292 AC.RICl'LTURE OF MAINE. 



the soil was formed, and its original source was /the air, 80% 

 of which is nitrogen. Its source, for most of our farm crops, 

 is the organic matter in the soil, as the nitrogen in the air is 

 unavailable for ordinary plants. The question might be asked : 

 By what means was the nitrogen contained in the organic mat- 

 ter taken from the air? A small amount of this is washed 

 down yearly in the rainfall, and soil bacteria also fix nitrogen 

 in the soil in a form that will become available to plants. By 

 means of these two factors, then, a supply of nitrogen was 

 taken from the air so that plant life might begin. These two 

 factors have kept up the nitrogen content of the soil. 



Let us consider for a moment Nature's method of handling 

 the plant food problem. Under natural conditions, plants died 

 and decayed where they grew and were carried away only in 

 small amounts. Thus the supply of plant food and of organic 

 matter in the soil was kept up, and the plant food in the decay- 

 ing plants was becoming available each year to supplement the 

 original source in the soil. This produced w^hat we might call 

 a revolving fund of soil fertility ; — that is, plant food was taken 

 from the soil by plants which decayed, and in the course of 

 time released this plant food for the use of other plants. It is 

 necessary that we have an additional source of plant food, 

 besides that made available in the soil each year by natural 

 conditions, as such a small amount will not produce profitable 

 crops. 



Man, however, must remove the crops he grows from the 

 soil ; thus the available plant food is removed from the soil 

 and the organic matter content of the soil cut down. Both of 

 these must be supplied, or the productivity of the soil is reduced. 

 The farmer may make two uses of his crops, he may sell them 

 from the farm or feed them to the stock and return the manure 

 to the soil. The purpose of this paper is 'to discuss the return 

 of plant food to the soil in manure. 



Let us here briefly look at the relation of the. method of 

 farming to the amount of plant food removed from the farm. 

 For each one hundred dollars' worth of product sold, the value 

 of the plant food removed from the farm will be as follows : 



