Fertilizer Garden Myths

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Robert Pavlis

A lot of the stuff on the internet is garbage when it comes to gardening advice. I am not surprised about that since many people just repeat what they have heard and give it very little thought. Some garden writers don’t actually do much gardening – they are writers, not gardeners. I rarely believe information unless it comes from experts in a field, government sites or published research articles.

This post is about an information guideline on fertilizers and soil amendments which is published by a government source – one you should be able to trust. Unfortunately it is full of incorrect or misleading advice. Let’s have a look at some fertilizer gardening myths.

fertilizer garden myths; Lobelia cardinalis X Lobelia siphilitica, by Robert Pavlis
Lobelia cardinalis X Lobelia siphilitica, grows just fine without added fertilizer, by Robert Pavlis

Fertilizer Garden Myths

The goal of this post is not to provide an extensive list of myths about fertilizers, nor is the goal to provide a detailed discussion of some specific myths. My goal in this post is to review a published guideline by the University of Maryland Extension office. This guideline, called ‘Soil Amendments and Fertilizers’ is all about fertilizers and was written specifically for the home gardener (ref 1).

Unless stated otherwise, the statements in quotes are taken from reference 1.

Balanced Fertilizer

“A fertilizer containing all three nutrients (N, P and K) is a balanced fertilizer.”

That statement is just wrong. A balanced fertilizer contains the same amounts of N, P and K. For example a 10-10-10 fertilizer is a balanced fertilizer and a 10-5-5 is not. For more information on this see Fertilizer Nonsence #1: Balanced Fertilizer and Fertilizer NPK Ratios – What Do They Really Mean?

Soil Science for Gardeners book by Robert Pavlis

Starter Fertilizer

Starter fertilizer specifically formulated for seedlings and transplants , are high in phosphorus to foster root establishment.

It is true that so called ‘start fertilizers’ and ‘booster fertilizers’ are higher in phosphorus, but it is incorrect to promote the idea that adding extra phosphorus will foster root establishment. Most garden soil has lots of phosphorus in which case adding more does nothing except cause pollution – if enough is added it will even become toxic to the fungi in soil. Adding phosphorus does not promote root growth, unless the soil has a deficiency. But that is true of any nutrient. If any nutrient is deficient, and you add it, it will foster root growth. There is nothing magical about phosphorus.

Ammonium Sulfate Fertilizer

“Ammonium sulfate, a dry fertilizer, …. is very acidic. ”

Any dry fertilizer is not going to be acidic. It would need to be dissolved in water to become acidic. At a 0.1M solution ammonium sulfate does have a pH of 5.5, which I don’t consider ‘very acidic’ when compared to vinegar at pH 2.4. It is also less acidic than the preferred soil pH for blueberries which is 4.8 to 5.5.

Bone Meal for Bulbs and Root Crops

“bone meal is especially good for bulbs and root crops, and lasts for 6 to 12 months.”

Bone meal has been discussed in Bone Meal Organic Fertilizer . It contains phosphorus and calcium – most garden soils are not deficient in either of these nutrients. High levels of phosphorus are not needed for growing bulbs.

Bone meal releases the phosphorus very slowly, and once released it moves through the soil very slowly at about an inch a year. It will be available to plants for many years – not 6 to 12 months. Bone meal should not be used unless a soil test shows a deficiency of either phosphorus or calcium, and in that case there are better options for providing either nutrient.

To better understand how phosphorus affects soil, see Rock Phosphate Fertilizer.

Compost Tea

“Compost tea is a good method of applying soluble nutrients to foliage and roots in the early part of the growing season when nutrients from soil organic matter are not yet available. Do not use farm animal manure compost.”

There is no good scientific evidence that compost tea is any better than just using compost. For more details see Compost Tea.

Building Natural Ponds book, by Robert Pavlis

The statement ‘nutrients from soil organic matter are not yet available’ is interesting. It is true that in early spring when the soil is still cold, decomposition of organic matter would be slow, but nutrients resulting from decomposition in prior years would be available.

