How much calcium is in your farms’ soils? This is a question many farms cannot answer. But, too low a level in your soils could have a big influence on how much milk goes in your tank, how much you spend on feed and fertiliser, and on the health of your stock.
Understanding soil Cation Exchange Capacity (CEC) and Base Saturation (BS) can be real game changers for your farm. The science can seem daunting at first but is pretty logical. The starting point is soil testing.
Testing Times
Traditional standard soil tests will give pH, P, K, Mg. While this is fine for following fertiliser recommendations out of RB209, they tell us next to nothing about the soil. They certainly don’t answer our question about calcium. We need to challenge our systems, and to do that, we need better data.
Most of the labs now offer more broad spectrum tests. These will give us the CEC and BS values we need.
Cation Exchange Capacity (CEC)
CEC is a measure of how many cations your soil can hold. These cations are essential for soil condition and plant nutrients. The higher the CEC the more of these cations can be stored.
This CEC comes from negatively charged clay soil particles and organic matter. We can build more CEC by building organic matter.
The image shows a representation of a clay particle. Scattered all across its surface are the cations (positively charged ions) of Calcium (Ca), Potassium (K), Magnesium (Mg) and Sodium (Na).
These are known as base cations. The percentage of the CEC that is taken up with the base cations is the Base Saturation.
There are other types of cations too. The relevant ones are the acid ones Hydrogen (H) and Ammonium (NH4), and the metal ones, Aluminium (Al) and Iron (Fe).
We are very familiar that the measurement of the hydrogen gives us the soil pH. The more there is, the lower the pH. But importantly, if hydrogen is taking up space on the CEC, then this has taken a place of a useful nutrient.
It is likely your farm soils could have a range of CEC values. If you have any peat soils, then the high organic matter content will give it a pretty large CEC of over 40 meq/100g (meq stands for millequivalents). Sandy soils with low organic matter may only has a CEC of <5meq. The typical range for loam soils is 5-15 and clay loams 15-30. But as they say, it’s not the size of your CEC that matters, its how you use it!
Base Saturation (BS)
The percentage of your CEC that is covered in base cations (Ca,Mg,K,Na) is a useful metric. At Dairy Club a threshold value we use is 75%. Anything below this puts up big flags!
So if we have a CEC of 10 meq, then the sum of adding up all the meq of the base cations should come to al least 7.5 meq/100g.
Below is a result from a farm with a CEC of 14 meq/100g. The sum of all the base cations comes to just over 8 meq/100g. This gives a BS value of 57.2%. This is well short of our baseline threshold of 75%. The chart shows the proportions of cations held around the clay particle. In the middle of the chart is a microscopic view of a clay particle. It is easy to see the huge surface area that it has due to the layers and layers of sheets of material that it is constructed of.
The analysis tells that the CEC has 1.4% of it covered in potassium (K). As you know K is essential for plant growth. Our target level should be between 2 and 5%. In this soil we are missing out on having our reservoir of K available for the plants to draw on. We could look at our strategic use of manures to correct this over time. In terms of RB209 values it is an Index 1.
Calcium (Ca) levels at 49.3% are also well below the target. Calcium is probably the most important one to get right and probably the most under appreciated plant nutrient. Too little calcium can lead to soils that compact, and compaction is the last thing we need!
The calcium is easily corrected by liming with calcium carbonate. For this soil we will get a win win by liming as the hydrogen level is far to high. The acidity of this soil is 5.8 pH. Below is the chemical reaction when we apply lime.
Lime CaCO3 + 2H+ = Ca2+ + H2O + CO2
By exchanging the hydrogen cations for calcium cations we have removed the acidic hydrogen and turned it into water. Having removed the hydrogen from the clay it leaves a space that is then filled by the calcium. It’s almost like magic 🙂
Where we get Ca saturation level > 55%, soils do tend to open up. The electrostatic positive charge that holds these cations to the negatively charged clay is the same charge that will repel another positively charged Ca ion. This is like trying to put two magnets together the wrong way round. They push each other away.
This is extremely useful as open soils: drain better, hold moisture better, allow root penetration better and very importantly, reduce the incidence of headaches in earthworms!
Magnesium (Mg) at 4.9% is borderline too low. Whilst this isn’t the end of the world in this soil, we need to be very mindful that chlorophyll (the green bit in plants that generates sugars) has Mg right at its core. So if the plant cannot access sufficient Mg from the soil pool, then our sugar yield and therefore ME could be reduced. An amber flag means we should watch this one.
Sodium (Na) is the smallest targeted portion. At 1.6% we don’t need to take any action. This isn’t always the case and many soils need extra sodium in the pool. Sodium is great at making grass taste palatable and can also be used by the plant where potassium is low.
Soil structure is fundamental in our farm systems
Soil structure
Soil structure is fundamental in our farm systems. Whilst most fertiliser programs focus on the needs of the crop, they fail to manage the medium in which the plants grow, being the soil. This should be the first thing we get right.
It’s easy to see that putting more rumen fermentable protein into a cow that has insufficient fermentable energy is wasteful and potentially damaging. Yet we do it frequently with soils and end up with very poor nitrogen recovery rates.
Time spent physically looking at soils as well as looking at analysis is well worth it. Be the ace of spades and look at the rooting, the texture, the smell, count worms, rusty spots, gold. All these things will bring you closer to a solution.
How much of your fertiliser and cash has been lost to the air or leached away because soil structure wasn’t a focus?
The appliance of science
There is a fair bit of material on the internet about the use of base saturation (BS). A portion of that information focuses on the work of William Albrecht who really helped highlight the processes. Some of the advice around BS is great, other bits are more ‘BS’ than BS 😉
Ultimately for you it’s about knowing your farm. You will have fields which are your favourites. In theory you should love all your fields the same, but it’s difficult not to have favourites. These are the ones that yield the best, don’t go rusty at the first sign of fog, don’t give cows hypomag when grazed, drain well, hold moisture in the summer, dry in the winter etc. If you test all your fields, get the results and lay them all out on the dining table, reference them all back to your favourite field and look for the opportunities to correct any problem fields.
Ultimately these results are clues to the puzzle of why there are underlying problems with plants and animals on your farm. The thought process is just as valuable as the numbers.