ORGANIZATION OF THE AGRICULTURAL ENTERPRISE 359 



115. ECONOMY IN THE USE OF PLANT FOOD' 

 BY CYRIL G. HOPKINS 



For practically all of the normal soils of the United States there 

 are only three constituents that must be applied in order to adopt 

 systems of farming that, if continued, will increase or at least perma- 

 nently maintain the productive power of the soil. These are lime- 

 stone, phosphorus, and organic matter. 



Where such natural materials as chalk and marl have not been 

 accessible, more or less use has been made of water-slacked or air- 

 slacked lime; because, by burning and slacking, limestone rock may 

 be reduced to powdered form and thus distributed over the land. 

 With the development of rock-crushing and rock-grinding machinery, 

 fine-ground natural unburned limestone can be had, and, where 

 material can be gotten at reasonable cost,. it replaces all other forms 

 of limestone used for the improvement of normal soils. The manu- 

 facturer in hydrating lime simply converts, at considerable expense, 

 the slower acting carbonate to the caustic hydroxid form. This is a 

 powerful agent in hastening the destruction of organic matter, stimu- 

 lating the soil at the expense of permanent fertility. 



So much has been said and written regarding the value of farm 

 manure that it is common talk that the manurial value of the food 

 is almost wholly recovered in the manure; and there is even a vague 

 notion in the minds of some that the manure is worth more for soil 

 improvement than is the food from which the manure is made. The 

 fact is that the most important and least appreciated method of 

 obtaining or increasing the supply of organic matter in the soil is by 

 the use of green manures and crop residues. A ton of clover plowed 

 under will add nearly three times as much organic matter to the soil 

 as can possibly be recovered in the manure if the clover is fed. Nitro- 

 gen may also (besides that secured from organic matter produced on 

 the farm) be bought in the market in such forms as dried blood 

 (14 per cent), sodium nitrate (15^ per cent), and ammonium sulphate 

 (20 per cent); but when we bear in mind that such commercial nitro- 

 gen costs from 15 to 20 cents a pound, and that one bushel of corn 

 contains about one pound of nitrogen, it will be seen at once that the 

 purchase of nitrogen cannot be considered practicable in general 

 farming, although in market gardening and in some other kinds of 



1 Adapted from Soil Fertility and Permanent Agriculture, pp. xxii, iS7-6 2 > r 99 

 200, 207, 237. (Copyright by Cyril G. Hopkins. Published by Ginn & Co.) 



