ORGANIC CONSTITUENTS OF SOILS 



153 



burned limestone or quicklime is that it tends to destroy the organic 

 matter of the soil, and most soils that need lime have too little 

 organic matter to begin with. At the Pennsylvania Station the plots 

 having burnt lime applied for 25 years showed less nitrogen by 

 375 pounds than the limestone plot. This difference is equal to 

 37.5 tons of barnyard manure per acre. At the Virginia Station it 

 has been determined that the applications of quicklime have reduced 

 the amount of nitrogen and organic matter when compared with 

 plots treated the same except that quicklime was omitted. 



(g) By Fallowing. Fallowing is leaving the land without a 

 crop for a season during which the soil is cultivated. This has been 

 a very common agricultural practice in European countries, but 

 more especially in England. The objects of the fallow were to 

 destroy weeds, to develop an abundance of nitrates for the succeeding 

 crop, to increase the moisture content of the soil, and to produce 

 good tilth in heavy soils. While all of the objects were accomplished, 

 yet in regions where heavy fall, winter, or spring rains occur much 

 of the soluble plant food which was produced at the expense of 

 organic matter was leached out of the soil and lost. King found 

 that in the spring of 1900 land fallowed the previous season con- 

 tained 245.7 pounds more of nitrates per acre than the cropped 

 land. The following table from Hall shows the effect of leaching 

 from fallowed land upon the wheat crop : 



Yield of Wheat Grown When Percolation loos Large and Small 8 



Fallowing should bo practiced only where the rainfall is not 

 sufficient to cause any loss by leaching, as in sub-humid and somi- 

 arid regions. 



Estimation of Organic Matter. Xo very satisfactory method 

 has been devised for determining the organic matter of soils, since 

 it is impossible to determine it directly. 



