Science Buddies: "Ask an Expert"

My brother is doing a science fair experiement using plants to see how they affect nitrate leaching. I wanted to do something a little different.

A farmer told me that they plant cereal rye in between their crop season to soak up nitrates in their field. When I was researching this, somehow I came across biochar and how some people think it helps uptake nitrates and help plants grow better. I thought I could do a project where the independent variable would be one pot plain potting soil, one pot potting soil with cereal rye, one pot with potting soil/10% biochar, one pot potting soil/20% biochar and one pot potting soil/50% biochar. (I would be doing five trials at the same time so really it is five pots of each). I would have the same amount of cereal rye in each container. I would water each on the same timeline. I want to see how fast it grows in each type of soil and then i would like to check the nitrate leaching. Is it best to check the water that runs through the pot for nitrates and maybe other things, or is their a way to test soil without a lab. I had seen other projects where they said they tested the soil retention of nitrogen, phosphorus and potassium but it never said how they did this. I want to find out that since cereal rye is good at taking up nitrates could it be made even better using biochar. Just wondering your thoughts on this project.

Thanks for your help!
Krys

Hi Krys,

I think you have a good idea to plant cereal rye and test whether Biochar improves its removal of nitrate (NO3) from the soil. That was smart to ask a farmer what they use for nitrate capture!

In terms of the groups you set up, I would add one more control pot—soil with 20% Biochar alone so you could see if adding Biochar alone affects how much nitrate leaches through the soil. Doing 5 pots of each is good because then you can do statistical analyses on your results.

For your dependent variables, I would measure the change in weight of the plants with time as well as the NO3. Maybe Biochar stimulates growth. You will need an electronic scale that measures in grams and has the capacity and sensitivity to weigh your pots with soil and plants. Use plastic pots filled with your soils and weigh them before watering to get the initial weight. Are you planning to plant seeds of rye? That would probably be the best way to do the experiment.

As to the NO3, I would measure it in the leachate rather than in the soil itself. The harm from excess nitrate in the environment comes from runoff and leaching so that’s where you want to measure it. I think you can use NO3 test strips to measure the amount in the leachate but you may need to check with the company to make sure they are sensitive enough (https://www.google.com/#q=nitrate+test+ ... afe=active). I have no idea how much NO3 will leach out of the soil compared to how much the plants will use.

You will need to measure NO3 leaching over time but I don’t know for how long. Once the rye is grown up, you will add the same amount of a liquid fertilizer to each pot as your starting amount of nitrate. How much you add is going to depend on how much soil there is. You might want to run some tests first with just soil to see how much NO3 comes out in the leachate for a certain amount of fertilizer added. You want to add enough but not too much. Before you add the fertilizer, weigh the pots with soil and plants and record this.

One week after adding the fertilizer, weigh the pots again then add an equal volume of distilled water to each pot and collect ALL the liquid that runs out the bottom. Measure the volume of each leachate and record this in your lab book. Set each sample aside so that the dirt can settle out before you measure the NO3. Water and measure weekly until no more NO3 is detectable in the water. Don’t add any more fertilizer!

This is basically how I would do the experiment. What do you think? You can modify anything you want so long as it is done scientifically. Using the different percentages of Biochar is a good idea and testing rye without Biochar is the correct control. I think weighing the plants is important because the larger the plant mass the more NO3 it can take up, so you need to be able to put the amount of nitrate you measure on a per kilogram-of-plant-mass basis.

I hope this gives you some of the details you need to think about in planning a good science project. We’ll be here to keep guiding you along.

Good luck!

Sybee

Thanks Sybee!

Yes I plan on planting from a seed because the farmer said it is a cool weather grain and now is perfect time to plant. I also have many months to do this. Thanks for the weighing idea because biochar is supposed to help in growth of plant.

I was wondering...when I plant the seeds, do I plant in potting soil like a miracle gro with the fertilizer already added? When you suggested adding the fertilizer, was that on top of the miracle gro it will be planted in? After thinking about it I am not sure if it should be planted in miracle gro which has slow release fertilizer or regular soil with no fertilizer? If I do plant in miracle gro (which says it fertilizes for 3 months), would I still add fertilizer? I was going to plant in miracle gro because I was assuming that the plants would take up nitrate in order to grow.

