124 CHYMISTRY APPLIED TO AGRICULTURE. 



criminately, nor in the same proportions, ail the juices and 

 salts that are presented to them; but that either vitality, 

 or the conformation of their organs, exerts an influence 

 over the nutritive action ; that there is on the part of 

 plants some taste, some choice regarding their food, as has 

 been sufficiently proved by the experiments of Messrs. 

 Davy and de Saussure. It is with plants as it is vi^ith ani- 

 mals, there are some elements common to all, and some 

 peculiar to each kind : this is placed beyond doubt, by 

 the preference given by some plants to certain salts, over 

 others. 



Principle 4. All plants do not restore to the soil either 

 the same quantity or the same quality of manure. 



The plants that grow upon a soil, exhaust more or less 

 of its nutritive juices, but all return to it some remains, to 

 repair a part of its loss. The grains and the oleaginous 

 seeds may be placed at the head of those which exhaust 

 a soil the most, and repair the least the injury done it. In 

 those countries where plants are plucked up, they return 

 nothing to the soil that has nourished them. There are 

 some plants, to be sure, besides those mentioned above, 

 that by forming their seed consume a great part of the 

 manure contained in the soil ; but the roots of many of 

 these soften and divide the soil to a considerable depth ; 

 and the leaves which fall from the stalk during the prog- 

 ress of vegetation restore to the earth more than is re- 

 turned by those before mentioned. There are others still, 

 the roots and stalks of which remaining strong and succu- 

 lent after the production of their fruits, restore to the soil 

 a portion of the juices they had received from it ; of this 

 kind are the leguminous plants. 



Many plants that are not allowed to produce seed ex- 

 haust the soil but very little ; these are very valuable in 

 forming a system of successive crops, as by introducing 

 them into the rotation, ground may be made to yield for 

 many years without the application of fresh manure ; the 

 varieties of trefoil, especially clover and sainfoin, are of 

 this sort. 



Principle 5. All plants do not foul the soil equally. 



It is said that a plant fouls the soil, when it facilitates or 

 permits the growth of weeds, which exhaust the earth, 

 weary the plant, appropriate to themselves a part of its 

 nourishment, and hasten its decay. All plants not pro* 



