DEPARTMENT OF BOTANICAL RESEARCH. 67 



rate of evaporation from soils in place were made, with results that are 

 in conformity with the physical textures of the soils in question. The 

 temperature of the lightest soil in the area (sandy outwash) and of 

 the heaviest (flood-plain) was followed for two months by soil-ther- 

 mograph readings at 3 inches. The latter soil showed higher maxima 

 and minima than the former, but in few cases was there more than 5° 

 difference between the stations, located at approximately the same 

 elevation. 



Laboratory work was carried on with samples of outwash, sandy 

 outwash, flood-plain, and playa soils, with particular reference to 

 determining their penetrability, the capillary rise of water in them, 

 and the rates of evaporation under laboratory conditions. The 

 results are in conformity with the physical textures of the soils, except 

 in the case of the flood-plain type. This is an adobe soil in which 

 from 22 to 51 per cent, by weight, consists of particles less than 0.01 

 inch in diameter. Although the mechanical analysis of these soils 

 has not been carried to further refinement, there is doubtless a con- 

 siderable portion of the finest material that is less than 0.001 inch in 

 diameter, resulting in a very slow rate of penetration and a low rate of 

 evaporation as compared with the other much coarser types of soil. 

 In cans of soil of the four types, which were saturated and allowed to 

 evaporate for 36 days, the graphs representing their behavior ran 

 closely parallel for the outwash, sandy outwash, and playa soils, but 

 the rate of fall was much slower for the flood-plain, and at the end of 

 the period it still contained 6 per cent of the original saturation content. 



Determinations were also made of the amount of salts readily dis- 

 solved from soil samples by hot water, covering the four types of soil 

 just mentioned and also the coarse outwash, a tj'pe which encircles 

 all of the larger hills. The results show the highest percentages of 

 soluble matter for the coarse outwash and the flood-plain — the soils 

 which stand at the two ends of the erosion c^^cle. The lowest per- 

 centages were found in certain samples of outwash, in which there 

 appears to be only one-tenth as much readily soluble matter as in the 

 flood-plain. Special determinations were made on samples from 

 areas in which the vegetation is not in conformity with the texture of 

 the soil, as is so generally the case over the entire area under investi- 

 gation. In these cases the amount of soluble salts appears to be the 

 determining factor and explains the exceptional vegetation. In view 

 of these results further investigation will be made of areas in which 

 the vegetation differs from that in adjacent locations appearing to have 

 identical soil conditions. 



In order to secure a more precise record of the differences in vegeta- 

 tion on the several soils in the Avra Valley and adjacent areas, a census 

 was begun of the plant population on typical areas 10,000 square 

 meters in size. The poorest vegetation, both in the number of species 



