CEC S20 audio

Added: 2026-05-10

[00:00:00]um so we discussed fertilizers last week um i tried to emphasize that they are salts that's what plants absorb is ions cations and anions and uh when we add a salt to water it dissociates into its positive and negative charged components okay um we talked about how we measured the total salinity of solution using an electrical conductivity meter uh but we measured total right not individual elements it's important to understand that that doesn't tell you what's in the water just how much of all the different charged particles um we discussed the different methods pour through saturated media extraction and two-to-one dilution methods and you should know how basically all three work and you should know the advantages and disadvantages and uses for different ones for example doing a pour through on a large tree root ball is not practical so you're going to be doing two to one or saturated media extraction on something like that we also discussed ph and how it plays a role in nutrient availability we'll touch on that again today um but this picture there's many of this on these on the internet really you should become familiar with them and to help you understand nutrient diagnoses and how ph impacts availability and we're looking at a compromise you know between giving up availability of major nutrients to keep minor nutrients happy like iron um so today i want to introduce a term called cat on exchange capacity and the best uh analogy i have is a sponge and this is a measurement of a soil property that measures its ability to store nutrients uh essentially charges so in the images here we've got positive and negative uh the salts that are being watered into your soil interact with the soil and they stick like on this side to the soil particles based on charge and that allows the soil to hold on to nutrients so you'll notice here that clearer water is coming out the bottom because the soil is holding onto the nutrients where in this side a lot of the nutrients are coming out the bottom right so it's the ability of the soil we're measuring the ability of the soil to hold on to cations that's why it's called cation exchange and to help you remember what a cation is i've got this goofy mean right they are positively attractive right that means that cations have a positive charge uh anions are negative charge so it's really important remember that as far as plants are concerned the major group of nutrients that we are concerned about are cations um well not all but a good chunk of them right uh anions like nitrate phosphate and sulfate are important um and we usually are supplying them in much larger quantities but cations is what we're talking about primarily positively charged ions um yeah so positively charged ions are attracted to negative charges on the surface of the soil so soil essentially is made up of negative charges which means it can only attract cations it does not attract anions very effectively so uh remember that it's the positively charged ions that are stored in soil i really like this picture and you'll probably see it if you take more courses with me because it really helps remind you of what's going on here so on this side we've got the root all right and we've got root hairs extending into the media and the root hairs are putting out protons and those protons will travel through water in the soil to the soil colloid and because they are positively charged they will dislodge cations which then the root can absorb so it's like they're cashing in their coins to get their magnesium and their potassium so all plants can really do is pump out protons that's really what they're they're capable of um most of the time there are some selective pumps that can pick up certain anions and cations but for the most part we're looking at proton exchange with soil surfaces so they essentially they produce an acid the acid goes into the media and dislodges the metals the nutrients that then the root can absorb so plants can only absorb what they need to grow at any particular time there are some nutrients they're just greedy with and they'll keep taking them up like nitrate but generally speaking you want to be able to dole out the nutrients as the plant needs them uh if you put a ton of nutrients into your soil knowing that the plant will need all that in its lifespan but your soil is not able to hold it it basically leeches out into the ground and it's lost and you've just wasted your money and you've polluted the environment so we have to understand what kind of soil we're working with and that dictates what we can and can't do as far as delivering nutrients to help the plants reach their full potential and that involves getting a measurement of cation exchange capacity uh the soil particles that i keep talking about are called colloids that's a term i'm going to use in the rest of the lesson so colloids are the soil structural elements that have charged surfaces that attract the nutrients okay those are called colloids um so when we're measuring electrical conductivity we were measuring the ions that were free in solution right not ions that were bound to soil colloids we were measuring ions that were in solution already whereas when we measure cec we're measuring the amount of storage capacity that soil has in fact in the lab they often will knock all the elements off the soil with an acid wash right and then they can measure what charges are remaining on the surface and that tells us what its full capacity is to store cations that's the cation exchange capacity um so again colloids are what stores the cations and they are where the nutrients are and the roots have to access them so think of it as like a bank you know you put your fertilizer in the soil it gets stored in the bank of colloids for later access by roots and again looking at clay you can see how with its thin wafers and really high surface area