398 POPULAR SCIENCE MONTHLY. 



they must be united to something, and so, as soon as old associations 

 are dissolved, new ones are formed. These new ones are those needed 

 by plants, and thus is plant food digested. 



The term " plant food " has been frequently used, and should 

 now be distinctly explained, for merely stating the chemical elements 

 is not describing the food. When a physician tells a nurse to feed a 

 patient he does not order so much carbon, nitrogen, phosphorus, and 

 the like, but specifies a soup, certain vegetables, and so on, detailing 

 every particular; and the same should be done for vegetable invalids. 



In medical practice a condition is recognized that is called scurvy. 

 It is not exactly starvation, but is produced by lack of some food 

 materials usually supplied by fresh vegetables. If scurvy appears at 

 sea, no amount of meat, bread, cakes, or pastry will stop it; vegetables, 

 and they only, will stay it. Sometimes a similar condition pre- 

 vails among crops : some ingredient in a soil is lacking, and the others 

 may be supplied indefinitely without giving the desired relief. To 

 this may be attributed much of the fault found with fertilizers ; for if 

 the soil does not need a particular compound it is useless to apply it, 

 and an excellent fertilizer is often blamed for not producing a crop on 

 land already overstocked with it and crying for something else. 



Let us suppose a field on which cotton has been grown for many 

 successive years until it has become exhausted. Analysis shows that 

 a crop yielding one hundred pounds of lint to the acre removes from 

 the soil: 



Nitrogen 20.71 pounds; 



Phosphoric acid 8.17 " 



Potash 13.06 " 



Lime 12 . 60 " 



Magnesia 4.75 " 



Total 59.29 " 



The weight of the whole crop from which these figures were 

 taken was eight hundred and forty-seven pounds, so that cotton 

 exhausts land less than any staple crop, if the roots, stems, leaves, etc., 

 be turned under and only the lint and seed be removed. Of these 

 the lint (one hundred pounds) takes 1.17 pound from the soil, and the 

 seed 13.89 pounds, making 15.06 pounds net loss.* But ignoring 

 returns that may be made in the shape of cotton-seed meal, etc., 

 and lime, with which our soils are abundantly supplied, we see that 

 nitrogen, phosphoric acid, and potash have been removed. Suppose 

 the owner puts bone meal on his exhausted land: the phosphoric 

 acid in the bone will supply one need, and an improvement results. 

 On the strength of this, bone meal will be loaded into the soil again, 



* United States Department of Agriculture. Farmers' Bulletin, No. 48. 



