io8 BACTERIA AND SOIL FERTILITY 



going to save something. When the boy gets older he loses this 

 primitive kind of destructiveness and uses a higher degree of more 

 thoughtful intelligence by burning the forests and exhausting the 

 soil." 



The quantity which should be present varies with the soil and 

 the use to which it is to be put. A safe rule is that there should 

 be sufficient present to keep the soil in good physical condition and 

 to liberate food as needed by the growing plant. 



Kind of Organic Matter. — The kind of organic matter pres- 

 ent may be even more important than the quantity. One soil 

 containing lo per cent of organic matter may be less productive 

 than another with only 2 per cent. Hence, the kind and condition 

 of the organic matter is important. It may be divided into three 

 classes: (i) Active or fresh plant tissues which rapidly decay 

 liberating their own plant-food and giving rise to acids which in 

 turn liberate plant food from the mineral portion of the soil. 

 From the viewpoint of the bacteriologist and that of fertility this 

 form is most important. (2) The inert which is old and slow to 

 decompose. The quantity of plant-food liberated from or by 

 this in unit time is small and often insufficient to mete the needs 

 of the growing plant. This form, together with the next or third 

 form, constitutes the so-called humus. This is composed of a 

 great variety of different compounds which have been produced 

 through the decay of plants. Their composition varies with the 

 original material from which they were derived and the condi- 

 tions under which they were formed. They are probably of more 

 value in modifying the physical properties of the soil than in the 

 furnishing of actual plant- food. (3) This is the very resistant 

 coal-like residue resulting from the action of bacteria, the atmos- 

 phere, and the soil, upon the original plant debris. It imparts to 

 a soil its black color but probably has little value as plant- food. 



Origin. — The origin of the organic matter of the soil is plant 

 tissue, either that which has grown on the soil or has been carried 

 to it by various agencies. That of the surface soil comes mainly 

 from the leaves and stems, whereas the organic matter of the 

 subsoil is the decayed and decaying roots of plants. The subsoils 



