274 IMPROVEMENT OF THE SOIL. 



we examine their ashes, we find that some species require phos- 

 phate of magnesia or phosphates, and others potash, and oth- 

 ers still, substances rich in nitrogen, such as nitre and ammo- 

 nia. We have also seen that these substances exist in the soil, 

 in small quantities, and hence are liable to be removed by a 

 continued course of cropping. 



1. If we take 100 parts of wheat straw they will yield 15.5 

 parts of ashes. The same quantity of barley straw will yield 

 8.54 parts, and 100 parts of oat straw only 4.42 parts. The 

 ashes of all are of the same composition. The principal salts 

 are phosphates, especially phosphate of magnesia, hence it is 

 evident " that upon the same field, which will yield only one 

 harvest of wheat, two crops of barley, and three of oats may 

 be raised," and this is due to the different quantity of phos- 

 phates, which they derive from the soil, and if wheat suc- 

 ceed wheat, these substances will be sooner taken from it. 



2. It is evident that if two plants grow beside each other or 

 in succession on the same soil, they will injure each other if 

 they withdraw the same alkalies from it; hence wild chamomile 

 and Scotch broom impede the growth of corn, because they 

 yield from 7 to 7.43 per cent, of ashes, which contain -{^ of 

 carbonate of potash, the very alkali which the corn requires. 

 If these plants succeed each other the same injury will be 

 done. 



3. But on the other hand if two plants grow beside each 

 other or in succession, which require different quantities of 

 any alkali for their development, they will flourish well ; 

 hence if a soil contain potash, wheat and tobacco may suc- 

 ceed each other although both are exhausting crops, that is, 

 both require potash ; yet they require different quantities of 

 phosphates ; thus for example, 10,000 parts of the leaves of to- 

 bacco-plant contain 16 parts of phosphate of lime, 8.8 parts 

 silica, and no magnesia ; whilst an equal quantity of wheat 

 straw contains 47.3 parts, and the same quantity of the grain 

 of wheat 99.45 parts of phosphates. {De Saussure.) Hence 



