106 A YEAR IN AGRICULTURE 



Liberation of soil fertility. After determining the total 

 amount of plant-food in a plot of soil, the next important 

 question is not how much is available, but how much can be 

 made available during the course of the crop season year after 

 year. We must liberate plant-food by practical methods. 

 We must convert it from insoluble compounds into soluble 

 and usable forms, for the plant-food must be made soluble 

 before the plant can take it from the soil. It has already 

 been stated in previous paragraphs that decaying organic 

 matter is the important factor in making plant-food avail- 

 able, and attention has been called to the fact that the decom- 

 position of this organic matter is hastened by drainage and 

 tillage, which permit the air to enter the soil and assist in 

 the decomposition of the organic material. The application 

 of limestone, as will be explained later, also assists in the 

 liberation of certain plant-foods. 



Loss of plant-food. These plant-food elements are lost 

 by cropping, erosion, and leaching. In a four-year crop 

 rotation under ordinary practices, the amount per acre of 

 calcium lost by leaching is 300 pounds; of magnesium, 30 

 pounds; of phosphorus, 2 pounds; of potassium, 10. It is 

 a well known fact that great quantities of our richest soil 

 are washed from the hillsides and valleys into the streams 

 and carried to the sea. Some of the richest of the corn belt 

 soils have found their way to help build the delta at the 

 mouth of the Mississippi. The table above gives some idea of 

 the extent to which the plant-food of the soil is lost through 

 removal of crops that are grown upon the soil. 



Sources of elements likely to become deficient. It was 



