Resumen
(1) Background: Previous research has demonstrated that the cation exchange capacity (CEC) of soil and the balance of exchangeable cations Ca, Mg, and K are key factors affecting plant growth and development. We hypothesized that balancing exchangeable cations based on increased CEC would improve plant growth and development. (2) Methods: This study conducted a two-phase experiment to evaluate methods for increasing soil CEC and the effects of increasing CEC and balancing Ca, Mg, and K on plant growth. Therefore, we first conducted a soil culture experiment using organic fertilizer, montmorillonite, and humic acid to investigate fertilizers that can effectively increase CEC in the short term. Then, a tomato seedling pot experiment was conducted using the control (CK) and OMHA fertilizer-treated soils collected from soil culture experiments. The CK and OMHA treatment soils were constructed with balanced exchangeable cations and an unbalanced control, respectively. (3) Results: The soil culture experiments revealed that the combination of organic fertilizer, montmorillonite, and humic acid (OMHA treatment) had the most significant effect on increasing CEC. The CEC of the OMHA treatment increased by 41.07%, reaching 27.10 cmol·kg-1. The tomato pot experiments demonstrated that balancing the exchangeable cations in OMHA soil improved the Mg and K nutrition of tomato seedlings and significantly increased SPAD, leaf nitrogen content, and dry weight, while balancing the exchangeable cations in CK soil improved only the K nutrition of tomato seedlings. (4) Conclusions: Overall, balancing exchangeable cations based on increasing CEC can improve soil nutrient availability and alleviate the competition effects of Ca, Mg, and K cations. Low CEC and imbalanced exchangeable cations can be detrimental to tomato seedling growth.