94 CHYMISTRY APPLIED TO AGRICULTURE 



All farmers know, that when they subject to tillage a piece 

 of artificial grass land, which has for several years been 

 constantly mown at the time of flowering, it will yield 

 several harvests without any dressing ; but if the grass has 

 been left to go to seed, it will be necessary to supply the 

 earth with manure before it will yield a good return. As 

 those plants that are cut at the time of flowering do not 

 exhaust the soil so much as those that remain for seed, 

 the belief has arisen amongst farmers, that before the 

 period of fructification, they are nourished by the con 

 stituent principles of the surrounding air and water ; but 

 that during the time of the formation of the seed, theii 

 support is almost wholly derived from the earth. But this 

 opinion will not hold in regard to all plants ; lettuce, tur- 

 nips, tobacco, woad, endive, cabbages, and onions exhaust 

 the soil greatly, though they are gathered before producing 

 seed. Potatoes, though they produce but few seeds, impov 

 erish land more than almost any other vegetable. Plants 

 raised in a nursery, and afterwards transplanted, exhaust 

 the soil in which they spring, more than the one in which 

 they complete their growth. 



Thus we see, that during the whole time of their vege- 

 tation, plants derive their nourishment from the air, and 

 from the substances contained in the earth ; but if they are 

 mown at the time of flowering, they leave in the soil their 

 roots and portions of their stalks, which restore to the earth 

 nearly as much as they have received from it ; whilst, it 

 they remain uncut till they have completed their course, 

 they return little or nothing to the soil to compensate it for 

 the nourishment they have received from it. 



It is well known to farmers, that ploughing in a green 

 crop of any kind whatever, prepares the soil for producing 

 well without any other manure; since, by this process, all 

 that the soil has yielded is returned to it, with some addi- 

 tions, resulting from the decomposed principles of air and 

 water, which are contained in the plants. 



In order fully to understand this doctrine, which appears 

 to me of great importance to agriculture, it is necessary 

 to consider the successive changes which take place in 

 annual plants during their growth ; first, they produce 

 green leaves, which, by coming in contact with the air, 

 receive from it the principles of which I have spoken; 

 subsequently the stalks increase in size and number, and 

 are covered with numerous leaves, which absorb from the 



