A new onsite soil analytical probe as an effective tool for applying nutrients at the right time

The root zone of high-density orchards is relatively small while the root density is high resulting fast depletion of nutrient sources in the period of intensive uptake of nutrients by plants. Checking the actual nutrient level of the soil would be an effective way of controlling over- or undersupplying, however, measuring soil nutrient content using classical analytical methods is labor intensive and expensive, therefore frequent soil testing is not applied in the practice. An onsite soil analytical probe has been developed to direct check of soil available nitrogen, phosphorus and potassium content. The probe contains two unique designed optical diffuse reflective spectroscopes measuring in the 420-730 nm intervals. The patented light collecting part of the sensor provides the required sensitivity. The measuring units are located at the depth of 15 and 45 cm. The probes were calibrated using laboratory standards for the nutrient intervals typical in orchard soils (e.g., 100-800 mg kg^‑1 phosphorus expressed as P₂O₅). After verification of the probe under laboratory conditions it was tested in isolated soil monoliths, which were fertigated and irrigated periodically to generate rapid nutrient level changes. The probe values exactly followed the nutrient concentration changes determined by lab analysis. The field test was carried out in a high-density apple orchard, where 12 probes were installed for 5 months and soil samples were taken in every two weeks to control the probe values. The probe values and lab results correlated well. In spring of 2019 a set of probes were installed in 10 different commercial orchards representing different soil conditions to determine the necessary number of probes depending on soil types and heterogenity. Based on the first field tests, the new onsite soil analytical probe would be an effective tool for plant nutrition, especially in intensive cultures with high nutrient demand.