Tomato Seedlings Have Light Colored Leaves

[Gerald51]

How can you tell if the very light colored leaves on your tomato seedlings is caused by, not enough sun light, over watering or not enough fertilizer?

Gerald

[dice]

You could try an eyedropper of dilute fish emulsion on the leaves.
See if they turn greener (if they do, they probably need a little
fertilizer).

[dice]

Interesting Dice. I always assumed the nutrients in fish
emulsion had to broken down to be available. I know the
nitrogen in urine (dilute 20x) is of the immediately available
type. Fish emulsion is the same then?

Judging by the response of plants to a soil drench with fish
emulsion, I would say that mostly they do need to be worked
on by bacteria/fungi in the soil before the nutrients in the
fish emulsion become available to the plant, that fish basically
supplies slow release nitrogen. It can take a week or more
before the plants seem to react visibly to the added fish
fertilizer.

However, there is apparently more than one type of nitrogen
source in most fish emulsions (fish emulsions generally list
ammoniacal nitrogen, soluble nitrates, and insoluble nitrogen
proportions on their label): since the leaves do seem to turn
greener a day or two after spray feeding with dilute fish
emulsion (if they do not already have dark green leaves),
I can only conclude that at least some of the nitrogen is
immediately available to the plant and can be absorbed
through foliage.

This fits with the comment at the beginning of
http://www.ars.usda.gov/SP2UserFiles...rUreaNMPRO.pdf
that

Leaves can absorb inorganic and
organic nitrogen sources. Small
pores within leaf cuticles can take
up urea, ammonium and nitrate.
These pores are lined with nega-
tively charged molecules. Therefore,
uptake of cations (such as ammoni-
um) is faster than anions (such as
nitrate). Uptake of small, uncharged
molecules, like urea, is fast.

I tend not to have fertilizers around that contain urea, so
how well that may work as a foliar feed is a moot point for me.
When foliar feeding tender seedlings in particular, the slower
rate of absorption of the mostly nitrate nitrogen in fish emulsion
is not necessarily a disadvantage. It can be a safety feature.