140 THE SOIL: ORGANIC MATTER 



working the soil, are all important factors for the farmer to 

 consider, but do not come within the scope of agricultural 

 chemistry. The study of the important compounds in the 

 soil and the changes which take place in them; in other 

 words the chemical reactions and their causes, which directly 

 or indirectly affect the growth of crops, do, however, com- 

 prise soil chemistry. The food of plants, from what derived, 

 how made soluble, how retained in the soil or made insoluble, 

 are particular points to be considered. 



Plant food is derived from the rock particles and from the 

 organic matter. Soil moisture, organic matter, bacteria, 

 fungi, and gases are factors influencing the changes taking 

 place in plant food. The mineral particles, and the organic 

 matter to a small extent, supply the compounds containing 

 phosphorus, potassium, sulphur, calcium, magnesium, and 

 iron. The organic matter is the source of nitrogen. In 

 taking up the subject of plant food in soils it seems best 

 to discuss first the organic matter which has been very 

 truthfully called the life of the soil. It is probably the most 

 important single factor in making plant food soluble, except 

 of course water, the solvent medium itself. 



118. Organic Matter. — The soil is separated horizontally 

 into two portions as it lies in the field: First, the surface 

 soil, or sometimes called merely soil, and second, subsoil. 

 For certain purposes, such as the scientific study of soils 

 in comparing types, it is advisable to arbitrarily assume that 

 the surface six, eight, or ten inches shall be the surface soil, 

 and all below that shall be the subsoil, but the natural 

 division lies at the place where the color of the soil changes, 

 frequently very abruptly, almost always very distinctly, 

 from dark to light (Fig. 32). This depth varies in different 

 soils, sometimes lying only a few inches below the surface 

 of the land, sometimes lying several feet below. 



It is in this dark soil that crops can grow best. It frequently 

 happens that when a light colored subsoil is turned up, 

 crops will not grow. This may be due to several causes, but 

 one of them at least is the absence of organic matter which 

 gives the dark color to surface soil. Organic matter in the 

 soil is composed of particles of roots, leaves, bark and other 



