Fertilizing Tomatoes

To fully understand the fertilization issue we must first understand how the plant uses nutrients. I'll use nitrogen here as an example.

Nitrogen is the central element in chlorophyll, the material in plants that makes them
green and performs photosynthesis. Nitrogen, in the inorganic form, is quite mobile in the soil, meaning it is carried down in the soil profile by rainfall and irrigation. This
nitrogen eventually becomes "out of reach" for the plant roots.

Nitrogen is also responsible for vegetative growth to a great extent. A lack of nitrogen will produce little in the way of leaves and stems. Too much and the plant will go into a solely vegetative mode and produce no flowers or fruit until a balance is reached.

Organic fertilizers typically have a low percentage of total nitrogen in the analysis equation. Bags of fertilizer of any type usually show the analysis in the form of N-P-K, which stands for Nitrogen, Phosphorus, and Potassium. Other micronutrients are listed separately in the analysis profile.

In the case of bagged compost, an example analysis might be 2-4-1. In simple terms these numbers represent percentages. So, 2% nitrogen would mean a 100 pound bag contains 2 pounds of total nitrogen.

Reports from university and privately funded trials inform us that 80 to 150 pounds of total nitrogen per acre is needed for optimal tomato growth over a season. This number varies depending on soil types and locations throughout the United States. II typically target 100 pounds for each growing season. Doing the math reveals that we would need 40 to 75 bags (or 4000 to 7500 pounds) of compost from the example above per acre. That's 2 to 4 tons!

Most of us are not growing our tomatoes on acre sized gardens so let's convert this down to 100 foot rows. If we assume the rows in the per acre example are 100 feet long and are spaced 4 feet apart, we have approximately 109 rows per acre (there are 43560 square feet in an acre). And, if we use 100 pounds of total nitrogen per acre as our target, we need about 0.92 pounds of total nitrogen per 100 foot row. For ease of calculations, let's round it up to 1 pound.

Going back to the bag of compost with an analysis of 2-4-1, we would need 50 pounds per 100 foot row or a half a bag.

In inorganic forms, we could use much less because the typical analysis is higher. Some examples are 21-0-0 for ammonium sulfate, 48-0-0 for ureas, and 10-20-10 or a complete mixture. As I stated above, though, these forms of nitrogen are much more prone to movement within the soil. A typical use would be to divide the total amount needed by 3 or 4 and apply each smaller amount at intervals throughout the growing season.

An advantage of organic fertilizers such as compost is that the total nitrogen is not immediately readily available. This allows us as gardeners to be able to apply the total amount at the beginning of the season knowing that it will become available as the season progresses. In addition, by using organic fertilizers, we have the opportunity to build the total organic matter content of our soils, which is a huge plus.

My recommendation here is two-fold. First, use organic fertilizers. If you are applying the total amount at the beginning of the season, be sure to incorporate it into the top 4 to 6 inches of your soil before planting. Organic fertilizers may cost more, but the benefits far outweigh the cost. Second, have you soil tested at least every few years by a lab. This will tell you how much nitrogen and other nutrients are available in your soil. Then you can adjust your organic fertilizer applications based on the results following the above guidelines.

Check your phone book under the government section to locate you nearest County Extension Agent. They will be able to provide you with information on how to take soil samples and where to send them, which is usually to your state's land grant university.

If you live outside of the United States, check with your local garden center or gardening club for soil labs in your area.