Soil Properties

本站添加: 2026-05-10

[00:00:01]hi there in today's lab your first lab for plant science 2.

[00:00:06]uh you're sort of looking at a a real rough assessment of soil characteristics um we're going to say texture analysis so as a preparation for the lab i want to go over some of the what we call physical properties of soil so structure density porosity and color and texture and texture is the one that today's lab will will focus on [Music] looking at how it feels how it forms and we use that as just a real rough initial guide to characterize soul so there's our snowman yet again but yeah anyways so starting off with if you you grab a handful of a particular soil you want to see how's that soil feel is it smooth is it gritty is it sticky what's the overall feel of the soil if it's gritty then we have an idea that there is sand in that soil if it's very smooth and sticky it's going to have clay content as far as structure the structure is uh relates to the individual particles right so soil is an amalgam of lots of different types of sizes and comp and ingredients and how they come together also also influences the structure so do they form hard chunks that are mixed in between soft chunks uh is it very uniform uh that kind of thing and when it comes to horticulture and agriculture those those properties come into play when it when you're trying to figure out how to work the soil and what's needed to amend it so that you can grow vegetables for example uh with decent germination and success so um the density is related to uh its ability to compact i got a picture here of a tire leaving some foot uh some tread prints in some sand um just about any sand will leave compression marks but how stable are those compressions are they ones that are difficult to break up or do they crumble easily so the density of the soil plays a really big role in that as far as equipment compacting it or the ability of equipment to get into it for seeding or planting it also impacts things like drainage and oxygen porosity is probably the biggest one when it comes to drainage and oxygen and as you will begin to see everything kind of is interplaying right um these cross sections of soil here are showing clay silt and sand and the same volume of water being deposited and the clay has it sort of pooling on the surface the silt has it sort of wicking down and the sand it gets it penetrates very deeply rapidly so in terms of porosity sand has the highest porosity meaning the water is able to drain readily and deeply where clay has the lowest porosity we have another lab coming up another i think it's the week after uh next week we're looking at micropores and macropore so we'll discuss that uh our next class i mean which would be what thursday so um so porosity refers to the size and quantity of pores or spaces in the soil color is another interesting uh property right the color gives us an indication of what makes up the soil is it dark brown that means it's got high organic content if it's light yellow or red it may have oxidized or hydrated iron in it which is an indication of maybe poor drainage and then lastly we look at separates in today's lab another exercise is taking a soil sample and mixing it into a volume of water and then putting it into a tall glass jam jar or whatever in your case at home and letting the soil settle into the different layers be it clay silt and sand the coarsest layers settle first followed by the silts and then the clay and by measuring the total height and then the percentage that each layer represents we can use that information to sort of classify the soil um whether it's a sandy loam or whatnot and we'll use the texture triangle for that as a whole though when all those individual elements are mixed into a soil they can clump and form what we call aggregates here's a cross-section of the different types of particles that you would come across in soil uh clay being the smallest uh less than.002 millimeters in size and it's not visible under a microscope so basically in the in the sort of geological scale of soil starting as a solid rock and then eroding and wearing and getting smaller and smaller and smaller clay is the final stage of uh soil evolution before elements are fully solubilized and washed away completely um in niagara we have a lot of clay soils this is one of the reasons why uh grapes seem to do fairly well in some areas not just microclimate but also the soil clay has lots of nutrients now clay soils are not great for fruit trees um so those tend to be in the more sandy soil areas but there's definitely a lot of clay in the niagara area clay under a microscope a scanning electron microscope gives you a lot of insight into the structure and you'll see these sheets these thin wafers they're like stacks of paper piled on each other this is what gives clay its unique properties clay is a phenomenal resource for plants to store water and nutrients the downside of course is with this tightly packed structure is there's not a lot of room for oxygen um so here we have a picture of clay particles that have different elements stuck to the surface later in the course we'll discuss the chemical properties involved in that and how soils can store nutrients but essentially roots reach out and will extract nutrients from those particles and clay has the highest nutrient content of all the soil elements obviously everything in soil science is a compromise and you could say well then i should just grow in clay but yes you'd have lots of nutrients but you don't have oxygen you don't have a place for water to escape so obviously the ideal soil has a percentage of clay and a percentage of sand and a percentage of silt and that's where in the middle of the texture triangle is that sort of classic loam which is a balance of all the different particles uh silt which is exactly what you probably thought it was is often