Sorry, this has been the part that is the most confusing for me everytime I do projects involving fertilizer. Thanks for all your help!

Krys

Hi Krys,

MiracleGro makes a great planting medium, but you don't want to use it for this experiment. You definitely do not want a slow-release fertilizer leaching out every time you water! I would get a bag of plain topsoil and use that for your seeding mix. Once the plants get going well you can apply the kind of MiracleGro fertilizer that you dissolve in water. I just don't know how much would be right. You may have to do a preliminary experiment in which you apply increasing amounts of fertilizer and measure how much comes out in the leachate. You want to apply enough so that you can measure the nitrate with your test kit but not so much that it overwhelms the plant's ability to use it.

It is common for scientists to have to do a series of experiments first to establish the right conditions before they can do the experiments that are specific for answering their hypothesis. You or I haven't done this before so we don't know what amounts and times to use. You are asking the question whether the Biochar enhances the ability of the rye to remove nitrate. You don't know if it does or not. The differences may be small so getting the right amount of nitrate to start with is critical.

Have you done some searching and reading online about nitrate utilization by plants? You should do that to try and get an idea how much nitrate various plants use over time. Maybe weekly measurements are too frequent. Monthly readings might give you a better chance to see a difference in nitrate leaching. I don't know.

Keep posting and we'll keep trying to help!

Sybee

Hi Sybee..

Yes I did try and my mom helped me but that is why I am having problems on fertilizer. We cant find anything that gives us an amount. I did a test of a pot of plain soil and I fertilized it with the amount the bottle said, with the liquid miracle grow. I tested it with a nitrate test strip and the strip was a llittle lighter than the 20ppm. I was afraid this wouldn't be a big enough concentration to show the decrease in nitrates. I then kept adding fertilizer to see how much it would take to get to 60 ppm but it was a huge amount and my mom said it could kill the plant when it is growing. I thought 60ppm would be a good start because it is enough to show the decrease and then I would be able to see the amount decreasing. At 20ppm which is the lowest reading on the nitrate strip it would be hard to see the decrease and to get a number unless the strip goes to all white and make the reading 0 ppm. When I used a pot of potting soil(miracle grow) the reading was 60ppm but my mom said she thought that the fertilizer would keep putting out nitrgeno every now and then so it would be hard to see a difference because there would always be nitrogen being released in the soil. So this is why I am confused. I guess even if the nitrate is close to 20ppm I would be able to see if it does go to almost 0. It would still show which one is taking leaching the least amount of nitrates. I am planting now and will use fertilizer when the plants grow a little and plant a few extra pots to do some tests to see what happens before I fertilize them all. Thanks a lot for helping me.

I'm happy to help! Science is always a team effort.

Can you give me more exact details on your method? When you got the 20 ppm nitrate reading how much soil did you use? Was it plain topsoil? What volume of liquid fertilizer did you add? Did you measure nitrate in the liquid fertilizer itself? How long did you wait before you added water to test the leaching and how much water did you add? What volume of water came out of the pot?

All these factors will affect your reading. I'm trying to figure out the best starting amount of fertilizer and the minimum volume of leachate so you can accurately measure a difference in nitrate by comparing the starting amount to what you get after plants have grown in the soil and used nitrate for some time.

PPM is equivalent to milligrams per liter [http://www.unitconversion.org/concentra ... rsion.html] so 20 ppm would be the concentration of nitrate in one liter of water containing about 20 mg of nitrate. You said this was the lowest amount of nitrate you could measure with your test strip. Is this sensitive enough? I don't know. You need to find out how much nitrate a plant of a certain size takes out of the soil in a certain amount of time. If the amount is much, much lower than 20 mg per liter then you won't be able to detect it with your assay.

I had an idea before about how to detect nitrate but didn't mention it because I didn't want your experiment to get too complicated; but if your test strips are not sensitive enough, you'll have to use some other assay. My idea was to use a bioassay. These can be much more sensitive than chemical assays. Have you heard of 'algae blooms'? These happen in ponds where fertilizer runoff increases the nutrient level and stimulates growth of single-celled algae (http://en.wikipedia.org/wiki/Algal_bloom) and can cause harmful effects to pond animals. This is one way that fertilizer leaching damages the environment so it would be a good way to measure the activity of your leachate in real terms.

