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PRINCIPLES OF AGRICULTURAL CHEMISTRY 



each size received no fertilizer, and one of each size received I, 

 2, and 3 grams fertilizer respectively. The results are as follows : 



WEIGHT OF CROP IN DIFFERENT SIZED VESSELS. 



The increase of fertilizer from 2 to 3 grams had little effect 

 upon the crop in the pots of either size. That is, the limiting 

 condition is the size of the pot. The crop in the large pot is con- 

 siderably larger than in the small one ; this shows that the space 

 occupied by the roots has considerable influence. However, this 

 difference may be due to differences in moisture content. 



Limitations of Soil Depth. The depth of the surface soil, as 

 stated, is largely dependent upon the depth of plowing. The sur- 

 face and subsoil together may be so shallow as to interfere 

 seriously with the productiveness of the soil. The limiting condi- 

 tion may be hard pan, the water table, rock, or heavy subsoil. 



Hard pan is a layer of hard earth, sometimes rock-like, which 

 cannot be penetrated by plant roots. Hard pan may be caused by 

 constant plowing at the same depth. The sole of the plow con- 

 solidates the layer of soil on which it slips. Such hard pan is 

 most liable to occur in heavy clay soils. Hard pan may also be 

 caused by deposition of matter from drainage or irrigation waters. 

 The deposited matter cements the soil grains into rocky masses. 

 The cement is usually carbonate of lime, but sometimes it is an 

 iron cement, or a humate. 



Hard pan may be prevented by varying the depth of plowing, 

 or by an occasional subsoiling. It is particularly liable to occur 

 in arid climates. In some localities it should be destroyed with 

 dynamite or other explosives before fruit trees are set out. 



Rock, when too near the surface, forbids the use of the soil for 



