Science project time: conditioning clay soil
October 26, 2021 4:40 PM Subscribe
Our junior high student has decided for their science project to compare different methods of treating/altering clay soil for better gardening. We could use some help deciding on: factors we could test; treatments we could compare; and how we might measure success.
For the factors we could test we are thinking how quickly water drains through the soil and/or pH. We do have a consumer grade soil pH/light/moisture tester.
For treatments we could compare, we are thinking something green-ish, like mixing in mulch or another natural additive. As a comparison, we might try a more chemical additive like liquid gypsum. Possibly we could do a third thing, if it seems worthwhile. (Maybe we could even simply offer some research on plants that like clay soil.)
Finally, we need to come up with standards by which we could measure relative success. The timeline of the project is only a few weeks, so trying to grow things in the soil to compare results is not really practical.
Any thoughts/tips are appreciated!
For the factors we could test we are thinking how quickly water drains through the soil and/or pH. We do have a consumer grade soil pH/light/moisture tester.
For treatments we could compare, we are thinking something green-ish, like mixing in mulch or another natural additive. As a comparison, we might try a more chemical additive like liquid gypsum. Possibly we could do a third thing, if it seems worthwhile. (Maybe we could even simply offer some research on plants that like clay soil.)
Finally, we need to come up with standards by which we could measure relative success. The timeline of the project is only a few weeks, so trying to grow things in the soil to compare results is not really practical.
Any thoughts/tips are appreciated!
You could send both samples to your local agricultural station for testing. One of the figures that they will return is cation exchange, which refers to a soil's ability to hold and exchange compounds and nutrients. Clay will have a high cation exchange. So will mulch or other organic matter, though. So for your third test, maybe try adding sand? Here is a fact sheet that refers to soil in New York but you'll get the basic idea.
posted by xo at 5:00 PM on October 26, 2021
posted by xo at 5:00 PM on October 26, 2021
We've got clay soil where I am and a common suggested amendment is adding sand to improve drainage. I don't know if it actually helps though.
posted by any portmanteau in a storm at 5:14 PM on October 26, 2021
posted by any portmanteau in a storm at 5:14 PM on October 26, 2021
Add compost/manure
posted by pinochiette at 5:31 PM on October 26, 2021 [1 favorite]
posted by pinochiette at 5:31 PM on October 26, 2021 [1 favorite]
We do things like this a lot in my (college level) biology and environmental science classes. Clay soils are often acidic and low in nutrients, so folks often add lime or gypsum to them.
If you're interested in plant growth, a lot of things will grow quickly from seeds. You're not going to harvest any veggies, but you could compare growth of something that sprouts quickly. Beans are always easy and reliable. My students have had good success with radishes, too.
posted by hydropsyche at 5:33 PM on October 26, 2021 [2 favorites]
If you're interested in plant growth, a lot of things will grow quickly from seeds. You're not going to harvest any veggies, but you could compare growth of something that sprouts quickly. Beans are always easy and reliable. My students have had good success with radishes, too.
posted by hydropsyche at 5:33 PM on October 26, 2021 [2 favorites]
I’m with teramala in thinking that a few weeks is way too short for the actual biological effects of amending clay. One last physical measurement is how much water the soil can hold at field capacity.
Half the people who discuss adding sand to clay say it will result in cement if compacted, so I definitely think kid should do that to find out which half is right.
posted by clew at 9:02 PM on October 26, 2021
Half the people who discuss adding sand to clay say it will result in cement if compacted, so I definitely think kid should do that to find out which half is right.
posted by clew at 9:02 PM on October 26, 2021
re: liquid gypsum- it's a molecule that displaces other molecules on soil particles and can take anywhere from a few days to a few weeks to be effective. And though adding organic material to clay is the best way to improve soil structure in a garden (compost, manure, &c.) this also takes weeks to months as aggregate particles form, increasing the porosity of clay soil. So if you had several weeks you could possibly do an experiment with these additives over a few weeks and see if you can increase porosity/change the soil texture. Kind of a bust though if nothing happens in the "lab" in that time.
