Q On the Theory of Manures. 



states himself at 15 to 20. It may also be matter of doubt, 

 that, though a certain quantity of phosphates and other saline 

 matters is needed, we are therefore entitled to affirm that 

 every addition to the quantity of these will produce a corre- 

 sponding result. Certain quantities are needed as constituents ; 

 and, when they are abundant, the plenty of materials may pro- 

 duce more action. Another portion, not so easily estimated, 

 may be needed to act chemically in the soil, in preparing the 

 food, and in the transformations going on in the vital juices to 

 fit them for the assimilating organs. It may be well to provide 

 abundance ; waste in these does not take place so readily as in 

 nitrogen and carbon, but there will be a limit. If, however, 

 we had many careful analyses of the different species of ma- 

 nures, conducted in the same way, they would come ultimately 

 to be of great benefit, in enabling practical men to know, when 

 there was a deficiency of ordinary manure, how much of the 

 other substances should be added to make up the deficiency. 

 When manure is required to be carried to great distances, a 

 considerable saving of expense might be found to result from 

 using the more concentrated manures. Peaty soils, or those 

 abounding in brushwood, turf, leaves, branches, &c., producing 

 carbon, are often situated at a distance from towns ; and con- 

 centrated, nitrogenous, and saline manures might be carried 

 there at much less expense. If tables were once furnished of 

 the quantities of all the different substances drawn off by the 

 different crops usually cultivated, and of the capability of the 

 different manures to supply these, such as those now publishing 

 by Professor Johnson in his Lectures; and if such tables, carefully 

 made out, and corrected by separate analyses of plants grown 

 in different soils and climates and in different seasons, and of 

 manures in different stages of decomposition and in mixtures, 

 were compared and digested; they would furnish data to enable 

 the practical man to know, in all circumstances, how to pro- 

 ceed, how to supply deficiencies in manure, how to make up 

 for exhaustion, and keep his ground properly supplied with 

 food, so that his crops might be duly benefited by the blessings 

 of Providence in sending good weather. To expect exact 

 mathematical results, where so many contingencies are at work, 

 might be Utopian ; but something sufficiently near the truth 

 may be hoped to be arrived at, so as to save a vast deal of 

 expense, and greatly increase the value of produce. When 

 substances are deposited as food, without a knowledge of their 

 contents, or the requirements of the plants they are intended 

 to feed, it is going blindly to work, and leaving to hazard 

 what it may be in our power to provide against. 



In endeavouring to elucidate the progress made in arriving 

 at correct theoretical views on the action of the different indi- 



