So, what's CEC then, is it important and how do you raise it? We don't have a clue! But here are four knowledgeable heads that do; Jerry Spencer, Liam Harper, Dr. Micah Woods and Anthony Asquith all give us their take on the message board's question of the month.
Check out the message board thread and debate here - www.pitchcare.com.au/message/message/502
NuTurf/ Amgrow Agronomy Services Officer
The cation exchange capacity of a soil is a measure of its ability to retain and exchange positively charged nutrients within the profile. This is largely affected by the presence of clay and organic matter in the material. As clay colloids and humic colloids (organic matter) are generally negatively charged, they have the ability to attract and retain positively charged nutrients such as calcium, magnesium and sodium.
These nutrients occupy what are known as exchange sites around the individual colloids. Once in place on an exchange site, the nutrient should be available for plant use through root absorption. The amounts of these exchange sites and their functionality is measured and recorded as a figure known as "Cation Exchange Capacity" (or CEC).
There is no right or wrong figure that the CEC should be at. However if the CEC is below 3 meq/100g (most sands are inherently below this figure), generally it is desirable to raise the level creating a more stable and robust soil environment that is resistant to dramatic changes. A sandy soil with a low clay content and little organic matter will naturally have a very low CEC - of say 1 or 2. In a turfgrass situation the desirable CEC level is about 5 for a sand based profile and about 8 for a heavier soil such as in a natural loamy profile.
To increase the CEC of a profile, the addition of humus based products such as organic fertilisers will help to create exchange sites and then applying essential nutrients such as Ca, Mg and K to fill them.
In a newly constructed golf green with a sand profile, it may take several years for the CEC to naturally rise to a level that is desired by the turf manager. To enhance this process turf managers can utilise various products, such as organic based amendments at renovation times along with regular applications of liquid organic products.
World Wide Turf Consultant
There has been a lot of information over the years about low CEC soils and amendments to increase CEC or moisture retention which are wide and varied. My own research over the years has been to evaluate many of these and whether these are practically economically and based on performance. The type of amendment promoted and used for such purpose would include peat, Zeolite, coconut husks, diomaceous earth rice hulls, gels, seaweed, paper crumble, Gypsum etc.
My evaluation work on these is beyond the scope of this information, however, the performance of many of these are economically and practically questionable. When you look at the maths, you soon realise that to make a difference to the soil matrix in terms of retention, it would require a huge amount of material and applied regular and ameliorated into the rootzone.
In my opinion the best way by far to combat CEC is to fertilizer little and often, high frequency spoon feeding with highly soluble fertilizers. There is absolute minimal leaching on established turfgrass regardless of the soil structure and the difference between using a spoon feeding approach little and often and using controlled release is interesting, under many trials NH4NO3 and Urea outperformed a magnitude of phased release fertilizers, especially when a thick carpet of grass was present. What has been observed on some trial plots is that under increased temperatures or moisture via irrigation or rainfall, there has been evidence of toxicity and leaching losses and some exhibited effectiveness only for a limited period due to it all being released very quickly.
Whereas the experiment using highly soluble fertilizers, little and often, showed spoon feeding approach tends to produce the best surfaces, with less toxicity and leaching as you are in total control. This means you can match your fertilizer programme due to the many important facets such as soil type, turf type, weather, standard of play etc.
(please contact me via email with further information about my work with amendments and low CEC soil feeding below)
Dr. Micah Woods
Director of the Asian Turfgrass Center
Cation exchange capacity (CEC) is important because it is a measure of the nutrient holding capacity of a soil. A great paper, entitled A Simple Method for Estimating Cation Exchange Capacity Across a Wide Range of Soils (Link Here), showed the results of experiments on estimating CEC on a range of fifty different soils from around New York state.
So what is important to know?