The recommendation not to use animal manure composts for making tea goes against most suggestions for making compost tea. Once the manure is composted it is not chemically different from other types of compost. Some people even prefer to make manure tea using fresh animal manure.

Foliar Fertilizers

” Plants take up nutrients more efficiently through leaves than through roots.”

At first glance this makes no sense. Leaves aren’t really designed to absorb nutrients. On further reflection I think it depends on how you define efficiency. Nutrients sprayed on leaves enter the stomata easily. I guess you could call that efficient. The problem is that most of the sprayed nutrient will stick to the leaf surface until the next rain or run off the leaf as it is being sprayed. That is not efficient.

Some nutrients do not move easily once inside the leaves so they don’t get to where they are needed eg iron, manganese and calcium. When calcium entering a leaf it will not be transported to developing fruits so it won’t prevent blossom end rot (BER) in tomatoes. Calcium entering roots can prevent BER. More on BER can be found in Blossom End Rot.

Foliar feeding for the home gardener is not recommended. It is of value in specific farming applications, but the process is too complex for a home gardener. For the long term feeding of plants it is less effective than putting fertilizer on the soil.

Humates and Humic Acid

“Humates are thoroughly decayed … so nutrients are available to plants. Contains up to 35% humic acid.”

The first part of the statement is correct- Humates are thoroughly decayed. What this means is that all of the nutrients that can be extracted, have been extracted, so nutrients are NO longer available to plants.

Adding humates have shown to have little effect on plant growth. Reference 6 reviewed many studies looking at the effect of humates and humic acid on turf grass and found no benefits. They do improve soil and act as a sponge to hold nutrients from other sources similar to most organic matter. Roots can then access these nutrients. The same goes for humic acid. So there may be some long term benefits.

Kelp – Magical Powers

“Kelp is valued as a growth stimulant because of rich concentrations of trace minerals (over 60 types), amino acids, vitamins, and growth hormones. ”

Garden soil is usually not deficient of trace minerals and most plant debris and compost contains them. All organic material contains amino acids – kelp is not unique in this regard. The vitamin B1 which was promoted extensively as a growth stimulant for plants has no effect on plant growth. Some other vitamins may have an effect.

Seaweed extracts have been shown to stimulate plant growth but the exact mechanism for this is not known (ref 2). It is too early in the research process to claim magical powers for kelp. There are also environmental issues if the kelp is harvested from the sea.

Azomite and Rock Dust

“Azomite or rock dust – an aluminum silicate clay mixed with over 50 minerals.”

It would seem to be a great addition to gardens, and it is used in agriculture in some countries. Based on the data I have seen, this material breaks down very slowly – we are talking hundreds of years. It is not considered to be a good fertilizer.

Tomato Blossom End Rot

“Blossom end rot of tomatoes is caused by a lack of calcium in the developing fruit. Prevent it by adding a small handful of finely ground limestone to each planting hole prior to transplanting”.

As explained in Blossom End Rot, the first sentence is correct. BER is due to a lack of calcium in the developing fruit. But this may have nothing to do with the amount of calcium in the soil or even in the plant. Plants can have trouble transporting the calcium in the plant to where it is needed, ie the developing fruit. This can be caused by irregular water levels.

Advising someone to add more calcium without knowing the calcium levels in the soil is bad advice.

Urea Fertilizer

“Urea: Must mix into soil to prevent conversion to ammonia and subsequent escape into the air. ”

Urea is a high nitrogen fertilizer (45,0,0) that is relatively inexpensive. It is true that urea converts to ammonia and if the ammonia is near the surface of the soil, it can escape as a gas into the air. It is not correct to say urea needs to be mixed into soil – although this can be done if you want to disturb the soil and promote weed growth. In permanent landscapes, flower gardens and lawns, it is much better to simply water it in. It is very soluble in water, and is easily washed into the soil. I apply it just before a good rain thereby preventing it from escaping into the air.

