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Soil Science – From the Textbook to the Practical March 3, 2019

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This article comes from the NOFA/Massachusetts 2019 March Issue Newsletter

This is the third edition of this Soil Science Mini Series with Noah Courser-Kellerman of Alprilla Farm in Essex, MA.

A Conversation with Noah Courser-Kellerman: What is Cation Exchange and why it is Relevant?

Interviewer: Julie Rawson, Executive Director, NOFA/Mass

Julie: What are cations/anions? 

Noah: An ion is any molecule that has any charge imbalance. A molecule with a neutral charge has an equal number of protons and electrons. If  a molecule is missing electrons it is positively charged and is called a cation. If the molecule has an extra one or more electrons it has a negative charge and is an anion.

Julie: What are mmeq or cmoles/kg- the units of CEC on soil tests? 

Noah: At one time the units that CEC are measured in were called meqs/kg and now they are called -  cmoles/kg. Cmole stands for  centi-moles.

Julie: Please explain cation exchange capacity.

Noah:  On the soil surface there are a good deal of negatively charged sites. The units of CEC are literally the number of negative charges in a given mass of soil. It measures how many electrons are “missing” from the particle surfaces of a kilogram of soil. A mole is a number in the same way that 1000 or 2000 are numbers. A mole happens to be 6.02 x 1023, or 602,000,000,000,000,000,000,000 - a really big number! A centimole is 1/100th of a mole. I don’t know why, centimoles are easier than moles, but that is the convention. They are spread over a really enormous surface of soil and one of these charges is so small that it is impossible for our human brains to visualize it.

Julie: What "parts" of the soil contribute to the CEC? 

Noah: Where does this charge come from? The main components of the soil that contain this negative charge are 1) the clays and 2) the organic matter.

 

1) The Clays

The phyllosilicates, or sheet silicates, come from the same root as or phyllo (Greek for leaf) dough. I like to think of clay mineral sheets like a piece of baklava – very thin layers that are loosely packed together and they can hold delicious nuts and are sticky on the outside. In clay, you have stacked sheets in different combinations of aluminum oxide – alumina, and silicon oxide - silica. Aluminum in an alumina sheet has a charge of +3. Silicon in a silica sheet has a charge of +4. Occasionally, there will be an Al3+ in the place of a Si4+ in a silica sheet, or a Mg2+ will be in the place of a Al3+ in an alumina sheet. The charges are no longer balanced in this case, and there is a permanent negative charge left on the sheet. This is called isomorphous substitution.

Julie: Why would something that is stable SiO2- silica or Al2O3, alumina – be prone to being knocked out by Aluminum or Magnesium?
Noah: It’s not so much that they’re knocked out. Isomorphous substitution happens as the sheets form. If there are lots of Al3+ around while a silica sheet forms, for instance, there will likely be more Al3+ substituted in the silica sheet.  Isomorphous substitution can only happen between ions of similar size.

Minerals like feldspar that don’t have permanent charges like clays also have isomorphous substitution. Their crystal structure distorts around the isomorphous substitution and packs in extra K+, Mg2+ or other cations which become permanently bound to the negative charges, balancing the charge. In the crystal there is no active charge – they are internally balanced.

The reason that clays have the active charge is that the alumina sheet is where most of the charge is. It is buried between two silica sheets and so the charge on the clay’s surface is kind of diffuse like with Velcro or a weak magnet. A positively charged ion will come close but not form an ionic bond; it just hangs out like a fly on the baklava. At any given time each negative site may or may not have a cation attached to it. Within the soil solution you have H+ or K+ or Ca2+ or NH4+ (ammonium) and they are constantly letting go and attaching depending on their relative percentages in the soil solution. It is like at a baseball game where there are 20 folks in line for hot dogs but they are not the same people at any moment during the game.

 

2)  The Organic Matter

From a practical soil management perspective, there is very little you can do to change the baseline of CEC, but organic matter (OM) is something that we do have some control over and that is a way to increase the CEC. Gram for gram we can have more CEC from OM than from most types of clay. The mechanism for CEC increases with OM is found on the humic acid molecule. This molecule is pretty poorly defined and may not really exist (according to some). The negative charge comes from the many carboxylic acid groups, COOH, on the humic acid molecule. Depending on the pH around it, the H might detach and float around as an Hand leave a negative charge behind, COO-, which acts similarly to the negative sites of the clay. This is called deprotonation and the result is that you get a CEC contribution. If the soil was really acidic, the H would not pop off of the humic acid molecule. Most soils are more basic so it contributes to the CEC. The more organic matter the more humic acid molecules are present. 

Julie: What types of soil tend to have a higher CEC? 

Noah: Usually the smaller the average particle soil, the higher the CEC. So heavier soils, which have more clay, tend to have higher CEC. It is easier to have higher OM in this case because they (the clays) can form a little slick over decaying material, making it harder for microbes to break them down. Not all clay soils have high CEC either. Tropical soils or old red clay in the American south – kaolin clays – have very little cation exchange. In a new soil that is relatively young as in New England, where our soil is only 10,000 to 20,000 years old, one can assume there will be a higher CEC. There is an easy qualitative test, at least for the Northeast. Take a handful of muddy soil and squeeze it. If you get ribbons you probably have a higher CEC. If it crumbles out, you might have a lower CEC.

