48 NATURE OF SOILS 



understood it will be necessary to consider the nature of soils 

 and the relation of the substances absorbed by the plant to the 

 soils. 



20. The Nature of Soils. — The soil is composed of minute 

 particles derived from decaying rocks and of organic particles 

 or humus derived from decaying plants and animals. The 

 mineral particles furnish several of the important crude plant 

 foods, as calcium, magnesium, potassium, phosphorus, sulphur, 

 iron and small amounts of other elements. These substances 

 occur in great abundance, comprising over thirteen per cent, of 

 the earth's crust. It is a singular fact that two of the most 

 abundant substances, silicon and aluminium, are utilized by 

 only comparatively few plants, although it will be seen later on 

 that the latter element is indirectly of great service to plant life. 

 The remaining elements used by the plant, oxygen, nitrogen, 

 carbon, hydrogen, are equally abundant. The first three may be 

 obtained from the air while the hydrogen is a constituent of water 

 and other compounds. It is an interesting fact, however, if 

 rocks containing all these mineral elements should be ground up 

 into the form of a good soil and the other necessary substances be 

 added thereto that it would not support plant life at all. The 

 reason is that these elements exist in the rocks in a form that the 

 plants can not utilize. They are largely combined with an ele- 

 ment silicon, .of. which substance pure sand or quartz is largely 

 composed. These compounds are termed silicates. The plant 

 can only utilize these elements when they are combined with some 

 element, other than silicon, and to which oxygen has been added. 

 The potassium, magnesium, etc., must be combined with nitro- 

 gen, phosphorus, sulphur or carbon to which oxygen is also added 

 in order to make a substance that the plant can utilize. These 

 crude foods of the plant are termed nitrates, phosphates, sul- 

 phates and carbonates. It is only in this form that the necessary 

 elements can be absorbed, silicates are rarely of service. The 

 question naturally arises: how do the silicates become trans- 

 formed into suitable compounds and how do the rocks become 

 changed into the form of a soil. Several factors are concerned 

 in the transformation and the reactions are complicated and 

 very slowly effected. 



