.Ch. VIII.] ON SOILS. AND COURSES OF CROPPING. 123 



yield bad ones ; for it would teacli us to reserve manure, and to make 

 arrangements regarding future crops better adapted for times succeeding 

 those which had been previously prolific, by which means a judicious 

 farmer might overcome the difficulties attendant upon an untoward season. 

 We have already adverted to some observations which he and other natu- 

 ralists have made upon the exhaustion of the earth occasioned by tlie 

 growth of grain * ; and, in order to further elucidate what he has said in 

 support of the above remark, we shall add some brief extracts from the 

 copious reasoning, and the numerous tables, which he has furnished re- 

 garding the effects of various rotations upon different soils. 



The vegetative power is supposed to be in proportion to the quantity of 

 humus, or mould, which is contained in the soil, and its consumption has 

 been found to be regulated according to the amount of nutritive matter 

 furnished by the crops which are grown upon it. The degrees of ex- 

 haustion thus occasioned, have only been fixed by naturalists with any degree 

 of certainty in so far as regards the usual species of cultivated grain and 

 pulse ; for, as to the other products of the earth, although they doubtless 

 have similar effects when frequently repeated, yet those which consist of 

 vegetable roots and grasses, and which are drawn from the land before 

 they have perfected their seed, are nevertheless — whether from the influ- 

 ence attributed to their shade upon the soil, from sustenance drawn from 

 the air and water, or from other causes with which we are unacquainted — 

 only viewed as ameliorating crops, and have therefore been assimilated to 

 the application of tare fallows. Corn-crops are however respectively con- 

 sidered to exhaust in proportions which render the production of about 

 4^ bushels of wheat equal to that of 6 bushels of rye, 8^ of barley, and 12 

 of oats. 



In certain seasons, however, one species of grain, though more ex- 

 hausting than another, may yet produce the largest crop ; but it will com- 

 paratively impoverish the ground, and the succeeding crops will be pro- 

 portionably deficient. If we estimate the produce of each crop according 

 to the quantity of sap which it contains, regard must be had to several 

 accessary circumstances ; thus, besides the temperature of the air, which 

 we can neither foresee nor regulate, a precarious crop or a species of 

 culture which has prepared the soil in a particular manner, will have pecu- 

 liar effects upon the vegetative powers of the earth, and its absorption by 

 the attractive strength of the grain. According to all the experiments 

 which have been hitherto made, there is however reason to suppose, that 

 upon a soil of moderate fertility, an average crop of wheat impoverishes the 

 land to the extent of 40 per cent., while one of rye only produces that effect 

 so far as 30. Although barley is more exhausting than oats, yet, upon 

 strong land, in a less perfect state of culture, the latter produces propor- 

 tionably larger crops, consequently absorbs more nutriment ; and, for this 

 reason, they may be both stated at 25 per cent. 



The exhaustion occasioned by these crops is proportionately repaired, and 

 the land is restored to its former nutritive jiowers in three ways ; namely — 

 By the application of putrescent manure ; according to its quantity 

 and quality. 



By the ground being left a certain time under pasture; according 

 to the number of stock which it can support. 



By the operation of a summer fallow, ; according to the manner in 

 which it is performed. 



The soil is never so utterly impoverished by cropping, as not to be still 

 capable of producing something. This productive faculty composes what 



* See the note marked % in the last chapter, at p, 91. 



