248 SOILS: PROPERTIES AND MANAGEMENT 



1. Crop. Differences due to different crops and to 



variations of the same crop. 



2. Climate. Rain, humidity, sunshine, temperature, 



and wind. 



3. Soil. Moisture and general fertility. 



170. Effect of crop and climate on transpiration. - 

 Not only do different crops show a variation of tran- 

 spiration in the same season, but the same crop may give 

 a totally different transpiration in different years. This 

 is due in part to inherent differences in the crop itself. 

 For example, leaf surface or root zone would totally 

 alter the transpiration relationship under any given 

 condition. However, a great deal of the variation ob- 

 served in the ratios already quoted arises from differences 

 in climatic conditions. As a general thing, the greater 

 {he rainfall, the higher is the humidity and the lower is 

 the relative transpiration. This accounts for the high 

 figures obtained by Widtsoe x in arid Utah. Mont- 

 gomery 2 found, in studying the water requirements of 

 corn under greenhouse conditions, that an increase in 

 the percentage humidity from 42 to 65 lowered the tran- 

 spiration ratio from 340 to 191. In general, temperature, 

 sunshine, and wind vary together in their effect on tran- 

 spiration. That is, the more the sunshine, the higher is 

 the temperature, the lower is the humidity, and the 



1 Widtsoe, J. A. Production of Dry Matter with Differ- 

 ent Quantities of Irrigation Water. Utah Agr. Exp. Sta., 

 Bui. 116. 1912. Also, Irrigation Investigations. Factors In- 

 fluencing Evaporation and Transpiration. Utah Agr. Exp. 

 Sta., Bui. 105. 1909. 



2 Montgomery, E. G., and Kiesselbach, T. A. Studies in 

 Water Requirements of Corn. Nebraska Agr. Exp. Sta., 

 Bui. 128, p. 4. 1912. 



