Effects of nitrogenous fertilizer application rates and intervals and prunings management on soil properties, mature leaf nutrients and tea yields in Kenya

Abstract

High tea production depletes soil nutrients through harvesting. Leaching, surface run-offs and fixation also degrade soils. To improve soil quality and tea production, lost nutrients are replenished. In Kenya, Nitrogen Phosphorus Potassium Sulphur (NPKS) fertilizer application at rates between 100 and 250 kgN/ha/year is recommended in single/split doses. Nitrogen application improves tea yields but degrades soils. Single annual application may be uneconomical. Biennial NPKS application gives similar yields as annual NPKS application under short term trials. It is not established if such interval of fertilizer application can influence soil and leaf nutrients to levels that can sustain high tea yields. Pruning rejuvenates growth and maintains plucking tables of tea bushes. Prunings left in-situ may return nutrients to soils to improve tea yields and soil quality. Farmers in smallholder tea sector have been getting low yields probably due to removal of prunings. It is unknown if removal of prunings reduces soil nutrients to levels that influence leaf nutrients and yields. This study assessed effects of rates and intervals of NPKS fertilizer application and pruning management on soil and mature leaf nutrients and tea yields in Kenya. Trial involved clone 12/12 in Kangaita (Kirinyaga) and seedling tea in Timbilil Estate (Kericho). NPKS was applied at rates (0, 60, 120, 180 kgN/ha/year) and intervals (12, 24months) in 4x2 factorial arrangement (NxF) with NPKS rates split for pruning management (prunings removed/left in-situ) in randomized complete block design, replicated thrice. Soil pH was determined digitally; soil and leaf N by Kjeldahl method; other soil and leaf nutrients (P, K, Ca, Mg, Mn, Al, Fe, Cu, Zn) spectrophotometrically. Yields were recorded over two years. Correlations were determined using Pearson’s product moment (p≤0.05). Soil pH levels significantly decreased with increasing fertilizer rates at Timbilil implying that high rates of NPKS fertilizer increase soil acidity. While soil N levels at Kangaita increased with increasing NPKS rates, soil N levels at Timbilil reduced. Other soil nutrients levels responded sporadically to NPKS rates with high CVs hence no conclusive trends were established. Mature leaf N levels and tea yields significantly increased due to increasing rates of fertilizer application in both sites. Biennial NPKS application significantly decreased soil N levels at Timbilil while increasing levels of soil N at Kangaita. However, the longer interval of fertilizer application significantly reduced mature leaf N levels and tea yields at Kangaita. At depth 0-15 cm, prunings left in situ increased (p≤0.05) levels of soil N, Ca, Mg and Mn at Kangaita and Al at Timbilil. Mature leaf Ca and Mn levels and tea yields increased (p≤0.05) due to prunings left in situ at Kangaita.Individual soil nutrient levels were not related to their levels in mature leaves of tea. Tea yields were positively correlated to Zn and Cu levels in lowest soil depth at Kangaita but negatively correlated to soil Al at depth 0-15 cm and Zn at depth 15-30 cm at Timbilil. It is recommended that NPKS fertilizer be applied at rates upto 120 kg/ha/year but still rates upto 100 kg/ha/year could be adequate. Immediately after pruning, tea prunings should not be removed from tea farms for improved soil quality and tea yields. Mature leaf analysis be used in establishing nutrients demands of tea. This study has shown that NPKS fertilizer application at rates upto 120 kg/ha/year can sustain high yields and reduce cost of production. Prunings left in situ improve soil quality and would reduce amounts of NPKS fertilizer requirements for tea. This would reduce costs while creating a more sustainable farming system.