SOIL TEMPERATURES IN THE UNITED STATES

DATA WERE COLLECTED ON SOIL TEMPERATURES BY WRITING TO ALL THE AGRICULTURAL EXPERIMENT STATIONS AND EXAMINING THE AVAILABLE LITERATURE ON THE SUBJECT. VARIATIONS IN THE CONDITIONS UNDER WHICH THE SOIL TEMPERATURES WERE TAKEN ARE DISCUSSED. VARIATIONS OCCUR IN THE YEARS, MONTHS, OR DAYS OF RECORD, THE METHOD OF TAKING THE RECORD AND OF COMPILING TABLES FROM IT, THE KIND OF SOIL, DEPTH, SOIL COVER, AND EXPOSURE. A STUDY OF THE TABULATED RECORDS AND GRAFTS SUGGEST A NUMBER OF CONCLUSIONS: (1) AIR AND SOIL TEMPERATURES NEAR THE SURFACE VARY IN A FAIRLY PARALLEL MANNER, (2) THE SOIL TEMPERATURES AT SLIGHT DEPTHS ARE GENERALLY HIGHER THAN THE AIR TEMPERATURES THROUGHOUT THE YEAR, (3) THE DIURNAL RANGE IN SOIL TEMPERATURES EXTENDS TO A DEPTH OF ABOUT THREE FEET, (4) THE ANNUAL RANGE IN SOIL TEMPERATURE IS QUITE APPARENT AT A DEPTH OF TEN FEET, THE GREATEST DEPTH FOR WHICH A RECORD IS OBTAINABLE IN THE UNITED STATES, (5) THE LAG OF MAXIMUM AND MINIMUM SOIL TEMPERATURES INCREASES WITH DEPTH, (6) A COVER CROP LESSENS THE DIURNAL AND ANNUAL TEMPERATURE RANGES, (7) IN THE WINTER TIME, NORTHLY STATIONS WHERE THE SNOW COVER IS MORE OR LESS PERMANENT SHOW HIGHER MEAN MONTHLY SOIL TEMPERATURES THAN STATIONS SOMEWHAT FARTHER SOUTH OR WEST BUT LACKING A GOOD SNOW COVER, (8) THE PRESENCE OF MOISTURE IN THE SOIL TENDS TO GIVE A LOW AND UNIFORM TEMPERATURE, (9) SOIL TEMPERATURE AND ITS ANNUAL RANGE DECREASES WITH ALTITUDE, (10) LOAM, CLAY, AND PEAT SOILS NEVER BECOME AS WARM IN SUMMER AS THE DRYER GRAVEL AND SAND SOILS, AND (11) SOUTH EXPOSURES AT ANY ALTITUDE HAVE HIGHER TEMPERATURES AND A GREATER RANGE THAN NORTH EXPOSURES.