A METHOD TO CALCULATE THE SIZE DISTRIBUTION OF ONIONS AND ITS USE IN AN ONION GROWTH-MODEL

A mathematical method is described and evaluated that enables size gra de distribution of onions to be calculated based on the total yield an d plant density. The method is based on a logistic distribution functi on for cumulative wt.% of which the two parameters are related to mean bulb weight. The calculation method is developed on the basis of data from seven trials with different plant densities, sowing times, varie ties, years and cropping methods and subsequently compared to independ ent data originating from 16 onion trials. On average the calculated w t.% for different size classes underestimated the observed data betwee n 1.2 and 6.7% depending on the size class. The underestimation was mo st observed in the wt.% range between 0 and 50%. Crops with heavy atta cks of leaf diseases, were not predicted correctly. The calculation pr ocedure was combined with a dynamic onion growth model. This combinati on was used to calculate the optimum plant density depending on the si ze grades desired and can thus be a tool to calculate the financial op timum plant density. (C) 1998 Elsevier Science B.V. All rights reserve d.