Field and Laboratory Evaluation of the Cardy Nitrate-Nitrogen Meter for Measuring Soil Solution Nitrate Nitrogen in Hawaiian Soils

The estimation of plant-available nitrate nitrogen (NO₃-N) is essential for any nutrient-management plan but can be time-consuming and expensive. However, the efficacy of rapid methods to determine soil NO₃-N levels designed for grower use has received mixed reviews in the literature. Therefore, the objectives of this study were to (1) evaluate the Cardy electrode-based meter for measuring soil solution NO₃-N concentrations under a perennial peanut living mulch in two mixed orchard systems on O'ahu and (2) determine the influence of soil type on measurement accuracy and precision under laboratory conditions. To achieve the first objective, 24 lysimeters were installed 15–30 cm deep at each of two fruit tree orchards with different soils and climate on Oahu island. For the second objective, a replicated column study was conducted, in which NO₃-N solutions of varied concentrations were leached through three representative agricultural soils (Wahiawa, Loleka'a, and Waialua series). Field soil solution and column leachate were analyzed using the portable electrode-based meter and a standard laboratory colorimetric method. In the field samples, soil solution NO₃-N ranged from <1 to 110 mg/L, and there was a strong correlation (r² = 0.92) between the portable meter and colorimetric values. Similarly, a strong correlation between the Cardy meter and the laboratory methods was observed in the column study, although r² values varied with soil type. The data suggest that the Cardy meter can be used to rapidly and accurately measure soil solution NO₃-N, if its concentrations are relatively high and concentrations of interfering ions such as chloride (Cl⁻) are low. Overall, the primary value of this rapid method may be in estimating relative changes in soil nitrate in response to nutrient management at a single site.