Are you aware of Magnesic soils?

You may have heard of ‘sodic soils’ and even seen instances where such soils have been permitted to interact with water and subsequently dispersed. But sodic soils are not the only soil chemistry problem which may result in a soils tendency to disperse. As a result, simply looking at your Exchangable Sodium Percentage (ESP) lab result is likely to result in application of incorrect soil management practices. Magnesic soils can pose similar problems.

Two recent projects have highlighted the occurrence of these soils and the photos below indicate the results of poor management.

PHOTO 1 – Significant soil loss from what started as a small roadside table drain, now over 4m deep.

PHOTO 2 – Deep incised rilling and tunnel erosion to an exposed road embankment. Note dumping rock directly on magnesic soils will not address the problem with the limited vegetation performing much better.

Soils like most things require balance (not the standing up kind, or the good balanced diet kind) but in terms of cations. An imbalance of say sodium, with the instance being too much is referred to as sodic soils. An imbalance in magnesium is you guessed it referred to as magnesic soils. A typical proportion of major cations (i.e. desirable for many, but not all plant species and soil conditions) is along the lines of:

Calcium – 65 to 80%
Magnesium – 10 to 15%
Potassium – 1 to 5 %
Sodium – 0 to 1%
Aluminium - <5%

Whist the concentration of magnesium (Mg % of cations) in soils required to cause dispersive conditions is an order of magnitude higher proportion than that of sodium it is an occurrence which occurs relatively frequently within certain locations and landscapes. The structural stability of soils containing greater than 10% clay is largely dependent on the balance between calcium, magnesium and sodium. 

The treatment for magnesic soils is very similar to sodic soils, given the problem is similar (i.e. imbalance in cations) and likely deficiency in calcium being the cation generally sought in the greatest concentrations. Application of gypsum has proved effective in treatment of magnesic soils, along with diversion of runoff, covering with non-dispersive material and establishing vegetation for long term soil stabilisation. Gypsum application rates should be calculated based concentrations of exchangeable magnesium relative to cation exchange capacity (CEC). Given the ‘target’ concentration for magnesium is much greater than sodium the amount of gypsum required for a given area is likely to be similar or less than highly sodic soils, however as always this is not a guarantee.

We have all heard the saying ‘the output is only as ever good as the input’ or the ‘findings/results are only as good as the data’. Experience shows this is 100% true, but having the data is one thing and understanding what it means is another. Too often we see poor soil management decisions leading to very expensive failures or rework onsite as a result of someone basing decisions off a single result (i.e. ESP result, or Emerson Class) in isolation and not having an understanding of the relationship between key soil properties. Getting specialist advice will likely save you doing unnecessary soil management and significantly improve revegetation productivity following works. It is possible to do…