Spatial-Temporal Variability of Soil Nutrients and Assessment of Soil Fertility in Nanyang Tobacco Planting Areas

The representative tobacco planting soil samples of Nanyang were separately collected in 2000 and 2020 by the fixed-point sampling method, and were used for analyzing the temporal and spatial variation characteristics of soil nutrients and evaluating soil fertility. The results showed that, the pH value of the tobacco soil decreased by an average of 1.26 units from 2000 to 2020, and the average content of available phosphorus and water-soluble chlorine decreased by 11.39% and 36.47%, respectively; Meanwhile, the average organic matter and alkali-hydrolyzable nitrogen increased by 19.69% and 90.92%, respectively. However, the average content of available potassium remained the same level. After 20 years’ continuous utilization, the soil fertility of tobacco planting fields in Nanyang had been significantly improved, the fertility index of tobacco-planting soil increased from 0.46 to 0.57 and the suitability index level increased from V to III. Moreover the medium and higher grade tobacco planting soil had occupied 70.70% of the total area. In 2020, soil organic matter, available phosphorus and available potassium content of tobacco planting fields in the city were all lower than the appropriate range, respectively less than 55.87%, 42.21% and 63.09%, and 52.13% of the soil water-soluble chlorine content was higher than the threshold. According to the temporal and spatial evolution of soil fertility and the current fertility level, it is recommended to adopt measures of "supplementing phosphorus and potassium, increasing application of organic fertilizer, and reducing chlorine" to improve soil quality.