Potassium management in tea plantations: Its uptake by field plants, status in soils, and efficacy on yields and quality of teas in China

Tea is one of the major cash crops in the tropical and subtropical areas of China. Insufficient potassium (K) supply is an important limiting factor to the productivity as the soils are highly leached and strongly acidic. However, information about effects of K fertilization and application techniques is very limited. This manuscript summarizes results of field experiments investigating K uptake, soil K status, effects and methods of K fertilization in China during the past two decades. The K stocks and uptake were investigated by soil sampling in plantations aged 1, 2, 3, 5, and 10 years. There was no harvest of young shoots, and most K was assigned to frame growth accounting for 59%–61% of the total uptake in the aboveground plant parts of young plantations aged 1–3 years. In plantations (≥ 5 years) such assignment to frame growth became relatively small (7.6%–11.9%) and a major proportion was assigned to young shoots accounting for up to 78% of the total. Analysis of 3396 soil samples taken in 2009–2010 from 54 counties of 16 main tea‐producing provinces showed that the exchangeable K (extracted by Mehlich III) in soils averaged 81 mg kg–¹ and in about 74% samples were below critical deficient level (100 mg kg–¹). The low activity ratio at equilibrium (AR ᵏ ₑ) and nonspecific absorbed‐K (ΔK₀) values of quantity‐intensity (Q/I) curves also indicated low status of labile K pools of tea soils. Field experiments at 16 of total 18 sites with black, green, and oolong teas during 1992–2002 showed a significant increase of yield after K application either as potassium sulfate (K₂SO₄) or potassium chloride (KCl) despite of the largely different initial soil exchangeable K contents, plucking standards, and yield levels among the tea types. The agronomical efficiency of K fertilizers was averaged at 8.8 kg fresh shoots or 1.71 kg tea per kg K fertilizer. The quality of harvested shoots was improved by K‐fertilizer application as revealed by increased concentrations of free amino acids, water‐extractable dry matter, and total polyphenols. There was little difference in the effects of K as K₂SO₄ and KCl on yield and quality. Field experiments at six sites evaluating variable K amounts demonstrated that the optimal K doses ranged from 124 to 160 kg K ha–¹ y–¹ for both K₂SO₄ and KCl and were little affected by initial soil K contents, tea types, and the yield levels. No privilege of K applications split into three times in tea seasons was observed concerning the risk of leaching in the soils of low cation‐exchange capacity and abundant rainfall in the production areas. A single K application as base fertilizer in autumn had similar or better effect. The present studies showed low soil labile K pools, incapable to meet plant K demand for quality tea production, highlighting the importance of K‐fertilizer application.