Productivity and resource use in ageing tea plantations

Keywords: Kenya, Camellia sinensis L., clones, seedlings, tea industry, management, N-P-K, biomass, made tea yields. The tea industry in Kenya is rural-based and provides livelihood to over three million people along the value chain. The industry which started in the first quarter of the 20th century has continued to increase in terms of production and total acreage. Tea is grown in prime agricultural and forest land and can be in production for up to 100 years if well managed. However, peak yields are obtained at 20–40 years after planting followed by a decline to a level where the plantations may become degraded and uneconomical. In the past, several hypotheses have been postulated, but the cause of this degradation largely remains unclear. The big question still lingers, is it the tea bush that degrades, the soil or both? In this study, trends in tea yields were first assessed by analysing long-term tea production data, from 1969 to 2006, for the two sectors of the Kenyan tea industry. The plantations are characterized by differences in age and genotypes (seedling or clonal). To explore plausible management options for tea productivity improvement, a simple decision-support (DS) model for Managing Ageing Plantations of Tea (MAP-Tea) was developed and scenario analyses were done to explore some promising management interventions. It was found that uprooting and replanting of degraded old seedling tea plantations with clonal cultivars would be profitable. However, management practices that prevent degradation are most cost effective. The experimental part of the study was carried out during two years, 2002/2003 and 2003/2004, in a chronosequence of existing tea plantations (14, 29, 43 and 76 years old), adjacent to a natural forest in Kericho, Kenya. Soil-plant-environment relations and effects on tea bush productivity, C and N-P-K stocks and soil quality traits were analysed. Younger clonal tea plantations established at high densities outyielded the older seedling plantations with a lower densities. Ageing per se did not reduce the N-response or the productivity of tea plantations. It was shown that seedling tea bushes acquire much higher C and N-P-K nutrient stocks with age than clonal bushes. This may enable seedling plantations to depend less on limiting nutrient(s) supply under adverse conditions, i.e. drought. The top soils of the four tea plantations showed small differences in chemical and biological characteristics, also in comparison to the natural forest. Soil pH and total organic carbon were weakly related to the productivity of the ageing tea plantations. Additions of tea prunings in incubated soils caused immobilization of N and lowered the net N-mineralization compared to the forest soil, but the differences were relatively small and unlikely to cause degradation of the ageing tea plantations. Further improvement in productivity and resource use of ageing tea plantations should come from a better timing and dosing of nitrogen, and from the transition from old low-yielding seedling plantations to modern higher-yielding clonal plantations taking into account Genotype × Environment × Management (G×E×M) relationships.