Question
We have started pelargoniums from seed. They have grown fantasticly and we have fertilized with a non-ammonium (nitrate) fertilizer on regular basis (200 ppm per watering).

6-8 leaves have developped and growth has been good and steady. I recently transplanted from 606's to 4" square pots. The transplant went well and growth continued, except now 2 days later the bottom leave of a few of them is half way yellow. like someone drained the color out of it.
(true leaf not cotyledon).

What could cause this ?
Transplant chock ?
Too dry soil during transplant ? (i watered thoroughly right after) ?
lack of fertilizer ?

My gut feeling tells me to water the plants well one more time in case some of the transplant mix was not quite wet enough ?

Have you ever experienced this ? e.g. bottom true leaf turning yellow from the edge and halfway inwards towards the stem ?

Tks
Kenneth  

Answer
Kenneth, when fertilization is not the issue, the primary cause of seedlings turning yellow is a depletion of nitrogen due to overwatering.

Seedlings should only be watered when the top 1/3 of the soil gets dry.  Excess watering not only depletes the soil of various nutrients, but also leads to certain fungus diseases specific to immature seedlings.  If this is the case, apply a 1/4 diluted solution of plant starter fertilizer for a quick boost.

Two other possibilities include excess fertilization and root binding.

Excess fertilization, especially with chemical or synthetic fertilizers, results in a salt buildup in the soil.  Excessive salt will neutralize the nutrients and can even burn the roots of the seedlings.  That is why I only use fish emulsion on young seedlings.  It's organic composition ensures that the plant will not suffer from salt buildup or go into shock from excessive nitrogen.

Plants becoming root-bound is caused by the seedlings simply outgrowing their individual cells or pots.  Once the roots become tangled and start wrapping around the perimeter of the soil, the roots can no longer absorb sufficient amounts of nitrogen and phosphorus, both needed for good growth and healthy plants.  To counter this condition, lightly scratch the root ball to loosen the roots prior to transplanting.  

I hope this gave you some ideas.  If I were to speculate, however, I find that the most common of these problems is overwatering, which deprives the plants of oxygen and nitrogen.

The good news is that the seedlings will recover with no adverse effects.  Losing their true leaves will not impede their future growth or affect the overall health of the plants.

Good luck, and please write again if I can ever be of assistance.

Regards,

Mike