A Response of Cassava to Magnesium Supplementation and Three-Year Applications of Gypsum in a Tropical Loamy Sand Soil Supplementation

Both calcium (Ca) and magnesium (Mg) are essential nutrients for plants; however, a high Ca application hinders the availability of Mg in the soil, which can possibly decrease crop yield. A 3-consecutive-year (2019–2021) field trial was carried out in a loamy sand soil in northeast Thailand objectively to investigate cassava's growth and yield responses to Ca (0, 50, 100 and 200 kg ha−1) from gypsum which in the first two years there was no Mg added while, in 2021, 0 and 50 kg Mg ha−1 were added to each main plot. For the 2019 and 2020 growing seasons, cassava responded best to the 100 kg Ca ha−1 addition. Calcium applied for the first 2 years increased the soil Ca-to-Mg ratio beyond 8:1; thus, Mg was added to narrow this ratio in the third growing season. This resulted in cassava responding more positively to the combined Ca-Mg than to the sole Ca when compared at each Ca rate, but with no statistical difference among Ca-Mg-combined treatments. The Ca50Mg50 treatment, however, gave better yields, nutrient uptake, and Ca-Mg use efficiency. The Ca and Mg balance was surplus in all treatments, with Mg showing a lesser extent than Ca, especially in the plots with no Mg addition. Our finding indicated that gypsum was an economical Ca source, and the amount can be lowered due to the Ca accumulation from previous applications, and added Mg can narrow the Ca-to-Mg ratio to sustain cassava yields.