FIELD EXPERIMENTS IN THE SOUTH 477 



soil. As a rule, the rainfall in southern Illinois is abundant and well 

 distributed during the growing season, and where the top soil is 

 kept fertile, severe injury from drouth is not common. 



From Table 15 it will be seen that the average surface soil of 

 this type contains per acre 2880 pounds of nitrogen, 840 pounds 

 of phosphorus, and 24,940 pounds of potassium, and it requires 

 an application of 2 to 5 tons of ground limestone. Compared with 

 the requirements for a practical crop rotation, this soil is very poor 

 in phosphorus and very deficient in lime. Compared with the com- 

 position of fertile soils, it is also deficient in humus as indicated by 

 the total nitrogen. 



If by the best systems of crop rotations, with proper use of green 

 manures, we can liberate, in favorable seasons, the equivalent of 

 i per cent of the phosphorus contained in the surface soil, it would 

 amount to about 8 pounds per acre for the first year for the type 

 of soil under consideration. This would be sufficient for a 25- 

 bushel crop of wheat. If with less perfect systems only half of i 

 per cent is liberated, it would amount to 4 pounds, or enough for a 

 i2-bushel crop of wheat, which is about the average yield for this 

 soil. 



On the Illinois soil experiment field near Odin, Marion County, 

 on this ordinary prairie land of the lower Illinoisan glaciation, 

 wheat is grown in a four- year crop rotation with clover, corn, and 

 cowpeas. By having four different series of plots, every crop may 

 be grown every year. 



As an average of four years (1904, 1905, 1906, and 1907), wheat 

 grown in this rotation produced 1 1| bushels per acre with no special 

 soil treatment, all crops having been removed. 



Where one cowpea crop and some catch crops (as cowpeas seeded 

 in the corn) had been plowed under during the rotation, the aver- 

 age yield of wheat was increased to 14 bushels. 



Where lime or ground limestone had been applied and the cow- 

 peas also plowed under, the average yield of wheat was 18^ bushels 

 per acre. On this set of plots better cowpea crops and catch crops 

 were produced and turned under as green manure, because the soil 

 acidity had been corrected by the lime, applied for the special 

 benefit of the legume crops. 



Where phosphorus was applied in addition to the use of lime and 



