## 4 Discussion  
Soil pH directly affects the growth of plants, and the soil with weak acidity is more suitable for the growth of tobacco [39; 40]. The pH of tobacco planting soil in seven towns of Debao County is generally at an appropriate level, which is conducive to the development of tobacco agriculture. The contents of soil organic matter, total N, available P, exchangeable Ca and exchangeable Mg and other nutrient factors are moderate. The average content of soil available K (270.5 mg/kg) is almost five times the average level of the national tobacco planting soil (57.5 mg/kg). As tobacco is a typical K-loving crop, K has an important impact on the growth, development and quality of flue-cured tobacco [41]. Compared with the national tobacco planting soil, the high soil K in the tobacco area of Debao County is more conducive to producing high-quality tobacco. The effective zinc content of the tobacco planting soil in the town is at a very high level, and the effective B content is slightly insufficient. The application of trace element fertilizer B can be slightly increased to improve the effective B content of the local tobacco planting soil.  
Based on the LightGBM model, the nutrient status of tobacco planting soil is evaluated. The accuracy of the model is 94.2%, the classification accuracy is high and the reliability is strong. The feature value of available K is the largest, its contribution rate to the evaluation of soil nutrient status is the largest and its importance is the strongest in the tobacco planting area. This also corresponds to the high content of available K in the tobacco planting soil in the study area. In the evaluation of nutrient status, Yantong Town, Du'an Town, Dongling Town and Jingde Town have good nutrient status, and the county is generally at or above the medium level, accounting for 81.03%, which has good tobacco planting value and is conducive to the development of tobacco industry. Some soil nutrients are slightly insufficient. In the future, we can improve the soil nutrients by formulating reasonable and scientific fertilization measures to provide more fertile soil conditions for the planting of tobacco crops.  
Applying the integrated algorithm to the evaluation of soil nutrient status, the multi-classification nonlinear mapping relationship between nutrient factors and nutrient status can be established. The regular and clear evaluation indexes improve the sorting performance of training samples and make the final sorting prediction more scientific. In this study, LightGBM, as an upgraded and improved version of XGBoost, obtained more accurate classification and recognition ability through its own full training. Automatic data processing can make the evaluation results of nutrient status of tobacco planting soil in the study area more objective and accurate. In addition, due to the reliability and flexibility of the LightGBM model, it can be widely used in the evaluation of various types of soil nutrient status and fertility, in order to provide more innovative research methods and means for the development of new agriculture in the 21 st century. At the same time, the LightGBM model also provides a new and innovative method for the research of other issues outside the agricultural field.