Response of tomato seeds fluid-drilled in low-phosphorus growth media to phosphorus incorporation in the carrier gel

Abstract

Germinated ‘Heinz 1350’ tomato seeds ( Lycopersicon esculentum Mill.) were fluid-drilled in peat-vermiculite prepared with or without 1.2 kg 0-9-0 m −3 (high-P and low-P media, respectively) using Natrosol 250HHR or Laponite 445 carrier gels containing 0, 10, 20 or 30 g l −1 of NaH 2PO 4, NH 4H 2PO 4 or KH 2PO 4. Percentage emergence was unaffected by P status of the growth medium and was little affected by salt species or concentration in either gel. Emergence rate, however, decreased as gel P concentration increased. Increasing gel P concentration did not increase seedling shoot dry weight in high-P media. In low-P media, however, 10–30 g l −1 of all salts in either gel, compared to 0 g l −1, gave an average 4.2-fold increase in shoot dry weight, with 10 g NH 4H 2PO 4 l −1 gel yielding the greatest shoot dry weight. In a further glasshouse study, germinated tomato seeds were fluid-drilled into low-P media using Natrosol or Laponite gels containing 2700 mg P l −1 as either monobasic (Na, NH 4, K) phosphate salts, or N + P + K salts (KH 2PO 4 + (NH 4) 2SO 4, KH 2PO 4 + NaNO 3, or proprietary 10-6.6-8.3). Low percentage emergence with KH 2PO 4 + NaNO 3 in both gels and with 10-6.6-8.3 in Laponite was associated with high gel electrical conductivity. N + P + K in gel, compared to single monobasic P-salt inclusion, did not give greater seedling growth. The cationic species associated with the phosphate did not significantly influence percentage emergence, shoot dry weight or shoot P concentration. All P-bearing gels gave greater growth and P concentration of shoots than the fluid-drilled (zero salt) control. The 22% greater shoot dry weight with Natrosol than with Laponite was not associated with increased shoot P concentration.