that clay can be an excellent nutrient resource it's like all things in in nature it's a compromise right so we have fantastic nutrient storage capacity but we have very poor aeration because there's a not a lot of macropores so obviously a clay loam or any soil that's a combination of clay is going to have excellent nutrition for the most part with some exceptions we'll talk about but it's going to have other challenges like drainage and aeration so everything in balance right uh just briefly hydroponics obviously there's no soil involved so that means there's no cation exchange which means that nutrients are are immediately available to the roots the chemistry has to be in such a way but also means there's no reserve so as soon as you forget to add nutrients the plants immediately show nutrient deficiency because there's nothing in the soil to buffer and start releasing nutrients that you may not have right on time so hydroponics you can achieve excellent yields and productivity but you need to be on the ball all the time to keep the crops happy uh we mentioned i mentioned this these two charts already and how there's a difference between hydroponics and soil in terms of nutrient availability if you look at this picture closely you'll see that down here it says also soilless mediums that's true to some extent but not quite so what you see here is like a true hydroponics chart uh soilless media has slightly less uh cec than actual soil you would find outside mineral based soil but it has some so it's somewhere in the middle but i think to play it safe people like to say well we should treat it as if it's hydroponic but it's somewhere between these two in terms of nutrient cation exchange capacity okay so again here's a really good uh picture uh outlining what the roots are doing so the roots are pumping out protons or acid which then dislodges charged positively charged nutrients like magnesium and potassium from the soil for them to be absorbed by the root uh another interesting thing which this picture is showing and i haven't brought up yet is that not only do the plants pump out acids but just by the fact that they're alive and undergoing respiration means that they are producing carbon dioxide and carbon dioxide mixes with water to form carbonic acid which releases a proton so what's really interesting about that is that as a plant becomes physiologically active i.e it's growing rapidly it will naturally acidify the water because of its respiration at the roots which will then also enhance the release of nutrients so it's sort of they're they're self-perpetuating in that sense so if the plants growing more rapidly it releases more nutrients from the soil if the plant slows down growth it releases fewer nutrients from the soil sometimes it has to actively pump protons out if it's really in trouble like there's a deficiency but more often than not it's just simply their metabolic activity that that releases the nutrients from the soil so you might be asking what happens to negatively charged nutrients i mentioned nitrate and phosphate and sulfate the reality is that sulfates and nitrates pass right through the soil there's really nothing to hold them there and they are especially nitrates one of the big causes for pollution in the sense that when uh agriculturalists over apply nitrogen because they think well if i just add more i get more yield but if they over apply at a rate that's beyond what the plant can use uh there's nothing in the soil to hold on to that so some people like to use slow release fertilizers but at the end of the day if you are not matching the plant's needs those nitrates and sulfates are just going to wash into the groundwater and pollute it phosphate is an interesting one in that it reacts with iron and aluminum in the soil and it actually becomes immobile even though it's a negatively charged nutrient it is actually very immobile in soil so farmers often have to over apply phosphate to compensate for that depending on their soil so soils that typically have that sort of look like they have high cattle exchange capacity even though this isn't a cation they tend to have high iron and aluminum content which means they also immobilize phosphate something to keep in mind phosphorus is an interesting nutrient it's sort of one we typically don't see deficiencies of in greenhouse context but in field contexts we do especially in the spring and fall when the soil temperatures are cold uh i'll talk more about that when we get into nutrient deficiencies but that's why young plants go purple purple is a symptom of phosphorus deficiency and often it's not because the phosphorus isn't in the soil because it's not available and as the soil temperature warms up the phosphorus becomes more available so some farmers will over apply phosphorus at the early stages to help the plant get the the best start possible but you have to be really careful that you don't have soil erosion because you will pollute waterways very quickly okay so to summarize cation exchange capacity is what cec stands for uh cations have a positive charge uh cations exchange capacity is talking about storing cations anions have a negative charge right and they leach from soil they're lost because they do not stick to the soil particles generally um looking at different soil types they obviously don't all have the same cation exchange capacity i've got the example of a unicycle here and i've got this uh chap who has a tremendous amount of boxes on here i'm assuming they're quite light but obviously he has a much higher capacity trying to create a memorable picture so looking at soils as far as increasing capacity going from sandy to clay we have increasing cation exchange capacity so peat moss is okay it's not stellar organic soils are definitely better and clay has the highest cation exchange capacity um looking at the solids the cola sorry the colloids that make up soil