Fertilizer nonsense #3: All Tomatoes Need the Same Fertilizer

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Robert Pavlis

Last week I showed that it does not make sense matching a fertilizer formulation to a plant type. For example, a 1-2-1 fertilizer for tomatoes does not make sense. In part this is true because every manufacturer has their own recommendation. But there is a more important mistake being made by anyone who recommends a certain formulation for a specific type of plant–read on.

Fertilizer nonsense #3 All Tomatoes Need the Same Fertilizer
Fertilizer nonsense #3: : All Tomatoes Need the Same Fertilizer

Filling the Fridge

Before I talk about plants I want you to think about your own shopping habits. When you go to the store to buy groceries, does the store manager tell you what you need to buy? Do they tell you that this week you need chicken, peppers and bread? No they don’t. Why not? Because they don’t know what you already have in your fridge. If you bought a lot of peppers last week, and you have lots left you don’t need to buy any this week. The store manager does not know this.

You might have used up all the bread and have a bread shortage in the house, and so this week you need to buy bread.

The important point here is that you buy the groceries that you need to restock your fridge. You buy the ones that are missing from your fridge, not some arbitrary list of groceries that the store manager might recommend.

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Nutrients Missing from the Soil

Plants use nutrients from the soil. The soil is the “refrigerator” for plants. The soil contains a variety of nutrients and as long as there are plenty of each type of nutrient, the plant can grow well. When one of the nutrients runs low in the soil refrigerator, it is up to the gardener to replace that nutrient.

If you knew that nitrogen was getting low in the soil you, the gardener, would go to the store and buy some more. You want to replenish the soil refrigerator so that the plant has access to the nutrients it needs. This is no different than buying groceries for yourself.

But buying fertilizer is different. Instead of buying what your soil needs, most people go online or to a book, and the expert (ie the store manager) tells them what they need. Oh—you want to grow tomatoes, you need a 1-2-1 fertilizer. Instead of buying what you need, you buy what the expert recommends. This makes no sense for your groceries and it makes no sense for your garden!

Instead, you should go to the store and buy some nitrogen because that is what is missing from the soil refrigerator.

Let’s look at it a different way. My soil has been created over millions of years based on the degradation of limestone. As a result of this process we have lots of phosphorus in the soil. I am not unique. Much of the north-eastern North America has similar soil. We have too much phosphorus in the soil.

When we go to buy fertilizer we don’t need any more phosphorus because we already have enough.

So when the so called ‘expert’ tells us we need a 1-2-1 for our tomatoes they are wrong. We don’t need any phosphorus. We might need a 1-0-1 fertilizer, but we don’t need more phosphorus.

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Fertilizer Recommendations are Wrong!

Any time an expert, who has not analyzed your soil, tells you that you need a certain fertilizer number you can be certain they are wrong. Why? Because they don’t know what your soil has or doesn’t have. They don’t know the nutrient profile of your soil.

In the same way that the grocery store manager can’t tell you what you need to replace in your fridge, a fertilizer or plant expert can’t tell you what you need to replenish in your soil, unless that expert has analyzed your soil. That means that any box of fertilizer that makes a recommendation of a certain formula is wrong–they don’t know anything about your soil. A web site or book that recommends a certain formulation of fertilizer is also wrong–they don’t know anything about your soil.

Match Fertilizer to Your Soil

When you  buy fertilizer you should be buying it to replenish the nutrients that are missing from your soil. In the same way that you buy carrots this week because you have run out of them, you should only buy nitrogen if your soil is low on nitrogen. If your soil has lots of nitrogen, don’t buy any more–you have enough.

You now have a real dilemma. Since most of us don’t do a soil test, we don’t know what is missing from the soil. If we don’t know what is missing, we have no idea which fertilizer to buy.

What we do know is that buying something because the store manager says we should is stupid. Buying something because we think it is better than nothing does not make sense either. It seems obvious that we should be doing a soil test. Join me next week, in Fertilizer Nonsense #4 – Soil Tests, to see why even this suggestion has a serious problem.

References:

1) Photo Source: Neil Conway

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

7 thoughts on “Fertilizer nonsense #3: All Tomatoes Need the Same Fertilizer”

  1. I couldnt agree with you more. I have my vegetable garden soil tested every spring. My soil has plenty or actually too much P&K. I only add Urea in the amount my local University recommends.I also side dress Urea as recommended.Plants have grown well for my 4 years of gardening. I would like to point out that you should only apply Urea in the amounts recommended as it is a very strong Nitrogen fertilizer. 46/00/00

    Reply
  2. Hi Robert. You says. “My soil has been created over millions of years based on the degradation of limestone. As a result of this process we have lots of phosphorus in the soil”. I mean limestone is calcium carbonate (CaCO3). Not phosphorus. Can you explain it please? Anyway plants can take nitrogen from air as well. What do you think?
    Thank you for good site 🙂

    Reply
    • Found this definition of limestone; “Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. All limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, clay minerals, pyrite, siderite and other minerals. It can also contain large nodules of chert, pyrite or siderite. “. It contains lots of other minerals including phosphorus.

      Plants cannot take nitrogen from the air. There are some plants – mostly the legumes – that form an association with bacteria which can take nitrogen from the air and convert it to nitrate (or maybe nitrite?). But plants can’t do this.

      Reply
  3. I very much enjoy your science based garden talk. I have a question. I have very high levels of Potassium and Phosphorus in my soil, probably from too much manure. My permaculture friend recommends urine which I have used but fear the salt [actual NaCl] levels might be too high. What do you think?

    Reply
    • According to a NASA study of human urine it contains 8 mg/l nitrogen, 0.8 mg/l phosphorus, 1.7 mg/l potassium and 2.8 mg/l sodium. The NPK value for this would be 0.8 – 0.08 – 0.17. So using urine will still add P and k to the soil – any organic material will do this. It also adds 30% of the nitrogen value in sodium.

      I don’t see the logic of permaculture friends in recommending urine in your case. Urea will add the nitrogen, and no P, no K and no Na.

      Reply

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