The Mole Truth and Nothing But the Truth

By Andrew Price FAIH RH0004

A common theme of advice has been given to me over the years by the gardeners, Horticulturists and Botanists whose work I admire is the very old adage “Fertilise your plants little but often”.

When I was younger and filled with misdirected youthful vigour, a client of mine was unhappy with the health of her roses and knowing I had exhausted all avenues that I could think of at the time she directed me to call a well-known rose nursery for expert advice. Having had my Horticulture qualifications for longer than a decade at the time I had to admit I was missing the mark and had to get better.

So I swallowed my pride, steeled myself to be open to direction and not be defensive or argumentative and nervously made the call. I must have called at a bad time, as I described what I was doing to help get the plant back to health so not to sound like a complete horticultural failure of a gardener. I was told what I needed to do was ‘Feed the Plants’. The conversation ended shortly thereafter. I hung up the phone thinking, do roses like rare or medium cooked NY Steak?

This interaction intensified my resolve to ‘Get Good’ at the application of nutrients countered by the medical Hippocratic Oath of ‘DO NO HARM’. I didn’t study then practice horticulture to make the plants in my care unhealthy nor did i want to contribute to the environmental problems associated by the misapplication of fertiliser.

Anyone who knows me well will attest to the fact that this subject is a personal near psychopathic obsession, so much so that my family and friends nicknamed my first nursery ‘The Meth Lab’. An apprentice of mine at the time was assisting me in making a batch of fertiliser when his girlfriend called, he said he couldn’t talk because he was Breaking Bad.

It has taken me awhile to acknowledge the biggest truth in horticulture that light is the ultimate fertiliser, plants convert photons/light (PAR – Photosynthetically Active Radiation) into carbohydrates then generously secrete about 50% of it to their root tips to feed the billions of specific microorganisms that support that specific plant. Nutrients only assist in this chain of events and plants are exceptional at compensating or adapting to available resources.

That being said there is a big difference between cultivation and nature, the smart money is in the grey area between the two. If you wish to cultivate a plant out of its habitat you must supply the nutrients that the species needs otherwise the result will not be one that makes your account manager or more importantly your clients happy.

This brings me to the point of this article which is how much and often should I dose nutrients and what nutrients? The Australian band You Am I penned a pertinent song called How Much is Enough, not a song about fertiliser but a great song nonetheless.


Image Source: Planet Permaculture


Molar Mass is not referring to a calorically-challenged spook but in this case to the concentration of elements in a compound. Synthetic or organic compounds are what we use on a daily basis.

To get the ball rolling I’ve selected an interesting compound that gets used often – Epsom Salts or Magnesium Sulphate to the chemically-educated amongst us. Referring to our trusty periodic table of elements Magnesium has the molar mass of 24.31 while Sulphur is 32.07 with Oxygen at 16 x4. When these three elements are combined to make a compound the atoms balance together to give us the equation of MgSO4 with a combined molar mass of 120.38 grams per mole.

It must be noted that usual Epsom Salts that gets used in the household is a hydrated compound – heptahydrate MgSO4·7H2O, but of course you know that!

Ok to get the Mg wheel hitting the road and getting those Chloroplasts to green up, how concentrated is 1 Mole of Epsom salts in 1 litre of water in Parts Per Million (PPM) or as I like to say how many cents or dollars in a million Dollars?

Now hold onto your hat based on the above weight of 1 Mole of 120.38 grams if you added this to 1 litre of water you would have a reading of 120,380 PPM. This is obviously way too high for plants or anything else unless you are on Mars.

1 gram of any salt compound in 1 litre of water will give you a reading of 1,000 PPM. It’s worth noting that Mg in seawater is around 1200 PPM, this should give us pause to think about getting heavy handed when applying any fertiliser.

1 gram of pure salt dissolved in 10 litres of water is a good starting point when looking at fertilising on a regular basis. This will give you a reading of 100 PPM, still pretty high but effective if you’re using the required salt compound.

The philosophies held close to my heart are ‘Don’t Chase the Numbers’ and ‘Does Nature agree?’

A game of observation I play when looking at anything in my care is the What’s different today game, this forces me to notice things that might get overlooked.

These are some curveballs to consider in fertilising:

  • Power of pH – check and balance your solution to somewhere between 6.5 – 7, consider wood vinegar on alkaline readings, PyroAg is my go to. Deficiencies can be caused by a pH shift that makes that element unavailable or inaccessible to the plant.
  • Liebig’s Law of the minimum – Growth is determined by the least available nutrient not the most abundant. Plants need at least 17 elements but could be as high as 28.
  • Mulder’s chart – nutrients affect nutrients in either a synergistic or antagonistic way, e.g. too much Mg will affect K and vice versa.


Mulders Chart Image Source: Nutri Ag


Brewed Fertiliser is a great way to magnify a fertiliser so you get a much better result for less input because you are cultivating the beneficial microbes that the plants need.

Time to get back to work cultivating that beloved grey zone between nature and cultivation -Happy Spring!


Andrew Price FAIH RH is a Registered Horticulturist and Fellow of the Australian Institute of Horticulture and Principal of Jungle Horticulture based in Sydney.


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