Boron Distribution and the Effect of Lime on Boron Uptake by Pansy, Petunia and Gerbera Plants

Reports of boron (B) deficiency have become more prevalent in pansy (Viola ×wittrockiana), petunia (Petunia ×hybrida), and gerbera (Gerbera jamesonii) plug production. When symptoms are observed in production the presence and severity of symptoms have no obvious pattern; symptomatic plants can be located adjacent to asymptomatic plants in the same plug flat. The availability of B in the soil decreases as soil pH increases. ‘Yellow Dynamite’ pansy, ‘White Storm’ petunia, and ‘Festival Apricot’ gerbera seedlings were grown in a peat:perlite substrate that was amended with pulverized dolomitic limestone at a rate of 3,560, 4,746, 5,933, or 11,866 g/m. As the rate of lime incorporated into the substrate increased so did soil pH and shoot tissue concentrations of Ca while shoot tissue concentrations of B decreased. Many commercially available germination substrates are amended with macroand micro-nutrient fertilizers as fertilizer granules. Relatively small amounts of each element are incorporated into the substrate and uneven mixing could result in varying amounts of nutrients in individual cells of a plug flat. This variability could lead to nutrient deficiencies early in the crop cycle. Six commercially available germination substrates were used to fill plug flats and individual cells were sampled and analyzed for B concentrations. Of the substrates tested, there was no variation in B concentration from cell to cell. INTRODUCTION Boron (B) is available to plants as boric acid [B(OH)3] (Marschner, 1995). As with most other elements, boron availability is pH dependent and is more readily available with pH values <6.0 (Gupta, 1993). The pKa of B is at pH 9.25, therefore B is found as boric acid at lower pH values and as unavailable borate at higher pH values. The application of lime has been observed to cause temporary B deficiency in plants (Goldberg, 1993). The pH of a substrate may increase over time due to the basic effects of alkalinity in irrigation water or the practice of liming substrate. Irrigation water alkalinity would only account for the occurrence of boron deficiency late in the production cycle due to the lag time required for the alkalinity to increase the substrate pH. In addition, if one would expect this to be the primary cause of boron deficiency, its prevalence should be much greater in the Midwest US and US Plains states where high alkalinity is a problem and practically nonexistent in the Southeast US where the water is of low alkalinity. Observations of pansy, petunia and gerbera crops over the past several years do not support this; severe incidents have been seen at a large North Carolina greenhouse with excellent water quality (pers. commun., Sim McMurray, Metrolina Greenhouses, Huntersville, N.C.). Liming of the substrate, if done well in advance of sowing, could cause boron to become unavailable earlier in the production cycle. The use of sphagnum Proc. IS on Growing Media and Composting