8 NATURE AND PROPERTIES OF SOILS 



ceedingly desirable condition. Incidentally the soil is ren- 

 dered thereby very much more difficult to study, especially 

 chemically. 



The incorporation of organic matter in any soil, either by 

 natural or artificial means, tends, if the proper decay occurs, 

 to make the soil more friable. The water capacity is markedly 

 increased and the vigor of the bacterial and chemical activ- 

 ities stimulated to a marked degree. As these two latter 

 actions progress, some of the organic matter passes into simple 

 combinations, allowing certain elements to become available 

 to crops. Nitrogen, which is held in the soil largely in organic 

 combination, emerges in the form of ammonia, nitrites and 

 nitrates. It is from a salt of nitric acid that most plants 

 absorb their nitrogen. Small amounts of sulfur, phosphorus, 

 potassium and calcium are liberated from the tissue as decay 

 proceeds. The largest product of organic decay, however, is 

 carbon dioxide (C0 2 ), which in the soil becomes important 

 as a solvent for minerals, thus hastening the decomposition 

 processes. 



6. Factors for plant growth. — The growth and develop- 

 ment of a plant depends on two sets of factors, the internal 

 and external. The latter may be classified as follows: (1) 

 mechanical support, (2) heat, (3) light, (4) oxygen, (5) 

 water, and (6) nutrients. 1 With the exception of light, the 

 soil supplies, either wholly or in part, all of these conditions. 

 Mechanical support is a function entirely of the soil. The 

 comparatively loose and friable condition presented by most 

 soils allows ample foothold to the ramifying roots. 



Air and water are easily supplied because of the open 

 condition of the soil, and its large pore spaces. Temperature 

 depends almost wholly on climatic relationships. The water 



1 Nutrients are materials from which food may be elaborated once 

 they have been absorbed by plants. The energy for this synthetic proc- 

 ess comes from the sun. A food is any substance from which the plant 

 may obtain energy for its normal processes. A large proportion of the 

 materials absorbed ty plants are nutrients. 



