Why Are the Tomato Leaves Yellow?

In the vegetable garden, some of lower leaves on the tomato plants have turned yellow. Why?

To answer that question, I researched and observed.

Research indicates the possible causes, in roughly descending order of frequency:

  • too little water
  • too much water
  • too much scorching sunlight and extreme heat
  • too little sunlight
  • pests
  • soil too acidic (PH)
  • soil too basic (PH)
  • weird chemicals in the soil

And don’t forget:

  • leaves yellow with old age
  • leaves yellow when it turns cold

Well, everything’s clear now. Not.

My observations:

  • The day I found yellow leaves on all three tomato plants, the dirt was dry down to about an inch. One time, one day, the day after they got soaked from a thunderstorm, I figured I didn’t need to water.
  • They’ve gotten regular water (not quite enough to run out the drainage holes in the bottoms of the containers every day for weeks. But some more leaves are yellow now.
  • This week it rained a lot, but the yellow leaves started before this week. The dirt is soaking wet but there’s no standing water because the drainage holes work.
  • This week it was cloudy a lot with highs in the 70’s.
  • The weather so far this season has been mild, maybe up to the mid 80’s.
  • The plants get about 9-10 hours of direct sunlight, which is optimal for growing vegetables.
  • I can’t see any bad bugs. I found two (harmless) spiders, one ladybug, and one ant. I assume the ant had a friend out of sight (cause when is there ever see just one ant?) I saw bumblebees and some sweatbees pollinating. None of the leaves look chewed or have holes. The plants are literally surrounded by anti-pest companion plants of radishes and marigolds.
  • It’s good potting soil, a store bought, PH neutral, compost with a little slow-release fertilizer.
  • There are more yellow leaves now than two weeks ago, but it’s still only a few.

It’s all becoming clear now. Not.

Most of the tomato leaves look very healthy, and they’re growing well.  They’re staked, and the tallest ones are at the four foot mark.  Lots of tomatoes have set on, lots are big for cherry tomatoes (though still green), and there are lots more blossoms. The yellow leaves are at the bottom of the plants, so they are the oldest and maybe getting the least light. Tomatoes put out smallish scattered leaves, so I just don’t see how the top ones would block enough light to matter. I snipped enough leaf lettuce for a salad, and pulled a few of the largest radishes I’ve ever grown. The beets are struggling to get their leaves into the sunlight below the squash and eggplants, so probably I won’t get any beets. The crookneck squash and eggplants have blossoms, and are much taller than the beets.  They aren’t as tall as the tomatoes (which is par since they grow a little slower.)

Action Plan:

  • Continue as I am because I don’t know what to do differently.
  • If you’ve eaten something today that you didn’t hunt, gather, or harvest yourself, then thank a farmer. The only thing clear here is that farming is tricky.
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