46 THE CULTURE OF FARM CROPS. 



contained among the particles of the soil expands and loos- 

 ens these particles and separates them from each other. 

 When the soil is thawed by returning warmth, a thin crust 

 is loosened from each clod and fragment and falls apart 

 into exceedingly fine particles, and as the warmth penetrates 

 the mass this gradually crumbles down and forms a larger 

 bulk of fine porous earth. Moreover the particles of soil 

 themselves are ruptured ; stones are gradually disintegrated, 

 and new soil is made; and these particles are thus subjected 

 to the solvent action of the'w r ater of the soil by which fresh 

 plant food is liberated and the soil is enriched. This effect 

 is the more remarkable when we learn that one single cubic 

 inch of clay when reduced to fine particles by the action of 

 frost presents in the aggregate superficial areas of the frag- 

 ments, a surface of at least 100 square inches. The bene- 

 ficial effect of this result of freezing upon the soil is of 

 incalculable value to the farmer, both as regards the me- 

 chanical condition of the soil and its fertility. On clay soils 

 both of these effects are produced to the greatest extent. 



In the form of snow too water exerts a considerable effect 

 which is beneficial to the farmer. The snow is exceedingly 

 porous, being made up of a mass of crystals formed like 6 

 rayed stars, which lie very loosely upon each other. The 

 interstices contain air and act as a non-conductor of heat, 

 and as a protection against the severity of the cold, and 

 also against sudden changes of temperature. In very, cold 

 countries, the snow covering the soil early in the winter, 

 prevents the freezing of it altogether, and tender plants such 

 as potatoes, often remain green under the snow the whole 

 winter, without injury in the warm soil. Such a climate 

 obviously favors very much the successful growth of winter 

 grain. Snow has also the power of absorbing ammonia, 

 oxygen, and nitrogen from the air. Ammonia held in the 

 snow is gradually taken in by the soil and is not lost as 

 when it is brought down by rain, which not only carries it 

 off in floods, but also washes a more considerable quantity 

 of it from the soil into brooks and rivers. The air which is 

 held in the open spaces in the snow holds only 17 per cent. 



