|Soil surface horizons in the Southeastern U.S. Coastal Plains region are physically favorable to peanut (Arachis hypogaea L.) production but are often depleted of calcium. Peanuts lacking calcium may form undeveloped pods called “pops” or have poor germination and vigor. Peanuts develop just below the soil surface in the “pegging zone,” and this is where the developing pods acquire calcium. Calcium requirements of peanuts are affected by seed size and genetics. Thus, calcium recommendations need to be continually re-evaluated as production shifts to new varieties. Furthermore, maintaining adequate calcium during high rainfall years is also a challenge due to the poor cation retention in the sandy Coastal Plain soils. The objectives are to (1) evaluate the effect of gypsum rate and calcium source (i.e., gypsum, lime, and liquid calcium fertilizer) on yield and seed quality of current varieties of small- and large-seeded runner peanuts and (2) evaluate the effectiveness of using electrical conductivity (EC) probes to observe gypsum leaching from the pegging zone and to quantify the amount of rainfall required to leach gypsum from the pegging zone of peanuts. All field sites were located at Headland, AL, and Tifton, GA, and research was conducted from 2008 to 2010. Gypsum treatments of 0, 560, and 1120 kg ha-1 were applied to small-seeded Georgia Green (GG) and large-seeded Georgia-06G (06G). Lime and liquid calcium fertilizer treatments were applied to 06G in Headland. Yield, total sound mature kernels (TSMK), seed calcium, and germination were analyzed at harvest. Gypsum generally improved TSMK, germination, and seed calcium in GG and 06G. Liquid calcium and lime treatments did not improve yield and TSMK of 06G above control, but lime did improve germination. Yield response to gypsum treatments varied with initial soil calcium levels for both GG and 06G. Improvement in yield of 06G and GG peanuts above the control plot was observed twice for both varieties when soil calcium was above the critical level (150 mg kg-1) established for traditional runners. This suggests that current recommendations for gypsum application to runner peanuts may slightly underestimate the needs of current runner-type peanuts. Gypsum generally increased germination across a variety of soil calcium levels. This supports the recommendation by the Alabama Crop Improvement Association for seed producers to apply gypsum regardless of pegging zone soil calcium levels. The liquid calcium fertilizer evaluated was not an effective calcium supplement to increase yield, which is in agreement with results of from studies evaluating similar products. Producers may not expect an increase in yield or with lime application on 06G when soil calcium levels are above 252 mg kg-1 or in a drought year.
For the gypsum leaching experiments, rainfall was continuously simulated at 5 cm hr-1 on columns of commercial sand, Dothan loamy sand, and Tifton loamy sand with surface-applied gypsum. Gypsum leaching was monitored by electrical conductivity probes installed 9 cm deep. Leaching of gypsum applied at 560, 1120, and 1680 kg ha-1 was also performed on a Dothan loamy sand. Leaching of gypsum in the field was evaluated in Tifton and Headland in the pegging zone of established peanuts. Probes were horizontally placed 9 cm below the soil surface with gypsum surface applied at 0, 560, 1120, and 1680 kg ha-1. Results of the column study indicate that increasing gypsum application rates did not increase the residence time of the gypsum, but did increase the maximum EC measured. The amount of rainfall required before the peak EC occurred was lower in the sand compared with the loamy sand. Electrical conductivity was near baseline after 12 cm of rainfall in the soil column. This was also observed in field experiments at both sites. Unlike the column experiment, the 1680 kg ha-1 gypsum rate provided an EC response to gypsum for a longer period than the lower rates. Using EC probes is an effective way to determine if gypsum still resides in the pegging zone, but may not be as useful in quantifying the amount of calcium leached as direct analysis of calcium.