78 FIRST PRINCIPLES OF AGRICULTURE. 



a food for stock, however, limits its use as a fertilizer 

 at points distant from place of production. It is fine and 

 dry, and decays rapidly in the soil, and is regarded as one 

 of the best forms of organic nitrogen. 



Castor Pomace. — Castor pomace is similar to cotton- 

 seed meal, both in its composition and in the quality of its 

 nitrogen. It is the refuse castor bean after the oil has 

 been extracted. It is not an important source of organic 

 nitrogen, though practically the whole product is used as 

 a manure. 



Organic Nitrogenous Materials are Variable. — As 

 has been indicated, organic nitrogenous materials are, on 

 the whole, variable products, both in respect to their 

 content of nitrogen, and to its availability. Those that 

 are uniform in respect to their composition are more valu- 

 able than the others, since their availability may be tested, 

 and an average arrived at. 



Those high-grade products, like dried blood, azotine, 

 dried fish, and cottonseed meal, which are fairly uniform 

 in these respects, show, both by chemical and field tests, 

 a high percentage of availability, which does not vary 

 greatly with different samples ; while those like tankage 

 sometimes show a high and sometimes a low availability, 

 because of the lack of uniformity in the proportions of 

 their component parts. 



The Use of Nitrogen. — Great care should be exercised 

 in the purchase and use of nitrogen, first, because it is an 

 expensive element ; and second, because when it is in a form 

 useful to plants it is entirely soluble in water and freely 

 movable, and, therefore, liable to be washed away and lost. 

 The other elements, phosphoric acid and potash, cost much 