You can physically add enough other materials to clay soil to immediately change the texture, which might be fine for experimental purposes as long as you keep in mind the feasibility and expense of doing that in a garden is impractical. If that has no bearing on the usefulness of the project, I do think pH is a good thing to measure (it's very important because pH regulates how well plants can take up nutrients in soil). You could also measure water holding capacity or speed of drainage.
posted by oneirodynia at 9:53 PM on October 26, 2021
You can physically add enough other materials to clay soil to immediately change the texture, which might be fine for experimental purposes as long as you keep in mind the feasibility and expense of doing that in a garden is impractical. If that has no bearing on the usefulness of the project, I do think pH is a good thing to measure (it's very important because pH regulates how well plants can take up nutrients in soil). You could also measure water holding capacity or speed of drainage.
posted by oneirodynia at 9:53 PM on October 26, 2021
I should add- gypsum only helps clay soils that are sodic, i.e. clay soils that have a high pH, low permeabilty and lots of soluble salts. Sodic soils are common only in the Western United States. Sodium keeps clay particles from flocculating, or forming aggregates.The calcium in gypsum replaces sodium on clay particles. Gypsum has no effect clay soils that are not sodic, and it actully has no effect on the pH of any soil.
Lime does nothing for clay soil but increase pH, it will not improve structure and is ineffective at aggregating clay particles.
posted by oneirodynia at 10:07 PM on October 26, 2021
Lime does nothing for clay soil but increase pH, it will not improve structure and is ineffective at aggregating clay particles.
posted by oneirodynia at 10:07 PM on October 26, 2021
That consumer grade tester is almost definitely worthless as a scientific instrument, I’m sorry to say. Is it the kind with two long spikes off a plastic rounded head that costs about $15?
Total junk if so alas. Worse than useless. I’m guessing that is the case because it “tests” light, soil moisture, and ph together. Supposedly.
Proper instruments, even for home gardeners, separate those functions. I’m afraid your son will have a frustrating experience with unusably inaccurate readings.
You can buy independent light and ph meters for not too much money. They yellow $17 ph pens all over Amazon recommendations are frustratingly hard to calibrate but they work. A real Ph tester suitable to commercial gardening costs a minimum of $50-60. I recommend one from Apera instruments (the A1209 Value Series is damn accurate for $50). Otherwise I’d be draining the water from the soil and measuring with good old fashioned ph paper strips — sure to be more accurate than the thing you’ve got. Don’t forget to use consistent water ph going in.
Light meters also start at around $20 for functional accuracy, and go way up from there. There’s a $20 Leaton unit on Amazon that many gardeners use and I own and find is nearly as accurate as my more expensive units.
Moisture testing is best done by weight differential. The thing you’ve got may be accurate enough for that otherwise, as it’s the least complicated to measure by instrument. But what you want to measured is water retention in the soil, and the best way to do that is to measure the weight of the soil at watering, and at fixed intervals after watering. Use small enough containers that you can easily place them on a scale. Or pull out equal sized small amounts from each container and weigh those.
If you know any cannabis growers you can probably borrow the right instruments from them. These variables are crucial for cannabis and we tend to get good instruments for the purpose.
You may also want to acquire a total dissolved solids meter.
Oh and the simplest additive to test, and most important for clay soil, is compost.
posted by spitbull at 12:54 AM on October 27, 2021
Total junk if so alas. Worse than useless. I’m guessing that is the case because it “tests” light, soil moisture, and ph together. Supposedly.
Proper instruments, even for home gardeners, separate those functions. I’m afraid your son will have a frustrating experience with unusably inaccurate readings.
You can buy independent light and ph meters for not too much money. They yellow $17 ph pens all over Amazon recommendations are frustratingly hard to calibrate but they work. A real Ph tester suitable to commercial gardening costs a minimum of $50-60. I recommend one from Apera instruments (the A1209 Value Series is damn accurate for $50). Otherwise I’d be draining the water from the soil and measuring with good old fashioned ph paper strips — sure to be more accurate than the thing you’ve got. Don’t forget to use consistent water ph going in.
