414 Mineral Nutrition of Plants 



osmotic pressure of the soil solution has been called the total soil mois- 

 ture stress (128) and can be used as a measure of the total amount of 

 work that a plant must do to absorb a unit amount of water from the 

 soil. The freezing point measurement directly gives an indication of 

 the total soil moisture stress. 



Furr and Reeve (46) have shown that the rate of growth of sun- 

 flower plants drops to zero as the soil moisture approaches the wilting 

 percentage. There is increasing experimental evidence that the growth 

 of crop plants ceases when the soil moisture stress rises into the range 

 of 15 to 20 atmospheres (/jo). 



The course of events during moisture depletion in field soil may be 

 set forth briefly by an illustration. Consider a newly planted hill of 

 corn after a thorough soaking rain. When the rain stops, the moisture 

 moves gradually out of the larger pores toward the water table which, 

 for best crop growth, should be below the root zone. This water move- 

 ment takes place primarily under the influence of gravitational force 

 because, when the whole profile is very wet, gradients in soil moisture 

 tension are small. 



Concomitant with the recession of water in the larger pores, air is 

 drawn into these pores. As the soil approaches field capacity there is a 

 reasonably continuous gaseous phase throughout the soil, but the mois- 

 ture films covering the soil particles become so thin that additional 

 moisture movement in these films is considerably restricted.* 



Thus, several days after the rain has stopped, the soil moisture ten- 

 sion may rise into the range of 0.05 to 0.15 atmosphere (50 to 150 cm. 

 of water) and moisture movement in the profile becomes considerably 

 restricted. Water vapor loss will occur from the top few inches of sur- 

 face soil due to air circulation and the elevated temperature of the 

 surface soil, but moisture loss from soil layers a few inches below the 

 surface is usually slow. When the corn seeds germinate and the plant 

 roots develop in the neighborhood of the kernel, the rootlets remove 

 moisture from the immediately adjacent soil and can build up soil 



* It should be kept in mind that the condition here referred to as field capacity 

 can exist only in well-drained soils. When soil horizons having low permeability 

 are present in the profile, or when a nearby ground water table exists, then the 

 root zone may remain excessively wet for long periods. 



