THE FOOD OF PLANTS. 141 



have become the better fitted, nevertheless, for some other kinds of 

 plants. A particular species of plant may and does in time, exhaust 

 the soil of a certain constituent, but another takes up, in preference, 

 some other constituent which the previous one rejected. Hence the 

 crops should be so varied as to take up all the elements furnished by 

 manures, otherwise these, containing various elements, will be of little 

 use to a soil, or to a particular crop, which already finds in the soil 

 an abundance of the elements furnished by the manure. Again the 

 manure may not and very often does not contain the elements which 

 the plant most wants. The tillage of the soil therefore has well 

 been called a branch of practical chemistry. 



Supplying the soil with food for plants is manuring it, whether the 

 materials be earthy' or saline food, gypsum, nitrate of soda, bone-dust 

 or the produce of the farm yard. Manures are therefore of vegetable, 

 animal or mineral origin. Vegetable manures loosen the soil, make 

 it lighter and yield those organic materials which plants should find in 

 the soil but which exist in the manures peculiarly fitted to enter into the 

 circulation of the plants. But this is to be in accordance with the kind 

 of plants and the parts of which they are composed, and consequently 

 with the quantity and kind of inorganic matter in the different vegeta- 

 ble substances. 100 Ibs. of the fermented saw-dust of the willow 

 add to the soil but 4 Ibs. of saline or earthy matter, while the same 

 amount of the dried fermented leaves of that tree adds to it 82 Ibs. of 

 inorganic matter. Green vegetable matter, having the juice in it, 

 decays most readily, and most readily imparts nourishment to the plant. 

 If mixed with the earth from the bottom of ditches, the compost soon 

 enriches the ground. The same occurs when a green crop is ploughed 

 in. This was much practiced by the Romans. Potato and turnip 

 tops buried in the soil where they are raised, best enrich the soil. 

 Thus buck-wheat and tares are ploughed in when the flower just 

 begins to open, and the land becomes richer thereby than when the 

 seed was sown. Three-fourths of the matter thus buried has been 

 derived from the air and is most equally diffused in the soil. Ammo- 

 nia and nitric acid are also produced to the greatest extent in this way. 

 Every green vegetable should therefore be buried, instead of being 

 cast away. 



Sea weeds are especially valuable for fertilizing the soil. Crops 

 have been trippled by its use. They decompose easily and yield 

 both organic food and saline matter to grass, or green crops. Dry 

 vegetables decay very slowly, straw is therefore mixed with other 

 substances that ferment more readily ; other dry vegetables thus fer- 

 ment, if much divided, such as saw-dust and shavings. The more 

 complete the fermentation, if not carried too far, the more immediate 

 will be its effects. The straw of the grain-plants, or the stems and 

 leaves of grasses are mostly used, but the seeds are much more 



