EDITORS PREFACE. V 



slower process of tlie decomposing action of tlic wcatlicr. 

 When the food is ehminated by disintegration (by fallow 

 and mechanical operations) from this inert state of 

 chemical combination, it passes into that of physical 

 combination with the eartliy particles before it is ab- 

 sorbed by the plant. Each kind of soil has its own 

 absorptive power for causing the food to pass into a 

 state of physical combination. When manure is ap]"<lied, 

 its greater or less dispersion throughout the soil will 

 depend on this power. In general it is absorbed and 

 lixed by the upper few inches of the soil, a smaller 

 quantity penetrates to the lower layers, and scarcely 

 any at all to the deep laj'ers and subsoil. Hence when 

 a subsoil is exhausted, manure cannot restore its fertility. 

 From this pecuhar property of soils of arresting the 

 cuTulatiou of solutions of the food of plants, arises the 

 necessity of employing means for the distribution of 

 food, and for the uniform mixture of the different layers 

 of the soil. The manner in which this is effected by 

 mechanical operations, by organic matter, by manures, 

 by certain chemical salts, &c., is pointed out in ciiapters 

 second, third, and twelfth. 



The quantity of food in a state of pJiysical combina- 

 tion in any fertile soil is only limited. Continuous 

 cultivation without replacement of all the mineral matters 

 removed in tlie crops destroys fertility, either by causing 

 tlie absolute loss of the assimilable food, or by altering 

 the proper relative proportions between the different 

 elements of food, to such an extent that the due growth 

 of all parts of the plant is altered. For the successful 



