MATTER TAKEN FROM THE SOIL. 97 



horse-chestnut bark contains 12 per cent, of potash and 76 

 of lime, the ash of the wood yields twice as much potash, 

 that of the leaf-stem nearly four times as much, and that of 

 the flower 61 per cent, of potash and 13 of lime. 



All the cresses, on the other hand, contain a large pro- 

 portion of sulphur, and though a crop of mustard may be 

 grown upon damp flannel, it is not to be supposed that it 

 lives and flourishes on air and water only ; for the water, 

 unless it be distilled, is quite certain to contain some 

 mineral matter, and from this source the plants derive suffi- 

 cient food for a time. If grown in pure water, they will 

 sprout and even grow at first ; but as soon as they have used 

 up the matter contained in the seeds, they will die ; and if 

 they be burnt, the remaining ash, when weighed, will be 

 found to be only the same in amount as that contained in 

 an equal number of seeds, showing that they have not been 

 able to find anything in the water. 



Every crop, therefore, takes a certain amount of mineral 

 matter from the soil,* and as one takes more of one thing 

 and another more of another, the farmer varies his crops 

 that the soil may not be exhausted. Sometimes he will let 

 it lie fallow altogether, that the carbonic acid and ammonia 

 washed down by the rain may dissolve the minerals and 

 prepare them for plant-food, and the same end may be 

 more quickly attained by the growing and ploughing in of 

 green crops. But, of course, neither plan will answer unless 



* In general, probably 99 per cent, of the soil without water does not 

 contribute to the support of vegetation. The hay crop takes up the largest 

 amount of mineral matter, but reckoning it at z\ tons to the acre, it takes 

 only 4oolbs. The weight of the soil, one foot deep, is 4,ooo,ooolbs. per 

 acre. (S. W. Johnson.) 

 H 



