Effect of Gypsum on Lettuce in Marana, 1988

Gypsum was applied preplant on spring lettuce at rates of zero, 2 tons per acre, and 4 tons per acre. There were no significant differences in carton yields or in carton weights. Postharvest soil analyses showed that the application of four tons of gypsum per acre significantly increased the calcium and magnesium levels in the saturated paste solution. There were no significant effects of gypsum application on ESP and SAR values, nor in pH or sodium concentrations. INTRODUCTION The history of this test site did not indicate particular problems with high sodium levels in the soil or with water infiltration. However, there was interest in finding out whether a gypsum application could benefit spring lettuce if heavy fertilizer applications had led to soil permeability problems. MATERIALS AND METHODS The test site is at the southwest corner of the intersection of Central Arizona Project aqueduct and Avra Valley Road in Marana. The previous crop was upland cotton; the soil type is Tubac clay. Gypsum was applied in 60 -foot wide swaths across the field, aligned with the crop rows. Application rates were zero (check), 2 tons per acre, and four tons per acre. The experimental design was a randomized complete block with 4 replications. Yield data were taken by measuring off 50 -foot lengths of row, and then counting and weighing the lettuce cartons packed by a trio (two cutters and one packer) as they made their 4 -row pass through the 50 -foot long area in the center of the plot. Soil samples were taken post -harvest and analyzed for pH, electrical conductivity, Na, Ca, Mg, exchangeable sodium percentage, and sodium absorption ratio. RESULTS AND DISCUSSION Carton yields per acre showed no significant response to gypsum applications at the 95% level of probability, though the 2 tons per acre treatment had the highest numerical yields (Table 1). Carton yields varied greatly within the plots; this undoubtedly contributed to the high coefficient of variation (Table 1). Average carton weights were not significantly different (Table 1). The market price was low at the time, and the harvest crews were selecting only the heaviest heads. Because of this, carton yields per acre varied more than carton weights. This selective harvesting technique could have masked any treatment differences. The trend