Ii6 SOILS AND FERTILIZERS 



ing free nitrogen to the air, a process called denitrifica- 

 tion. Through this process animal manure, left exposed 

 to the weather and not packed down, loses much of its 

 nitrogen and therefore its fertilizing value. Soil often 

 loses soluble nitrates through the operation of this cause 

 and thus becomes less able to produce good crops. 

 Cover crops, that is, crops left unharvested so that 

 they may protect the surface of the ground, will retard 

 denitrification. 



Acidity of Soil. Farmers have long been familiar 

 with what they call a sour soil. A poorly drained 

 soil will easily become acid, but it is equally true 

 that a well-drained one may also reveal the same 

 condition. Sometimes this acidity is due to lack 

 of ventilation in the soil, sometimes to the pres- 

 ence of too many trees shading the ground, some- 

 times to the excessive use of fertilizers, and sometimes 

 to the leaching of' the soil. It is also true that some 

 plants through the action of their root tips render the 

 soil acid, and that decay of organic matter will produce 

 the same result. 



Note. Investigations by Professor Whitson of the Wisconsin Ex- 

 periment Station have shown that long-continued cropping on upland 

 clay soils renders them acid, and that these acid soils respond readily to 

 an application of rock phosphate. 



Suggestive Experiment. Collect from a few inches below the 

 surface small quantities of soil from different places where there might 

 be reason to think that acid soils might be found. After thoroughly 

 moistening the soil, place a small piece of blue litmus paper in each 

 sample, cover the paper completely with the soil and leave it for five 

 minutes. If, on examination, the paper has become distinctly red, you 

 may know that the soil is sour, for an acid always has this effect on 

 litmus. (See page 23.) 



