272 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



sufficient to saturate the soil, and pass into the ground water, 

 there is no percolation. Plants, by withdrawing water from the 

 soil and causing it to evaporate, diminish the quantity which 

 percolates. As more of the rainfall will penetrate an open, 

 porous soil than a compact soil, there would be a greater surface 

 off-flow from the latter. Soils which retain water near the sur- 

 face suffer greater losses by evaporation. See Chapter VII. 



(2) The Composition of the Soil Extract. This depends upon 

 the fixing power of the soil and the solubility of its constituents. 

 It also depends on the kind and quantity of the various additions 

 made to the soil. The application of fertilizers almost always in- 

 creases the quantity of potash and nitrogen in the soil extract and 

 consequently increases the loss of plant food. Ammonium sul- 

 phate and potash salts also increase the quantity of lime in the 

 soil extract. 



(3) The Presence or Absence of Vegetation. Besides affect- 

 ing the amount of percolation, vegetation withdraws plant food 

 from solution and thereby diminishes the loss in percolating water. 



Study of the Loss. The loss by percolation may be studied in 

 two ways, both of which have their limitations : 



(i) By determination of the amount and composition of the 

 percolating water. For the purposes of this experiment, a sec- 

 tion of the soil must be enclosed in a water-tight receptable, so 

 arranged that all the water which percolates may be collected, 

 measured, and subjected to analysis. 



A section of soil in its natural condition, may be isolated by 

 trenches, enclosed by brick walls, and then separated from the sub- 

 soil, so that the percolating water may be collected in a suitable 

 vessel. This was the method used in preparing the drain-gauges 

 (as they are called) at Rothamsted. See Chapter VII. 



Soil may also be placed in boxes of cement 1 or cans of gal- 

 vanized iron, or other material, arranged with suitable tubes and 

 collecting vessels. In such case, the soil is stirred and aerated 

 and is otherwise under unnatural conditions. The apparatus 

 1 New York, Cornell Station Report, 1909. 



