Effect of Rainfall on Applied Gypsum and Its Relationship to the Calcium Demands of Developing Peanuts In the Pegging Zone.

It is well documented that peanut (Arachis hypogaea) requires calcium in the upper 7 cm of soil (pegging zone) for proper seed development. The objectives of this study are to (1) determine the peanut developmental stages that absorb calcium, (2) quantify the amount of rainfall required to leach calcium from the pegging zone, and (3) evaluate the potential for calcium depletion during the developing seed’s demand. Peanuts were planted in a randomized complete block design in Headland, AL and Tifton, GA without irrigation. Gypsum was applied at rates of 0, 600, and 1200 kg ha^-1 40 days after planting to Georgia Green and Georgia-06G peanuts with four replicates of each treatment. Representative peanut and soil samples were harvested bi-weekly and analyzed for calcium.  Prior to analysis, peanuts were sorted by developmental stage using the Williams and Drexler hull scrape method.  To quantify the rainfall required to leach calcium from of the pegging zone, electrical conductivity (EC) probes (ECH₂O^® 5TE, Decagon Devices) were installed 9 cm below the soil surface and within 15 cm of the peanut stem in the field at both sites.  Treatments of 0, 600, 1200, and 1800 kg gypsum ha^-1 were applied to the surface. Each block had four replicates. Rainfall was measured at a weather station adjacent to each field.  The no gypsum treatment was used to normalize EC data from the gypsum-applied plots. Results showed that gypsum can be leached from the pegging zone during a growing season.  Calcium continued to be absorbed by the seed until harvest (i.e., white to black stages of the hull scrape method). When rainfall is plentiful, calcium may not be adequate resulting in reduced yields and/or quality of seed.