|dc.description.abstract||Wasabi (Wasabia japonica (Miq.) Matsumara) is a new crop for New Zealand and its fertiliser requirements are unknown. New Zealand growers of soil-grown wasabi apply large quantities of organic matter which is expensive and may immobilise N. Organic matter is applied to improve soil physical properties, in particular allowing more oxygen to transfer to the root zone. If free draining soils are selected for wasabi production then applied organic matter may not be necessary, and could possibly be replaced with inorganic fertiliser, which may result in large cost savings to the wasabi industry in New Zealand. Large quantities of gypsum are also applied. Gypsum applied as the sulphur component is thought to increase the quality of wasabi.
Three experiments at Lincoln, Canterbury, New Zealand examined the yield and quality response of ‘Daruma’ wasabi to different nitrogen and sulphur treatments. The first six month glasshouse experiment determined that 25% gravel: 75% soil was the appropriate free draining experimental medium to grow wasabi in and that at least 400 kg N ha⁻¹ was required to optimise top dry weight yield.
The second (main pot) and third (field) experiments were run concurrently between May 1999 and November 2000 in a shadehouse. The main pot experiment was a 5 x 5 factorial with N rates from 0 to 840 kg ha⁻¹ and S rates from 0 to 1120 kg ha⁻¹. The highest total biomass, stem dry weight (SDW) (405 g m⁻²) and number of stems over 30 g fresh weight (FW) (exportable) were all obtained from 420 kg N ha⁻¹. This treatment also produced a linear growth rate of 3.9 g m⁻² d⁻¹ over the 18 month crop cycle.
For the field experiment the treatments were organic matter at the commercial (178 t ha⁻¹) and half commercial rate and gypsum at the commercial (4 t ha⁻¹) and half commercial rate. However, stem yield (SDW 364 g m⁻²) was unaffected by halving the rate of applied organic matter. It was concluded that applied organic matter could be substituted with applied inorganic N fertiliser in an Eyre shallow fine sandy loam soil medium amended with 25% gravel.
High quality for wasabi relates to high isothiocyanate (ITC) concentration. The highest ITC concentration measured in the stem was 2486 ppm in the main pot experiment from an application of 560 kg S ha⁻¹, 380 days after planting (DAP). The ITC concentration decreased by 20% over the last six months of growth. The ITC concentration was three to four fold higher in the stem and roots compared with the leaves and petioles.
The average stem ITC concentration was 2280 ppm at final harvest for the field experiment. The ITC concentration in all plant parts was unaffected by halving the rate of applied gypsum and organic matter in the field experiment.
It was concluded that at this site, to obtain a high SDW, 420 kg ha⁻¹ of N fertiliser was required. In addition, an acceptable ITC concentration in the stem was obtained from 560 kg ha⁻¹ of S. These results suggest that current commercial rates of fertiliser application could be halved without adversely affecting yield or ITC concentration.||en