Among the non-leguminous green manure plants are rye, wheat, oats, mustard, rape, buckwheat. Of these the rye and buckwheat are most generally used, the rye being a winter crop and the other a warm weather plant. They are both strong feeders and can use tough plant food. They do not add new nitrogen to the soil though they furnish humus and prepare food for the weaker feeders which may follow them. CHAPTER XXI COMMERCIAL FERTILIZERS THE RAW MATERIALS Next to the soil itself, the farmer's most important sources of plant food are the farm manures. But most farms do not produce these in sufficient quantities to keep up the plant food side of fertility. Therefore the farmer must resort to other sources of plant food to supplement the farm manures. There is a large class of materials called Commercial Fertilizers, which, if judiciously used, will aid in maintaining the fertility of the farm with economy. We learned in a previous chapter that the plant foods, nitrogen, phosphoric acid, potash and lime, are apt to be found wanting in sufficient available quantities to supply the needs of profitable crops. We learned also that lime is useful in improving the texture of the soil and in making other plant foods available. Now the commercial fertilizers are used to supply the soil with these four substances and they may be classified according to the substance furnished as follows: Sources of nitrogen, " " phosphoric acid, " " potash, " " lime. SOURCES OF NITROGEN Nitrogen is the most expensive of plant foods to buy, therefore special attention should be given to producing it on the farm by means of barn manures and legumes plowed under. The principal commercial sources of nitrogen are: Nitrate of soda, sulphate of ammonia, dried blood, tankage, dry ground fish, cotton-seed meal. _Nitrate of Soda_ or Chile saltpetre containing 15.5 per cent. of nitrogen, is found in large deposits in the rainless regions of western South America. In the crude state as it comes from the mine it contains common salt and earthy matter as impurities. To remove these impurities the crude nitrate is put into tanks of warm water. The nitrate dissolves and the salt and earthy matter settle to the bottom of the tank. The water with the nitrate in solution is then drawn off into other tanks from which the water is evaporated, leaving the nitrate, a coarse, dirty looking salt which is packed in three-hundred-pound bags and shipped. Plants that take their nitrogen from the soil take it in the form of nitrate. Hence nitrate of soda, which is very soluble in water, is immediately available to plants and is one of the most directly useful nitrogen fertilizers. It is used for quick results and should be applied only to land that has a crop or is to be immediately planted, otherwise it is liable to be lost by leaching.