ORGANIC NITROGEN FERTILIZERS 107 



investigations show that the plant can obtain its nitrogen 

 in other forms such as ammonia or urea, both of which may 

 come from cyanamide. Cyanamide containing dicyano- 

 diamide should not be applied to a soil immediately before 

 or after applications of sulphate of ammonia, as the nitrifica- 

 tion of the latter will be checked. 



Ammonium sulphate can be made from nitrolim by 

 treatment with superheated steam, when all the nitrogen 

 is given off in the form of ammonia, which can be condensed 

 with sulphuric acid. Alternatively the ammonia so produced 

 can be converted into ammonium nitrate, as described on 



p. 101. 



Oil Cakes. Some of the oil cakes, which are obtained as 

 a by-product of pressing oil seeds, are not suitable for use as 

 cattle food, but may be used for manure. When the 

 resulting cake is only intended for use as manure, vety crude 

 and imperfect methods of pressing for oil are legitimate. 

 The preparation of oils and fats from small oleaginous 

 seeds, such as castor and rape, was originally performed by 

 crushing the seeds and grinding them between stones, or 

 grinding them in a large wooden pestle and mortar. 

 Frequently some system of heating the meal was used, together 

 with a certain amount of moistening. Presses which were 

 driven home by means of wedges or screws were also used in 

 primitive methods. On the larger scale, when the seed is 

 received at the mill, there is first a preliminary operation 

 of freeing the seeds from dust and sand, although this 

 procedure is not of great consequence where the cake is 

 only intended for manurial purposes. The seeds and 

 nuts are sometimes decorticated, the shells being thrown 

 aside and only the kernels being crushed and ground. 

 The rollers used for crushing are generally finely grooved, 

 although they are sometimes smooth. The material, after 

 heating and moistening, is conveyed to the press, operated 

 by a hydraulic ram, which presses the cakes upwards against 

 a fixed roof. The meal is generally delivered, packed 

 into closed measuring boxes, or into measuring boxes with 

 cloths, open at two sides. In some systems the seed is 



