This blog is a bit of a soil chemistry lesson. I hope its not too painful!
On the Swan Coastal Plain we live mostly on sand. Sands have almost no ability to hang on to nutrients. Most of the fertiliser you put on today is washed away by your watering tomorrow unless it is slowly soluble or controlled release. We can measure the ability of a soil or potting mix to hang onto nutrients – that is its cation exchange capacity or CEC. CEC is usually measured in me/100 g and the CEC of our sands is generally around 3. Compare that to a loamy soil which is has a CEC of 5-15 or a clay which is over 30.
But you can add things to a soil to help this. Clay minerals such as zeolite which has a CEC or between 120-145, bentonite (70-120), attapulgite (25), vermiculite (120-150) and spongelite (1-10) are some clays sold for amending soil. Many of these are quite absorbent and so they are used for other purposes such as spillage kits and, kitty litter. However just because they absorb and hold water, don’t think they give it back up easily. Plants actually find it hard to extract water from clays and so, although a clay soil may seem damp, don’t think that that remaining water is all available to the plant. It is also possible that the release of nutrients back to the plant is also not exactly easy! That issue is seen commonly in the form of phosphorus adsorption where clays hang onto up to 90% of the phosphorus applied to the detriment of the plant.
Claying sandy soils for managing water repellence increases the CEC of the surface layers by a small amount depending on type and amount of clay added. Usually, CEC is increased by less than 1 me/100 g.
Some people promote the idea of managing cation ratios, claiming ideal ratios for Ca:Mg or Ca:K. for optimum plant nutrition. This is open to debate, a more critical factor is whether the net amount of Ca or K in the soil is enough in its own right, for plant growth.
Organic matter has a very high CEC. It is dependent on pH (ie how acid or alkaline the soil is). Humus has a CEC of about 200 at pH 8 but only 120 at pH 5. Spagnum peat moss is about 100-250 but cocopeat is only around 50. Adding organic matter to soil does increase the CEC of a soil but not as much as you’d think, especially in sands because much of the organic matter is burnt off over the course of the growing season. Its also very hard to add meaningful amounts of organic matter to soils without running into nutrient leaching problems and potentially polluting the groundwater, paradoxically. Potassium from composts is very readily leached and surprisingly much of the phosphorus is also water soluble and therefore also easily leached. Nitrogen is little better!
Potting mixes, especially those that are based on composted bark, generally have quite a reasonable CEC of about 200 me/L (note the different unit) and maybe up to 300. The easiest way to increase CEC in potting mixes is to add something like zeolite. The new CEC can be calculated mathematically using the proportions of each material (if you are so inclined). Much is said about the ability of zeolite to fertilise. Zeolite is NOT a slow release fertiliser. Its ability to break down and release nutrients is of little real value to a plant trying to grow at any reasonable pace. But what it can do is hang on nutrients added to a potting mix or soil so they aren’t watered away so quickly.
Manipulating the ingredients in a potting mix or soil is one way to help your plants grow better by allowing them to access nutrients for longer. Another thing you can do is use controlled release fertilisers but more on that at another time.
Gardening WA style 