ILLINOIS FIELD EXPERIMENTS 465 



produced very marked increases, the average value being, as a 

 rule, more than double its cost in steamed bone meal, the form 

 in which it was applied to these fields. On the other hand, the 

 use of phosphorus without nitrogen will not maintain the fertil- 

 ity of the soil (see plots 104 and 106, Sibley field), and a liberal 

 use of clover or other legumes is suggested as the only practical 

 and profitable method of supplying the nitrogen, the clover to be 

 plowed under, either directly or as manure, preferably in connec- 

 tion with the phosphorus applied, especially if raw rock phosphate 

 is used. 



From the best treated plots, 100 pounds per acre of phosphorus 

 have been removed from the soil in the seven crops. This is equal 

 to 10 per cent of the total phosphorus contained in the surface 

 soil of an acre. In other words, if such crops could be grown for 

 84 years, they would require as much phosphorus as the total 

 supply in the surface 6| inches of soil. The results plainly show, 

 however, that without the addition of phosphorus such crops 

 cannot be grown year after year. Where no phosphorus was 

 applied, the crops removed only 75 pounds of phosphorus in seven 

 years, or nearly 1 1 pounds a year, equivalent to almost i per cent 

 of the total amount (1260 pounds) in the surface soil. (See also 

 Table 50, giving results of raw rock phosphate on brown silt loam.) 



The yellow-gray silt loams are found on the undulating upland 

 areas that are, or were originally, timbered. The topography 

 varies from nearly level to gently rolling, corresponding to the 

 topography of the brown silt loam prairies. The yellow-gray silt 

 loam varies from yellow to gray in the surface, and, as a rule, there 

 is more or less " gray layer" in the subsurface (especially in the 

 older formations). On the late Wisconsin glaciation, the loess 

 covering being shallow, glacial material containing more or less 

 gravel is frequently found in the subsoil within 40 inches of the 

 surface. 



As shown in Table 15, the late Wisconsin yellow-gray silt loam 

 (1034) contains in the surface 6| inches about 2900 pounds of 

 nitrogen, 800 pounds of phosphorus, and 47,600 pounds of potas- 

 sium. Compared with the more productive, more durable, and 

 more valuable soils (as the early Wisconsin black clay loam), 

 this soil is very poor in phosphorus and quite low in humus as 



