ABSTRACTWe conducted quantitative point count determinations of phosphorus, potassium, calcium, and magnesium by electron microbeam analysis and scanning electron microscopy of soil-root interfaces with soil-root samples from 30-day-old peanut and soybean plants, grown in Davidson and Cecil soil. Scanning electron micrographs showed undistorted cellular details of the plant roots embedded in the soil mass. Electron microbeam counts revealed that the soil contained lower concentrations of P, K, and Ca than the root tissue. The Mg concentration showed a tendency to be higher in the soil than in the roots. An ascending ion concentration gradient was noticed in the soil from 0.40 to 0.10 mm from the root surface. Nevertheless, a slight decrease in ion concentration was observed 0.05 mm from the root, which was attributed to the presence of a mucilage layer bridging the soil with the root surface. On the basis of a sharp break in element distribution with distance to the root, we estimated the soil rhizosphere to be 0.20 mm thick. Keeping this thin layer of soil saturated was thought to be more beneficial than keeping the bulk of soil saturated with nutrients. Specific accumulation zones of K and Ca were noted in root tissue. Highest concentrations of K and Ca were measured in cortex and epidermis, respectively. Lower levels of K, Ca, and Mg were detected in soybean than in peanut roots, indicating different nutrient requirements between plant species. The results of element counts by electron microbeam analysis in peanut and soybean roots were within ranges of concentrations measured by conventional methods with dry-ashing and atomic absorption spectrophotometry.
We conducted quantitative point count determinations of phosphorus, potassium, calcium, and magnesium by electron microbeam analysis and scanning electron microscopy of soil-root interfaces with soil-root samples from 30-day-old peanut and soybean plants, grown in Davidson and Cecil soil. Scanning electron micrographs showed undistorted cellular details of the plant roots embedded in the soil mass. Electron microbeam counts revealed that the soil contained lower concentrations of P, K, and Ca than the root tissue. The Mg concentration showed a tendency to be higher in the soil than in the roots. An ascending ion concentration gradient was noticed in the soil from 0.40 to 0.10 mm from the root surface. Nevertheless, a slight decrease in ion concentration was observed 0.05 mm from the root, which was attributed to the presence of a mucilage layer bridging the soil with the root surface. On the basis of a sharp break in element distribution with distance to the root, we estimated the soil rhizosphere to be 0.20 mm thick. Keeping this thin layer of soil saturated was thought to be more beneficial than keeping the bulk of soil saturated with nutrients. Specific accumulation zones of K and Ca were noted in root tissue. Highest concentrations of K and Ca were measured in cortex and epidermis, respectively. Lower levels of K, Ca, and Mg were detected in soybean than in peanut roots, indicating different nutrient requirements between plant species. The results of element counts by electron microbeam analysis in peanut and soybean roots were within ranges of concentrations measured by conventional methods with dry-ashing and atomic absorption spectrophotometry.
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