Calibration of Soil Test for Mineral Nitrogen in Poland

Abstract

The mineral nitrogen (Nmin) method is generally accepted as the universal soil test for determining crop fertilizer requirements and predicting the possibility of groundwater contamination by excess nitrates (NO3). However, it must be properly calibrated both for agronomic and environmental purposes. Since 1997, the program of soil monitoring for the content of Nmin has been running in Poland. In 5,000 randomly selected sites on arable land, soil samples are collected twice a year (in early spring and autumn) from soil layers 0–30, 30–60, and 60–90 cm deep. The frozen samples are sent to the one of 17 regional agrochemical laboratories and analyzed for the content of nitrogen (N)–NO3 and N‐ammonium (NH4). The results, expressed in kg of Nmin in the given soil layer and/or in the soil profile at 0–90 cm, are collected in the data bank at the Institute of Soil Science and Plant Cultivation, State Research Institute, Puławy. These data are made available for farmers and environment protection services. On the basis of about 75,000 data already collected, the calibration of Nmin method was accomplished, and the results presented as the normatives of Nmin content in spring and autumn. The distribution of data was far from normality, and therefore the median value was used as a characteristic of central point and percentile as a measure of distribution. Similar to the five classes of soil reaction and Phosphorus (P), potassium (K), and magnesium (Mg) contents officially used in Poland, five intervals of Nmin content were proposed. Each interval contains 20% of ordered observations, and thus five classes of Nmin were distinguished: very low, low, medium, high, and very high. Because of the significant Nmin dependency on soil texture (very light, light, medium, heavy), altogether 20 normatives of Nmin are included in the calibration table. In the article, the method of simulation of nitrate concentration in soil solution in the soil layer at 60–90 cm, assuming soil saturation to full water capacity, is presented as well.