Manures.— Soils gradually become poorer in plant food as crops are removed every year. If the crops removed are fed to farm animals and the manure returned to the land the soil may be maintained in a fertile condition. The manure from an adult animal not giving milk contains practically all the nitrogen, potassium, phosphorus and calcium that was present in the food, but growing animals and those giving milk use a certain amount of these elements, so the amount in the manure is generally about 80 per cent of that in the food. On a farm where milk is the only product sold and where some feed is bought for the animals the manure will return to the soil all that the crops take from it, providing the manure is used in such a way that nothing is lost from it by fermenta tion or leaching. In addition to returning plant food to the soil manure furnishes humus and so greatly improves the physical condition of most soils. This beneficial effect is lasting, so that the soil remains fertile for several years after the application of manure.
The condition of a soil is improved by green manure as well as by farm manure, especially when the latter is not obtainable. It has been known for ages that the growth of leguminous crops such as red clover is to a great extent equivalent to the application of nitrogen fer tilizers for the growth of cereals, and in IN. Hellriegel and Wilfarth demonstrated that this was due to the fact that micro-organisms in the nodules on the roots of leguminous plants assimilate free nitrogen from the air and make it available for plant food. Growing and plow ing in leguminous crops therefore not only returns to the soil the plant food used by the crop but it also increases the supply of com bined nitrogen in the soil.
Commercial Fertilizers.— When crops are continually taken from the soil and plant food is not returned in manure, it is necessary to supply the necessary elements in commercial fertilizers. These are used mainly to supply nitrogen, phosphorus, potash and lime, as the other elements are required in such small amounts that most soils have a sufficient supply. The principal nitrogen fertilizers are either organic matter such as dried blood, meat waste, fish scrap, tankage, sewage sludge and cotton seed meal, or inorganic compounds including ammonium sulphate recovered from coal nitrates from the natural deposits in Chile, and artificially fixed nitrogen in the form of cal cium nitrate, ammonium sulphate and calcium cyanimide. Nitrogen is assimilated by plants in the form of nitrates, so the nitrate salts are most readily available for plant growth, hut in fertile soils there are bacteria to ammontfy organic nitrogen and to nitrify ammonia. Some kinds of organic matter such as leather and wool waste decompose very slowly and so the nitrogen is not readily available.
The phosphate fertilizers are phosphate rock, bones, superphosphate and basic slag. The first two are insoluble, consisting mostly of tricalcium phosphate, and it was Liebig who in 1840 first suggested making a more soluble product by treatment with sulphuric acid to make superphosphates which consist of monocalcium phosphate and calcium sulphate. In the last few years some phosphate rock has been treated in the electric furnace to get a soluble concen trated product which may be used either in combination with ammonia, potash or lime. Basic slag contains the phosphorus removed from iron by limestone in the blast furnaces.
Except for tobacco stems and wood ashes, most potash fertilizers have been salts from the Stassfurt mines in Germany. Since 1914, when the supply from Germany was cut off, many attempts have been made to obtain potash sup plies in this country. Two methods of obtain ing large amounts at a reasonable cost appear to be successful. The first is the recovery of potassium salts from the kelp of the Pacific Ocean by processes in which other by-products are also recovered. The second is the recov ery of potash from feldspar and clay in con nection with the cement industry by the use of the Cottrel process for precipitation from the flue dust. By this method a large amount of potash which has previously gone to waste is now recovered at a slight cost.
Applications of lime are often of benefit on soils which are not deficient in calcium, on account of the effect on the physical condition of the soil and the chemical action on the soil constituents. Lime floculates clay and so im proves the drainage of a heavy soil, it liber ates potash from feldspar, makes certain phos phates more available for plants and aids bac terial nitrification by neutralizing acidity and maintaining a neutral or slightly alkaline con dition. Lime is applied either as quick lime, slaked lime or finely ground limestone.
Crops.—The different fertilizers have dif ferent effects on the growth of plants and dif ferent crops require different amounts of the plant foods, so commercial fertilizers should not be used indiscriminately. Analyses of plants show how much of each of the necessary plant foods is removed by any crop and this gives an indication of how much fertilizer should be added to the land to keep the soil from becoming deficient in any element. Ni trogen generally tends to delay maturity by causing more growth. Maturity is also delayed by a lack of phosphorus which is especially needed for the growth of the seeds. While potash tends to delay the maturity of grains it hastens the maturity of root crops. While the use of fertilizers may materially increase the yield the composition of the mature crop is influenced very little by the amount or kind of fertilizer used.