um helps us understand why we arrive at different cation exchange capacities and a lot of it's to do with weathering so the age of the soil uh i mentioned this earlier in the course and we're going to talk a little more about it today with this idea that tropical soils are more weathered and have lower cation exchange capacity if we're looking at organic or clay particles they have these negative charges you can't see it's really small here but what looks like this initially and the soil quickly turns into the hydrogens are lost quickly turns into lots of negative charges so things like humus is huge adding compost which is high in humus helps increase these negative charged uh things in your soil so adding compost to a garden may not add a lot of nutritional value but it really improves properties like water holding capacity and cation exchange capacity it doesn't mean there aren't nutrients there but you know there are a lot of other benefits to adding compost clay particles obviously have maximum amount of negative charged particles so they have very high uh cec for a colloid where sand and silt which are not as weathered uh and don't have a large surface area um are typically poor cattle exchange so saying these soils just don't hold on to their nutrients right if you go through niagara our soil in this region is quite interesting there's pockets of land that are very sandy and there's pockets that are very heavy clay uh we're all over the map uh there's pockets that are high in limestone there's pockets that are not it's really quite variable you can have one farm set another that has vastly different productivity and yields and challenges with root disease and past pest pressure a lot of it related to the soil that is on the land so it's definitely worth knowing what you're buying if you buy some land for a garden or a farm because the value will be dictated by the quality of that soil you may think well i can change it but you know realistically to change quite a few acres takes a lot of time and energy and money and not everyone has that so they typically grow lower value crops that are less sensitive to poor quality soils so sandy soils which are excellent drainage right so in the spring they warm up quickly uh they have good aeration but they have poor cattle exchange and they have a lot of leeching problems so farmers have to be very careful how they apply nutrients so typically fruit farmers that are in sandy soils will be adding compost and manure as often as they can not overdoing the manure because your salt levels can start to go up from all the the salts and the cow urine um looking at tropical soils though they tend to be quite weathered and that seems to impact their their cattle exchange right so sandy soils are low cec tropical cells are low cec and clay and human soils have excellent cec looking more closely at tropical soils they have what looks like good structure and drainage but what happens is the clay has broken down so much that the layers actually flatten to the point where they are treated as a solid object which means you no longer have access to those layers for the cation exchange and you don't have the charges available to hold onto the nutrients and so they leach through quickly that type of cell's called kaolinite clay my best analogy would be like a stack of peanut butter and jam sandwiches so clay that's not too old has got lots of good sticky layers between the the clay structure that can exchange lots of nutrients and hold on to them but as that clay wears and weathers it ends up tightly packed like over here with no stickiness uh between the layers and that means that they they leach nutrients very rapidly and they don't store anything and it's very difficult to achieve uh decent yields it doesn't mean it can't be done but there's things that have to be done to improve the soil if you're just looking at uh fresh land soil uh looking at clay in ontario where we have what was called tempered so our soils are not nearly as old the clay looks like a sponge these are close-ups here of what our clay looks like so it's a it's much more sponge-like and open there's definitely a lot of surface area there uh but it's not flat sheets like a like a like a ream of paper there's there's air pockets and surface area in there to hold lots of nutrients um can we fix soil like this can we fix the tropical soil absolutely 100 so adding compost and organic matter is is a huge uh boost to soils that are heavily weathered so for example farmers that have been existent in existing for a long time in tropical climates that have been managing their soil will have decent yields it's not like because someone's in a tropical country they don't have any yields that's not true but looking at the base soil there are challenges um so we've talked a lot about compost but we haven't talked about biochar and i want to bring that up because it's a it's a oh my where's my eraser it doesn't want to click it's a a really fascinating uh story um so yeah adding review again here organic matter so farmers will spread manure and compost on their fields here's someone putting compost in a garden in some sort of arid climate and this is an enthusiastic composter [Music] and as far as organic matter goes in ontario a lot of farmers do what's called no-till farming if you look at this this is a relatively young corn crop and in the ground there is all sorts of dead corn matter from the previous year and this matter will break down and form humus which works its way into the soil increasing its cation exchange capacity and water holding capacity and all sorts of beneficial properties same thing over here last year's crop rotting on the top of the soil so no-till farming is is very popular for preserving soil prevents erosion and increases your organic matter content which improves your cat exchange capacity in recent years as in the last couple years farmers