News release: Fertilizing Tomatoes

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640

Release Date: May 26, 2016
Headline: Fertilizing Tomatoes

In my last column, I discussed the decision points in selecting a tomato variety for your home garden.  Once you have made that important decision, you next need to select a site where they will thrive.

As with most garden plants, tomatoes do best on a well-drained soil.  This not only keeps the soil from becoming waterlogged, but it allows good aeration in the root zone.  Heavy soils which do not drain well should be avoided, because this can lead to root diseases and other problems.

Tomatoes prefer full sunlight.  Having said that, if your yard does not have a spot that receives full sunlight all day long, just pick the sunniest spot you can find.  A little shade won’t hurt, but the more sunlight, the better.

Some plants may cause toxic reactions to tomatoes.  The classic bad mix is black walnuts and tomatoes.  The roots of black walnuts produce a compound called juglone, which is toxic to many kinds of plants, including tomatoes.  So it’s best to keep your tomatoes at least 50 feet away from the drip line of the tree.

You may want to improve the soil in your garden site by adding leaf mold, well-composted manure, or similar organic amendments.  However, some caution is in order.  I have seen situations where people have added a lot of manure, thinking it would grow big tomatoes.  Well, it did grow big tomato plants, but few tomato fruits.  The reason was that it was actually adding too much of a good thing.  Under very high fertility, tomatoes produce a lot of vegetation, but do not set fruit well.  So take it easy on those organic amendments.

You may need to add lime, but then again, you may not.  This is where a soil test is really a good idea.  It will tell you the nutrient status of your garden site, as well as the soil pH.  If your pH is already high enough, adding lime is not a good idea, and may make the situation worse.  A soil test is something that I always recommend.  Tomatoes need a pH of 6.5 to 7.0.  Ideally, lime should be added well before you plant, so it has time to affect the soil pH.

Your soil test will also give you an idea of how much nitrogen, phosphorus, and potassium should be used.  It’s best not to randomly add fertilizer.  I have seen soil tests come back to my office with P and K off the charts.  That’s not a good situation, and if you add more P or K, you’re not helping yourself.

One other nutrient that a soil test will report on is calcium.  Tomatoes are subject to a physiological condition called blossom end rot.  This is caused by a calcium deficiency in the fruit.  This may or may not be due to a low calcium level in the soil, and again, this is where the information provided by the soil test really helps.  If you need to add lime, you will get some calcium there, but if the pH is already too high, you will need to add gypsum instead.  The soil test will tell you which one and how much.  You also may have plenty of calcium.  In this case, there are other causes leading to blossom end rot.  I’ll cover those in a future article.