what comes out in rivers it's sort of the last stages before soil and rock has eroded to the clay it's the next size up and in rivers the rocks that are tumbling and washing over each other knock little bits off and the smaller particles will fill up the river in what we call silt in estuaries where the water slows down the silt settles and starts to fill the bottom of the river so silt is still a very small particle anywhere from 0.05 down to close to the dimensions of clay it is visible so you can see the individual particles under a regular microscope unlike clay this is a scanning electron microscope view of silts so you can see the particles are starting to look kind of sheet like but they're not as defined as individual sheets as clay is they're still kind of clumpy bits then there's sand sand is uh sort of the last fine particulate before we get into things like rocks and gravel uh sand we're all familiar with it a lot of the sand we go to at the beach is quartz or silicate rock but sand is really just a dimension that can be found with lots of different rock uh types you can make sand from limestone but it's highly soluble and will eventually dissolve and you can make rocks from granite and you can make rocks from quartz so now often other minerals will dissolve or break down sooner from granite which would leave much more quartz behind as the grant as the sand is aged so it's not uncommon to see more quartz in sand than you would normally have found in the original rock because the other parts of the rock have since dissolved [Music] so clay to summarize particles are not visible under a microscope it is a probably the most important component of soil because of its ability to store nutrients and a storehouse for water uh it is also what imparts stickiness to soil the clay is the part that makes soil hold its shape so later in the lab when you do your uh making ribbons and rolling soil in your palm looking at how sticky it is will help you sort of to give a qualitative assessment of how much clay is in that soil um so whoops silt on the other hand which is visible under a microscope it's decent small spaces so you have a little more drainage it's got moderate water holding capacity again it's sort of halfway between sand and clay it's fairly fine to the touch it will stay suspended in water for um maybe anywhere from a few minutes to an hour where clay takes many many hours to settle sand which is visible to the naked eye is usually fairly large rounded particles large pores so excellent drainage but a small surface area which means it doesn't have a lot of water holding capacity and does not store nutrients but it is good for aeration so certain crops that like good drainage do very well in uh in sandy soils it does erode easily right so it's not a very stable soil it doesn't have that clay to bind it so it will wash away more quickly it's not always ideal though as far as aggregates that's these chunks they can be bound together with all kinds of organic molecules as well right so organic fertilizers or weight dyeing organic matter will also bind up those sore soil particulates into aggregates um in more mineral based soils we often see things like iron oxide silica clay or lime calcium carbonates will also bind up soil into chunks um as far as aggregates go there's two groups uh there's what we call peds which are aggregates that form naturally and then there are clods which is from tractors tilling the ground that's two fancy little terms there so peds are naturally formed aggregates and clods are aggregates formed through a human activity right a plow going down the road or you take a shovel and you dig up soil so aggregates concern in terms of uh plant growth we need something that will take water and store it but not collapse we need a certain amount of structure but we also need some porosity we need a variety of particle sizes so that we get that structure in the soil that gives us air stores nutrients also holds water that balance so when we're looking at pores we're talking about both air storage and water storage and large pores are what we call macro pores and small pores are micropores and in the video that is linked in today's lab folder i have a couple of three different size screens that i fill with water and hold up and you can see the water drain and the idea is that the the coarse screen the water is able to span the gaps between the wires because of surface tension but as the the core screen has bigger distances the water drains down the screen and eventually it leaves those holes open that would be an example of how macro pores would work in soil right so the the idea is that water can be applied to the soil but it will drain out of the macropores into the lower parts of the soil more readily whereas the the finer screen where the the pore spaces are closer you'll notice that the water doesn't drain from the screen it actually stays suspended in the screen because the surface tension of the water is holding it in those tight spaces so when we're talking about soils and trying to balance the ability to have air and water we're talking about micropores and macropores so macropores being where air is kept and micropores where water is kept so all that plays into soil texture and density and porosity and what makes crops soil suitable for various crops here's a picture of an orange grove which is interestingly enough oranges really like to grow in sand uh which is a very porous soil so often orange trees have irrigation i don't know if you can see under here there is a black line they use drip trickle irrigation in orange groves because of course sandy soil doesn't hold water very well and then they have to supplement with fertilizers because sand doesn't store nutrients very well because there's not much clay in it so interesting though right large pore spaces provide the drainage and oxygen that certain