56 THE CULTURE OF FARM CROPS. 



makes a heap of manure, or a hot bed, the mass soon begins 

 to heat and in time is changed from its previous condition 

 into a black powdery substance having no resemblance to 

 vegetable tissue. The heat produced by the chemical ac- 

 tion which has resulted in this change has been precisely 

 equal to that which would have been required to drive off 

 the moisture; set free the gases; and reduce the matter to 

 its mineral, carbonaceous, and nitrogenous elements which 

 remain in the mass. In like manner heat is produced 

 by every chemical change. The union of water with sul- 

 phuric acid is accompanied by violent heat; so is the solu- 

 tion of a piece of copper in nitric acid. And as the decom- 

 position of a vegetable cell in a manure heap is accompanied 

 by heat so is its decomposition in the soil; and its formation 

 in the plant. 



The effects of heat and cold upon the soil are great 

 and varied. It is the sun's heat, penetrating the soil which 

 causes the germination of the seed. At low temperatures 

 seeds will remain in the soil for many years unchanged. 

 The heat of the sun does not penetrate very deeply and at 

 a very moderate depth the heat of the soil is constant, dur- 

 ing summer and winter. This is caused by the effect of 

 evaporation, as well as by the nonconducting property of 

 the air spaces between the particles of the soil. Seeds of 

 weeds and plants which remain at some depth in the soil 

 are thus kept dormant for many years, starting into growth 

 whenever they are brought under the influence of the 

 warmth of the sun's rays. 



The heat of the sun also causes the evaporation of w r ater 

 from the soil and dries it and makes it fit for the labors of 

 the farmer. But this result has also another effect which 

 is unfavorable. It cools the soil and reduces the temperature, 

 and when the soil contains an excess of water and the evap- 

 oration is copious, this cooling is exceedingly hurtful to the 

 crops. There are soils which are called cold clays; and 

 swampy lands are always cold and unproductive of the bet- 

 ter class of crops, favoring the growth of mosses and ferns 

 and other useless plants. This is due to the constant evap- 



