132 



SOIL CONDITIONS AND PLANT GROWTH 



a crop. But the puffing up or " lightening " may go too far, 

 and sometimes causes much trouble in old gardens that have 

 long been heavily dunged. 



j 4. It increases the water-holding capacity of the soil. The 

 [amounts of moisture present in adjacent plots at Rothamsted 

 jare shown in Fig. 17, from which it appears that the plot 

 .'annually receiving farmyard manure contains normally 3 or 4 

 'per cent, more water than the adjoining plot receiving no 

 organic manure. 



The variations in water content follow very closely the 

 variation in the amount of organic matter present. So marked 

 are these physical effects that if 1 5 or 20 per cent, of organic 

 matter is present in a soil the operation of other factors ceases 

 to count for much, and the distinctions between sands, loams, 

 and clays are obliterated. Thus, much of the famous Red 

 River prairie soil of Manitoba is identical in mineral composi^ 

 tion with certain poor infertile Wealden soils, but the presence 

 of 26 per cent, of organic matter completely masks the harm- 

 ful effect of the clay and fine silt. A similar pair of soils, 

 owing their difference in agricultural properties to their different 

 organic matter content, have been analysed by C. T. Giming- 

 ham (106) (Table XXXVIIL). 



Table XXXVIII. 



-Effect of Organic Matter^ on the Texture of 

 Soils. 



1 Measured by the loss on ignition. 



