240 PERUVIAN GUANO, ETC. [chap- 



which remain after the fat has been melted down and 

 expressed as far as possible. The resulting waste 

 material is still very fatty, and contains anything from 

 1-5 up to 6 per cent, or even more of nitrogen, with 

 phosphates varying from 5 to 12 per cent, of phos- 

 phate of lime. As a rule, the mechanical condition of 

 greaves is bad and much against its proper distribution 

 in the soil ; the price is also often higher than its 

 nitrogen content would warrant, because reasonably 

 clean samples can be used as poultry food. The 

 amount of fat present is again possibly detrimental to 

 its availability. Since greaves is extremely variable 

 in its composition, according to the kind of material 

 which happens to be treated at the factory from day 

 to day, it is difficult to buy any large bulk on a 

 guarantee, just as is the case with shoddy. It is 

 difficult also to judge a consignment from a small 

 sample, so that, as with shoddy, it is best to fix the 

 price on the agreed unit value for nitrogen, taking the 

 mean of several analyses from the bulk. 



Rape dust and otJier cake residues. — In the manu- 

 facture of oil cake the oil-bearing seeds are subjected 

 to great hydraulic pressure, either in bags or in metallic 

 moulds which permit of the escape of the oil. The 

 pressure is increased, aided sometimes by a little heat, 

 until as much oil as possible has been obtained, there 

 being left behind a cake consisting of the other parts 

 of the seed, the proteins, carbohydrates, fibre, etc., 

 together with a certain amount of oil which cannot 

 be expressed. The remaining cake is usually a valu- 

 able cattle food and is sold as such. In crushing 

 rape seed, however, the resulting cake is apt to be 

 very impure ; rape seed not only contains a large 

 proportion of impurities, but often also a good deal 

 of wild mustard seed, from which, when the cake is 



