How could cow manure threaten a successful spring flower display?

Most gardeners understand that well dug, compost rich, freely draining soil suits the cultivation of many plants, so what could possibly go wrong when adding cow manure to a flower bed?

In this example, digging industrially produced cow manure resulted in a series of unexpected issues for an inexperienced gardener as he tried to continue his mother’s tradition of growing a spring flower display.

David’s (not his real name) mother was an experienced gardener. Each year she planted a large, prominent display of flowering annuals in support of the community spring flower show in northern NSW (Australia).

When his mother died, David decided to continue her gardening family tradition. Growing a spring display would now honour his mother’s memory AND support the community flower show. Bravo!

Cow manure isn’t particularly rich in nutrients, it contains 0.6% nitrogen, 0.4% phosphorous and 0.5% potassium. Like compost, it is more of a soil improver than a fertiliser, and both can be ideal for preparing a seed bed or flower bed.

David bought sixty 30 litre bags of cow manure. He remembered his mother also checked the soil pH – a measure of the acidity/ alkalinity level of the soil – before sowing. You can buy a pH test kit for around $35 from a good garden centre. The kit explains how to do the tests and how to interpret the results.

Soil pH influences plant health and the availability of nutrients

Ideally, soil should be slightly acidic to neutral. That is when most plant nutrients present in the soil become available for plant use. A soil pH of 6.5 – mildly acidic – is ofter referred to as the gardeners’ ‘holy grail’ because that is when the widest range of nutrients become available for plant use.

When David tested the prepared bed, the results revealed the soil had a pH of 13. Knowing this was too alkaline for sowing, he contacted two separate soil scientists for advice. The bad new was that while the pH could be adjusted to a suitable level, he couldn’t sow or plant while the soil chemistry was adjusting. That meant he wouldn’t have time to sow and grow flower to coincide with the flower show, and that was when he contacted me for advice.

There may be a profound difference between conventionally produced cow manure and certified organic cow manure

The aim of a conventional dairy, also known as an industrial dairy or factory farm, is to maximise the speed of animal growth and the yield of milk. The diet includes copper and zinc which are not assimilated and these heavy metals end up in the manure. Industrial cow manure can also be very alkaline. The alkalinity and heavy metals can influence plant growth.

In alkaline soils, nutrients such as phosphorous, potassium, sulphur, molybdenum and boron also become more available. This availability increases the more alkaline the soil becomes, and in excess they can harm plant growth.

Other nutrients, such as calcium, magnesium, iron and manganese – all essential for healthy plant growth – become locked away. The less available they are for plants to use, the more likely nutrient deficiency becomes. This also affects plants.

Soil pH has a scale of 1 to 14. Soil at pH 1 doesn’t occur naturally, because it would be like battery acid – extremely acidic – likewise soil at pH 14 doesn’t occur naturally because that would be like oven cleaner. Soil at pH 7 is neutral.

The pH scale isn’t linear, it is logarithmic so each increase rises by a factor of ten, therefore soil at pH 8 is ten times more alkaline than pH 7, and so on.

By adding industrial cow manure, David had managed to prepare a flower bed at pH 13 – which is 100,000 times more alkaline than an acceptably alkaline soil at pH 8. Yes, it’s shocking.

Test additives before you apply

One way of avoiding this problem is to test a sample of a soil additive prior to digging them in.

Australian certified organic

Organic produce is not just chemical free, it is a holistic means of producing and handling food. The whole system is linked: soil, plants, animals, food, people and environment.

Certified Organic products are grown and processed without the use of synthetic chemicals, fertilisers, or Genetically Modified Organisms.


Cows in an Australian certified organic dairy have better welfare and better diets than conventionally (factory) farmed cows, and this affects the nature of their manure, its contents, and its impact on soil health. Certified organic cow manure is alkaline, but not extremely so.

Gaining organic certification takes three years, and once certification is achieved, the standards must be maintained. Audits and inspections occur to ensure compliance.

Writing ‘organic’ on a product does not guarantee quality

When David bought the manure, he recalled the bag stated it was ‘organic’. This is not the same as Australian Certified Organic. Consumers must look for the certification logo as proof.

It is a ruse to state a product is organic, rather like adding the term ‘eco’ or ‘green’ to a product, it implies it might be more environmentally benign than competing products.

I am a human therefore I am, biologically speaking, an organic creature. Plants and manures too are organic. However, I and they are not Australian Certified Organic. The difference is distinct.

Deeply concerned, David contacted me for advice. The standard way to correct alkaline soil is to dig in powdered sulphur. This is highly effective, but it is a gradual process. Too slow for a display that needs sowing by mid-August.

Applying iron sulphate is also very effective. It is faster than powdered sulphur, however it too requires more time to work than is available. Watering in vinegar is the fastest of all, but the dilemma with any soil conditioner is that you cannot safely sow or plant in soil while its pH is changing. Changing soil chemistry can harm developing young plant roots. Three weeks is the standard time that should elapse between adding a soil conditioner and sowing or planting. Two weeks is rushing the process.

To check the chemical reaction is complete, the soil should be tested in various locations at least once to confirm what the resultant pH is.

David didn’t have the time, so we agreed that removing the soil to the depth of one spade deep (roughly 30cm) and re-soiling the bed with a quality topsoil would work.

Where to find quality planting mix or soil

There are two ways to source quality soil: buy agricultural loam gathered from a greenfield housing development site, or buy from a trustworthy commercial source. Before purchasing, it is very wise to pH test the soil or planting mix.

The lessons learned here is that a cascade of problems can result in:

* assuming ‘organic’ manure is ready and safe to use;
* not knowing the difference between ‘organic’ and Australian certified organic; and
* not testing the pH of soil improvers – like cow manure – before purchase or use.

Feedback from David

“Thanks indeed for all this info and for your assistance. I will follow your blog and ABC Radio North West Slopes and I look forward to learning more about gardening. I’m awaiting soil tests from a Lismore lab to see a deeper analysis of my soil. Thanks for the deep knowledge you freely share with others. Greatly appreciated”.




Jerry Coleby-Williams
Patron, National Toxics Network
5th August 2021

5 Comments Add yours

  1. nologner says:

    Thank you so much for this article Jerry,

    I had several problems with bought bags of cow, sheep & horse manure last year – all of these seemed to affect my soil biology in a negative way. So this year I am going back to my never-fail green manure crops and home-made compost.

    Thank you again, Jo

  2. Stephen Besnard says:

    How interesting thank you

  3. John Lee says:

    Thanks, Jeremy, for clarifying the use of soil testing; and the logarithmic nature of the pH scale (which I’d forgotten since I ceased work in the now defunct School of Chemistry at UWA!).John L.

    Sent from Yahoo Mail on Android

  4. bernice says:

    Wow, this is a bit of a revelation ☹
    In starting our community food forest we hauled a load of very old cow manure from the showground (being free!) – but some of the plants in that section are not looking so happy. I keep forgetting to take the PH test kit when I go, but I will definitely do that now!
    Loving the blog and GA! Thank you 😊

  5. Margaret Sadler says:

    Wonderful advice, Jerry. The detail of your knowledge is doing in my feeble mind but there is obviously so much science in our basic efforts to grow our own food and the breadth of your generosity is truly heartening. Thank you so much.

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