Posts Tagged ‘soil magnesium’

Farm soil testing – major nutrients

Tuesday, May 6th, 2014

Up to now we’ve looked at some of the main physical factors in the soil at the Gembrook horse property. Now its time to look at the other main nutrients and the soil’s fertility.

Soils have an ability to loosely hold nutrients to prevent leaching and to make them slowly available to plants. This applies mostly to the positively charged nutrients like calcium, magnesium and potassium. This is called the Cation exchange capacity and it depends on the type of clays but is higher in good structured soils like loams. Because organic matter also holds these same nutrients the amount of organic matter affects CEC.

This soil has a CEC of 17.76 meq%. This value is around mid range with sandy soils at around 1 meq% and organic soils around 30 meq%.

Nutrients are assessed in terms of absolute level, relative proportions, percent of nutrient holding capacity and levels compared to non nutrients and acidity.

Major nutrients weight for weight in the soil.

Nutrient Result Recommended range Level

ppm ppm
calcium 1859 1200 – 2000 good
magnesium 297.5 150 – 300 good
potassium 400.7 180 – 300 high
Horses at Gembrook eating hay during Autumn. Good nutrition is essential for growing or working horses and this can start with good quality pasture. A soil test can identify problems that can lead to nutritionally poor pasture and further health problems.

These horses at Gembrook are being fed hay during Autumn. Good nutrition is essential for growing or working horses and this starts with good quality pasture. A soil test can identify problems that can lead to nutritionally poor pasture.

The overall level of the major nutrients calcium, magnesium and potassium is good but a look at the makeup of the exchange capacity tells a different story.

Nutrients as proportion of (CEC) with comparison to non nutrients
Nutrient % of nutrient Recommended Level
holding capacity range
calcium 52.3 60 – 70 low
magnesium 13.8 12 – 20 ok
potassium 5.8 4 – 6 ok
Non nutrients
sodium 1 < 4 ok
exchangeable acidity 27 13 – 20 too high

The proportions (of CEC) of calcium and magnesium are low. At the same time, exchangeable acidity proportion of CEC is very high. So the soil has a too high proportion of acidity taking the place of more desirable nutrients. Its like saying, the soil has the capacity to hold more nutrients but they are not there. Instead, their place is being taken by acidity.

For similar soils, the higher the exchangeable acidity, the lower the pH.

Another way to look at this is to say that in this soil desirable nutrients (plus some low levels of sodium) occupy 73% of available capacity. This figure is called the base saturation. The base saturation represents the proportion of nutrient holding capacity CEC that is actually occupied by desirable nutrients. Around 80 – 87 % is considered desirable. The non desirable proportion is exchangeable acidity.

Sulphur was also measured in this soil. The result was 17.6 ppm. Around 10 – 40 ppm is considered desirable so the sulphur level was OK.

To change the pH of the soil some of the exchangeable acidity will need to be neutralized. This has to be done gradually, usually by applying lime. As the lime neutralizes the acidity, calcium and magnesium (if using dolomite lime) slowly occupy more of the exchange capacity and will slowly raise the pH by increasing the base saturation.

Acidity and major nutrients in dairy farm soil

Tuesday, December 3rd, 2013

What is the connection between soil pH, acidity, nutrients and amount of lime required to raise the soil pH?

When we think of soil acidity most people think of pH. But pH is a measure of active acidity which can be measured with a meter, test strips or indicator solution or powder. In simple terms they measure hydrogen ions in water that’s in the soil.

But there is a ‘pool’ of acidity that is held in the soil. This is called exchangeable acidity and it creates a balance with pH in the soil solution.

An important property of soils is their ability to hold nutrients such as calcium, magnesium and potassium and make them available to plants. This is called the ‘exchange’ capacity and it is generally larger for soils with more clays and organic matter. But this capacity can be partly taken up by exchangeable acidity.

For agricultural soils generally, as pH increases (less hydrogen ions), exchangeable acidity decreases. But also with increasing pH the total exchange capacity of the soil increases and this capacity is taken up with a larger proportion of desirable nutrients. In soils with pH around 5.5 to 7 exchangeable acidity should taper off as pH rises with more of the available exchange capacity occupied by nutrients.

Soil samples were taken on a West Gippsland dairy farm at the same 3 sites described in previous entries. Exchangeable acidity was extracted with KCl salt solution. Exchangeable calcium and magnesium were extracted using Double Acid (Mehlich 1).


Exchangeable Exchangeable Exchangeable

acidity calcium magnesium
Site pH meq% meq% meq%
1 5.5 0.76 7.24 0.93
2 6 0.13 17.00 2.46
3 6 0.32 26.10 9.83

Typical figures for exchangeable acidity reported for other soils range from 0.5 to 1 meq% so Sites 2 and 3 have low exchangeable acidity.

Typical values for exchangeable calcium can range from 0.23 to 12.5 meq%. Typical values for exchangeable magnesium range from 0.25 to 4.2 meq%. Calcium levels are moderate at Site 1 to high at Sites 2 and 3. Magnesium levels are low / moderate at Site 1, moderate at Site 2 and high at Site 3.

West Gippland dairy farm Site 1. Of 3 sites tested on this farm, Site 1 has lowest pH, organic matter, phosphorus, calcium and magnesium. But exchangeable acidity is highest here.

West Gippland dairy farm Site 1. Of 3 sites tested on this farm, Site 1 has the lowest pH, organic matter, phosphorus, calcium and magnesium and it has the highest exchangeable acidity.

The unit meq% used to express acidity and nutrients is designed to allow a direct comparison between the amounts of each held on exchange sites in the soil. It also provides the mechanism for working out how much lime to apply to soil.

As lime is applied to soil it slowly reacts with the soil acidity. The active (pH) acidity is constantly replenished from the exchangeable acidity but in the process some of the calcium (and magnesium for Dolomite type lime) becomes attached to the exchange sites. The lime will displace some of the exchangeable acidity. This raises the proportion of desirable nutrients to acidity and in doing so, raises the pH.

One approach for working out how much lime to apply to raise the pH is to calculate how much would be required to neutralize the exchangeable acidity. At least this takes the guess work out of liming. Tests like the ones shown here can be used to monitor progress.

Another related approach is to estimate or measure the occupied exchange capacity then by using a diagram of pH vs exchange capacity decide the percentage change required to raise the pH a particular amount. See the reference below for more details.

So far, tests for organic matter, pH and some major nutrients have shown significant differences in fertility between paddocks on a dairy farm.

Further reading: Soil test interpretations by Apps Labs.