DIFFICULTIES NOT ALWAYS EXPLAINED. 153 



latter from appropriating the nutritive substances, 

 which the ground offers to other plants in a perfectly 

 available condition ? Finally, how comes it that this 

 very soil, after a few years, will again yield a^ remune- 

 rative crop of peas, although by intervening' harvests 

 we have rather impoverished than enriched its store of 

 nutritive substances ; and that peas, when sown among 

 oats, barley, or summer corn, will often yield a higher 

 crop than when they grow alone upon a field, and have 

 not to share with other plants the store of mineral con- 

 stituents ? 



Analogous facts are observed in the cultivation of 

 clover. In many districts, a field, after producing many 

 clover crops, will become almost unfruitful for that 

 plant. 



In such cases, manuring fails in restoring to the field 

 the power of producing clover ; but after several years, 

 during which the same field continues to give remu- 

 nerative crops of cereal and tuberous plants, the soil 

 again becomes for a while fruitful for clover. 



For a considerable number of our cultivated plants 

 we have a pretty accurate knowledge of specific manur- 

 ing agents, i.e. those which have a peculiarly favourable 

 influence upon the majority of fields. Farm-yard ma- 

 nure, as a rule, acts beneficially in all cases ; salts of 

 ammonia are especially valuable for cereals, superphos- 

 phate of lime for turnips ; bone earth and ashes will 

 perceptibly increase the produce of fruitful clover-fields, 

 and, in like manner, a supply of lime will often make a 

 field fruitful for clover, though otherwise unable to 

 bear it. 



But upon fields which have become, as it is termed, 

 peas or clover sick, that is, have lost their power of pro- 

 ducing these plants, all these matters otherwise favour- 

 able for their growth exercise beyond a certain time no 

 further beneficial action. It is this fact in particular 

 which embarrasses the practical farmer, and makes him 

 doubt the lessons taught by science. 



When the farmer is compelled to give up for many 

 years the cultivation of plants which he had found re- 

 7* 



