Effects of long term fertilizer use on a high yielding tea clone S15/10: Soil pH, mature leaf nitrogen, mature leaf and soil phosphorus and potassium

Abstract

Harvesting (plucking) the young tender shoots of Camellia sinensis (L.) O. Kuntze to make tea beverages results in removal of high amounts nutrients from the soil. AHPS15/10 is a high-yielding tea clone with average annual yield of approx. 6,000 kg made tea/ha/yr (kg mt ha-1 yr-1) in a four-year pruning cycle, but has yielded over 10,000 kg mt ha-1 yr-1 when there was good cropping weather. Economic production of the crop can be sustained by replenishing the lost nutrients through addition of fertilizers. NPKS 25:5:5:5 or NPK 20:10:10 are the recommended fertilizers for tea in Kenya. An experiment comparing different rates of the two fertilizer formulations was conducted for 18 yrs to establish changes in yields, leaf nutrients and soil chemical properties. The experiment demonstrated that soil pH declined with increasing nitrogenous fertilizer rates. The NPKS fertilizer reduced the pH to the same extent as the NPK formulations, suggesting that the pH reduction was mainly caused by the nitrogen, not sulphur. The application of increasing rates of the fertilizers increased mature leaf nitrogen (N) but reduced mature leaf phosphorus (P) and potassium (K) and had no effect on leaf sulphur levels. The available P, especially in the upper soil layer, increased with increase in the rates of the fertilizers. This was more pronounced after repeated applications for several years, but less pronounced at the lower soil depths indicating the low mobility of the nutrient element. Despite the increase in available soil P, leaf P declined with increase in the nitrogenous fertilizers rates. The available P levels did not change much with time, suggesting that with decrease in soil pH; applied P was being fixed and made unavailable. Despite the increasing amounts of K in the fertilizers with increasing rates of the NPK fertilizers, there was also decrease in soil exchangeable K with increasing higher rates of the fertilizers. This suggests that K may not be available to the plant when applied concurrently with nitrogen. There seemed to be increased leaching of K with increase in N applied. However, the decline in the uptake of P and K with increasing applied N had no effect on yields. It may be necessary to review the need to apply the NPK fertilizers as compound mixtures. Application of increasing rates on NPKS fertilizer increased the soil available sulphur, but NPK fertilizer had no effect on soil S.