Abstract:

Proper soil nutrients content plays an important role in agricultural production—undernutrition would reduce crop yield and quality and overnutrition would cause environmental pollution. Though the traditional approaches based on sampling and chemical analysis can comprehensively and accurately measure soil nutrients, but the soil sampling and pretreatment process are cumbersome, complicated, time-consuming, and costly. Therefore, rapid and accurate measurement of soil nutrients is of great significance for precise fertilizer application, which can increase yield, improve crop quality, and alleviate environmental pollution. Toward this objective, a rapid soil nutrients detection method based on modulated near infrared spectroscopy for active near-field telemetry was proposed, which could effectively minimize effect of sunlight during the measuring process. Eight channels narrow-band laser diodes with wavelengths of 1260, 1310, 1350, 1410, 1450, 1510, 1550 and 1610 nm were selected as active lighting sources for measuring the reflectance of soil samples. Eight channels narrow-band laser diodes were symmetrically placed on a concentric circle. A photodetector with a circular photosensitive area of 5 mm in diameter was placed at the center of the concentric circle to maximize the reception of laser beam reflected by soil. A focusing lens was placed in front of the photodetector to collect the laser beam reflected from the soil sample to increase the sensitivity. The sensing area of the photodetector was located at the focus of the lens. seventy four groups of soil samples with known N content were divided into training set (54 groups) and prediction set (20 groups) for data analysis. The spectral reflectance significantly correlated with soil N content was screened by analyzing the training set based on a general linear model and a quantitative measurement model with R² of 0.97 between the screened spectral reflectance and soil N content was achieve. The predicted soil N content obtained from prediction set based on the established model and the referenced soil N content of the prediction set had a R² of 0.9, indicating that this method has an ability to quickly, as well as accurately detect soil nutrients.

Key words: soil nutrients, near infrared spectrum, near-field telemetry, lock-in amplifier, photoelectic detection