Objectives There exists an antagonistic effect in crop absorption of calcium (Ca) and magnesium (Mg). This study investigates the effects of Mg fertilizer application rates on exchangeable Ca content in crops and acidic soil, as well as on the uptake and distribution of Ca and Mg nutrients in wax gourds. The aim is to optimize Mg fertilizer management for wax gourds in South China.

Methods  A 5-year consecutive field experiment on magnesium fertilizer application rates was conducted at the Guangdong Vegetable Experimental Station using ‘Tiezhu No. 2’ wax gourd as the test vegetable variety. Five magnesium fertilizer levels (expressed as MgO) were established: 0, 30, 60, 90, and 120 kg/hm², and were sequentially designated as Mg0, Mg30, Mg60, Mg90, and Mg120. Wax gourd yield, dry matter, and plant Ca and Mg content were investigated during the wax gourd harvest in 2020 and 2021, while soil exchangeable Ca and Mg contents were determined.

Results With the increase of Mg fertilizer dosage, there was no significant change in wax gourd yield in 2020, while stem, leaf, fruit, and total biomass reached their maximum in Mg90 treatment, which was significantly higher than that in Mg0 and Mg30 treatments. In 2021, the highest yield was observed in the Mg90 treatment, which was significantly higher than the no-Mg treatment, but there were no significant differences among the magnesium treatments in terms of parts and total biomass. With the increase of Mg fertilization, the Mg content and accumulation in stems, leaves, and fruits at the harvesting stage gradually enhanced, and the Mg content in stems, leaves, and fruits in the Mg120 treatment was increased by 20.8%−41.7%, 36.1%−75.3%, and 16.5%−29.4%, respectively, compared to the Mg0 treatment over two years; and the Mg accumulation in stems, leaves, and fruits of Mg90 treatment was increased by 45.7%−127.8%, 74.5%−89.5%, and 27.1%−80.4% respectively, compared to the Mg0 treatment. However, with the increase of Mg fertilization, the Ca content in stems and leaves gradually decreased at harvest, and the Ca content in fruits first increased and then decreased, which in turn led to the change of the total Ca accumulation of the plant in an increasing and then decreasing manner, and the total calcium accumulation reached a maximum at an Mg fertilizer rate of Mg90 treatment in 2020, which was 18.7% higher than that of the no-Mg treatment, while there was no significant increase in total Ca accumulation in wax gourd in 2021. Mg application increased the proportion of Mg allocation in leaves and decreased the proportion of Mg in fruits; on the contrary, the proportion of Ca allocation in fruits showed an increase and then a decrease with the increase of Mg fertilizer dosage, whereas the overall proportion of Ca allocation in stalks decreased and then increased, but with a small range of change. Ca and Mg contents in leaves and stalks showed a significant negative correlation, while Ca and Mg contents in fruits showed a quadratic curvilinear relationship, and Ca and Mg ratios in stems and leaves decreased significantly with the increase of Mg fertilization. In addition, with the increase of Mg fertilization, soil exchangeable Mg and Ca content enhanced significantly; soil exchangeable Mg and Ca content after magnesium application was increased by 30.2%−80.9% and 13.0%−47.1% compared with no-Mg treatment, and they were significantly positively correlated. The Mg content of wax gourd stalks, leaves, and fruits raised with the increase of soil exchangeable Mg content, while the Ca content decreased with the increase of soil exchangeable Mg content, and the sensitivity of leaf Mg and Ca content to the change of soil exchangeable Mg content was higher than that of stalks and fruits.

Conclusions Mg fertilizer application can significantly increase the exchangeable Mg and Ca contents in soil, so stimulate the yield, biomass, and Mg-Ca accumulation of wax gourd. In stem and leaves, the Mg content is negatively correlated with Ca content, while in fruits, the Ca content and allocation are increased first and then decreased, with the highest Mg and Ca distribution rates at MgO 60 kg/hm². So, the MgO application should within 60−90 kg/hm² for maximize yield-enhancing effect of Mg fertilizer in wax gourd production.