Question
I live in north east pa. many of the lower leaves on my healthy tomato plants are turning very yellow what can i do?

Answer
Kathy, poor soil is the primary reason plants fail to do well.  Tomatoes need magnesium and calcium in addition to a small ratio of nitrogen to phosphorus.

You should add as much organic matter to the soil as possible. These materials would include compost, aged manure, and peat.  Then I would spray the plants with a water-soluble fertilizer designed especially for tomatoes.  Mix in one tablespoon of epsoms salts to supplement any magnesium deficiency in your soil.  Apply this mixture once every 10 days until the conditions improve.  Then you can cut the soution to ?the recommended strength every 10 days for the remainder of the growing season.

It is not uncommon for bottom leaves to yellow or brown in dry conditions.  Even though you may be watering several times per week, the watering is most likely shallow which means the roots are reaching to the surface for water.  It is much better to water very deeply once or twice per week than to water shallow everyday.

Another cause of yellow leaves is too much water.  This is especially common in the spring after heavy storms.  The excess water leeches the nutrients from the soil, and saturates the roots.  The plants cannot take in the nutrients and oxygen needed for strong growth.  Vegetables should receive no more than 1.5" of water per week.  It's not an exact science, but too much water is actually worse than too little.

Although I doubt you are experiencing fungus diseases, the most common types are fusarium wilt and verticillium wilt.  If the plants have contracted either of these, the leaves will gradually curl and drop off.  Blight, on the other hand, results in blackened patches on the leaves.  Most current day hybrids are resistant to these diseases.  That is why I am skeptical that your plants have contracted these disorders.  On the other hand, if you are growing heirlooms or open-pollinated varieties, disease is a stronger possibility.

As a preventive measure, you can spray the plants with a general fungicide.  I always recommend copper for tomatoes, but a good homemade solution that works very well is 2 tbs. of baking powder mixed with 1 tbs.  of non-concentrated dishsoap (not detergent) and 1 tbs. of vegetable oil.  The baking soda is an excellent fungicide, the vegetable oil not only helps the baking soda adhere to the foliage but also smothers eggs of insects, and the dishsoap kills insects within hours.

To avoid diseases in the future, my advice is to water less often, but deeper.  Also, avoid overhead watering, especially in the evening.  Water on the leaves can cause fungus diseases, as can water splashing up from the soil.  If you must water overhead, do it in the morning.  Otherwise, use soaker hoses.  Finally, mulch the plants heavily with dried grass clippings or straw.  This will not only prevent soilborne diseases from splashing up onto the leaves, but it will also retain the moisture which means less need for watering.

In the meantime, I would go ahead and apply the water-soluble fertilizer in addition to the application of epsoms salts. As I mentioned earlier, make sure you purchase a fertilizer specifically recommended for tomatoes.  General-use fertilizers, such as Miracle-Gro, are very high in nitrogen, which will promote lush foliage at the expense of blooming and fruiting.  Fruit-bearing plants need a higher concentration of phosphorus and potassium than they do nitrogen.  A tomato fertilizer will provide the proper nutrients.  If a lack of nutrients is your problem, this should correct it.

If the problem does not improve within two weeks, apply a copper-based fungicide to combat any potential diseases. Between these two measures, the problem should correct itself within a few weeks.

I hope this helps.  Good luck, and please write again if I can be of further assistance.

Regards,

Mike