Abstract: The problems of soil quality decline, nutrient imbalance, and unreasonable fertilizer application are becoming increasingly prominent in the grain fields of the central Hebei Plain, seriously restricting the improvement of grain production capacity. Clarifying the controlling factors and distribution characteristics of soil nutrient barriers is of great significance for improving farmland quality and increasing production capacity. Based on soil data from 2005 to 2019, with organic matter, total nitrogen, available phosphorus, and available potassium as evaluation indicators, we constructed an evaluation index system for soil nutrient barriers in grain fields in the central Hebei Plain by using the boundary line method to explore the types and distribution characteristics of soil nutrient barrier factors. The results showed that under crop relative yield (nutrient-deficient plot/optimized fertilization plot) thresholds of ≥95%, 90%-95%, 75%-90%, and <75% as standards, the barrier degree of organic matter (wheat season: ≥16.2, 15.0-16.2, 11.3-15.0 and ＜11.3 g·kg^-1; maize season: ≥15.5, 13.6-15.5, 8.1-13.6 and ＜8.1 g·kg^-1), total nitrogen (wheat season: ≥0.8, 0.7-0.8, 0.3-0.7 and ＜0.3 g·kg^-1; maize season: ≥0.9, 0.8-0.9, 0.4-0.8 and ＜0.4 g·kg^-1), available phosphorus (wheat season: ≥18.7, 15.6-18.7, 6.1-15.6 and ＜6.1 mg·kg^-1; maize season: ≥22.0, 17.4-22.0, 3.8-17.4 and ＜3.8 mg·kg^-1) and available potassium (wheat season: ≥113.1, 95.8-113.1, 44.1-95.8 and ＜44.1 mg·kg^-1; maize season: ≥117.3, 97.5-117.3, 38.3-97.5 and ＜38.3 mg·kg^-1) in grain fields of the central Hebei Plain could be divided into four levels: barrier free, mild, moderate, and severe. Based on the obstacle level evaluation index, the farmlands in the central Hebei Plain mainly exhibited mild obstacles (low available potassium) and moderate obstacles (low organic matter, total nitrogen, available phosphorus). The nutrient barrier factors differed across crop planting seasons. The main barrier factor in wheat season was low organic matter, which accounted for 55.1% of the cases. Those in maize season were low organic matter, total nitrogen and available phosphorus, and the proportion of samples were 53.8%, 53.2% and 54.0%, respectively. Across the whole region, the degree of nutrient barriers increased gradually from west to east. Among the four sub-regions (Baoding, Langfang, Cangzhou, and Shijiazhuang), the degree of nutrient barriers was the lightest in Shijiazhuang and the most serious in Cangzhou. In the production process of grain fields, chemical fertilizer inputs should be optimized and the application of organic fertilizers should be increased according to the types of nutrient barriers, crop types and spatial distribution characteristics, so as to reduce nutrient barriers and improve the production capacity.

Key words: boundary line analysis method, yield, soil nutrient, barrier factor