Discussions about fertilizing tomatoes is mostly about selecting the right NPK and rarely discusses the amount to use. When quantity does come up, most comments suggest following the instructions on the box. But are the instructions on the box correct? Are the instructions complete enough so that you can apply the correct amount?
This post will compare some commercial products to see what they suggest and then I’ll compare that to reliable agricultural recommendations to see how much fertilizer gardeners should be using to grow great tomatoes.
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Table of Contents hide
1. Best Fertilizer For Tomatoes
2. Nitrogen is The Most Important Nutrient
3. Applying The Right Quantity of Fertilizer
4. Commercial Tomato Fertilizer
5. What Do The Experts Say?
6. Why Are Commercial Rates Lower?
Best Fertilizer For Tomatoes
Fertilizing correctly requires two bits of information: the NPK ratio and the quantity to add.
The tomato is the most popular food crop and several manufacturers make so-called tomato fertilizer. But they all use a different NPK. The three products compared in this post are a 18-18-21, a 3-4-6 and a 3-6-4. The first one is essentially balanced with the same amount of nitrogen, phosphate and potash (potassium). The second one is high in potash and the third is high in phosphate.
How can all of these be tomato fertilizer?
Because the right NPK depends on the soil, not the crop. You replace nutrients that are missing in soil. You do not feed plants. There really is no such thing as tomato fertilizer.
In a soilless mix, as used in seedlings mixes and containers, there are no nutrients and so you must add all of them. In that case you want an NPK ratio of 3-1-2. That is what plants use.
In real soil, the right NPK depends on your soil and every garden is different. Without a soil test, you can’t be sure which NPK to use. If your soil is quite good and contains a reasonable amount of organic matter you probably have enough P and K, in which case you only need to add nitrogen.
Nitrogen is The Most Important Nutrient
Plants need enough of all of the required 20 nutrients but the nutrient that is almost always limiting growth is nitrogen. There are several reasons for this:
- Nitrogen is very mobile. It moves with water and is washed through soil quickly.
- Everything living in soil needs it, including plants, insects, worms, and microbes.
- It is not provided by the minerals in soil.
Nitrogen is also the nutrient that is not measured by most commercial soil tests. It can be measured by home soil kits, but they are so inaccurate that the results don’t mean much. That means gardeners never know how much nitrogen to add, but this post will provide some guidance.
Nitrogen in fertilizer is provided in different forms. Some is soluble (plant available) and can be used by the plant right away. This includes nitrate, ammonium and urea. Other forms are in large organic molecules, like proteins, that need to be decomposed before the nitrogen is available to plants. Manufacturers tend call this “insoluble” nitrogen.
The problem with insoluble nitrogen is that you don’t know how quickly it becomes available to plants. Some might be available in a few weeks, but it probably takes several years for all of it to become available. Insoluble nitrogen can’t be counted on to provide nitrogen to plants in this growing season.
Applying The Right Quantity of Fertilizer
Too little or too much nitrogen results in poorly growing plants. It is therefore important to get the nitrogen level correct and in order to do that you need to know the application rate for the fertilizer you are using. The application rate depends on knowing three things.
- the NPK value of the fertilizer (gives you %N)
- the area to be fertilized
- the quantity of fertilizer to apply per given area
If the fertilizer is to be made up as a liquid you will also need to know the ratio of fertilizer to water used to make the mixture.
As you will see in the comments below, this information is rarely provided in complete detail by fertilizer manufacturers.
Commercial Tomato Fertilizer
What do commercial tomato fertilizers recommend?
I selected 3 gardening products based on the fact that they are popular brands and that I could find most of the data I needed to do the calculations. Several of the other brands I looked at did not provide basic information about their products. Some didn’t even provide an NPK value. Don’t buy such products.
Each of the selected products used different methods of application, different coverage areas and different units of measurement, making it difficult to compare products. Each also contains different amounts of soluble nitrogen.
For the purpose of the comparison, I have normalized the recommendations and calculated the amount of soluble nitrogen that is applied to one tomato plant, growing in a 2 x 2 foot space, during a 4 month season. The following table summarizes the data.
The total amount of nitrogen added by each product was about the same, around 7 g per plant. The synthetic fertilizer (Miracle-Gro) added this amount in the form of soluble nitrogen so it was immediately available to plants. The organic fertilizers added the nitrogen mostly as insoluble forms. If they are similar to compost, the insoluble nitrogen will be released slowly over the next 5 years.
