36 FIELD CHOPS 



36. Sources of Plant Food. The rocks from which the 

 soil is made are the principal source of all the mineral ele- 

 ments of plant food. Decaying vegetable matter is also an 

 important source, for all the elements of plant food are taken 

 up by plants and naturally they are returned to the soil 

 when these plants decay. The two most important sources of 

 decaying vegetable matter are the plants themselves, either 

 the roots and stubble which are left when the crop is har- 

 vested or the entire plant which is turned under as green 

 manure, and barnyard manure. Barnyard, or stable, manure 

 is made up of bedding and parts of plants which are not 

 eaten by animals and also of the material in the food they 

 consume which they are unable to digest and assimilate, so 

 that it is all vegetable matter. When this matter is incor- 

 porated in the soil, it is acted upon by bacteria and molds 

 and reduced to forms in which it can again be used by plants. 

 Another important source of plant food and one which is 

 largely used in the eastern and southern states is commercial 

 fertilizers. These are composed mostly of refuse animal mat- 

 ter from stockyards and of mineral matter which is taken 

 from certain soil deposits containing the desired elements. 



37. Humus. The partially decayed vegetable matter in 

 the soil is usually called humus. The term humus, however, 

 as commonly used, has so many different meanings that it is 

 confusing. On this account the term vegetable matter is 

 used here, because it includes the fresh suppUes of vegetable 

 matter, such as roots, stems, and manure, as well as that 

 which is partially decayed. The properties usually credited 

 to humus are found also in the fresher forms of vegetable 

 matter. In addition to supplying a source of plant food, it 

 has considerable effect on crop growth in other ways. Soils 

 which contain plenty of vegetable matter are easier to work 

 than those which are lacking in it, for they hold moisture 

 better and are less likely to bake and become cloddy. The 

 acid developed by the decomposition of vegetable matter 



