SOIL MOISTURE 15 



begin to wilt, and growth practically ceases. The per- 

 centage of moisture corresponding to this point is called 

 the uniting coefficient. 



According to the researches of Briggs and Shantz, 

 the wilting coefficient is about one and one-half times the 

 hygroscopic coefficient. That is, in a soil with a hygro- 

 scopic coefficient of 6 per cent of water, the wilting coeffi- 

 cient would be about 9 per cent. This relative value of 

 the wilting coefficient appears to be confirmed by field 

 experiments conducted at the California and Utah 

 Experiment Stations. 



The wilting coefficient, like the hygroscopic coefficient, 

 varies with the soil used. In sandy soils it is low, often 

 less than 1 per cent of moisture; in clay soils higher, 

 often more than 16 per cent, and in extremely heavy clays 

 as high as 30 per cent; in the average loam, about 10 per 

 cent of moisture. It is ordinarily thought that plants 

 differ markedly in their power of reducing the soil mois- 

 ture before wilting. Recent researches do not bear out 

 this view. On a given soil, under like meteorogical con- 

 ditions, the wilting coefficient is within 1 to 3 per cent 

 of each other for all the ordinary plants at the same 

 period of growth, whether grown under arid or humid 

 conditions. 



While growth undoubtedly ceases at wilting, yet the 

 plant may slowly take up some of the moisture held 

 in the soil below the wilting point. On the other hand, 

 under proper methods of agriculture the soil moisture is 

 seldom reduced to the wilting point, especially on deep 

 soils, if irrigation has been practised regularly. At the 

 Utah Station, several crops, in their medium stages of 

 growth, were allowed to go for long periods, from twenty- 

 seven to fifty-five days, without irrigation. At the close 



