GREEN MANURES. 315 



salts (199), are useful additions in the state of saw- 

 dust to manures rich in ammonia. 



850. There are a few manures of vegetable origin 

 which contain a considerable quantity of nitrogen, 

 and which consequently approach very near in nature 

 to those formed of animal substances : such, for ex- 

 ample, is yeast or barm ; this consists of matter rich 

 in nitrogen in a state of incipient putrefaction (366), 

 and is a very strong manure. 



851. So also is oilcake, the residue left after ex- 

 pressing the oil from seeds like linseed, which are 

 rich in oil ; it contains much gluten and albumen, 

 and is for that reason a strong and valuable manure. 



852. Green manures, as they are called, always 

 contain enough nitrogen to insure their own decom- 

 position, though it goes on far slower than does that 

 of animal manures; the value of green manures is 

 shown in the ploughing up of an old pasture ; and 

 even more strongly in the case of those crops which 

 are sown, merely to be ploughed in, when they have 

 formed a quantity of organic matter. 



853. Some plants decompose with far more readi- 

 ness, than others; for the most part, water plants, 

 both sea and land, decay less easily than land plants; 

 they notwithstanding are useful manures. Seaweed, 

 employed alone, decays but slowly ; it is, however, 

 valuable for its mechanical effects, and likewise for 

 the alkali which it contains; it may be used with 

 great advantage together with strong minimal manures 

 (802). 