Julie: What is the relationship between CEC and pH? 

Noah: pH is a measure of the concentration of H+in solution. Hydrogen is the smallest element. You have one electron and 1 proton, and that is your atom. Once you strip off the electron you have a proton. In a water based solution like the soil environment, protons (H+) that come free of the COOH group on a humic acid, for instance, join up with a water molecule to form hydronium, or H3O+.

pH is a scale with which you measure the concentration of hydronium ions in a water based solution. It is a negative log scale, which means that each unit of pH increase, you have 10 times fewer moles/liter of hydronium ions.  With pH of 7, which is neutral,  you have 0.0000001 moles of hydronium/liter. Lemon juice has a pH of 2, which means that its concentration of hydronium ions is 0.01moles/liter. Ammonia has a pH of 11, which means that its concentration of hydronium ions is 0.00000000001 moles/liter. Higher concentrations of hydronium equate to a lower pH. I have a hard time with math, but I found it really helpful to think of pH as the number of units to the right of the decimal place. Most vegetable and grain crops perform best at a pH around 6.5 to 6.8.

CEC acts as an acidity sponge. You can hold hydronium on the negative sites. As acidity is being generated in the soil, hydronium might knock off Ca or K and will stick to the clay but the pH won’t go up. If you lime and start to neutralize the hydronium and turn it into water, more hydronium with pop off the clay. With more clay you might need to add more lime than if you were on a sandy soil. This property is called pH buffering.

Julie: What is Base saturation?

Noah: Base saturation is the percentage of the negative sites in the CEC taken up by the major cations – H, Ca, K, Mg, and Na and the traces.

There is a ratio that William Albrecht posited as ideal –

Ca – 70%

Mg – 14- 20 %

K – 3-5%

H, and other cations – Na and the NH4+ and traces – the rest up to 100%

The larger the CEC the more of a reservoir for the nutrients is available. The cations will be held in the soil. In a really sandy soil with a low CEC there are limitations to how having a balanced based saturation will affect performance.

Julie: What CEC is too low? Or too high?

Noah: Below 3 – 5 the soil has a harder time hanging onto nutrients. Some clay soils have very high CEC’s but they can have too much clay and thus have poor drainage. With a sandy soil more side dressing or fertigation and building organic matter is a higher priority so that your soil can supply enough nutrients throughout the growing season. Nutrients, especially positively charged ones, are more prone to leaching in a low CEC soil.

Julie: Why does CEC matter for liming and fertility management? 

Noah – You need more lime in a higher CEC soil for a pH change, but once you have the reserve your plants have more nutrients available. It takes longer to fill up your gas tank but it will run longer than one with a low CEC.

Julie:  Why is lime important for more than pH?

Noah: Lime is a fertilizer; plants use Ca for most of their growing processes along with Mg and all of the cations that are held in the soil. When we get a soil test back from the universities and they suggest changing the pH with the cheapest source – wood ash from power generation, for example – the primary cation that is available is K. Too much K can cause poor growth of the plants and health problems in livestock fed on them. Dolomitic line can bring in too much Mg over Ca. Calcitic lime will provide a higher Ca:Mg ratio. You can have an ideal pH and a really imbalanced soil at the same time.

Julie: What role do base saturation ratios play on soil structure? 

Noah: Base saturation matters to soil structure because it impacts the way that clay particles interact with each other. In a well structured soil with a crumbly texture and plenty of aeration, clays are stuck together in little blobs. This property is called Flocculation. It is defined as the process by which individual particles of clay aggregate into clotlike masses or precipitate into small lumps. Flocculation occurs when the negative charges on the clay soil surface are fully balanced with exchangeable cations so that they no longer repel, but they stick together. 

Clays where the charges are properly balanced with Ca have better structure. Clays in sodium rich soils tend to disperse, and have poor structure.

Julie: Why is Calcium better than Sodium for flocculating clays?

Noah: There is the hydration layer around each cation- weakly held water molecules in a sheath around the cation. They keep the cation from getting too close to the clay molecule. The negative charge on the clay is not strong enough to break the cation free of its hydration layer.  The larger the charge vs the size of the ion and its shell, the more completely the negative charge will be neutralized on the clay. Na only has a charge of +1 but ends up with a large water shell, gunks up the negative sites and doesn’t neutralize them as effectively as Ca, which has a charge of +2.  Clays with a high percentage of their base saturation have a remnant negative charge, and thus repel each other, preventing aggregation.

Luckily, here in New England, where precipitation is abundant, sodium tends to leach from the soil. High tunnels are the main areas where sodium can become an issue around here. However, soils with too much magnesium can have poorer flocculation than those with a good balance of Ca:Mg. One can improve soil structure with gypsum if pH is on the high side but calcium is needed. Gypsum adds the needed calcium without raising the pH.

If you enjoyed this article be sure to sign up for the full e-mini series here where you’ll be the first to receive the next individual installation of this monthly “Soil Science- From the Textbook to the Practical” throughout 2019.  Or go back and read the first article in this series here.

Interested in learning in person from Noah?  Register to attend our upcoming NOFA/Mass educational event, Soil Science for Gardeners.

April 14, 2019 - 9am to 5pm
Allyn Cox Reservation
82 Eastern Avenue
Essex, MA

NOFA Members: $72 / Non-members: $90*

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