166 LECTURES ON 



serves to separate the particles from each other, and thus remedies 

 too great cohesiveness. 



When water freezes its volume increases, as is well known. The 

 alternate freezing and thawing of the water which impregnates the 

 soil during the colder part of the year plays thus an important part 

 in overcoming its cohesion. The effect is mostly apparent in the 

 spring, immediately after "the frost leaves the ground," but is 

 usually not durable, the soil recovering its former consistence by the 

 operations of tillage. Fall-ploughing of stiff soils has been recom- 

 mended, in order to expose them to the disintegrating effects of 

 frost. 



In turning now to the chemical characters of the soil, we have 

 first to notice its composition. It being understood that the soil is 

 the exclusive source of mineral food to the plant, we of course expect 

 to find all the ingredients of the ash of plants in every soil that is 

 able to maintain vegetation. Great differences however, are found 

 to exist in the proportions, and especially in the condition as regards 

 solubility of these matters, as seen from the following analyses: 



1st. Analysis of a productive wheat soil (clay) from Renfrewshire, 

 Scotland, by Dr. Anderson: 



Soluble in water — 



Silica 0.0221 



Lime 0.0475 



Chlorid of calcium 0.0205 



Chlorid of magnesium 0. 0061 



Chlorid of potassium 0.0003 



Chlorid of sodium 0.0015 



Sulphuric acid 0.0309 



Organic matter ©.2084 



0.3373 



Soluble in acid — 



Silica 0.0838 



Alumina 1.6104 



Peroxyd of iron 3.4676 



Lime 1.0771 



Magnesia 0. 1262 



Potash 0.0469 



Soda 0.0920 



Sulphuric acid 0.0039 



Phosphoric acid 0. 0749 



6.5828 



Insoluble in acids — 



Silica 74.4890 



Alumina 7. 2540 



Peroxyd of iron 1.4167 



Lime 0.3150 



Magnesia 0.4043 



• 83.8790 



