Alfalfa was grown on Superstition loamy fine sand on the Yuma Mesa for the 4-year period, 1949–52, to evaluate the effects of moisture and phosphate variables on hay production and associated factors. The three moisture treatments were based on tension levels, tensiometers being installed at 9 depths from 6 to 120 inches. A total of about 125,000 tensiometer readings were recorded during the 4 years. Ten phosphate (superphosphate) rates and/or frequencies varying from 100 to 1,300 pounds P2O5 for the 4-year period were employed as subplots in the split plot design.
Average annual hay yields for the 4-year period varied from 6.4 to 12.3 tons per acre, being greatly affected by either moisture or P level. Small frequent irrigations and an initial fertilizer application of more than 200 pounds P2O5 plus 100 pounds P2O5 annually appeared to be the goals to approach. Phosphorus movement was largely limited to the surface 18 inches, which was less than expected. As measured by tensiometers the depth for the greatest relative activity of roots decreased with decreasing moisture tension, increased with decreasing supplemental phosphate, and increased with increasing temperatures.
Soil sampling during a 32-month period revealed that for “dry”, “medium”, and “wet” treatments respectively, 80, 74, and 27% of the available moisture and 72, 82, and 85% of the total water utilized by the plant was removed from the surface 48 inches of soil. During a 3-year period the mean water use per day varied from 0.21 to 0.25 inch, the greatest amount being used in the “wet” or low-tension treatment. Water used per ton of hay produced varied from 7.1 to 16.5 inches, high rates of moisture and phosphate giving greatest efficiency.
The alfalfa stand thinned rapidly, correlation analysis indicating a reduced stand was an important cause for declining yields. Root grading analyses at the termination of the experiment demonstrated that moisture and phosphate levels had no effect upon root shape and nematode infection. Moisture and/or phosphorus had significant effects on crown and root weight, root diameter, amount and distribution of feeder roots, and crown- and root-rot. A barley crop was grown following the alfalfa to measure residual productivity levels due to previous moisture and phosphate variables. The only significant difference was caused by moisture, the “dry” treatment resulting in the greatest grain yields.