X TALKS OX MANURES. 



Mining for coal is a very simple matter, but how best to get 

 the greatest quantity of plant food out of the soil, with the least 

 waste and the greatest profit, is a much more complex and 

 difficult task. Plant food consists of a dozen or more different 

 substances. We have talked about them in the pages of this 

 book, and all I wish to say here is that some of them are much 

 more abundant, and more readily obtained, than others. The 

 three substances most difficult to get at are: nitric acid, phos- 

 phoric acid and potash. All these substances are in the soil, 

 but some soils contain much more than others, and their rela- 

 tive proportion varies considerably. The substance which is of 

 the greatest importance, is nitric acid. As a rule, the fertility 

 of a soil is in proportion to the amount of nitric acid which 

 becomes available for the use of plants during the growing 

 season. Many of our soils contain largo quantities of nitrogen, 

 united with carbon, but the plants do not take it up in this 

 form. It has to be converted into nitric acid. Nitric acid con- 

 sists of seven pounds of nitrogen and twenty pounds of 

 oxygen. It is produced by the combustion of nitrogen. Since 

 these "Talks" were published, several important facts have been 

 discovered in regard to how plants take up nitrogen, and es- 

 pecially in regard to how organic nitrogen is converted into 

 nitric acid. It is brought about through the action of a minute 

 fungoid plant. Thera are several things necessary for the 

 growth of this plant. We must have some nitrogenous sub- 

 stance, a moderate degree of heat, say from seventy to one 

 hundred and twenty degrees, a moderate amount of moisture, 

 and plenty of oxygen. Shade is also favorable. If too hot or 

 too cold, or too wet or too dry, the growth of the plant is 

 checked, and the formation of nitric acid su3pended. The 

 presence of lime, or of some alkali, is also necessary for the 

 growth of this fungus and the production of nitric acid. The 

 nitric acid unites with the lime, and forms nitrate of lime, or 

 with soda to form nitrata of soda, cv with potash to form 

 nitrate of potash, or salt-petre. A water-logged soil, by exclud- 

 ing the oxygen, destroys this plant, hence one of the advan- 

 tages of underdraining. I have said that shade is favorable to 

 the growth of this fungus, and this fact explains and confirms 

 the common idea that shade is manure. 



The great object of agriculture is to convert the nitrogen of 

 our soils, or of green crops plowed under, or of manure, into 

 nitric acid, and then to convert this nitric acid into profitable 

 products with as little loss as possible. Nitrogen, or rather 



