[lynde-dxjpré] on osmosis IN SOILS 115 



Does underdrainage bring about an upward movement of water by 



osmotic pressure? 



It is well known that crops on drained land receive more moisture 

 than crops on undrained land. This may be due to a deeper root 

 system, but it may be partly due to osmosis, as follows. 



In drained land the water level is lowered to a depth of 3 or 4 

 feet below the surface. This permits air to enter to this depth. This 

 makes the conditions more favorable to the growth of bacteria. The 

 bacteria thus procure more plant food, that is, more salts soluble in 

 water. If these salts increase the concentration of the soil solutions 

 in the upper 3 or 4 feet of soil, they thereby increase the osmotic pres- 

 sure of these solutions. This should cause an upward movement of 

 moisture by osmotic pressure, as noted above. 



Does the use of the soil mulch bring about an upward movement of mois- 

 ture by osmotic pressure? 



A soil mulch conserves the moisture in the soil. Our theory 

 suggests another way in which the moisture under the mulch may 

 be increased. Bacteria thrive where there is moisture as well as 

 heat. If the soil is dry the bacteria remain dormant. Under the 

 mulch the soil is moist and warm. These are the ideal conditions 

 for growth of bacteria. Under the soil mulch, then, the bacteria 

 thrive and produce an abundance of plant food in the form of salts 

 soluble in water. If these salts increase the concentration of the 

 soil solutions under the mulch, they increase the osmotic pressure 

 of these solutions. This should cause an upward movement of mois- 

 ture through the subsoil by osmotic pressure, as noted above. 



Do the operations of dry farming cause water which has percolated below 

 the reach of the roots, to be drawn back by osmotic pressure? 



In dry farming it is the practice, in some cases, to plant a crop 

 every other year and maintain a mulch on the land in the off year. 

 The explanation of the benefit of this is that the moisture is held over 

 for the crops of succeeding year. 



It is possible that osmotic pressure plays a part here. The 

 deeper subsoil in dry farming areas is usually very dry, the water 

 available for crops being rain water which has percolated into the 

 soil. It is possible that some of this rain water penetrates below the 

 reach of the roots, and is brought, back to the roots by osmotic pres- 

 sure as follows. In the fallow year the surface soils become rich in 

 soluble salts. If these increase the concentration of the soil solutions 



