THE CARE OF THE SOIL. 



27 



Chemical. 



It is obvious that soils differ chemically just as we have seen them 

 differ mechanically ; in fact, far bigger variations occur. This is easily 

 understood when the chemical constituents of a soil, as shown by 

 chemical analysis, are examined. A moment's reflection on the widely 

 differing origin and composition of soils will show us that this difference 

 will be evident in their chemical constituents. The only way to 

 ascertain this difference and to learn the percentage of the various 

 essential ingredients in a soil is by soil analysis. Chemically, soils 

 consist of humus and minerals : from the former is obtained the essential 

 plant food — nitrogen — and among the latter are the essential mineral 

 plant foods of which the chief are phosphoric acid, potash, and lime. 

 There are others that are known as the minor essentials, but for all 

 practical purposes we need only examine a soil chemically for its 

 organic matter, or humus, and the nitrogen therein contained, and the 

 mineral ingredients above referred to. The value of a soil analysis 

 as showing the percentage of plant food a given soil contains, and 

 its bearing on the manurial treatment of that soil, is far too big a 

 subject to be dealt with here. The problem is not so simple as it 

 appears, and it is certainly safe to say that a soil analysis requires very 

 careful interpretation by a skilled agricultural chemist. This subject 

 is touched on in " Some Hints on Manuring of Garden and Other 

 Crops " above referred to. It is sufficient for our purposes to know 

 the possibilities of a soil analysis as an aid to the care of the soil, and 

 especially with regard to the estimation of lime, which we have seen 

 plays so important a part in the correct management of all soils. Again, 

 soils differ very considerably in the freedom with which they give 

 up the chemical food materials they contain, and a plant can more 

 easily obtain this food on a sandy soil than in a close-textured clay. 

 Light soils will frequently grow heavy crops with far less food material, 

 as shown by analysis, than heavy soils supporting poorer crops. This 

 is a question of root activity and ease with which water percolates 

 in a light soil. Further, crops differ in the food material taken and 

 required, as is fully explained in the article last above referred to. 

 All these questions are the province of the agricultural chemist, and 

 a most interesting study is open here for the agricultural student 

 It is sufficient for our purposes at present to glance at this study to 

 show that, to know a soil, the fullest chemical information regarding 

 it should be obtained. 



Biological. 



Not very long ago the soil was held to be an inert mass containing 

 the food materials of the plant referred to above. It is only recently 

 that we have learned of the biological side. We now know that the 

 soil is peopled by countless millions of bacteria, that these bacteria 

 may be roughly divided into two classes — the beneficial and the 



