Plant Food in the Soil 91 



guano of to-day comes from a different set of islands and 

 has a far lower percentage of nitrogen, though a high 

 total of phosphoric acid. One of the greatest sources of 

 nitrogen of late years is the nitrate of soda found in Peru. 

 This, being already in the form of a nitrate, is at once 

 available to plants, and is always to be applied to growing 

 plants and not to dormant ones in winter, since if not 

 used at once by plants it is readily leached by the soil. 

 Another important source of nitrogen is the sulphate of 

 ammonia. This salt when pure contains 21.2 per cent, 

 of nitrogen, ammonia being a hydride of nitrogen, and in 

 the ordinary commercial forms it has 20 per cent, of nitro- 

 gen. This makes it far richer than nitrate of soda. In 

 the manufacture of boneblack for the sugar refineries by 

 distillation it is obtained, and also from the waste ammo- 

 niacal liquor from the gas works. Of late years im- 

 proved methods of making coke have increased the 

 amount of sulphate of ammonia saved in the process. 

 The sulphate of ammonia becomes more quickly avail- 

 able to plants than the organic nitrogen in animal refuse 

 since in that form the ammonia needs first to be released 

 before the process of nitrification or the forming of nitrates 

 can be accomplished, while the ammonia is ready at once 

 for the work of the bacteria that carry on this process of 

 making nitric acid. But it has been found that on soils 

 that are to any extent acid the application of the sulphate 

 of ammonia is actually poisonous to plants until the 

 acidity is corrected by lime. 



We have seen that phosphorus is the element most 

 generally deficient in old cultivated soils, and since we 

 cannot get it from the air as we can nitrogen, as will be 



