129 High Nitrate Fertilizers Control Shoot Growth through Low Phosphate Stress

Abstract

The greenhouse industry successfully uses high NO3 fertilizers to produce plants with short, compact shoots. It is commonly assumed that NO3 results in compactness while NH4 or urea stimulate large shoot growth. However, high NO3 fertilizers contain little or no phosphate. Four sets of treatments were applied to five species of bedding plant plug seedlings in two experiments to differentiate the effects of N source vs. phosphate supply on growth. Seedlings were established on 20-4.4--16.6 fertilizer until 10 days into stage 3, when the following treatments began. Set 1: phosphate-P was held at 22 mg/L and total N at 100 mg/L with NH4 comprising 40%, 13%, 7%, or 0% of total N, the remaining being NO3. Differences in shoot size did not occur as a consequence of the shift in NH4:NO3 ratio. Set 2: N was supplied at a concentration of 100 mg/L from 40% NH4 plus 60% NO3 while PO4-P was varied over the series of concentrations of 21.9, 6.6, 3.3, and 0 mg/L. Set 3: the same as Set 2 except that N was supplied entirely as NO3. Height and weight of shoots in Sets 2 and 3 were positively related to PO4 supply. Set 4: three commercial fertilizers containing 0 PO4-P and 8, 13, or 20% of N in the NH4 form. Compact shoots developed in these treatments. When 22 mg phosphate-P/L was added to one of these fertilizers, compactness was reversed. Shoot suppression by high NO3 fertilizers was concluded to be a function of low phosphate and not N form.