An Introduction to Basic Soil Chemistry for the Home Gardener

If you are like me you would rather be out in the garden or having bamboo strips inserted under your fingernails than trying to remember your high school chemistry. Phrases like “cation exchange capacity”, “cations and anions” and “bric” give you an instant headache and leave you running for aspirin, a good glass of red and if you’re lucky a pack of TimTams. So for me to put you (and me!) through reading (and writing!) this article you know I must think that understanding basic soil chemistry is very important to your garden. Too many people think that throwing a slow release fert at their plants twice a year is all they need to do to maximize growth and flowering. And some even think if twice a year is good 4 times a year even better, Right?
Wrong, Wrong, Wrong.
Yes, NPK are important but they aren’t the only nutrients your plants will need and they need to be used wisely. For any of you that don’t know or may have forgotten:
N: Nitrogen important for the formation of chlorophyll and very important for growth.
P: Phosphorus important for healthy roots and shoots. Yellowing of the leaves is a common sign of P deficiency.
K : Potassium Vital to photosynthesis and protein synthesis and improves water use efficiency ( in English, makes plants more drought tolerant) Regulates about 50 enzymes and heliconias love it. Also seems to contribute to better coloring up of cordylines.

If you are over fertilizing you are throwing your money away as plants only pick up what they need and what is “available” to them. Most likely you would be better off spending your money on a simple soil test kit.
At minimum you should know the PH of different areas of your garden. The PH is very important and affects the plants ability to use available nutrients. PH is a measure of hydrogen ions and is the result of total cations in the soil. I’m only telling you this because CEC or Cation Exchange Capacity is a measure of your soil to store nutrients. Clay soils have lots of stored nutrients but they are not always “available” to your plants. . In heavy clay soil you can add calcium (usually in the form of lime) to lower PH which allows for better nutrient exchange. Sandy soils need fertilizing more often (a little, a lot) because they can’t store nutrients well.
Aside from PH it is important to know the level of minerals in your soil and ideally work towards correcting any imbalance. That is because an excess of one element can cause a deficiency in another by inhibiting the plants ability to utilize it.
The most common problems occur when people add too much nitrogen to the soil. This can lead to burning through humus, potassium deficiency and sappy growth with thin cell walls that are more attractive to sap sucking insects. DPI studies on bananas in North Qld have indicated excess nitrogen in the Wet can feed Phytophthora. Also remember that at different parts in a plants growth cycle plants will take up extra nutrients. There’s no point on putting extra potash on a plant when it’s just starting growing it. It would be much more effective to use before flowering occurs to encourage big healthy flowers. Remember Sulphate of Potash is a much better form of potash as Muriate of Potash is %0% chloride (Salt) and can kill your soil. Wood Ash and Kelp are also good sources of K.

Often gardeners may also need to improve the structure of their soil. Fertilizer alone will not do this for you. If you have sandy soil amend your soil with organic matter and zeolite. Natural zeolite has an open box work crystal structure which is occupied by cations and water molecules.  These ions and water molecules can move within the large cavities in the structure allowing ionic exchange and reversible rehydration plus they have a very high micro-porosity.  These special features enable zeolite the ability to attract and absorb cationic materials such as plant nutrients for slow release. If you have heavy clay soil adding organic matter will help over time but adding gypsum can help get you there quicker. Gypsum breaks up heavy clay soils by chemical exchange between the clay and gypsum, causing soil to become more friable. The nutrients in the clay become more available to plants as well as an improves drainage.

I could go on and on dazzling you with soil chemistry information that I’ll plagiarized from the internet but I think it’s time to wrap this up succinctly and say if you are serious about your garden buy a simple soil test from your local hardware store or even better splash out and get a qualified agronomist or horticulturist to do a soil analysis for you. In the long run it could save you heaps of money and improve your overall soil health.
If this is all too scientific for you the bottom line is that by adding as much mulch as humately (get it, humately) will improve your soil structure and increase nutrient uptake. And when in doubt kelp and fish emulsion are excellent for giving your soil and plants heaps of yummy micronutrients.

If for some reason you are still awake at the end of this article and would like to find out more about soil chemistry I recommend the book Soils Alive published by Lawrie and 61 sustainable agriculture S.A. It goes into great detail about mineral management in your soil but is accessible to the layman.
And if you decide to ignore everything I’ve just written and throw slow release at your plants twice a year at least water it in! And choose a blend with lots of micronutrients. Mulch! Mulch! Mulch!
Now if you’ll excuse me there’s a TimTam waiting for me.
Happy Gardening