Potential for Using Plant Xylem Sap to Evaluate Inorganic Nutrient Availability in Soil: II. Comparison between the Xylem Sap Method and the Extraction Method

The “extraction method” that is commonly used to evaluate the availability of nutrients in soil often involves complex, problematic and simultaneous assays of multiple components and comparisons of analytical values. In a previous paper using pot experiments of plants, we confirmed the possibility of using nutrient concentrations in the xylem sap of Luffa cylindrica Roem. seedlings as an indicator of nutrient availability in soil and suggested that the “xylem sap method,” might represent a solution of the above-mentioned problems and inaccuracies characteristic of chemical extraction. To determine whether a method enables to evaluate available nutrient contents in soil, the criterion should probably be that the relationship between the values obtained by that method and the amounts of nutrients absorbed by plants does not vary depending on the types of soil. In this regard, we compared the conventional extraction method with our proposed xylem sap method. In the present paper, we attempted to change the cation exchange capacity (CEC) in soil by the addition of zeolite and/or humic acid to soil to change the nutrient availability without changing the total quantity of nutrients in soil in the pots. Our results showed that the amounts of nutrients absorbed by plants changed under these treatments. However, in some cases, changes in the amounts of phosphorus, calcium, magnesium, and zinc obtained via different extractants did not correspond to the changes in the amounts of nutrients absorbed by the plants. Thus, the amounts of nutrients absorbed by the plants were compared with nutrient concentrations in the xylem sap and the amounts of nutrients inferred by each extractant to examine their relationship. A highly positive correlation was consistently found between the amounts of nutrients absorbed by the plants and their concentrations in the xylem sap for every nutrient. By contrast, there was a lower correlation for the extraction method with some extractants. For phosphorus in particular, no close correlation was obtained between the amounts of nutrients absorbed by the plants and the amounts extracted, regardless of the extractants. In the present study, because we attempted to change the nutrient availability by artificially controlling the soil CEC value, without changing the total amount of nutrients present in the culture medium, the changes in nutrient availability were not conspicuous. Thus, it is possible that the extraction method itself generates large errors for such small changes. Therefore, we concluded that the xylem sap method is superior to the extraction method, as an indicator of nutrient availability in soil, at least under the conditions of the present experiment.