufactured from it, bonemeal, fish, and guano (bird droppings) are used to provide phosphorus. The potassium needed is usually already in clay soils, soils of volcanic origin, and desert soils, but it is deficient in peat soils and in sandy soils of the Atlantic coastal plain. A clay dressing restores potassium when it has been depleted.
In the old days farmers used to put fertilizers on the soil at random, often adding material of which there was already enough and not giving the soil the things in which it was deficient. After years of persistent experiment in the federal and state agricultural stations and colleges in this country and abroad, the type of soil needed for growing any particular crop is now generally understood with great exactness. Nowadays a farmer often has his soil tested. From the analysis of its contents he can know what fertilizer his soil needs for each crop.