S06 - Session P2 - Optimal spacing distance in open air hydroponics of leek is determined by auction and seedling price

Authors: Tomas Van de Sande *

Open air hydroponics (Deep float technique, DFT) of leek allows a substantial yield-increase. This is caused by an increased growing speed and can be further increased choosing the optimal spacing density. In soil bound leek growing, leeks are planted in rows. Within rows, plant distance is often 10 cm. The distance between the rows can vary between 60 an 75 cm depending on several causes. On average per hectare, about 160000 seedlings are planted. When leeks are grown hydroponically, spacing density can be drastically increased and net yield will increase accordingly until a maximum is reached at about 1000000 plants/ha. Economically this maximum is not the most desired scenario. Selling prices (auction system,€/kg) of leeks highly depend on leek diameter. Small leeks (- 2 cm) are not desired and generate low prices. Medium sized leeks (2-3 cm and 3-4 cm) generate the best prices. Large leeks (+ 4 cm) generate intermediate prices. Furthermore, the price of seedlings becomes a large variable cost with increasing spacing density. In three consecutive experiments lasting a little over a year, leeks were grown hydroponically on a DFT system. Leeks were transplanted on floats with 4 different spacing densities. Nutrient supply was sufficient and identical for all floats as was the use of crop protection products. At harvest leeks were cleaned, sorted in diameter classes and weighted. Combining the obtained yields with the average weekly selling prices per diameter class (2013 - 2017, REO auction), a theoretical revenue was calculated for each harvest and for every float. The relation between optimal spacing density and theoretical revenue was a second degree equation (R²=0.672). For current market prices of 0.03€/seedling, the economically optimal spacing density is 605688 plants/ha. Furthermore, there was a linear relation between optimal spacing density and cost price of seedlings.