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  • Author or Editor: Mohamed Errebhi x
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Applying appropriate rates of nitrogen fertilizer during the growing season for potatoes on irrigated sandy soils is an important concern from both a production and environmental standpoint. Although potatoes on sandy soils are responsive to nitrogen fertilizer, high rates of nitrogen applied early in the growing season have been associated with nitrate leaching due to unpredictable rainfall. Use of lower nitrogen rates applied more frequently through the season is one strategy to minimize nitrate losses and improve nitrogen use efficiency. Portable nitrate electrodes were used to measure nitrate concentrations in petiole sap. Diagnostic criteria based on final yield and nitrate sap concentrations at various growth stages were developed over a three year period. This rapid test can now be used to make an immediate assessment of nitrogen status of the plant and a prediction for whether supplemental nitrogen will be needed. On-farm trials are currently being carried out to demonstrate the use of the saptest as a best management practice.

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An important aspect of establishing critical sap nutrient concentrations for diagnostic purposes is to determine the accuracy of the analytical method used. We compared a Cardy flat membrane NO3 electrode, a Hach portable NO3 electrode, and a Wescan N analyzer for their ability to determine NO3 concentrations in sap of potato (Solanum tuberosum L.) petioles. The Hach and Wescan instruments require diluted sap, while nondiluted sap can be used with the Cardy. Nitrate-N concentrations in nondiluted petiole sap measured with the Cardy electrode were 90 to 120 mg·L–1 higher than the other two methods. Using sap diluted with 0.075 m aluminum sulfate tended to lower Cardy NO3 readings to concentrations closer to the other methods, but made the procedure more complicated for practical use. We also compared a Cardy K electrode with flame emission spectroscopy for determining K concentrations in sap. Using nondiluted sap with the Cardy procedure resulted in K concentrations 200 to 2500 mg·L–1 lower than those determined by flame emission, depending on K concentration of the sap. Diluting sap with 0.075 m aluminum sulfate or deionized water for use with the Cardy electrode resulted in K concentrations similar to those determined by flame emission.

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An important aspect of establishing critical sap nutrient concentrations for diagnostic purposes is to determine the accuracy and precision of the analytical method used. We compared a Cardy flat membrane NO3 electrode, a Hach portable NO3 electrode, and a Wescan N analyzer for their ability to determine NO3 concentrations in sap of potato petioles. The Hach and Wescan instruments require diluted sap, while nondiluted sap can be used with the Cardy. Nitrate-N concentrations in nondiluted petiole sap measured with the Cardy electrode were 100 to 200 mg·liter–1 higher than the other two methods. Using sap diluted with 0.15 M Al2(SO4)3 tended to lower Cardy NO3 readings to concentrations closer to the other methods, but made the procedure more complicated for practical use. We also compared a Cardy K electrode with flame emission spectroscopy for determining K concentrations in sap. Using nondiluted sap with the Cardy procedure resulted in K concentrations 1000 to 1700 mg·liter–1 lower than those determined by flame emission. Diluting sap with 0.15 M Al2(SO4)3 for use with the Cardy electrode resulted K concentrations similar to those determined by flame emission. Implications for using the electrodes for diagnostic purposes will be discussed.

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