Objectives The problems of potassium (K) and magnesium (Mg) scarcity are increasingly highlighted in Latosol of rubber plantations in Southern China. A study of K and Mg deficiency affecting on roots morphology and nutrients absorption of rubber seedlings was conducted to provide theoretical basis for balanced fertilization and high-quality and high-yield rubber cultivation.

Methods A hydroponic culture experiment was conducted and the seedlings of Hevea brasiliensis ‘7-33-97’ were grown as the tested materials in artificial climate chamber. Four treatments were setup, including: control (CK), no K supply (–K), no Mg supply (–Mg), no K and Mg supply simultaneously (–K-Mg). The dry matter, root morphological parameters, root activity, and nutrients content of plant samples were measured after three months of rubber seedlings grown in the treated solution.

Results 1) Compared with CK, the single plant dry matter amounts were significantly decreased by 8.4% and 27.5%, and the root/shoot ratios decreased by 20.4% and 26.9% under –K and –K-Mg treatments, while the dry matter amounts and the root/shoot ratios were not significantly affected by –Mg treatment; –K-Mg treatment accelerated the adverse effects of –K and –Mg on the dry matter accumulation of stems, roots and total plants and root/shoot ratios (P < 0.05). 2) Compared with CK, the root length, root surface area, root volume, total root tip number and absorption activity of root (< 2 mm) were significantly reduced under all the three nutrient deficiency treatments (P < 0.05), but the average root diameter per plant was increased. Variance analysis showed significant interaction-effect between K and Mg on root length, root surface area, root volume, total root tip number of absorption root (P < 0.01). 3) The distribution proportion of N and Mg in leaf, P and K in root, Ca in stem bark were higher than those in other organs. Mean whiles compared to CK, the proportion of shoot to whole nutrients showed an increased trend under nutrient deficiency. 4) Compared with CK, –K resulted in a significant increase of N and Mg accumulation per plant, –K-Mg treatment resulted in a significant reduce of N accumulation per plant, while all the three treatments resulted in significantly increased Ca accumulation. K and Mg had significant (P < 0.05) or extremely significant (P < 0.01) interaction on N, P, Ca and Mg accumulation.

Conclusions Both K and Mg deficiencies will significantly affect plant nutrient absorption, and then stunt the growth and development of shoots and roots of rubber seedlings, and the interaction of simultaneous K and Mg deficiencies will accelerate the adverse impaction. So more attention should be paid on the balanced supply of K and Mg nutrition during the rubber tree production.