have sort of taken a bit of a break from no-till farming to try to break some disease cycles so it looks like constantly doing this isn't necessarily the best either but obviously the majority of the time doing this is the idea would be good but every once in a while you need to till the whole field to break cycles because bugs can nest in here fungus can sporulate over the winter as we get milder winters and there's less kill these disease outbreaks become more common so um in this picture you can actually see how organic matter from the surface is slowly eluviating out or down into the soil and proving the soils can exchange capacity as you go further down down here we've got sandy soil which has poor cec and as the organic matter leeches down into the soil it improves the cec profile of that that particular cross section or of the horizons um okay go back to biochar so biochar essentially is activated charcoal you've used it in your brita filter uh you know any any kind of water filtration that purifies it uses activated charcoal and it's exactly the same uh property that does that it's cation exchange capacity so by having activated charcoal it's got tons of negative charges on the surface all those negative charges will attract positively charged molecules and atoms and take them out of the water which helps purify it now as far as biochar is concerned it's taking typically straw or some other biomass that's easy to burn and you burn it in a controlled environment if you look carefully this is a steel kettle they put a lid on and they limit how much air it gets in there so they can kind of smoke it for a while and then shut it down and that means instead of getting a pile of ash when you open you get actually charcoal that's how that's done [Music] in the old days people would pile a bunch of combustible material and then cover it with dirt and then they would set it on fire and then cover it with dirt again and it would smoke for a few days as it slowly burned but because it was burning in low oxygen you ended up with char charcoal instead of ash so obviously you didn't get all the heat and flame you would get if it was well oxygenated you lose some carbon obviously from the combustion but you're left with a biochar and the next picture so there's lots of evidence that biochar was used thousands of years ago in south america in amazonia and it was known as terrapreta uh that's probably when uh colonialists came in because i don't think that's in the language that would have been used but tara preda is what we have documented and this these pictures on the side are from a paper there's a link down here to a paper that i pulled out this is a couple years old but it's still a really good paper um that shows how uh recent fields that have been recently deforested have low yields and by adding the biochar they are able to improve the productivity so here you see what a corn field looks like on a soil that is been recently cleared and then after adding biochar you can see all the black alluviation that's happening um the yields are substantially improved uh and because the soil in the south american countries that are close to the equator are heavily weathered this practice has gone on for a long time and has shown to improve productivity substantially and what's really cool about this is that the biochar has a long lasting impact on the quality and yields um now looking at biochar it may be impressive and it does increase cad and exchange but there is always something else right so but wait you don't want to just run out and go buy a bunch of biochar and add it to your ontario soil okay if your soil is already productive i.e it has good cation exchange capacity already and then you add biochar having too much cat on exchange capacity actually makes it very hard for your roots to get the nutrients away and you actually decrease the nutrient availability so biochar is reserved for soils that are low on cation exchange capacity it's not something you'll automatically add to your soil that's going to have some negative impacts so it's best for poor weathered heavily weathered soils this is a chart i pulled from that paper that was looking at farms in colombia and this is over a three year span four year span 2003 to 2006 and you'll notice the gray line the middle one is where they add eight tons per hectare of biochar and the black one is 20 tons per hectare and what's interesting is you'll notice the first year the differences are minimal and then of course as time goes on the difference in yields improves now you should understand farming is what it is every year is not the same total yield this is grain yield so for example 2006 would have been a bad year hot and dry with poor yields but relative to the control so the white one is control the first year there's no difference second year there's already a difference third year there's even a larger gap and third year there's even a bigger gap in fact more than double the control's overall yield where you know here we're looking at maybe a 25 percent increase now we're looking at 125 percent let's say why is that um partly because when you add the biochar at the top of the field and then you try to till it in it takes time for that to alleviate into the soil fully into the root zone so the plant can take advantage of the extra cation exchange capacity the roots generally don't like growing on the upper surface alone they're going to go down and so if the soil below that hasn't really had the biochar alleviated down you don't get that full cation exchange capacity boost then of course adding a fair amount is also helpful but there's probably an upper limit right so 20 tons per hectare is quite a bit i'm wondering if they'd gone to 30 if you would have seen actually a decrease in yield relative to the 20 because now it's going to be hard for the roots to get the nutrients out so too much of a good thing right