Nitrogen for Tomatoes: Are You Making This Fertilizer Mistake?

Added: 2026-05-24

[00:00:00]We had an interesting question on our Facebook group called Garden Fundamentals. It goes like this. If you give a tomato plant too much nitrogen, it grows lots of leafy stuff, but not many fruits. Apparently, for maximum production, you have to intentionally starve the plant of nitrogen to force it to make more fruits. What is the right amount of nitrogen and when should you apply it? I've seen this claim many times. If you give a tomato plant too much nitrogen, it produces lots of leaves and very little fruit. So, the common suggestion is to reduce the amount of nitrogen as the plant matures.

[00:00:44]That way you get more fruit. Is that really true? Now, this claim stems from another claim I see a lot. Nitrogen is for making leaves. Phosphorus, the middle number in NPK, is for flowers and roots. And potassium? That's for the overall health of the plant. I've had a look at that claim before and and completely debunked it. It makes no sense. Nitrogen, phosphorus, and potassium are needed for every type of growth. For every cell that gets developed, it needs all three nutrients.

[00:01:20]Roots, flowers, seeds, fruits, leaves, doesn't make any difference. Every single cell in a plant needs all three nutrients. So, here's the common advice gardeners have given for tomatoes. Once the plant is large enough to make flowers, cut back on the nitrogen. You want the plant to make more fruit, less leaves. But, what do the experts say?

[00:01:44]Well, there are lots of studies online that look at the amount of fertilizer to apply to soil, but that gets kind of complicated because the amount you apply really depends on the soil that you have. And that's different for every farmer. However, there is one group of people that uses nitrogen in a fairly consistent manner, and that's hydroponics. And a lot of the tomatoes we have these days are grown hydroponically. They know how to grow tomatoes and maximize their yield. So, what do they say? Florida State had this to say, "A common problem with tomatoes is giving them too much nitrogen in the early stages of growth, when the plants are small. They become too bullish."

[00:02:31]That's their term. "They produce too many leaves." So, what this suggests is that you should keep the nitrogen level lower at the beginning of the growth cycle, which is the opposite of what gardeners are being told. The IFAS at the University of Florida has been researching tomatoes for a very long period of time. And here's what they recommend for nitrogen and a few of the other macronutrients. In the transplant to first cluster stage, you want to give them 70 parts per million. First cluster to second cluster, you increase that to 80. Second to third, up to 100. Third to fifth, up to 120. And from fifth to termination, 150 parts per million.

[00:03:16]That's the exact opposite of what gardening experts are telling gardeners.

[00:03:21]They use a low amount of nitrogen in the beginning, and as the plants start producing fruit, they increase the amount of nitrogen. Now, if we have a quick look at phosphorus, they keep that number about the same. This emphasizes the myth that phosphorus is a bloom booster. People believe that if I give the plant more phosphorus, it makes more flowers. That's simply not true, and the data from Florida confirms this. We give the same amount of phosphorus. You don't need to add more once the plant starts flower. How about potassium? Well, potassium is also increased as the plant gets older. Potassium and nitrogen are needed for the growth and health of the plant. And so, we increase that as the plant gets larger. All right, so that's the University of Florida. That's one expert. Let's look at another one. Ohio State University has this to say. Young plants should be provided with lower nutrient concentrations than mature plants. This prevents them from becoming too vegetative. Plants in early fruiting stages require increased levels of specific nutrients like nitrogen, calcium, and potassium because the developing fruit demands greater amounts of these nutrients. Mature fruiting plants require the highest level of nutrients to promote plant growth and fruit development. Now, their numbers aren't exactly the same as the numbers from Florida, but the ranges are the same, and for this discussion, the important point is they both suggest an increase in nitrogen as the plant grows.

[00:05:06]The exact opposite of what gardening gurus are telling you. Now, the numbers I presented in this chart here are in parts per million. Now, some of you may not know how to calculate that. But, that's really easy. If you hop on over to my website gardenmyth.com, there's a calculator there, and it'll tell you how many parts per million you're using for your fertilizer, and how to make up a particular type of solution. So far, we've been talking about hydroponics, but what about real soil? Well, I found a study in Italy that looked at giving plants different amounts of nitrogen as they grow, and they found that as plants mature, they do better with more nitrogen. You get more fruit, more weight of fruit. So, that's yield. You get more tomatoes if you increase nitrogen as the plant grows. Now, their particular numbers may not apply to your soil, but the trend of increasing nitrogen will be true for everybody's soil. So, gardening gurus tell you to decrease nitrogen as the plant matures, and experts in the field are actually studying this, say the exact opposite.

[00:06:22]Why does this make sense? Why should you give plants more nitrogen as they grow?

[00:06:28]You have to remember that nitrogen is a key nutrient, and it's used in everything that's going on in the plant.

[00:06:35]As a seedling, it's needed to make leaves and roots. As the plant gets a little older and starts to flower, nitrogen is critical for making flowers, and it's critical for developing the fruit that results from those flowers.

[00:06:49]As the plant gets larger, it has more leaves, it has more fruits, it has more flowers. All of these things are going on at the same time. So, as the plant gets larger, it needs more and more nitrogen. That seems to make common sense, and that's exactly what the experts say. Throughout the season, provide more and more nitrogen. Now, there are a couple special cases. I grow in zone five, it's a cold climate, and we tend to pick off the flowers late in the season. So, around the third, fourth week of August, we pull off any new flowers. And the reason for that is that the plant will not mature those before frost. So, there's no point in the plant putting energy into growing more fruit that will never develop. I pull them off so that the plant puts the energy into the existing fruit and gets it growing to full size. And that's a fairly common practice in colder climates. So, if you're doing that, you might as well stop feeding nitrogen at the point where you're pulling off these flowers.

[00:07:53]There's another scenario we do have to consider. If you're the type of gardener that puts on a ton of nitrogen early in the season, then the recommendation to cut back on nitrogen in the middle of the season may actually make sense. But, that's only because you've over-fertilized early on. If you're using a reasonable amount of nitrogen, you should be increasing the amount throughout the season. This little story about nitrogen is important cuz we all grow tomatoes, but it's also important because it illustrates a common problem.

[00:08:25]Advice from gardeners is not always right. Gardeners will tell you, "Well, I've been doing this for 50 years. It's the way to grow tomato." We have to push back and say, "It may grow tomato, but unless you've done some good studies, unless you've done some control, and you've actually measured things, you don't know that your way of growing tomatoes is the best." And that's why we have to go back to the science. The scientific studies do large numbers of plants. They compare things. They use proper controls. They measure things.

[00:09:00]They do statistics on the data. That is the data we should believe, not what gardeners are telling us. Happy gardening.

[00:09:09]Thank you very much for your support. If you'd like to connect directly with me, the best way is through my Facebook group, Garden Fundamentals. We'll answer all of your questions there. I also have a blog called Garden Myths, where I've debunked over 2,000 different myths.

[00:09:25]I'll put links to all of these in the description below. Happy gardening.