Crop Production and Security in Ningjin County of the North China Plain.

Our results also found that the yields of wheat and maize did not improve but instead showed a decreasing trend in Ningjin County when fertilizer exceeded 550 kg ha −1 , which is consistent with the research results of Zhen et al.

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From the perspective of different years, the wheat yield presented a slightly increasing trend from 2000–2006 at a rate of 1.73%, then showed a rapid increase with a rate of 2.84% in 2007–2016, and after 2016 decreased at a rate of 3.39%.

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the self-sufficiency rate of wheat also showed an increasing trend, from 192.87% to 617.37%, which indicates that wheat and maize can meet food self-sufficiency and the per capita grain yield is in a safe state.

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