Effect of nitrogen application and pumice as a soil amendment on the growth of wasabi plants, (Wasabia japonica.): A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Horticultural Science with Honours

Abstract

Wasabi plants grow naturally in stony riverbeds, in free running water but can be grown in soil provided it is well drained and well aerated. Wasabi growth responds well to nitrogen (N) fertiliser application, but optimum rates of N fertiliser were unknown. A pot trial experiment was carried out on wasabi (Wasabiajaponica) to determine the effects of five rates of nitrogen (N) fertiliser (0, 150, 300, 450, and 600kgN/ha) and four soil media (100% soil; 25% soil and 75% pumice; 50% soil and 50% pumice; 25% soil and 75% pumice). Pumice additions were intended to increase the porosity and aeration of the media. Measurements leaf size (leaf width and petiole length) and number of leaves and side shoots per plant were taken. SP AD measurements were taken and correlated positively with leaf chlorophyll content and leaf N concentration. Plants were weighed at harvest, and fresh and dry shoot weights, and root dry weights were recorded. Increasing the N rate from 0 to 150kgN/ha increased root dry weight from 2.6g to 7.9g but no further increase in root weight occurred as the rate ofN was increased up to 600kg/ha. Top fresh weight increased from 79.6g at OkgN/ha to 210.8g at 150kgN/ha, then increased to 268.0g at 300kgN/ha, after which no significant increase in fresh weight was observed. The 75% pumice medium was the only soil treatment to cause a significant increase in root dry weight, compared with the 100% soil, 25/75 and 50/50 mixes, (8.1g compared with an average of 5.6g). Soil type had a major impact on plant growth in response to N application. Plant dry weights from the 100% soil ranged from 13.3g at 0kgN/ha to 24g at 600kgN/ha, while plant dry weights from the 75/25 mix ranged from 7.4g at 0kgN/ha to 39.4g at 600kgN/ha. It was concluded that the application N fertiliser up to 300kgN/ha greatly increased wasabi dry matter production, but only slight increases were obtained as the level was increased to 600kg/ha. The addition of between 25-50% pumice to soil increased porosity and decreased bulk density, with positive effects on plant growth. Comments are made on differences in plant phenology and morphology, and suggestions are made for further research.