Effects of long-term single application of chemical fertilizers and their combined application with organic fertilizers on total salt content and ion composition in the surface soil of greenhouse tomato cultivation

Objectives The effects of single chemical fertilizer application and the combined application of chemical fertilizer with different amounts of organic fertilizer on the total salt (TS) content and ion composition of surface soil were studied, aiming to provide a theoretical basis for fertilization management to mitigate soil secondary salinization in greenhouse tomato cultivation.

Methods This study was based on a long-term tomato experiment established in 2012 in Shenyang, Liaoning, using brown soil as the test soil. Treatments were set up by cultivation combining organic fertilizer at rates of 15000 (M1), 45000 (M2), and 75000 (M3) kg/hm² with each NPK treatment (M1NPK, M2NPK, and M3NPK). Except for the control treatment, the annual application rates of N, P₂O₅, and K₂O in all other treatments were 375, 225, and 450 kg/hm², respectively. Soil samples (0−20 cm) were collected in 2023 during the pre-ploughing period (S1), seedling period (S2), first and fourth fruit expansion periods (S3 and S4), and after uprooting of tomatoes (S5). The soil samples were analyzed for TS content, pH, and the contents of salt ions (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, and NO₃⁻).

Results Compared with CK, the NPK treatment significantly reduced soil pH, while the other treatments had no significant effect on soil pH. Fertilization significantly increased soil TS during the five sampling periods, and this increase was in proportion to the application rate of organic fertilizers. The M1NPK treatment reduced soil TS by 43.7% at S2 relative to the NPK treatment during the five sampling periods, whereas the M3NPK treatment increased soil TS by 39.4% at S4 compared with NPK treatment. In all treatments, the total contents of soil water-soluble cations and anions increased, with Ca²⁺ and SO₄²⁻ being the main components, respectively. The total contents of soil water-soluble cations and anions in the treatments with combined NPK and organic fertilizer application increased with the rise in the application rate of organic fertilizer. During the fallow season (S1 and S5), water-soluble K⁺ and SO₄²⁻ were important ions affecting soil TS, and the contents of water-soluble K⁺, Ca²⁺, NO₃⁻, and SO₄²⁻ were significantly positively correlated with soil TS (P<0.01), while soil TS was significantly negatively correlated with soil pH (P<0.05). The relative contribution rate of water-soluble K⁺ content to soil TS was the largest (31.2%), followed by that of SO₄²⁻ (12.7%). During the growing season (S2, S3, and S4), water-soluble Ca²⁺ and K⁺ were important ions affecting soil TS. The contents of the eight salt ions were significantly positively correlated with soil TS (P<0.05), and the water-soluble HCO₃⁻ content was significantly positively correlated with soil pH (P<0.001). The proportions of water-soluble Ca²⁺, K⁺, and HCO₃⁻ increased in the treatments with combined NPK and organic fertilizer application, while those of NO₃⁻ and SO₄²⁻ decreased. The relative contribution rates of water-soluble Ca²⁺, K⁺, NO₃⁻, and SO₄²⁻ contents to soil TS were relatively large, at 31.6%, 18.3%, 12.9%, and 12.5%, respectively.

Conclusions The important ions influencing soil TS were K⁺ and SO₄²⁻ during the fallow season, while they were Ca²⁺ and K⁺ during the growing season. Long-term continuous application of chemical fertilizers, especially those containing urea, significantly promoted soil TS accumulation. Co-applying an appropriate amount of organic fertilizer effectively reduced the buildup of water-soluble ions, thereby alleviating secondary salinization and acidification. However, since commercial organic fertilizers are inherently rich in salt, excessive application increased rather than decreased soil TS, ultimately leading to soil secondary salinization. In the brown soil region, under the condition of annual application rates of 375 kg/hm² of N (urea), 225 kg/hm² of P₂O₅ (superphosphate), and 450 kg/hm² of K₂O (potassium sulfate), the combined application of 15000−45000 kg/hm² of organic fertilizer can effectively reduce the risk of secondary salinization in greenhouse tomato cultivation.