* Normal soil test procedures and the CEC values that you see on your soil reports are overestimated, especially for sand rootzones
* Cation ratios or percentages based on these erroneous CEC values are meaningless
* Addition of calcium, magnesium, and potassium fertilizers should be based on the amount of that element in the soil, and not on the percentage of that element
* A nearly foolproof method of applying the right amount of nutrients to turfgrass, even without having any idea of the soil CEC, is to base the nitrogen application rate on the turfgrass growth potential (varies with turf species and temperature) and then to apply an 8:1:4 ratio of nitrogen, phosphorus, and potassium
For more details and a technique to predict the actual CEC of your soils (because the value on your soil report is almost certainly wrong), read on. Take this quote from the abstract of a paper presented: The CEC sum estimates based on Mehlich 3 or Morgan greatly overestimated the CEC of the fifty soils tested in this experiment. This is exactly what I found in my research on soil testing during my time at Cornell. The CEC estimates that you see on a soil report from your laboratory are almost certain to be very different from the actual CEC of the soil. And these errors in CEC as shown on soil reports are especially evident when the tested soils come from golf course putting greens or other sandy sites.
In my research (you can download the chapter about CEC from my dissertation here) I looked at 54 sand and soil samples from around the world. Many samples were collected from golf course sites in Asia. We found that the best way to estimate CEC in sand rootzones is to use a 0.01 M solution of SrCl2 in a rapid five minute extraction. The standard soil tests such as Mehlich 3 produced an inflated value of the CEC. Because commercial soil testing laboratories do not use the SrCl2 method, the best way to estimate CEC for the average turfgrass manager is to use an equation with only two variables (soil pH and soil organic matter) to predict the CEC. Surprising, for sand rootzones, this equation predicts CEC more accurately than does a conventional soil test.
CEC (mmolc kg-1) = soil organic matter/100 * (-311 + (268 * soil pH))
If you are more comfortable in CEC units of cmolc kg-1 (or meq/100 g), then divide the soil organic matter by 1000 rather than 100 as is shown in the equation above.
The poster above Micah prepared for the Oregon State University turfgrass field day: It shows some interesting results on the measurement of plant-available nutrients in sand rootzones. In this poster I showed data looking specifically at calcium, magnesium, potassium, and sodium. Download poster by clicking on above image or here
The soil nutrient analysis (soil testing) procedures used at commercial laboratories were developed for soils that contain clay and silt particles. The sands used on golf courses and other fine turf areas are expressly selected to be relatively free of clay and silt. The data in this poster show that conventional soil test extractants such as Mehlich 3, normal ammonium acetate, and sodium acetate (Morgan extractant) overestimate the available nutrients in sand rootzones.
For the purpose of making fertilizer recommendations, it doesn't really matter whether the extracted nutrients are plant available or not, as long as the fertilizer recommendations are accurate. But for estimating cation exchange capacity (CEC), it is clear that conventional soil tests overestimate the CEC of sand rootzones. Sand has an effective CEC of zero. Therefore, we can use an empirical relationship based on the contribution of soil organic matter to CEC. For a quick and accurate estimate of the CEC of a sand rootzone, use this equation:
CEC = (-311 + (268 × pH)) × (OM ÷ 1000)
CEC units will be cmolc kg-1 which is equivalent to meq/100 g
pH is the sample pH
OM is the organic matter percentage of the sample, in %
In tests conducted at Cornell University on sands collected from golf course putting greens around the world, we found that the simple equation above is a more accurate predictor of the sample CEC than are conventional soil tests. So if you really want to know how many nonacid cations (calcium, magnesium, potassium, and sodium) can be held in a sand rootzone, use that equation. All you need to know is the soil pH and organic matter content.
Endeavour Turf Products (ETP)
What is CEC: The cation exchange capacity (CEC) of a soil is a means of measuring the ability of a soil to bind onto or exchangeable cations such as potassium, , sodium, calcium and magnesium. In other words, it is a measure of the number of negatively-charged binding sites in the soil. Both soil texture and the actual amount of organic matter have a direct influence on the CEC with a high level of clay or organic matter generally being associated with a high CEC, with different clays having different CEC's.
Source: McLaren and Cameron (1996).
Rather than go into an in depth discussion of what can be regarded as a very broad topic I will try and stick to the question: How do you increase the CEC? As a rough guide the following is a means of categorising soils based on their CEC and in the case of sand constructions we are really focusing on increasing the CEC from say 1-2 into the 5-12 range shown below.