If the soil is low in nitrogen and organic matter, as is the case for new gardens, the plants are probably under fertilized and need to rely on the applied fertilizer for all their nitrogen needs. That means organic fertilizers need to be applied in higher amounts.
In a mature garden that has lots of organic matter, plants will get some of their nitrogen needs from the soil and the rest from the fertilizer. Organic fertilizer can then be used.
Miracle-Gro Tomato Plant Food
This is an 18-18-21 fertilizer that is all soluble nitrogen based mostly on urea. The instructions say to mix it at the rate of 1 1/2 tablespoons per 1 1/2 gallon and to apply it twice a month by “soaking the base of the plant”.
Issues:
- there is no rate of application. Should you apply the whole 1 1/2 gallon per plant? Do you apply it to 10 plants? A statement like “soaking the base of the plant” is useless to the gardener for ensuring the right amount of nitrogen is applied. I have used 1/2 gal per tomato plant in the calculations.
Burpee Organic Tomato + Vegetable
This is a granular 3-6-4 fertilizer that has 1.8% insoluble nitrogen, meaning that only 1.2% is immediately available to plants. It is applied at 1/2 cup per 10 sq ft for bedding plants and at 1 1/3 cup per 10 ft row for established vegetable gardens. A cup = 1/2 pound. Apply every 2 months.
Issues:
- 2/3 of the nitrogen is not plant available, and there is no data to let you know how quickly it will become available. This is a problem with organic products.
- The width of the 10 ft row is not defined.
Espoma Tomato Tone
A granular organic 3-4-6 fertilizer that is 2.1% insoluble nitrogen. One pound = 3 cups. Apply 3 tablespoons per plant, twice per month. Application should be in a narrow band 3″ from the stem.
Issues:
- 2/3 of the nitrogen is not plant available, and there is no data to let you know how quickly it will become available. This is a problem with organic products.
- Fertilizer is applied “3 inches from the stem”. That is not were the feeder roots are, except right after planting.
What Do The Experts Say?
I also checked with 3 extension offices in the US to see what they recommend for nitrogen applications. Most of their recommendations use soluble nitrogen.
The first thing to notice is that there is quite a variation between extension offices, which was quite a surprise to me. Some of this may be due to region soil differences, but I suspect other factors are at play. If you can explain these differences, let me know in the comments.
The science of applying nitrogen is certainly well studied for agriculture, but we are still fine tuning the system. A study in 2006 looked at increasing yields by applying more nitrogen in Ontario, and found that application rates of 200 lb/acre (8 g/plant) produced the best yields. This value was 2-3 times higher than the recommended amount at that time and it is at the high end of the recommendations in the above table.
If you compare extension recommendations to commercial products, the extension offices are starting plants off with 3 times as much nitrogen, with the total seasonal amount being 50% higher.
In soilless mixes like peat moss, the recommended concentration for nitrogen is 200 ppm.
Colorado State
Work 4 ounces of nitrogen per 250 foot of row into the soil at planting; Side-dress with another 4 oz when fruit are about 1/3 grown. After picking the first ripe fruit, side-dress with another 4 oz.
Kansas State
Add 2/3 lb per 30 foot row using a 16-0-0. “In order to yield well, tomatoes need to be side-dressed with a nitrogen fertilizer three times during the season,” said Upham, a horticulture specialist with K-State Research and Extension.
Missouri
Before planting, apply 1 to 1.5 lbs nitrogen per 1,000 sq ft. An additional 3.2 oz per 1,000 sq ft can be added each week. These values are for high tunnel tomato growing.
Why Are Commercial Rates Lower?
Why are the nitrogen application rates for commercial products geared to gardeners lower than the rates suggested by experts in agriculture? I suspect there are a couple of reasons. Gardeners tend to over fertilize and they don’t measure very accurately. By recommending a lower rate, the fertilizer is less likely to harm the plants.
Gardeners are also less interested in maximizing yield. One less tomato per plant won’t make much of a difference in a backyard, and won’t even be noticed by gardeners. It is more significant on 50 acres.
There is another key difference between gardeners and agriculture. Gardeners use a lot more organic material to amend soil which in turn increases the available nitrogen level. That means less fertilizer needs to added to provide a good level of nitrogen.
If you are careful about measuring the fertilizer, you can add more than the box suggests, provided you have not already added a lot of organic material. This might be of value in newer gardens but as the soil improves over time, you don’t need higher levels.
Fertilizing Other Crops
What about other crops? They are all a bit different but for backyard gardens, you don’t need to worry about the differences. Many of the commercial fertilizer mention other crops right on the label. Remember, there is no such thing a tomato fertilizer.