Light meters also start at around $20 for functional accuracy, and go way up from there. There’s a $20 Leaton unit on Amazon that many gardeners use and I own and find is nearly as accurate as my more expensive units.
Moisture testing is best done by weight differential. The thing you’ve got may be accurate enough for that otherwise, as it’s the least complicated to measure by instrument. But what you want to measured is water retention in the soil, and the best way to do that is to measure the weight of the soil at watering, and at fixed intervals after watering. Use small enough containers that you can easily place them on a scale. Or pull out equal sized small amounts from each container and weigh those.
If you know any cannabis growers you can probably borrow the right instruments from them. These variables are crucial for cannabis and we tend to get good instruments for the purpose.
You may also want to acquire a total dissolved solids meter.
Oh and the simplest additive to test, and most important for clay soil, is compost.
posted by spitbull at 12:54 AM on October 27, 2021
I will add that if you have the combined meter I’m thinking of, it only has an analog scale meter. Even if it’s accurate it is very hard to get precise readings from it. The instruments I mentioned above are digital. You’ll have exact numbers to feed into your spreadsheet of results. And even if the meters aren’t well calibrated they will give you correct relative values.
posted by spitbull at 1:05 AM on October 27, 2021
posted by spitbull at 1:05 AM on October 27, 2021
I think I was not quite clear. You test soil ph by measuring the ph of the water that drains from it.
posted by spitbull at 1:14 AM on October 27, 2021
posted by spitbull at 1:14 AM on October 27, 2021
Self correct: I said “your son” above but you didn’t mention the gender. My apologies.
posted by spitbull at 3:19 AM on October 27, 2021
posted by spitbull at 3:19 AM on October 27, 2021
I agree with hydropsyche about plantlets generating measurable data in a short time: make sure there are lots of replicates [Latin square to see off confounding variables] within treatment and Team DirtyOldDirt can compare mean shoot length after a set time. It is a better surrogate for the effective value of soil amendment than physico-chemical "sciency" measures. Null results are good too, so long as the experiment is well enough powered to say we now know there is no difference. Please post results.
posted by BobTheScientist at 4:12 AM on October 27, 2021 [1 favorite]
posted by BobTheScientist at 4:12 AM on October 27, 2021 [1 favorite]
He could test the usual treatments for high pH (sulphur, organic matter) and low pH (lime). These are spread on lawns, not mixed in. Testing over a couple weeks might show how fast or slow they are have an effect.
posted by SemiSalt at 5:39 AM on October 27, 2021
posted by SemiSalt at 5:39 AM on October 27, 2021
Half the people who discuss adding sand to clay say it will result in cement if compacted, so I definitely think kid should do that to find out which half is right.
I agree with this and would be a great experiment. It's a very common trope. Though I can tell you it doesn't, unless instead of sand you add a sand that is mixed with cement, like for example sand used to glue patio pavers together.
posted by The_Vegetables at 8:00 AM on October 27, 2021
I agree with this and would be a great experiment. It's a very common trope. Though I can tell you it doesn't, unless instead of sand you add a sand that is mixed with cement, like for example sand used to glue patio pavers together.
posted by The_Vegetables at 8:00 AM on October 27, 2021
I think to test it, you'd add sand to various clay soil batches in increasing percents (in like a cut 1 liter soda bottle maybe?). And then repeatedly wet and dry them, with a heat lamp I guess. Then as a bonus you could send them to your agricultural extension for soil testing after X rounds of wetting and drying.
posted by The_Vegetables at 8:02 AM on October 27, 2021
posted by The_Vegetables at 8:02 AM on October 27, 2021
Half the people who discuss adding sand to clay say it will result in cement if compacted, so I definitely think kid should do that to find out which half is right.