crops need like oranges where the clay provides the moisture and the nutrients that crops like grapes would desire so yeah sandy soils will dry rapidly after a rain they definitely have oxygen but not a lot of moisture content where clay soils have very small pore spaces and very good at holding water and nutrients so there's our our grapes interestingly there is an irrigation pipe there as well that's not common uh certain grapes do need irrigation later in the summer so they don't stress out but most grapes their root system goes incredibly deep so moisture is not a problem for them so as far as permeability that's looking at um again this all sort of sounds repetitious right the ease of both air and water to pass into soil right so roots can penetrate and access both air and moisture clay soils being very low permeability and sandy soils having high permeability so looking at soil texture that's in today's exercises which are outlined on the the pdf sheet that's at the top of your folder there um texture tells us how well the cell could drain or hold water right so if it's the texture tells us that it's a sandy soil we know it's going to have good drainage if the soil if the texture analysis tells us that it's sticky and holds its form really well that it's got clay content and it's going to be very good at holding water which of course it's always a trade-off right so if you have good drainage that means you have good aeration if you have low drainage high water holding that means you've got poor aeration less air for your roots also if you have good drainage and good aeration you have poor nutrient retention in the soil because there's low clay content whereas in soil that has higher clay content you've got good water retention low porosity less oxygen but much more nutrient content so usually we're looking at all the different mixtures of the clay and silt and and sand and that helps us to classify the soil and we use a texture triangle for that so on here you can see on one side there is our percentage clay content and over here we have our percentage silt content and down here we have our percentage sand content so if you can identify at least two of these which is usually sufficient trying to measure three kind of gets pointless um you can triangulate where your soil type is so let's say for example we have 20 percent or 25 percent let's say we got 30 let me erase these lines let me clear let's say you've got 30 clay okay that's up to here that means we're going like this across here and let's say we've got 40 percent silt so we gotta go down to forty there's forty draw our line and where the two lines intersect that tells us what kind of soil we have a clay loam right so loam or clay loam these are the soils that are most commonly found and best for crops to grow on on average let's say loam is sort of your classic ideal soil it's got a good balance of everything where the clay loan might be very good for nutrient [Music] hungry crops that are also moisture sensitive right so or need more moisture so what's important to learn from the texture triangle is that looking at the different contents forty percent sand in any soil sample makes it a sandy soil 40 silt makes it a silty soil but if you have anything over 20 clay it is a clay soil so really want to pay attention to that clay content right so anytime you go over 20 percent you're into clay-like soils so if you have a crop that's particularly sensitive to excessive moisture in the root zone you want to make sure you're at below 20 percent clay content right so sandy loam loam silt loam you don't want to go anywhere over that 20 clay content in today's lab uh if you can go out and find a soil sample maybe a a variety of them you would moisten some soil and you would attempt to squeeze it into your fingers and form a ball and then squeeze that out to make a flat ribbon depending on how the ribbon forms if it begins to crack and crumble that tells us that there's some sand content if it forms a red ribbon easily and doesn't fall apart at all it's very high clay content also while you're squeezing the ribbon you can feel the texture of it's gritty or smooth uh typically you have to moisten your soil sample slightly to get it to sort of become like a dough doughy consistency for this to work and so then we look at whether it's a rim a ribbon if it crumbles or if it's a partial ribbon depending on where our clay sand or silk content is um another option is the uh same sort of idea and you can do this maybe before you do the ribbon it's just take that soil sample and squeeze it into a ball in your palm and try to see does the soil crumble easily after you form a ball does it does it form some structures from your hand or is palm prints from your palm really visible in the soil that gives us a high an indication that there's high clay content down here whereas this would be high sand content um in the non-audio version of this this ribbon test is a link to assist you in trying to understand how that ribbon test works so i would encourage you to go out and and grab some soil samples and practice assessing if it's a sandy or a clay soil it's basically you're looking at the extremes uh in terms of what sort of soil it is you're looking at and then once you've decided if it's a clay or sandy soil then you need to say okay is this suitable for what i want do i want to amend it so that's the ribbon test and there's my final slide right so sandy loamy clay soils um we'll talk a lot more about this as the term goes on so if you got more questions probably going to answer them in another lesson so um i'll show up with a video after this looking at my sedimentation results from my experiment okay and otherwise have a look at the videos where i go over my ribbon tests with the soil samples that i have one happens to be sandy and the other is clay so you can see what i'm talking about