The summer maize yields and partial factor productivity of nitrogen (N) fertilizer (PFP_N, grain yield per unit N fertilizer) on smallholder farms in China are low, and differ between farms due to complex, sub-optimal management practices. We collected data on management practices and yields from smallholder farms in three major summer maize-producing sites—Laoling, Quzhou and Xushui—in the North China Plain (NCP) for two growing seasons, during 2015–2016. Boundary line analysis and a Proc Mixed Model were used to evaluate the contribution of individual factors and their interactions. Summer maize grain yields and PFP_N ranged from 6.6 t ha⁻¹ to 14.2 t ha⁻¹ and 15.4 kg kg⁻¹ to 96.1 kg kg⁻¹, respectively, and averaged 10.5 t ha⁻¹ and 49.1 kg kg⁻¹, respectively. The mean total yield gap and PFP_N gap were 3.6 t ha⁻¹ and 43.3 kg kg⁻¹ in Laoling, 2.2 t ha⁻¹ and 24.5 kg kg⁻¹ in Xushui, and 2.8 t ha⁻¹ and 41.1 kg kg⁻¹ in Quzhou. A positive correlation was observed between the yield gap and PFP_N gap; the PFP_N gap could be reduced by 6.0 kg kg⁻¹ (3.6–6.6 kg kg⁻¹) by reducing the yield gap by 1 t ha⁻¹. The high yield and high PFP_N (HH) fields had a higher plant density and lower N fertilization rate than the low yield and low PFP_N (LL) fields. Our results show that multiple management factors caused the yield gap, but the relative contribution of plant density is slightly higher than that of other management practices, such as N input, the sowing date, and potassium fertilizer input. The low PFP_N was mainly attributed to an over-application of N fertilizer. To enhance the sustainable production of summer maize, the production gaps should be tackled through programs that guide smallholder farmers on the adoption of optimal management practices.

Potassium (K) is the second most important plant nutritional element and is used for numerous physiological processes. We established an eight-year experiment comparing the effects of five K fertilization treatments (0, 48, 84, 120 and 156 kg K ha⁻¹) on crop yield, K use efficiency and soil apparent K balance under the maize–wheat rotation system in the North China Plain. The highest maize and wheat yields were achieved in the K₁₂₀ treatment, increasing by up to 16.7% and 25.1%, respectively. The increase in grain yield and K agronomic efficiency (AE_K) with K application was greater in wheat than in maize. The K recovery efficiency (RE_K) and K accumulative recovery efficiency (ARE) significantly decreased with the increase in K fertilization in the maize and wheat seasons. However, the soil apparent K balance and soil available K content increased; the former was deficiency (−24.3 kg ha⁻¹ yr⁻¹) in the K₀ treatment, but the latter did not decrease significantly compared with that in the initial year of the experiment. The soil available K content increased by 10.9 mg kg⁻¹ per 100 kg ha⁻¹. In conclusion, the yield response to K fertilization was greater in wheat than in maize and alleviated soil K depletion, but the K fertilizer efficiency was lower. We believed that K fertilizer can be increased moderately in the wheat season and decreased in the maize season.