2 On the Theory of Manures. 



powerful, may be the manures deposited in the soil, if chemical 

 action is not kept up in the soil, the stomach of the plant, both 

 by the free admission and retention of heat and air. We may 

 have much greater produce from a small quantity of manure 

 under judicious cultivation, than from a great quantity when, 

 by improper cultivation, or by those baffling tids of weather 

 which occur so often to paralyse the efforts of the most skilful, 

 the soil has got out of order. It is difficult to lay down rules 

 stating how much should be ascribed to all these causes ; the 

 skill acquired by practice, and great observation and discrimi- 

 nation, with repeated trials on a large scale, will all be needed 

 to elucidate and harmonise conflicting statements. There are 

 particular periods in the stages of existence of plants also, when 

 nourishing weather is more requisite, and the reverse does more 

 harm ; as in turnips, carrots, &c., newly above ground, when, 

 if they are stunted and set up, no after nourishing weather will 

 altogether remove the defect ; or in grain crops at the time of 

 setting the flower, in potatoes at the time of germination, &c. 

 The different kinds of manure, also, differ very much ; some re- 

 quiring much more chemical action to render them soluble than 

 others. 



In experiments, also, conducted on a small scale, on small 

 measured portions of ground and manure, allowance must be 

 given for variations in the state of the soil, at very short dis- 

 tances, in the same field. Where immense level plains or 

 mountainous tracts of great extent occur, the soil is more uni- 

 form ; but where the land, as in many districts, is undulated 

 through its whole extent, with hill and vale, knoll and hollow, 

 it is found that different portions of a field, within a few yards 

 or even feet of each other, differ much in many respects. One 

 piece, having a stratum of gravel running through it, may 

 be parched and burned up with drought in a dry season ; 

 while in a wet season it may retain only its proper quantity. 

 Another piece may be stiff sodden clay, suffering much when 

 the seasons are wet and cold at the time of working, and greatly 

 improved by working dry, and having moist weather after- 

 wards. Another portion may have once been wet and marshy, 

 and accumulated a kind of peaty deposit, which, if drained after- 

 wards, may bear fine crops in ordinary seasons, but will suffer 

 in the extremes both of drought and wet. The field may have 

 had an excellent soil some feet deep deposited on it, but the good 

 soil may have been washed away from some pieces by partial 

 floods, and a stiff obdurate clay exposed : or the reverse may 

 have taken place ; the good soil may have been washed from 

 the^ hills and knolls, and deposited in the hollows. All these 

 varieties occur in the district around this, frequently in the same 

 field. The prevailing rocks are sandstone and greenstone, above 



