Organic Soil Can Be Nutrient Deficient Soil

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Robert Pavlis

Everyone loves organic soil! It is the best soil you can have because it is full of nutrients and all the things your plants need. Or at least that is what many people think.

In this post I will describe a situation that is very common. It illustrates that organic does not equate to nutrients. Using ‘100% organic soil’ can in fact create a garden where nothing grows well due to nutrient deficiencies.

To be clear, not all organic soil is nutrient deficient – most is full of nutrients and great for plants. But you can’t rely on the word ‘organic’ on the package to figure this out.

Organic Soil Can Be Nutrient Deficient Soil
Organic Soil Can Be Nutrient Deficient Soil

Why Don’t My Plants Grow?

Nutrient deficiency in raised bed due to organic soil
Organic Soil Can Be Nutrient Deficient Soil, used with permission

This all started with a discussion on a Facebook Group. Someone posted this picture of their garden looking for help. This gardener has been growing in the ground successively for a number of years, but decided to try a raised bed. They made a new raised bed and filled it with potting mix from Costco called ‘Premium 100% Organic Potting Soil’. To that they added a bit of garden soil, but it was mostly the potting mix. New soil, a raised bed, plenty of water, experienced gardener, but things were just not growing. The plants looked very yellow and stunted.

Soil Science for Gardeners book by Robert Pavlis

Of course lots of people made suggestions to fix the problem, and a very few even asked some important clarification questions, which determined the following facts. The potting mix is advertised for both indoors and outdoors. It contains aged bark, sphagnum peat moss, and perlite.

Several people commented that they tried this potting soil and also had problems with it. They concluded there was something wrong with it and that it was “just not good” for growing plants. Others used it every year and it worked great.

Several people thought the 100% organic content was too high, resulting in a very nutrient rich soil – to many nutrients for growing plants. In fact this was the most popular conclusion reached in another Facebook group that was also consulted.

After seeing the ingredients, I commented that “it is normal to use potting soil in pots, and to feed the plants in the pot. My first guess is that the plants are not getting enough nutrients – but that is just a guess. You could add a small amount of fertilizer to one end of the bed and see if those seedlings become greener.” You can’t really reach a firm conclusion with the limited information provided on a Facebook discussion – although people do it all the time.

Get a Soil Test

Soil test kits for gardens are not worth buying - use a certified lab instead
Soil test kits for gardens are not worth buying – use a certified lab instead, used with permission

This gardener was quite concerned and wanted an answer, so she went out and bought a soil test kit. The results seemed to confirm the opinion of the majority of people – nutrient levels were off the chart.

I have written about the inaccuracies of these garden soil test kits before, and I warned her that she should not rely on the results.

I know the picture is not great but if you look at the color chart behind the test tubes you can see that the color difference between high and low nutrients is very small. It is one reason these tests are not worth buying.

Get The Facts

Facebook Groups are a great way to have a discussion, but you have to be very careful about any data you find there. This gardener decided to get some real facts. She contacted the manufacturer of the product who told her that, “this soil is an excellent base but you need to add fertilizer to make things grow.” He went on to say, “this information is not clear to everyone by reading the ingredients and that when I change the packaging I’ll add a comment about adding fertilizer.”

The ingredients did indicate this. Peat moss contains very few nutrients and perlite has none. Aged bark may or may not add nutrients depending on how well it is aged. Newer bark will be robbing the soil of nutrients, especially nitrogen.

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Nothing Wrong With This Product

Let me be clear – there is absolutely nothing wrong with this product. It is a soilless mix that is very similar to other peat moss based potting soils. It just needs to be used correctly.

Incorrect Soil for Raised Beds

The first problem here is that this soil should never have been used for a raised bed, although soil mixtures similar to this are routinely recommended. When this type of soil is used, the raised bed becomes a large container that requires regular fertilizing, just like any other container. Water quickly washes nutrients out the bottom of the raised bed so they need to be reapplied.

It is much better to use real soil for raised beds and then add some organic material as needed. The clay in real soil holds on to nutrients better than bark and peat moss.

Incorrect Interpretation of ‘Organic’

People see the words ‘100% organic’ and associate it with good soil and high nutrient levels. It can mean that, but in this case they reached the wrong conclusion because they ignored the fact that organic does not equate to high nutrient level. This potting soil is 100% organic with zero nutrients.

Raised Beds Are Not That Great

There is a real trend towards using raised beds and people make all kinds of incorrect claims for them. In my book, Garden Myths, I reviewed the claimed benefits and found that except for a neater-looking garden they don’t add any benefits. High walled ones do make it a bit easier to work in them.

Problem Solved

A week after adding fertilizer the plants started looking better.

She has decided to go back to gardening in the ground, rather than raised beds – a choice I agree with.

 

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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!

21 thoughts on “Organic Soil Can Be Nutrient Deficient Soil”

  1. The answer garden products make great growing mediums. I used their Nutramulch product for my cedar hedges and dwarf bamboo and they absolutely loved it. Worked great for weed control as well.
    Note: I did not used any other growing medium than the Nutramulch.

    Reply
  2. Hi! I wanted to comment about the coffee grounds post from some years ago. I noticed that person had mentioned using coffee grounds that had already been brewed (they said something like wet)
    Anyway, I believe they were referring to the use of caffeine to get rid of ants. So the caffeine is released due to the brewing process.

    I still could not find any peer reviewed articles regarding ants and caffeine. However, I did find a study that involves mosquitos /eggs and caffeine. Maybe the whole caffeine aspect made it’s way to being a myth due to some aspect of this caffeine correlation.
    I just wanted to mention this, as it is interesting. Here is the link to the study :

    https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4436121/&ved=2ahUKEwi7goP_o4bdAhXFV98KHc28Bt0QFjAAegQIABAB&usg=AOvVaw3JVDypRvHqFfNT21asB2ib

    Sorry the entire link address is shown as a proper link.

    Also, for the diatomaceous earth, it will absolutely dehydrate a slug if it comes in direct contact with it, killing it. It will dehydrate to death many pests, both with and without an exoskeleton. It does not work if it’s wet. Without diatomaceous earth, I wouldn’t have success with my gardens. Due to the extreme humidity and rain during growing season in AL, harmful fungi is an ever present threat. The balance of beneficial and harmful pests is messed up from long term city pesticides etc.
    Fungi and disease is spread more easily via the damage causee by pests.

    And I despise fire ants !!!! It’s completely the commenters right to their opinion regarding not wanting to *kill anything* .. .. but in my own opinion, when it comes to fire ants, oh my! They are so viscious and they just destroy my plants.
    The only thing that will work for my case is the granules sprinkled around . It is yet another obstacle to remove them because they don’t have just one or two mounds, but instead can be just all over the place.

    Thank you for sharing great information via your blog.

    Reply

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