On account of the fact that different crops use different amounts of the several plant foods and so deprive the soil of different elements, a plan of crop rotation exhausts the soil less rapidly than growing a single crop continuously on the same land and allows the soil to be en riched occasionally by the growth of a legu minous crops. Some plants have short roots which take their food from the surface soil, while others have long roots which bring up their food from the deep soil and when the crop is harvested and the root decays it en riches the top soil by a certain amount of potas sium, phosphorus and nitrogen.
That the climate influences the composition of crops more than other factors has been shown by several Among them is LeClerc's experiment with wheat in which it was found that the same wheat planted in different parts of this country gave crops with very different amounts of protein and starch, while wheat from different parts of the country when planted in one place gave crops which were approximately alike and quite different from the seed from which they were planted. With regard to the effect of rainfall on the composition of wheat it was found that an abundant water supply produces a large starchy grain, while a dry climate produces smaller grains with a larger proportion of protein. \K iley's experiments with sugar beets showed a considerable variation in the composition of crops grown in different parts of the country from the same seed, a larger yield of sugar being obtained in the northern part of the country. In experiments with sweet corn Wiley found that the climate has less influence on the composition of the crop than the amount and distribution of the rainfall. From such chemical studies it is possible to find out in what part of the country to grow certain crops with the best results and also to select for de velopment the varieties that will produce more of a given desirable constituent, either protein, fat or carbohydrate. It is evident that for the improvement of crops by selection it is neces sary that the experiments be carried on in the climate in which the crop is to be grown.
Insecticides and Fungicides.— Plants have many insect enemies and it is often necessary to use poisons to kill them. These poisons must not be such as to harm the plant and must. not be used in a way to make it poison ous as a food for the higher animals. The
poisons used for spraying on leaves which in sects eat are mainly insoluble arsenic com pounds such as Paris green and lead arsenate. Among the sprays that poison by contact are lime sulphur and tobacco extract or nicotine solution. For destroying insects in stored grains poisonous gases are frequently used, such as hydrocyanic acid and carbon disulphide. Copper salts are commonly used for destroying fungi and the most generally used fungicide is Bordeaux mixture which is made from copper sulphate and lime.
Animals.— The animal body contains be sides from 50 to 70 per cent of water, inorganic salts and organic matter which is mainly pro tein and fat. The inorganic salts include phos phates and chlorides of calcium, sodium, potas sium and magnesium in the skeleton and body fluids. The organic matter includes compounds containing carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, iron and iodine. Small amounts of fluorin and silicon are also found but it is not known that these are essential to animal life. The organic matter required by animals comes directly or indirectly from plants which have synthesized their organic compounds from the inorganic matter in the air and soil. The organic matter required for growth and repair of tissues is protein, and the energy required for keeping the animal warm and for performing the functions of the organs and muscles is obtained by oxidation of organic compounds, mainly fat and carbo hydrate. Protein in excess of that required for building new tissue or replacing waste tissue is also used as a source of energy, but protein foods are a more expensive source of energy than fat or carbohydrate, and they are generally used in a diet only in amounts neces sary for tissue-building. Any excess of food over that required for growth and for energy is stored as fat. Not only must a diet contain protein, fat, and carbohydrates enough to fur nish the necessary energy, but it must have a sufficient variety and amount of inorganic salts, and there are also certain other organic com pounds, known as accessories or vitamines, which are present in small amounts in food from plant and animal sources and without which animals either do not grow or they de velop certain diseases such as polyneuritis or scurvy, which are therefore known as deficiency diseases.