CONDITIONS FOE KENDEEING FOOD AVAILABLE. 85 



manure in time of fallow will promote the process. 

 The dense shadow cast by a leafy plant tends to retain 

 moisture longer in the ground, and thus increases the 

 action of the disintegrating agencies during the fallow 

 season. 



In a porous soil abounding in lime the putrefactive 

 process of organic matter proceeds much more quickly 

 than in a clay soil ; the presence of the alkaline earth, 

 under these circumstances, serving to oxidise the car- 

 bonaceous matter, and to convert the ammonia present 

 in the soil into nitric acid. 



All kinds of lime, when lixiviated, give up nitrates 

 to the water. Nitric acid is not retained by the porous 

 earth, as is ammonia ; but it is carried down combined 

 with lime or magnesia by the rain-water into the 

 deeper layers of the soil. While the formation of nitric 

 acid taking place in the ground is useful for plants 

 which, like clover and peas, draw their food (here in- 

 cluding nitrogen) from a greater depth, yet for this 

 very reason fallowing has a less beneficial eifect, with a 

 view to the culture of cereal plants, upon a lime soil 

 rich in animal remains ; for by the conversion of am- 

 monia into nitric acid, and its removal, the ground be- 

 comes poorer in one of the most important elements of 

 the food of plants. The case is conceivable that a field 

 of the kind, if not cultivated for a number of years, may 

 ultimately have its productive powers impaired by a 

 deficiency of nitrogenous food in the soil. 



The cause of the exhaustion of a field by the culture 

 of any plant is always, and under all circumstances, 

 dependent upon a deficiency of one or more nutritive 

 substances in those portions of the soil which are in 

 contact with the roots. A field in which these portions 

 are deficient in phosphoric acid in the state of physical 

 combination, will be found unsuited for the production 

 of a proper crop, though it should contain abundance 

 of available potash and silicic acid. The same results 

 will follow from a want of potash, even though phos- 

 phoric and silicic acids be plentiful ; and equally so 

 from a want of silicic acid, lime, magnesia, or iron, 



