Suitability of soil electrical conductivity as an indicator of soil nitrate status in relation to vegetable cultivation practices in the Yangtze River Delta of China.

摘要

The suitability of soil electrical conductivity as an indicator of soil nitrate status in relation to vegetable cultivation practices in the Yangtze River Delta of China was studied using soil samples from sites where vegetables were grown in foil tunnels and open fields. Chemical analysis of soils comprised the parameters such as electrical conductivity (EC), pH, NO₃-N, NH₄-N, total N and organic matter. Cultivation practices had no influence on soil pH values. In comparison, EC was significantly higher with 462.5 micro S/cm in soils under foil tunnels than in open fields with a mean value of 251.2 micro S/cm. Approximately one third (35%) of all samples showed secondary salinization with EC values >500 micro S/cm under foil tunnels, and still 9% of all samples from field-grown vegetables. Soil EC values increased with the time period of vegetable growing and exceeded 500 micro S/cm after more than 4 years of continuous cultivation. A highly significant correlation (P<0.001) was determined between soil NO₃-N and soil EC irrespective of the cultivation practice if EC was <500 micro S/cm, while no relationship existed in soils with EC values >500 micro S/cm. The results suggest that NO₃-N is a main factor influencing EC values, but NO₃-N is of minor relevance for the complex phenomenon of secondary salinization as found in soils under foil tunnels. This implies that a reduction of the N fertilizer input is not sufficient to decrease EC values in these production units. Soil EC values might be used as an indicator for excessive NO₃-N soil contents and as a benchmark for calculating site-specific N rates. Thus, crop productivity and quality parameters of vegetables can be improved and environmental burdens, for example N loads to water bodies significantly reduced.