Increased Yields from Peppermint Crops through Improved Micro nutrient Nutrition (Stage 2)
Peppermint yields in Southern Australia are highly variable and this is impacting on the long term viability of the industry. The average yield is approximately 50 kg / ha, despite attempts to improve management practice. Peppermint is also an important crop that provides the necessary throughput to sustain the viability of field distillation units.
Micro nutrient deficiencies have been observed in peppermint namely iron (Fe), zinc (Zn), copper (Cu) and manganese (Mn) (Zeinali et al., 2003, Zheljazkov and Warman, 2004). These authors observed that responses in peppermint were similar to some vegetable crops such as potato, spinach, tomato and lettuce. Menary (1967) made a similar observation about the similarly between red beet and peppermint. Further studies in solution culture have demonstrated essentiality for boron (B), cobalt (Co), aluminium (Al), zinc (Zn) and copper (Cu) (Bode, 1940).
Boron is the one micro nutrient which produces a range of development and physiological effects on peppermint. Fischer and Bussler (1984) have shown that boron deficiency influences the development of oil glands and hairs, oil accumulation and vascular bundle development and their extension. It has also been observed that boron is most effective at high concentrations (Nandi and Chatterjee, 1986). In addition, Boichenko (1970) has demonstrated field responses in peppermint to applications of boron (B).
Micro nutrient responses influence oil quality and yield in decreasing order from nitrogen (N), phosphorous (P), potassium (K) to sulfur (S) (Baird, 1957). In recent years the tendency in fertiliser practice is to use high analysis fertilisers which are low in sulfur. Field grown crops in Tasmania often display a bright crimson colour which is characteristic of sulfur deficiency. This observation highlighted the need to include sulfur (S) in the study.
Peppermint yields as high as 140 kg / ha have been achieved in Tasmania using a double cut system (Clark and Menary, 1984). In the last 10-15 years there has been a general decline in vigour of crops and a double cut has not been possible.
A preliminary study was undertaken to establish the presence of nutritional disorders in peppermint crops in Tasmania (Menary, 2005). Soil and plant analysis surveys were undertaken in all production areas to measure macro and micro nutrient levels. The survey indicated that deficiencies of sulfur (S), molybdenum (Mo), Boron (B), and Copper (Cu) could occur. Plant symptoms expressed were also consistent with those described in the literature for peppermint and related crops (Weir and Creswell, 1993) Based on these preliminary observations a fertiliser mixture containing granulated forms of micronutrients was formulated in consultation with a major fertiliser company and made available to growers in 2005 (Menary, 2005).
During the 2006 / 2007 seasons greenhouse trials were undertaken to establish critical levels of nutrients in peppermint shoots. As well, a field trial was conducted over two seasons to assess and monitor nutrient responses in terms of oil yield and composition and dry matter production. The soil at the field site was a clay loam which was deficient in sulfur (S), boron (B), copper (Cu), molybdenum (Mo) and zinc (Zn). This combination of greenhouse and field trials was considered adequate to establish a diagnosis and fertiliser recommendation system for peppermint production.