VIII. APPLICATION OF MANUEES 167 



soil which is injurious to the plants, unless lime or other basic sub- 

 stance be used to correct it. 



(c) Calcium cyanamide can be used in much the same way as sul- 

 phate of ammonia and appears to be capable of giving the same results. 

 Its injurious effect upon plants, noticed in the earlier pot experiments 

 which were regarded as indications that it should only be applied some 

 considerable time before the seed is sown, do not appear to be of any 

 importance on the large scale, and unless excessive quantities be em- 

 ployed, no injury from this cause is likely to occur. Any unchanged 

 calcium carbide which may be present, is probably injurious to plants, 

 but this would soon disappear after application to damp soil. Calcium 

 cyanamide absorbs moisture and carbon dioxide on exposure to air ; 

 hence the percentage of nitrogen is lower in samples which have been 

 exposed to the air for some time, than in the fresh material. The 

 variety containing calcium chloride (known sometimes as "nitrogen 

 lime " as distinguished from the other variety " lime nitrogen ") increases 

 in weight on exposure more rapidly than the other. 



(d) Nitrate of lime. This substance closely resembles nitrate of 

 soda in its action and the remarks made about that substance apply 

 almost entirely to this manure. It differs, however, in being more 

 hygroscopic and should therefore be preserved as much as possible 

 from exposure to the air and used in the fresh condition. The fact 

 that it contains lime gives it certain advantages over the sodium salt. 

 It is used in about the same quantities per acre as nitrate of soda, and, 

 in many experimental trials, has been found to yield equal or slightly 

 better results. 



Phosphatic Manures. These manures differ from nitrates of 

 soda or lime in one important respect that they are retained tena- 

 ciously by soil and are thus in little danger of suffering loss by 

 drainage. They can therefore be applied before the crop requires 

 them, but here again, it must be remembered that comparatively fresh 

 applications are far more effective than residues from dressings 

 applied to the soil some time before. In the latter case, some of the 

 phosphates are " reverted " and pass into states of combination, 

 perhaps with the iron oxide or alumina of the soil, which do not 

 readily yield them up again to the plants' roots. 



As already explained, there are three chief forms of phosphates 

 present in the various phosphatic manures : 



(a) Soluble phosphates as in superphosphates and dissolved 

 bones. In these manures, the distribution of the valuable ingredient 

 is to a large extent accomplished by the solubility, in the water of the 

 soil, of the monocalcium tetrahydrogen phosphate, though this 

 substance is soon converted into insoluble forms by the action of 

 certain soil constituents mainly by calcium carbonate or ferric oxide. 

 But its initial solubility in water secures for the phosphoric acid a far 

 more complete distribution through the soil, than could be obtained by 

 any mechanical stirring. 



But as these manures are acid, their repeated application to soil 

 seriously diminishes its store of basic material and renders necessary 



