SOILS AND FERTILIZERS 391 



The beneficial effect of stable manure is due principally to the 

 organic constituents of the manure rather than to the mineral salts 

 which it contains. We know, too, that the same food passing through 

 a man, horse, cow, pig, or poultry gives, with the same mineral con> 

 situents, manure of very different properties and values, because of 

 the difference in the character of the organic constituents resulting 

 from the difference in the digestive processes and metabolic processes 

 in the animals through which the food has passed. 



In the case of horse manure it is advisable, and for some soils, 

 as well as for some plants, necessary, to give the manure a prelimin- 

 ary fermentation before it is applied to the soil. In other words, on 

 some soils we can safely use fresh manure and even have it contain 

 a great deal of coarse litter, while on other soils we can safely use 

 only well-fermented manure in which even the litter originally 

 present has lost its form and has become well broken down. Straw 

 as such is often deleterious when applied directly to the soil and 

 turned under, but straw mixed with manure and partly fermented in 

 the manure pile is beneficial to most soils, indicating a difference in 

 the way decomposition proceeds, whether this starts directly in the 

 soil or in the manure pile. 



There is evidence, therefore, that the fertilizers do something^ 

 to the soil to put it into better condition for crops. In disappearing 

 the fertilizers have performed some function, have acted on some 

 constituent of the soil perhaps that had heretofore retarded the 

 growth of plants, and, by its removal, oxidation, or other change, 

 lave rendered the soil a better medium for the growth of the plant. 



The results of fertilizer tests and the experience of farmers 

 seem to indicate that in a majority of cases nitrates increase vegeta- 

 tive growth of plants ; phosphates tend rather to check this vegetative 

 growth or to produce relatively more seed, while potash appears to 

 promote starch-forming activities (develop tubers in potatoes) and 

 to improve the quality of fruits (for example, apples). Combinations 

 of these ele'ments often seem to show in their effect the results which 

 would be expected from each alone, that is to say, a mixture of nitro- 

 gen and phosphorus may increase the vegetative growth and also in- 

 crease the amount of grain produced on the wheat plant. By adjust- 

 ing the proportion of these elements it has doubtless been possible in 

 many cases to modify the crop within the limits as desired. So, in 

 the same way, with potatoes, a moderate supply of nitrogen, either 

 in the form of nitrate of soda or of organic matter, in conjunction 

 with potash, often produces an increased growth of vine necessary 

 for an increased development of tuber. 



Another fact which must be borne in mind is that these effects 

 are greatly influenced by weather conditions. It is a common ex- 

 \ perience that a brand of fertilizer or a single fertilizer constituent 

 which has a decidedly beneficial effect this year may have little or 

 no effect on the same soil and the same crop another year when the 

 weather conditions are markedly different. 



Of the organic matters used directly or indirectly as manure, 

 the most effective are those containing a large proportion of nitro- 



