ANALYZING AND TESTING SOILS 569 



organic matter becomes adequate to liberate sufficient plant food 

 from the abundant supplies contained in the soil. Of course, if the 

 absolute invoice of the soil shows that the total potassium is low, as 

 in most peaty swamp soils, then that element should be regularly 

 provided in systems of permanent agriculture; and likewise, if 

 the total magnesium is low, it is certainly. advisable to apply liberal 

 amounts of magnesian limestone. Even calcium as an element of 

 plant food (especially for clover, which requires 29 pounds of 

 calcium per ton) may become deficient, as is plainly the case with 

 the extensive Leonardtown loam of southern Maryland. 



Plot Experiments. The following outline is one of the simplest 

 and most practical series of plot experiments: 



PLAN FOR PLOT EXPERIMENTS IN SOIL IMPROVEMENT 



For a four- year rotation, such as wheat, corn, oats, and clover, 

 there should be four different series similar to the 100 series, in 

 order that every crop may be represented every year. The use 

 of manure should be in amounts such as could easily be produced 

 in independent systems of live-stock farming not involving the 

 purchase of feed in excess of the crops sold. The use of crop resi- 

 dues should include all products except the grain or seed to be 

 sold in grain farming. Thus, the clover should be clipped once or 

 twice in May or June and left on the land, the cornstalks should be 

 cut and plowed under, and the threshed straw (from wheat, oats, 

 and clover) should be returned to the land either immediately 

 or subsequently. 



