2 BULLETIN 151, IT. 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 "nortKers" 
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 in 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 l 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 i-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 obtahied 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 
1 Field Operations of the Bureau of Soils, 1904. 