It doesn't make "cement" but it does make a soil that has less porosity than clay alone and has a similar structure to concrete, i.e. aggregates in a finer grained matrix. Clay soil has the most porosity: even though the pores are very small, there are many more of them because smaller particles have more surface area. clay soils are often described as "heavy" because these pores hold water, which is bound more tightly to clay particles than silt or sand. The rule of thumb for porosity in soils is: clay approx. 75% porosity, silt approx 50%, sand approx 35%.
The problem with adding sand to clay is that the clay particles are small enough to fill the pores between sand particles, so you lose the porosity between the sand particles. It's even worse if the soil is compacted by walking or being worked when too wet.
As an experiment, you could have equal volumes of clay soil and sand, measure the porosity of each, and then make some combined sand/soil samples of the same volume. then measure the porosity of those and see if porosity has increased or decreased.
one method is here: There are many different porosity measurement methods.The simplest one is the determination of po-rosity by saturation method. In this method, the beakers are first filled to the same mark with gravel, sand, silt or mixture of these three materials. Then the water is poured into each of the beakers until it reaches the top of each material. Measure the remaining water and subtract.
You'd want to slowly dry your soil samples in a very low oven to get the best results- especially the clay. Any water remaining before the experiment would skew the results.
posted by oneirodynia at 2:20 PM on October 27, 2021 [1 favorite]
It doesn't make "cement" but it does make a soil that has less porosity than clay alone and has a similar structure to concrete, i.e. aggregates in a finer grained matrix. Clay soil has the most porosity: even though the pores are very small, there are many more of them because smaller particles have more surface area. clay soils are often described as "heavy" because these pores hold water, which is bound more tightly to clay particles than silt or sand. The rule of thumb for porosity in soils is: clay approx. 75% porosity, silt approx 50%, sand approx 35%.
The problem with adding sand to clay is that the clay particles are small enough to fill the pores between sand particles, so you lose the porosity between the sand particles. It's even worse if the soil is compacted by walking or being worked when too wet.
As an experiment, you could have equal volumes of clay soil and sand, measure the porosity of each, and then make some combined sand/soil samples of the same volume. then measure the porosity of those and see if porosity has increased or decreased.
one method is here: There are many different porosity measurement methods.The simplest one is the determination of po-rosity by saturation method. In this method, the beakers are first filled to the same mark with gravel, sand, silt or mixture of these three materials. Then the water is poured into each of the beakers until it reaches the top of each material. Measure the remaining water and subtract.
You'd want to slowly dry your soil samples in a very low oven to get the best results- especially the clay. Any water remaining before the experiment would skew the results.
posted by oneirodynia at 2:20 PM on October 27, 2021 [1 favorite]
Make several mixes of clay + sand, clay + compost, clay + gravel, whatever. Wet each mixture and pack it tightly into the bottom of a plastic water bottle. Punch holes in the bottom of the bottle. Pour a cup of water into the top. The amount of time until there is no standing water left is your percolation rate. Compare between soil amendments.
posted by agentofselection at 10:33 PM on October 27, 2021
posted by agentofselection at 10:33 PM on October 27, 2021
To compare results between methods, I'd choose a simple, fast-growing plant like a marigold and try growing it in each amended soil. And I'd use a plant in un-amended soil as a baseline.
posted by summerstorm at 11:48 AM on October 29, 2021
posted by summerstorm at 11:48 AM on October 29, 2021
> For treatments we could compare, we are thinking something green-ish, like mixing in mulch or another natural additive.
They could make up some biochar and mix it in. That would also give your kid a chance to engage with the history/archeology and ecology of it, which might be intellectually interesting to them, while also impressing the eventual judges of the project.
Plus, it's always fun to play with fire. Carefully and thoughtfully, of course.
posted by sebastienbailard at 1:06 PM on October 29, 2021
They could make up some biochar and mix it in. That would also give your kid a chance to engage with the history/archeology and ecology of it, which might be intellectually interesting to them, while also impressing the eventual judges of the project.
Plus, it's always fun to play with fire. Carefully and thoughtfully, of course.
posted by sebastienbailard at 1:06 PM on October 29, 2021
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posted by teremala at 4:58 PM on October 26, 2021