Calibration and Categorization of Plant Available Silicon in Louisiana Agricultural Soils.

Calibration of crop responses to applied silicon (Si) serves as a basis for developing Si fertilizer recommendation guidelines.  A soil survey and greenhouse study were conducted to document, calibrate, and categorize plant-available Si in Louisiana soils. Silicon concentrations were determined in soils collected from 27 parishes of Louisiana based on different extraction procedures (0.5 M acetic acid-1 hr, 0.5 M acetic acid-24 hr, 1 M sodium acetate, deionized water, 0.5 M ammonium acetate, 0.5 M citric acid and 0.01 M CaCl₂). In a greenhouse set-up, six soils with distinct physico-chemical properties were processed, potted, and planted to rice. The experiment was set up in a randomized complete block design with five replications, two sources of Si (wollastonite – 23% Si and slag – 17% Si), and four Si rates (0, 170, 340 and 680 kg ha^-1). The relationships of dry matter yield and Si uptake by straw and panicles with soil Si levels using different extractants were evaluated. Compared with other extractants, the soil Si extracted by 0.5 M acetic acid-24 hr and 0.01 M CaCl₂ had higher positive correlation (r² values >0.45) with shoot biomass yield, straw and grain Si uptake. Based on quadratic model (P<0.03), the estimated soil Si critical level using 0.01 M CaCl₂ for Sharkey clay soil was 110 mg kg^-1while for Crowley silt loam and Commerce silt loam, levels were 37 and 43 mg kg^-1, respectively. For 0.01 M CaCl₂ extraction procedure, all surveyed soils (n=212) fell below 56 mg Si kg^-1. For five of the procedures about 50% of surveyed soils in Louisiana had Si level below the critical Si level that thus far established in other regions. Certain regions in Louisiana may likely benefit from Si fertilization especially those areas planted to high Si-accumulating crops such as rice and sugarcane.