Spring weather is finally breaking and vegetable transplants are being readied for field planting. We received a grower’s call about a leaf abnormality in some greenhouse grown tomato plants.
“The transplant tomatoes I’ve got hardened off, ready to go in the field have dead spots on the leaves.
What’s going on?”
We looked the plants over…
The tender leaves appeared like they had sunscald injury…and, it had been intensely sunny the few days prior.
(Ag Agents tend to keep a mental note – or sometimes even a journal – on the weather. Occupational OCD).
A closer inspection of the potting mix made us do a double take: granular fertilizer. Talking to the grower a bit more revealed the real cause of the problem; a hasty decision was made to use a product on-hand rather than making the trip to the farm supply to buy the correct product. Granular fertilizers work well for pre-plant applications in the field, where a massive amount of soil buffers the salts in the fertilizer. In a transplant pot, the small amount of media mix will not protect young roots from salt injury.
Tomato transplant: note the granular fertilizer.
Electrical Conductivity Meter.
Our field diagnosis was confirmed in the lab. Removing media from the root areas of the pot (not the top where fertilizer was still visible), we used a truncheon nutrient meter to determine the electrical conductivity in the soil – a measurement of salinity. It’s old school, soil salinity has been measured using electrical conductivity for more than 100 years. Salts in water reduces its water potential making it less available to the plant, thereby causing water stress in the plant. Water stress equals desiccated/damaged plant tissue. Salts in soil can come from the fertilizer we apply, but also from irrigation water and dissolved soil minerals naturally present. In this case, using too much of the wrong fertilizer created electrical conductivity levels in the potting mix more than triple what they should have been.
Using the Right Product for the Job
Using a dilute liquid fertilizer in transplant mix containers during transplant growth is the way to go. Caution must be taken not to overuse starter fertilizers on young transplants. High rates can cause burning of roots and leaf margin scorch. Even using recommended rates can cause injury if drought conditions occur when transplants are set out. When using starter fertilizers, irrigate the crop to reduce plant stress.
Getting the Best Performance from Transplants
During the hardening off process, fertilizer, temperature, and water is limited to “toughen up” the plants to ready them for harsh field conditions. Many growers will make one last liquid fertilizer application within a day or two of setting plants in the field. Is this the best option? Back in the day, before plastic and drip irrigation, having a “hungry” plant going to the field was thought to encourage increased growth after transplanting.
However, research conducted by Infante-Casella and Garrison points toward a better method and time to feed new transplants.
- Transplants that received the complete starter fertilizer in the trays just prior to planting had restricted root growth (roots stayed near the root ball where the starter fertilizer was applied) when evaluated two and four weeks after transplanting.
- Transplants with no starter fertilizer applied to growing media, but instead in the water wheel transplant water, had roots that quickly foraged into the soil. By using a complete starter fertilizer (N-P-K) in the transplant water lettuce and pepper plants showed increased root growth and earlier maturity.
