Investigations at Peru, Nebraska. 



33 



account largely for the observed deficiencies of moisture for crops in seasons 

 where the recorded rainfall would indicate an abundant supply. The mean 

 annual precipitation at Peru is about 34 inches. In table 4 is given the 

 monthly precipitation from March to August 1919, together with the mean 

 for the five preceding years. 



Table 4. — Precipitation in inches. 



These data were obtained from the reports of the U. S. Weather Bureau 

 station at Nebraska City, Nebraska, 15 miles north of Peru. March and 

 June were months of rather normal rainfall; April was unusually wet, while 

 May, July, and August had a precipitation far below the average. The only 

 deviation of any importance from these Nebraska City records was July 4 

 and 5, when two very heavy local showers occurred at Peru. About 2 inches 

 of water fell during a period of 2 hours or less. This resulted in a high run- 

 off. Nevertheless, these showers were very beneficial to the growing crops. 

 However, precipitation is only a very general indicator of conditions for plant 

 growth. Because of differences in time and manner of distribution, amount 

 of run-off, which in turn is influenced by soil structure, rapidity of evaporation, 

 etc., rainfall alone, indeed, yields data of httle value in a study of the water 

 relations for crop production. Although it modifies the temperature of both 

 air and soil, and especially the humidity, as well as having a profound effect 

 upon soil aeration, its major influence upon the activities of plants is exerted 

 through its power to replenish soil moisture. The soil may be compared 

 to a great reservoir of water from which growing plants are constantly draw- 

 ing their supply. When water becomes less abundant in the upper portion 

 of this reservoir, plants must either suffer from lack of water or extend their 

 absorbing organs into the deeper soils. Hence it may be that during this 

 season of drought the roots penetrated somewhat deeper than normally. 

 However, similar results were obtained at this station during 1921, but this 

 was also a year of deficient rainfall. Further studies may show this root 

 habit is quite normal for these rather mellow and relatively well-aerated 

 soils of loose texture, where the subsoil is moist to very great depths. In 

 fact, judging from the root habits of native species, this seems highly probable. 



Water-content of the soil, with its effect upon aeration, is the most im- 

 portant factor affecting root development under field conditions. Because 

 of its importance, a rather extensive series of determinations was made. 

 In taking the samples of 100 to 150 grams of soil at various depths, a Briggs 

 geotome was employed. The soil was dried to a constant weight at 105° C. 

 and the water-content calculated in per cent of the dry weight. For con- 

 venience, the water in excess of the hygroscopic coefficient is considered avail- 

 able for plant growth. In another place (Weaver, 1920 : 28), it has been 

 pointed out that soil upon which native prairie or short-grasses grow fre- 

 quently shows a water-content below the wilting coefficient of Briggs and 