Wood Ashes

“Wood ashes contain from 1 to 2% phosphorus and from 4 to 10% potassium and increase soil pH. The recommended yearly application rate is 25-50 lbs/1,000 sq ft. ”

Wood ash does contain phosphorus and potassium. Most garden soil contains enough of both nutrients. You should never apply a fertilizer unless you know it is required. Without a soil test you might already have high levels of potassium or phosphorus and just make the situation worse. Even if you need to add one or the other of these nutrients, wood ash should not be added to soil that is already on the alkaline side. Instead select a fertilizer that will not increase pH.

High Organic Matter is Good For Soil

“Most plants perform best in soils high in organic matter (greater than 2% organic matter by weight).”

Native top soil contains about 5% organic matter, by weight (or 10% by volume). As organic levels get over 8%, by weight, they start to cause problems including nutrient levels that are too high. You can have too much organic matter.

Plants perform best with the right amount of organic matter – not high levels.


“Humus can be purchased.”

I have discussed humus before in What is Humus? It is true that most organic matter, like manure and compost, contain some minor level of humus, but except for this, I have never seen ‘real’ humus for sale. I have seen a lot of bogus marketing which claims their product is humus, but all that means is that they are using the term incorrectly – referring to anything organic as being humus.

Use Manures Wisely

“Apply uncomposted manure in fall only. Mix into the top 4-6 inches of soil. Don’t leave it on top of the ground.”

Adding manure to the garden is a great idea, and adding fresh manure can burn plants. However, old manure that is still uncomposted can be added directly to the garden and one of the best ways to do this is to leave it on the ground. That way it continues to decompose and add nutrients for plants without burning roots.

Digging manure into soil is not a good idea. It brings weed seeds to the surface allowing them to germinate. The digging also destroys soil structure. Add the organic matter to the surface and let nature add it to the soil.

Mushroom Compost

“Mushroom compost can have high soluble salt levels and should be fully incorporated and watered prior to planting. ”

It is true that most mushroom compost contains high levels of salt – that is why it is not recommended for gardens, except in very small amounts.

Watering may remove some of the salts, but only the soluble ones. The problem with this recommendation is that watering will also leach out the good nutrients, in particular nitrogen. It makes no sense to me to add a product that has too much salt, and then try to wash away the salts. Why not just use a better organic source that does not have high salt levels?

Sand Added To Soil

” To improve water drainage and aeration of clay soils you need to add a minimum of 50% sand by volume.”

Adding sand to clay soil is a complex issue that needs  a separate blog post. Some experts, mostly on the western half of the US claim that adding sand to clay soil will create a hard substance like concrete. Other experts, in the UK, Europe, and Canada (me) routinely recommend adding sand to clay.

In this post I won’t argue for or against adding sand, except to say it works for me. However, the idea that you need to add at least 50% sand is ridiculous. The formulas for several soil properties change linearly with the amount of sand in the soil. Any amount added will start to change the properties of soil. I’ll provide details in a future post.


“Bioactivators: various commercial products containing: bacteria, growth hormones, nutrients, and vitamins may be useful as a tonic for lawns, seedlings, transplants and plants languishing in cool soils in the spring”.

Adding nutrients will help, provided the soil has a deficiency. Adding bacteria and vitamins does not help plants except in some special cases. For example, nitrogen fixing bacteria on pea seeds does work. Do growth hormones work? They probably have some effect, but they have not been shown to be needed for garden situations.

Don’t bother with bioactivators.

Lowering Soil pH With Sulfur

“At a pH above 6.0, iron sulfate lowers pH more quickly than sulfur”.