Any CEC levels shown in coarse sand such as those found on sand greens or sportsgrounds is almost entirely due to the presence/absence of organic matter. The idea is that to increase the ability of such a profile to retain nutrients the CEC is maintained or actually increased in some instances.
One of the easiest means of achieving this is to raise the soil pH. Soils contain two sources of negative charge, permanent and variable. Permanent charge is located within the structure of the clay particles. Variable charge is located on the edges of clay and organic matter particles.
The primary factor affecting the variable charge is pH and by increasing the pH it increases CEC. Changing pH from 5.5 to 6.5 may actually double the CEC of a sand-based field.
Another commonly used approach is to apply well decomposed organic matter (humus). For example assuming a soil has a CEC of 2.5 and contains 1.5% by weight humus with a CEC of 150 cmol/kg, then it contributes 1.5/100*150 = 2.25 cmol/kg of CEC; thus 2.25 cmol/kg of the CEC comes from humus and the balance from inorganic sources. If the humus content was doubled to 5% by weight it would contribute 7.5 cmol/kg of the CEC.
The total CEC is the sum of the humus fraction plus the inorganic fraction which is then 7 + 0.25 = 7.25 cmol/kg.
Commonly during establishment well decomposed organic matter is added to sands at a rate of 1-3% by weight (5-15% by volume) in the top 30cm of the profile.
A hectare of soil measured to a depth of 30cm weighs approximately 3,986,400kg. Based on this figure, adding 2% by weight into the top 30cm would equate to approximately 79,728kg/Ha of organic matter. These figures will vary if for example you are adding organic matter to a sand rather than a soil due to the different bulk density figure.
This simple calculation shows how difficult it is in reality to actually increase the CEC on existing surfaces by applying well decomposed organic matter especially when a surface is already established as how do you apply 79.7 tonnes of material to an existing surface and work it down to a depth of 30cm?
In reality there are two means of the CEC increasing in this situation. Either the turf itself producing organic matter or alternatively adding well decomposed organic matter over time.
Let's use an example of two clubs in Victoria with differing organic matter contents which are 1.5 and 3.5% respectively. The assumptions are as follows:
• The bulk density is 1.5g/m3;
• The weight of soil in the top 15 cm/Ha is 2,000,000kg.
• Approximately 60-80% of the organic matter breaks down to humus per annum
• Approximately 2- 5% of the humus decomposes each year
This means that for organic matter to remain constant it would have to be replaced at a level that equates to that being lost as shown above i.e. 9-12kg and 21-28kg/100m2 respectively.
Depending on where the site is, over time there can actually be an accumulation of organic matter such as in cooler wetter climates with periods of weather 0-12° or a net decrease where breakdown occurs quicker than accumulation, which tends to occur in hotter drier climates.
Calculating rate of amendment
Carrow and Waddington (2001) have actually determined a formula to calculate the amount of organic amendment required
1. Dry weight of amendment required /Dry weight of amendment required plus dry weight of sand or soil being amended
2. Multiplied by CEC of amendment = desired CEC in cmol/kg
Example: assume for the sake of this example 21000kg of sand is being amended with an amendment having a CEC of 25cmol/kg with minimal water content and you want to raise the CEC 4 cmol/kg. (The amount of sand/soil, the CEC of the amendment, the water content and the amount you want to raise the CEC are all going to vary)
a. Dry weight of amendment required /Dry weight of amendment required plus 21000
b. * 25
c. = 4
d. Dry weight of amendment required * 25 = 4 * Dry weight of amendment required plus 21000
Dry weight of amendment required * 25 = 84000 * Dry weight of amendment required
e. (25-4) * Dry weight of amendment required = 84000
f. Dry weight of amendment required = 84000/21 = 4,000kg of dry amendment is required
Thanks to all three of our turf panel. To support or talk to any of the three more please see the contact details below.
Agronomy Services Officer
Mobile: +61 418 475 326
Amgrow │ www.amgrow.com.au
Nuturf │ www.nuturf.com.au
Dr. Micah Woods
Director of the Asian Turfgrass Center
Turf Consultant ETP Turf
Mobile - 0439 019 050