8 



BULLETIN 151, U. S. DEPARTMENT OF AGRICULTURE. 



For the most part the curves show only slight variations in the 

 amount of moisture present in the fallowed and continuously cropped 

 plats during the period when crops were on the land. There was a 

 somewhat higher moisture content in the soil of the fallow plats at 

 the time when crops were growing on the other plats; but, as already 

 stated, the difference generally disappeared by the next planting 

 time. 



RUN-OFF FROM FALLOWED PLATS. 



The uniformity in soil-moisture content at planting time, already 

 noted, is probably accounted for by the higher loss by run-off from 

 fallow plats than from those which were cropped every year. Dur- 

 ing the years covered by this report the precipitation during the 

 winter and early spring was comparatively heavy. Consequently, 

 so far as the rainfall during the winter and spring immediately pre- 

 ceding corn and cotton planting was concerned, land cropped each 

 year and plowed as soon as possible after the removal of the crop 

 had the same opportunity to store moisture as fallowed land had 

 during the same period. Even though the fallowed land contained 

 a larger amount of moisture at the time of seeding oats in the fall, 

 a larger amount of run-off from the fallowed plats during the winter 

 would result in approximately uniform moisture conditions in all 

 the plats at the time of planting corn and cotton the following 

 faring. That there is a difference in the run-off from the different 

 plats is proved by the results of determinations shown in Table II. 



On February 16, 191 2, three days after a rain of 3.3 inches, soil 

 samples were taken on one plat of oats and on five fallow plats where 

 the length of time since plowing varied from 3 to 18 months. Table 

 II shows the moisture content at the last sampling before the rain and 

 again three days after the rain, together with the increase in moisture, 

 the run-off in inches, and the percentage of rainfall lost by run-off. 



On February 26, samples were again taken on the same plats after 

 a 2-days' rain of 2.9 inches. The results are also given hi Table II. 



Table II. — Absorption and run-off from 'rains in February, 1912, San Antonio Experi- 

 ment Farm. 





Fallow period 

 or crop. 



Samples taken on 



Feb. 16, three days after a 3.3-inch rain. 



Plat No. 



Average moisture 

 content in 3 feet. 



Increase. 



Run-off. 





5 days J 3 days 

 before j after 

 rain. 1 rain. 



Per cent. 



Inches. 



Percent- 

 Inches, j ape of 

 i rainfall. 



A4-1 



3 months 



Per cent. Per cent. 

 15.8 19.9 



19.1 i 21.6 



17.2 ; 19.8 



20. 23. 

 18.4 | 20.6 



18.1 | 22.3 



4.1 

 2.5 

 2.6 

 3.0 

 2.2 

 4.2 



1.92 

 1.17 

 1.22 

 1.40 

 1.03 

 1.96 



1.38 41.8 



A4-2 





2.13 ' 64.5 



A4-3 



5 months 



2.08 ' 63.0 



A4-4 



A4-5 



18 months 



6 months 



1.90 . 57.5 

 2.27 ! 68.8 



A4-6 



Oats 



1.34 ; 40.6 