Sulfur is the go-to product for lowering pH in soil, so I found this statement to be a surprise. Since it is the sulfur molecule that bacteria convert to sulfuric acid which in turn causes a drop in pH, I would expect you need less pure sulfur than iron sulfate to cause the same change in pH – why buy iron and oxygen molecules if you don’t need them?

A little Googling and I found that the University of Illinois Extension (ref 3), Purdue Extension (ref 4) and Colorado State University (ref 5) all disagree with the quote. In fact, you need 6 times as much iron sulfate as sulfur to cause the same pH drop.

High Levels of Nutrients

“Nutrient levels are often in the ‘excessive’ range in older and well-tended landscapes. This is not a problem for plants.”

What are they saying here? High levels of nutrients don’t bother plants? If that is the case why does the same article say the following in the Boron section; “Can be toxic to plants if applied in excess?”

High levels of nutrients become toxic to plants. That is why you should not add fertilizers unless you know the soil needs them and you should not add excessive amounts of organic matter for the same reason. Too much nutrient is bad for plants.

Soil pH Levels

” Pay close attention to your soil pH readings and be prepared to adjust them according to your soil test”.

The suggestion is valid if you have very high (over 7.8) or very low (below4) pH. If you are inside of these extremes it is better to accept your soil pH and plant things that will do well in your soil. Changing soil pH is difficult, requires continual attention and is just not a good idea for home gardeners.

I don’t grow blueberries or azaleas because my pH is 7.4. I have learned to live with that and my life and the life of my plants is better for it.

Before buying a pH tester for home use, read; Soil pH Testers – Are They Accurate?

Fertilizer Garden Myths Conclusion:

Wow – what a list, and I did not even touch on water-absorbing polymers and mycorrhizae. It is no wonder gardeners are confused about what do do in the garden. One would hope that a US university extension office – who is the states expert on such matters – would get it right. Not so in this case.

To be fare, the article discussed here does contain a lot of useful information. It is too bad that the good is mixed in with the bad and the very bad. The average home owner will not know which is which.

Most other university extension articles are quite good and I trust them, most of the time!


  1. Soil Amendments and Fertilizers, by University of Maryland Extension
  2. Seaweed Extracts as Biostimulants of Plant Growth and Development: Seaweed Extracts as Biostimulants of Plant Growth and Development
  3. How to Lower Soil pH: https://web.extension.illinois.edu/cfiv/homeowners/080818.html
  4. Lowering Soil pH for Horticulture Crops: Purdue University
  5. Soil pH: http://www.ext.colostate.edu/mg/gardennotes/222.html
  6. Humate and Humic Acid: http://aggie-horticulture.tamu.edu/newsletters/hortupdate/hortupdate_archives/2002/jun02/art4jun.html
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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!

31 thoughts on “Fertilizer Garden Myths”

  1. When I did a google search for “azimite research” several studies came up that seem to indicate that azomite has substantial benefits for some crops. I’m not an expert so I’m just wondering what you think of studies like
    https://www.researchgate.net/publication/317339057_Evaluation_of_the_effect_of_Azomite_fertilizer_on_the_growth_and_some_physiological_traits_of_two_tomato_Lycopersicun_esculentum_cultivars_under_drought_stress and https://azomite.com/pepper-study/

    • The first paper does not show that ” azomite has substantial benefits for some crops” – it shows there are some benefits in two cultivars of drought stressed tomatoes. And the study was a pot study – not a field study. These generally do not involve normal soil. It tells us very little about what happens in the field, especially in a garden that is usually irrigated.

      The second study showed some increase in weight of tomatoes with 100 lb application, but not at the 200 lb rate, and then only for one of the two cultivars tested. There was no data for the change in nutrient levels in the soil. This is published by azomite. I cannot find this study published in a scientific journal – so not peer reviewed and therefore not of much value.

    • Do they really use as a fertilizer on farms? – yes
      Do they use bloodmeal – not usually – far too expensive
      Do they use bonemeal – maybe it is cheap and organic certified (I believe)


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