Objectives The effects of nitrogen (N), phosphorus (P), and potassium (K) fertilizers on soybean yield, intrinsic quality, and soil nutrient contents were compared using a long-term field experiment. The aim was to address the issues of soybean yield and quality enhancement and soil quality preservation of the black soil region of Heilongjiang Province.

Methods The field experiment started since 1990, the used soybean cultivar was Dongsheng 17. Five fertilizer application treatments were established: no fertilizer (CK), P and K fertilizer (PK), N and K fertilizer (NK), N and P fertilizer (NP), and N, P and K fertilizer (NPK). The soybean yield, and the content of P, K, Ca, Fe, and isoflavone in grains were measured, and the organic matter, total and available N, P, and K contents in soil were systematically analyzed.

Results Compared to CK, all the fertilizer treatments increased soybean yield, with the increase ranging from 5.1% to 18.6%. Fertilization increased grain nutrient and isoflavone contents significantly, except for P. The nutrient and isoflavones increase effect of the treatments was in descent order of NPK > NP > NK > PK. Compared to CK and two-fertilizer treatments, NPK treatment increased the grain Ca, Fe, K, and isoflavones contents by 6.6%−12.6%, 19.9%−54.6%, 8.8%−16.3% and 14.1%−58.0%, respectively. The soil under NPK treatment had similar organic carbon and total N contents to those under all the other treatments, but significantly higher alkali-hydrolyzable N, total and available P and K, with a maximum increase of 80.0%. Among them, the available phosphorus content under NK treatment, as well as the total potassium and readily available potassium content under NP treatment, resulted in significant reductions of 44.4%, 14.7%, and 29.2%, respectively, compared to the NPK treatment. Among the four fertilizer treatments, PK produced the lowest yield and benefit of soybean production, which were even not significantly higher than CK. The agronomic efficiency of nitrogen fertilizer in the experiment was as high as 14.7 kg/kg, which was much higher than that of phosphorus and potassium fertilizer (1.1 kg/kg and 1.3 kg/kg). The addition of N fertilizer promoted the economic benefit significantly.

Conclusions In the black soil region of northeast China, nitrogen fertilizer application is necessary to soybean production, balanced application of N, P and K fertilizers can improve the yield and economic benefits of soybean production, enhance soybean quality, and effectively improve soil fertility.