354 CHEMISTRY. 



of carbonate, as limestone, chalk, etc. Soils are named ac- 

 cording to the proportions of these ingredients. Thus if 

 an ordinary soil, dried, is found to contain but 10 per cent, 

 of clay, it is a sandy soil; if from 10 to 40 per cent, a 

 sandy loam ; if from 40 to 70, a loamy soil; if from 70 

 to 85, a clay loam; and if from 85 to 95, a strong clay, 

 fitted for making bricks. If a soil contains from 5 to 20 

 per cent, of carbonate of lime it is called a marl, and if 

 more than 20 per cent., a calcareous soil. Sometimes the 

 only difference in the character of two soils may be me- 

 chanical, while the one is barren and the other fertile. 

 Thus there are sandy soils in Ohio which for fifty years 

 have yielded, without manuring, eighty bushels of corn 

 to the acre, and yet they do not differ in chemical char- 

 acter, so far as inorganic matters are concerned, from sandy 

 soils in the Eastern States which are nearly barren. The 

 only difference discovered between the two soils is that 

 the barren consists mostly of coarse grains, while the other 

 is a very fine powder. Commonly, however, when there 

 is a marked difference in fertility, there is a considerable 

 difference in chemical composition. 



494. Rotation in Crops. The differences in soil are af- 

 fected by the crops which we raise. If, for example, a crop 

 be raised year after year upon a soil which contains in due 

 quantity a chemical ingredient particularly adapted to that 

 crop, the ingredient will be at length exhausted. Hence 

 comes the good policy of rotation of crops. Potassium 

 compounds are particularly needed in the raising of tur- 

 nips, but if turnips be cultivated on the same field year 

 after year, the potassium salts will finally become deficient, 

 and you will have poor crops of turnips. So, also, if pease 

 be raised successively on the same land, the soluble lime in 

 the soil will be at length exhausted. But change these 

 two crops on the two fields, and there will be no difficulty. 



