Water use efficiency in a high magnesium soil

Does a soil’s cation(Ca Mg K Na levels) balance have the ability to control how a soil accepts  water ?

My thoughts on the  Murray River Symposium, on “A soils ability to accept or repel water”

Introduction: Working with irrigation clients in Victoria, NSW and SA, with some on the Murray River at Murray Bridge, in many of these cases we initially found irrigation water ponding and slow to filter into the soil profile. On analysis of these soils we found that most contained high levels of exchangeable magnesium with very low exchangeable calcium, Eg Ca 40% Mg 38%, and as a result of the excess magnesium there was high to very high soil sodium. The pH was always ideal to excess even with the absence of calcium. Once soil calcium was increased to 60-70% BSP which naturally decreases exchangeable Mg we see water being quickly absorbed into the soil and leaching sodium, so the application of lime (Ca) even on these good to high pH soils, had a three fold beneficial effect – decreased Mg improving soil structure, improved water utilisation and reduced soil sodium

It is my belief that by encouraging a better understanding of the soils nutrient balance we can improve soil water utilisation reducing water usage.

Soil Nutrient Balance

If we understand a soils cation balance (positive charged elements Ca, Mg, K, Na) and the associated beneficial or adverse effects of this balance, we see a greater window of opportunity to optimise the utilisation of irrigation water. If the soils cation balance is poor then water use efficiency is low, if the balance is good then water use efficiency increases and a smaller amount of water is required for a better result. As 67% of Murray water is used on pasture and cotton there is an opportunity to significantly reduce the volume used and still maintain or increase production. Another problem is the high nitrogen applications for cotton production and moderate to high applications for pastoral areas, especially dairying. These applications deplete exchangeable calcium which in many soils is replaced by exchangeable magnesium and as a result the soils become tight, impermeable and have a high water shed, so more water is required to wet the same soil. An increase in the exchangeable magnesium often results in an increase in soil sodium which significantly compounds the problem. By restoring the cation balance by increasing soil calcium closer to the desired 68% base saturation percentage (BSP), soil magnesium is reduced and as a result soil sodium will leach.

Ideal Cation balance

In evaluating the cation percentages (BSP) of a soil we can partly understand many of its characteristics. Firstly for the purpose of this exercise we must accept that the following are the ideal exchangeable cation percentages. As these percentages change then so does the soil structure




If the cation balance is close to or ideal then we see a good soil structure, good root development, good moisture movement into the soil and good potential for optimum pasture/crop production.

Magnesium: When magnesium dominates the BSP the soil will be wet and sticky in the winter or during irrigation and hard setting in the summer. Soils crust easily, soil air space is limited and soil microbial activity is below optimum. The pH however is normal good to high due to the influence on pH by magnesium irons. Water utilisation is very poor in these soils.

Pasture and crops develop a shallow root system. Elements sulphur, boron, iron and sodium are often high due to the soils poor permeability.

Potassium: High potassium soils can also be poor in structure and permeability. They normally show a good pH due the pH forming effect of the potassium ion.

Calcium: If calcium is in excess the soil will show a very loose soil structure with very poor water holding capacity.

Sodium: Sodium can show high readings due to two factors [1] High levels of sodium moving into the root zone with an increased water table. As the level of exchangeable sodium increases exchangeable calcium is displaced. High sodium will also result in a good to high soil pH. [2] High nitrogen applications depleting soil calcium which is often replaced with magnesium. As soil magnesium increases, increases in soil sodium follows due to reduce permeability.


These are the soils cation levels from a dairy farm on the Murray River at Murray Bridge. See the calcium and magnesium ratio at 1:1. Irrigation water pooled and most ran off. After the application of 5 tn of lime per hectare irrigation water now penetrates and the farmer reports a reduction of 30 to 40% in the amount of water required to wet the soil. Follow-up soil samples are about to be taken to evaluate soil changes.


      Example 2 shows the cation levels on a irrigated dairy farm both before and after a lime application correcting soil calcium.

The farmer reported that prior to treatment and in 2002, following irrigation dairy cows were with-held from the area for up to 7 days before grazing due to the very wet soil conditions. Following treatment with lime, with time to react, cows were able to graze almost immediately following irrigation. Plus irrigation water required was significantly less.





The real issue here is soil structure, if we are able to change the soil structure by changing the balance of the cations we are then able to see beneficial changes by the way water behaves in that soil.

These are only a few anecdotal cases but I believe that there is a place for research funding to further study this issue. If we can change the way a soil accepts and utilises valuable irrigation water and if reduced water can result in similar production then more water will be available for the river.


Bryan L McLeod

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