THE SOURCES OF PLANT FOOD. l7 



sub-soil 0.05 per cent. A good surface soil may contain 0.20 

 per cent, of phosphoric acid, or not unfrequently a smaller 

 quantity. Potash varies much, rising to 1.0 per cent, or 

 more in some clay soils, but being generally much smaller. 



The weight of soil on an acre of land is, however, so 

 enormous, that small proportions of plant food may amount 

 to very considerable quantities. Nine inches' depth of 

 arable soil (clay or loam) will weigh, when perfectly dry, 

 about 3,000,000 or 3,500,000 lb. A pasture soil will be 

 lighter, the first 9 inches weighing when dried and the 

 roots removed about 2,250,000 lb. Supposing, therefore, 

 a dry soil to contain 0.10 per cent, of nitrogen, phosphoric 

 acid, or potash, the quantity in 9 inches of soil will be 

 from 2250 lb. to 3500 lb. per acre. 



A large part of the elements of plant food contained in 

 soils is present in such a condition that plants are unable 

 to make use of it. A soil may contain many thousand 

 pounds of phosphoric acid or of nitrogen, and yet be in a 

 poor condition ; while a small dressing of readily available 

 food, as superphosphate or nitrate of sodium, may greatly 

 increase the fertility. 



The nitrogen contained in humus is not in a condition 

 to serve as a general plant food ; cereal crops are appa- 

 rently unable to appropriate it; leguminous crops, how- 

 ever, possibly assimilate some humic matters. By the 

 action of a minute Bacteriur)i present in all soils, humus 

 and ammonia are oxidised, and their nitrogen converted 

 into nitric acid. Nitrification only takes place in moist 

 soil, sufficiently porous to admit air. It is also necessary 

 that some base should be present with which the nitric 

 acid may combine : this condition is usually fulfilled by the 

 presence of carbonate of calcium. Nitrification is most 



