Fertilizer Nonsense #2: Match Fertilizer to Plants

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Robert Pavlis

Last week I pointed out that the concept of a balanced fertilizer did not make sense. When scientists measured the amount of nutrients in plants they found that the levels of N, P and K were not equal.

Fertilizer Nonsense #2 Match Fertilizer to Plants
Falling Garden

Match Fertilizer to Plants

Some plants like grass contain a lot of nitrogen relative to P and K. Other plants had higher levels of phosphorus and still others had higher levels of potassium. Clearly, not all plants required the same amount of nutrients.

Fertilizer companies started creating formulas specific to a type of plant. A grass might need a 20-5-5 formulation. It was known that plants needed a lot of phosphorus when they flowered, and so a 10-15-10 would be good for flowering plants. Vegetables needed a different formula again to help them develop fruits and roots. The marketing departments of fertilizer companies started to market specific formulations for special groups of plants.

Gardening experts liked the idea. Soon they had a special fertilizer numbers for roses, and another one for daylilies. Bulbs, they needed high phosphorus. You can find places all over the internet that provide special fertilizer recommendations for each type of plant. Books are full of this type of information.

Soil Science for Gardeners book by Robert Pavlis

Reality

The formulation for each type of plant was based on plant analysis–what is the plant made up of when it is analyzed. This misses a very key point. Just because a plant has a high phosphorus level today, does not mean the plant needs to absorb a lot of phosphorus from the soil. Nutrients in a plant are not static. They are used for building plant parts, and they are reused and moved around the plant as needed. Potassium, for example, is not used very much as a building block. It is critical for many plant processes to take place, but it is not used up when a plant grows. A plant reuses the same potassium over and over again. So a plant might have a lot of phosphorus in the leaves when they are analyzed, but the plant may not need to collect more from the soil.

There is a second reason why fertilizer recommendations based on the nutrients found in plants does not make sense, but I’ll leave that until the next post. For now let me conclude by stating that none of the recommendations for fertilizer, for a particular type of plant make any sense.

If you don’t believe me, have a look at the available fertilizer in stores or the internet. I did a quick search on the net for tomato fertilizer. I found the following fertilizer numbers; 1-2-1, 1-1.5-0.5, 1-2-2.5, 1-1-1, and 1-3-1. These have all been normalized so that the amount of nitrogen is 1, to make comparisons easier. How can all of these be correct? They can’t be. At most 1 is correct and the others are wrong. In my next post, Fertilizer Nonsense #3 – All Tomatoes Need the Same Fertilizer, I will explain why not even 1 of them is correct, except by accident.

References:

1) Photo Source: Pyewacket’s Garden

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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!

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