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water may be demonstrated, for soils of different texture, by direct 

 measurement of the wilting point (72) or from moisture potential- 

 moisture content curves, which may be obtained by means of pressure 

 membrane apparatus (5S) and by other methods (72). 



The permanent wilting percentage and moisture equivalent (a func- 

 tion of particle-size distribution) values published for a number of dif- 

 ferent surface soils by Richards and Weaver ($7) and by Veihmeyer and 

 Hendrickson (69) indicate a high positive correlation. This may be 

 explained by the existence, within soils of high clay content, of very 

 fine pores that retain water against plant withdrawal forces and that 

 are present in greater numbers in the clay-rich soils than in those of 

 lower clay content. Although the functional relationship between 

 permanent wilting percentage and soil is far from a simple one and may 

 involve mineralogical and physical characteristics, the proportion of 

 very fine particles present is possibly the most important single factor 

 affecting this property in nonsaline soils. For a given total water con- 

 tent below the wilting point the energy of water retention generally 

 increases considerably with increasing fineness of texture. 



Many fine-textured highly water-retentive soils also show a marked 

 tendency to be very plastic and to remain wet for a comparatively long 

 time. Such soils are too frequently worked while they are still wet, 

 with the result that puddling occurs and the soils thus become even 

 less satisfactory for plant growth. Strikingly harmful effects on plant 

 growth have been observed both in the field and the laboratory as the 

 result of soil puddling: the process of mechanically working a soil and 

 altering its structure so that, by dispersion of the aggregated particles, 

 the coarser pores are destroyed. 



For a definite expenditure of mechanical work of compression and 

 shear, the amount of dispersion of aggregates by puddling treatment 

 and the resultant diminution in apparent specific volume appear to 

 depend upon the moisture status of the soil, the amount of mechanical 

 work already expended in reducing the apparent specific volume, and 

 the particle-size distribution (/j). This last, of course, within the 

 limitations stated earlier, represents the ultimate in mechanical disper- 

 sion for a given soil. 



It has been shown by Thomas (67), Day (32), and Buehrer and 



