Young tomato seedlings will often show signs of nutritional deficiencies in their leaves, and if you know what to look for you can remedy it fairly easily. Whether planting your heirloom tomato in the ground or in a pot, starting off with a healthy plant is the best way of preventing disease in tomato seedlings, and warding off disease that may plague the plant as the summer goes on.
If your seedlings are compact and not leggy, have green leaves, and a short distance between each set of leaves (short internodes), you’re good to go. If somewhere along the line before you’re ready to plant out in the garden your seedlings start to show signs of trouble, treating it right away will often save the health of the plant.
Most vulnerable time in preventing disease in tomatoes
If you are starting your own tomatoes from seed, there are 3 keys to preventing disease in tomato seedlings:
- Use clean potting soil rather than garden soil. You can purchase good garden soil from most stores. Miracle Grow Moisture Control is a good one. If you want to go completely organic with your potting soil, see this post for a formula.
- If you start your seedlings in a flat with small cells, or a small container, pot up your plants when they have two sets of true leaves. I go from a 196-cell flat to a 4″ pot to the garden and it has been a successful gradation for 15 years now. There is not much soil in a small cell, hence the nutrient supply is rapidly depleted from the growing plant.
- Carefully control your watering regime. Dryness and water logging can both make it difficult for plants to take up soil nutrients. I water the seedlings once a day or less, taking my cue from how dry the soil looks (i.e., if the soil is light in color, then water; if it is dark it is still moist and doesn’t need more water). I also use an indoor watering hose to water, as you can regulate the amount of water you’re giving the little cells much easier than a watering can with a spout. I’m currently using this small indoor coil hose, as it has a very small nozzle that you can easily regulate the flow of water released onto the small cells.
Click here for more in-depth information on how to successfully germinate tomato and pepper seeds
Symptoms & Solutions for preventing disease in tomato seedlings
A. Symptom: Yellowing of lower leaves.
- Probable Cause: Magnesium deficiency or overfertilizing.
- Solution: Decrease the amount of fertilizer you are giving the young plants. If you used the Miracle Grow Moisture Control potting soil, you probably won’t need any extra fertilizer until they are planted outside. If you haven’t given any fertilizer, it could be a lack of magnesium and you can water with a weak solution of Epsom salts. Over-use of high-potassium fertilizers can cause magnesium deficiency, as plants take up potassium in preference to magnesium.
B. Symptom: Pale green leaves
- Probable Cause: Not enough light or a Nitrogen deficiency
- Solution: If the seedling is getting plenty of light (16 hours of light/day is good), transplant the seedling to a container with fresh potting soil that contains nutrients; mix some compost in with your potting soil to ensure a nutrient supply.
C. Symptom: Purple leaf-tints with bronze or brown leaf edges.
- Probable cause: Plant is overwatered or has a Potassium deficiency
- Solution: If you are not overwatering (see above tips), give the plant a dose of fertilizer that contains trace minerals or transplant to a new medium with compost.
D. Symptom: Reddish purple undersides of leaves, accompanied by slow or stunted growth.
- Probable Cause: Phosphorus deficiency due to cold soil or acid soil
- Solution: Soil that is too acid or too cold can make it difficult for the plant to uptake phosphorus. Transplant to new soil and do not water with cold water.
Graphical Portrayal of Deficiencies:
The graphic below is helpful in pictorially describing the symptoms I outlined above. Unfortunately, I copied the graphic a while back for my own information, and can no longer remember where it came from, so I am at a loss as to who to credit. If anyone knows which site this graphic came from, please comment below and I will credit them.
Resource for More In-Depth Information:
A very in-depth look at deficiencies in essential minerals of plants can be found here. Although Mr. Berry’s book is not focused on the young seedlings before they are planted out, he goes into great scientific detail on the causes and remedies of nutritional deficiencies in plants. The most helpful part of his work might be the photos however. They are great photos showing in detail what some of these deficiencies look like on the plant (fortunately I could not take these photos, as my plants have not succumbed to the diseases on our farm).
Comments and questions are welcome.