HAS THE WORM TURNED?

Gypsum is widely touted as being a ‘clay breaker’ but does the humble earthworm actually do a better job of improving the soil?

PART ONE OF A TWO PART BLOG

Let’s start by looking at what gypsum is and what it does.

Gypsum is calcium sulphate (in dry form the formula is CaSO4) and is a naturally occurring mineral found in arid, inland areas of Australia.

There is a significant difference in the purity of Australian gypsum deposits.

A 2015 study of the gypsum purity from multiple different mined products along the east coast of Australia by Pacific Fertilisers showed a range of purity from 60 to 98%.

There are also some industrial processes (including high analysis fertiliser production) that result in the formation of synthetic forms of gypsum* with the most common being where calcium carbonate (lime) is used in a ‘scrubbing’ process to remove sulphur dioxide from gas pollutants in fossil-fuelled power plants.

 

Gypsum is neutral pH and will not raise soil pH levels.

So, although it will supply calcium to the soil, if you need to raise your soil pH you should be using agricultural lime or dolomite in preference to gypsum.

But gypsum as a calcium compound is significantly more soluble than both ag lime or dolomite in the soil and this can have interesting ramifications, particularly if the gypsum is finely ground.

In this situation, the quick ‘flush’ of calcium into the soil can potentially depress plant availability of cations^ other than calcium, the particularly important ones being potassium and magnesium.

This can be exacerbated where the soil may already be high in calcium.

This quick flush of Calcium also means that any positive effect from gypsum applications are relatively short-lived.

It’s a rabbit rather than a turtle, to coin the analogy.

 

Basically, how gypsum works as a ‘clay breaker‘ is that, after application, as the sulphate sulphur from the gypsum moves through the soil profile with rainfall/irrigation it attracts and binds sodium to it and takes it with it on its’ trip ‘south’ through the soil profile^^.

Effectively, it removes sodium (which if present in sufficient quantity negatively impacts soil structure) out of the upper soil profile.

So, the answer as to whether you need gypsum for your garden is largely determined by whether you have a soil type at your place that requires it and will respond to applications.

Firstly, gypsum has no value as a ‘clay breaker’ if there is no clay!

(Or even low clay).

Sounds obvious of course, but sandy soils will not respond to gypsum applications with improved structure.

And, in fact, because nutrients can be very mobile through sandy soils, the sulphur from gypsum could significantly hasten the loss of important cations out of the upper soil profile.

The soil at our own property is very sandy for the first 0.5-0.8m depth and when we first purchased the block we soil tested to check the status and confirm (what we suspected) that there wasn't much of anything beneficial in the soil.

The results are shown in the image below.

 

I have highlighted the Cation Exchange Capacity (CEC) which is basically a measure of the soils ability to hold on to cations and not leach them through the soil profile.

The reading is only 1.81, which to give you some idea is what the Department of Primary Industries NSW would class as ‘dune sand’!!!

By way of comparison, soils vary in CEC but a high pH, high organic matter clay soil might have a CEC of between 30-40.

(Organic matter has a CEC of 250-400).

Note also the sodium % of cations (exchangeable sodium percentage or ESP).

This reading of 2.81% is interesting.

It shows a very low percentage of sodium (relative to the other cations) in the soil.

Some overseas work has suggested an ESP of 15% is the threshold for sodium to start affecting crop yields in agricultural situations.

Australian research has shown (on heavy clay soils) diminished crop yields from levels as low as 4%.

The bottom line to all this is, one can't expect gypsum to do a lot for you on sandy soils and, in fact, it may even be detrimental by hastening the leaching of some nutrients beyond the plant root zone.

So, we come back to the soils where potentially you might get some benefit from the use of gypsum.

And those would be high sodium (sodic) dispersive clay soils.

They present as having surface crusting, poor aeration, low microbial activity, poor draining and poor structure.

Further, if puddles of water in your soil are cloudy or muddy it might indicate your soil is dispersive clay and therefore responsive to gypsum

So, do you have these soils naturally occurring on your property?

And if you have established garden beds with either imported garden soil and/or good organic matter levels and lots of microbial activity does it even matter?

For the first question, here in Queensland, sodicity is a common trait of our naturally occurring soils, with 45% considered sodic to some degree.

The Queensland Government have a technical bulletin on sodic soils that includes a map of Queensland showing the general distribution of sodic soils within our state.

Its a useful map, but it's a macro view as opposed to what you might want, a close up of your specific property.

But it does show the wide distribution of sodic soil in our state.

So, without soil testing through a laboratory or having a site specific soil map to refer to, how do you determine whether you are going to get a response from gypsum?

Sometimes the KISS principle of keeping things simple can't be beat, and nowhere is this better demonstrated than with an ABC Gardening article from back in 2006 where  noted soil scientist Simon Leake explains how to do simple home tests to determine whether you will get a response from gypsum applications.

As for established garden beds with high organic matter content, that's where earthworms enter the picture and is a subject for Part 2 of our blog (coming soon).

 

^Cations are positively charged ions. Soil cations include Calcium, Potassium, Magnesium and Sodium. Clay and organic matter have a net negative charge, so these cations are held by the soil in the same way a magnet attracts metal. Sandier soils have less negative charge, and therefore these cations are potentially more mobile through the soil profile (beyond the plant root zone) on sandy soils.

^^Sulphur is a negatively charged ion (as is phosphorous) As well as the amount of clay and organic matter in a soil affecting how well the soil holds on to cations, cations themselves vary in their ‘holding’ ability. Calcium is the most strongly bound followed by potassium, then magnesium and finally sodium. This is why the sulphur binds with sodium (as opposed to the other cations) to strip it out of the soil.

*Most of the gypsum I have seen for sale has been from naturally mined sources, but if you do come across a by-product source for sale just check the heavy metal cadmium and fluoride levels of the product before proceeding with purchase.