2 BULLETIN" 151, U. S. DEPARTMENT OF AGRICULTURE. 



evaporation the effect of the precipitation is much lessened. The 

 mean annual evaporation from a free water surface, as measured at 

 the experiment farm for the 7-year period specified, is 65.88 inches. 

 The winters are mild, yet periods of cold weather or "northers" 

 are not infrequent during the winter season. The thermometer 

 seldom registers a temperature below 15° F. in winter, and conse- 

 quently plant growth continues practically throughout the year. 



SOIL CONDITIONS. 



The San Antonio Experiment Farm is located on what is called 

 locally black "hog-wallow land. " This local name is due to the fact 

 that the soil, when drying, shrinks and opens long, wide cracks, and 

 the filling of these cracks with loose surface soil results hi irregular 

 depressions, which resemble hog wallows. The soil is a black clay 

 loam, having a rather small proportion of sand and becoming very 

 sticky when wet. It is classified by the United States Bureau of 

 Soils * as Houston black clay loam and San Antonio clay loam. 



The first 3 feet of soil is fairly uniform in character and is under- 

 lain with a white gravelly material which is rich in lime. This under- 

 lying gravel has a relatively low moisture-holding capacity, while the 

 surface soil has a high moisture-holding capacity, averaging from 

 25 to 30 per cent. When wet, the soil has a tendency to pack 

 and become impervious, so that during torrential rains the loss of 

 water from run-off is high. The soil is rich in mineral plant food and 

 produces abundant crops when supplied with sufficient moisture. 



FALLOWING EXPERIMENTS. 



In 1910 experiments were inaugurated for the purpose of studying 

 the effect of producing a crop only on alternate years, as compared 

 with producing a crop every year on the same land. The crops of 

 1910 were grown on land which had not been previously fallowed, 

 so that the results for that year are not considered here. The results 

 here presented are from the years 1911, 1912, and 1913. 



The crops used in these experiments were corn, cotton, and winter 

 oats. For this purpose six ^-acre plats were used, as follows: Plats 

 A4-1 and A4-2 were used alternately for cotton, one plat being 

 cropped and the other fallowed each year. In a similar way plats 

 A4-3 and A4-4 were used for corn and A4-5 and A4-6 for winter oats. 

 For purposes of comparison with these biennially cropped plats, use 

 has been made of results obtained from three plats which are part of 

 another experiment. These three plats are cropped each year and 

 are given the same tillage treatment as the alternately cropped plats, 

 except that the fallow period is 12 months shorter. The plats that are 

 cropped annually have been under test since 1909, when the large 



i Field Operations of the Bureau of Soils, 1904. 



