FOOD OF PLANTS. 



119 



of tlie most important elements of vegetation, the proteine com- 

 pounds. The analysis of the ash of one of our common plants 

 will convey an idea of the constitution of ashes generally as to 

 the number of elements : their proportion will vary very much in 

 'different plants. The wheat, in 1000 lbs., gave 11 J lbs. of 

 ashes, with the following composition : 



Potash 2.25 



Soda 2.40 



Lime 96 



Magnesia .90 



Alumina 26 



Silica 4.00 



Sulphuric acid 50 



Phosphoric acid 40 



Chlorine 10 



Iron a trace. 



217. When plants of the same species perfect their seed the 

 analysis will give the same proportion, although growing in 

 very different soils ; and in different species of plants, although 

 growing in the same soil, the elements will be in very different 

 proportions, showing that plants require definite quantities of 

 the inorganic elements in order to perfect growth, and in the 

 soil that does not yield these elements a deficient growth only 

 can be obtained. 



These facts lie at the foundation of the rotation of crops and 

 manuring. When any given plant has exhausted the soil of the 

 soluble elements requisite for its growth, another plant requiring 

 other elements, or the same in different proportions, may grow 

 with luxuriance and in perfection in the same soil. Generally 

 the grasses, such as wheat, oats, etc., require larger quantities of 

 silica. Peas, clover, and tobacco, much lime; turnips, beets, 

 corn, and sweet potatoes, potash and soda. The stalk and fruit 

 often require the different elements in different proportion : both, 

 of course, must be supplied. These elements might exist in the 

 soil, but not in a soltfble condition, and of course yield no benefit 

 to the plant. 



218. Silica is soluble in the alkalies, and by them it is ren- 

 in fur the absorption of the plant. 



Phosphorus and Sulphur are acidified by oxygen, and com- 

 bined with some base, forming appropriate food lor plants. 



The soluble salts of lime and iron are formed spontaneously 

 in the soil. These changes, from an insoluble to a soluble state, 

 are continually going on in the soil ; and for a continuation of 



'..e constitution of the ashes of the wheat?— 217. When plants 



perfect the!: will the constitution of the ashes be ' What i< said 



[low explained ? What do ^ruins require ) Peas, clover, 



Turnips, beans ? What of the stalk and fruit l In what condition 



most their elements be ? How is silica rendered soluble ? Phosphorus and 



fculphur ? Lime and iron ? 



