Accumulation of NPK nutrients tend to decrease the effectiveness of calcium in greenhouse soil in the long term

Objectives Physiological diseases induced by calcium deficiency frequently occur under continuous vegetable cultivation. The accumulation of nitrogen, phosphorus and potassium in vegetable soils affects the forms and availability of calcium, which further impacts the uptake of calcium by plants. In this paper, the differences of calcium availability in greenhouse vegetable soil during various planting years were compared, and the relationships with N, P, and K were studied.

Methods Thirty-eight vegetable greenhouses were chosen for the investigation from Haicheng City, Liaoning province. The planting years lasted from 3 to 30 years. The contents of organic matter, available N, P and K in 0–20 cm deep of soils were analyzed. The contents of water soluble, exchangeable, acid-soluble and non-acid soluble calcium were extracted, and their relations with the concentration of N, P, and K were calculated with linear regression analysis.

Results The contents of soil organic matter, N, P, K, total Ca and non-acid-soluble Ca in all the tested greenhouses had kept increasing with the extending of planting years, the water-soluble Ca contents reached maximum of 104.1 mg/kg on the 9th–11th year and the exchangeable Ca contents reached maximum of 611.9 mg/kg on the 12–14 planting years, respectively, and then declined. About 94.3%–96.4% of total Ca was non-acid-soluble Ca. A significantly negative relationship existed between exchangeable Ca and NH₄⁺-N (r = –0.5451, P < 0.01, n = 38), and available K (r = –0.4809, P < 0.01, n = 38), which indicated that accumulation of N and K promoted desorption of exchangeable Ca. P accumulation accelerated the formation of insoluble calcium phosphate, and the concentrations of octocalcium phosphate and decacalcium phosphate were significantly and positively correlated with that of non-acid-soluble Ca (r = 0.5884, P < 0.01, n = 38). After 30 years cultivation, the ratios of available Ca to available N, P and K in greenhouse soil decreased by 70% compared with their open field controls.

Conclusions Long-term vegetable cultivation leads to the poor soil nutrient balance between available Ca and available NPK contents. The increased N and K mainly stimulate the release of exchangeable Ca by NH₄⁺ and K⁺ into soil solution, while increased P transfers these available Ca to low effective non-acid soluble form.