Fertilizers and Fertility. 349 



While the phosphoric acid of dissolved rock is soluble in water, it 

 cannot be leached out of a soil. As soon as the phosphoric acid comes 

 in contact with the soil, it becomes " fixed " in a form that entirely 

 resists loss by leaching even with heavy rains. 



In many soils the potash is also " fixed," though in a few there may 

 be some loss. It is more difficult to tell how the potash will be affected. 

 It depends on the constitution of the particular soil, and the chemical 

 changes that take place therein. 



Hozv do Fertilisers compare zvith Manures^ 



The commercial fertilizers supply practically no humus or humus- 

 forming materials. Even if they were one-half humus, the amount sup- 

 plied in any ordinary application would be insignificant. The natural 

 humus of the soil gradually tends to diminish in amount through decay. 

 Humus, more than any other constituent, gives to soils their water- 

 holding capacity. Hence, it follows, that the continued use of fertilizers, 

 without the use of humus, will eventually bring the soil to a condition 

 in which it will have less water-holding power and less ability to resist 

 drought, though it may be richer in added plant-food constituents. On 

 the other hand, the best results from fertilizers are obtained on soils 

 well supplied with humus, which helps to hold the water to dissolve 

 them for the roots. 



Manures are nearly all matter that will form humus and contain 

 relatively small amounts of plant-food constituents. This humus-forming 

 material readily decays. As the humus decays, it yields some carbonic 

 acid to the soil-water. This carbonic acid increases the power of the 

 soil-water to dissolve the necessary plant-food constituents in the soil. 

 There seem also to be other products resulting from the decay of humus 

 that make plant-foods available that, without the presence of humus, 

 are not so easily rendered of value. It is in the humus that the soil 

 supply of nitrogen is stored. This humus nitrogen is made available 

 by the same process that renders the nitrogen of dried-blood and tank- 

 age available. 



We have in the case of manures, then, substances high in humus- 

 forming materials which directly supply nitrogen and through their decay 

 in the soil, indirectly, furnish available phosphoric acid and potash from 

 the mineral part of the soil. All manures supply some phosphoric and 

 potash which become available, when the manure decays. 



Fertilizers supply practically the same elements of plant- food that 

 are furnished by decaying humus, but, in a form, that can be more 

 quickly used by the plants. They furnish no humus, and their continued 

 use without stable manures or some form of green manuring, leads to 