In order to test your leach water you would add it to a culture of freshwater algae, let them grow for a few days and see if they grew better with the leachate compared to plain water. Measuring the amount of algae exactly would require some specialized equipment so I would recommend just photographing the algae solutions to show a difference visually.

I don't know if you have the time to do this sort of a bioassay, but it would bring your project into the real world and make it really cool. The tricky part, I'm sure you are about to ask me is--where do i get the algae? You can buy algae cultures from Carolina Biological, but if i were doing a project like this i would try to get algae from a pond. You'll have to do some reading on how to grow algae but it is doable.

Let us know what you decide. We'll be here to help you on the way regardless of the project. Find out how much nitrate plants use and ask around to find out if there's a more sensitive way to measure nitrate that you can get access to. Call your agricultural extension service and ask them. They should be experts in soil analysis and they might think your project was so interesting that they would allow you to use their lab and equipment.

Good luck!

Sybee

I can't thank you enough for your help.

I realized I read the instructions on the fertilizer wrong. I tried it again and I used it like the package said. 1tsp per one gallon of water per 12 cups of plain top soil. I watered with 1 1/3 cups water with only a tiny bit of runoff.. When I fertilized with miracle gro in the water and I took an immediate reading of the runoff which gave me 20ppm. I realized that I was getting the immediate release of nitrates from the liquid fertilizer. I love the idea of the algae but don't think I can do that this time(I have decided that will be great for next year). So i have decided to plant my seeds (I am now using winter wheat) and wait for them to grow and then fertilize them. How long after I plant should I fertilize them? The farmer told me the wheat would be a foot and a half to two feet in two months. I was thinking two weeks. Then I will concentrate mostly on the size of the plant, amount of runoff, root growth at end of experiment. I am going to try the fertilizer and see if after everything is growing if the amount of the nitrates can be measured. I will just see what happens and if I get results great but if not I can just use how biochar helps plant growth. Even if the first reading of nitrates is 20ppm I can maybe just note which one got lighter or close to 0 or maybe I would see something unexpected then that would be great. When I talked to gardeners, they said if I over fertilize I may kill the rye since it is in a pot so I am afraid of adding too much fertilizer. I am going to the extension office this week and see if they can help or see if they know of a lab that can help. I live in the middle of nowhere so resources are limited. I am using 12 cups of soil for the experiment so I would add 10% biochar or 1.2 cups to the soil? I started to doubt myself as to whether I keep the same amount of soil per experiment or cut back on soil when I add biochar so that there is 12 cups of either soil or soil/biochar in the pot. I am pretty sure I add to the 12 cups. Thanks again!

Hello, again. You are thinking through the project very well. As a scientist I learned early on to always read the labels on containers of chemicals several times before I used them! Misreading a label is one of the commonest sources of error in a lab.

Three quarts [12 cups] of soil is a good amount for one pot. How many seeds are you going to plant per pot? I would think 5 would be about right. You are using plastic pots, right? I wouldn't worry about over-fertilizing because you are only applying fertilizer one time. In reading your post I wanted to make sure I understand what you did. You mixed 1 tsp of fertilizer in 1 gal of water, applied 1 1/3 cups of that to 12 cups of soil then measured the NO3 in the liquid that came out of the pot? Did you repeat this a couple of times to see if the NO3 readings were all similar? That's important statistically because you need to know the degree of deviation from the mean and you can calculate that if you have 3 readings from 3 different pots all treated identically. You should also be careful to measure the volume of leachate from each pot because that volume will vary. To compare the pots you need to calculate how much NO3 you recover after each watering. You would do that by multiplying the volume of the leachate in liters by the ppm [milligrams per liter, mg/L] you measure with the test kit. This will give you the total mg of NO3 in the leachate. You can't use the ppm reading by itself because it depends on the volume.

As to when to add the liquid fertilizer, that depends partly on the project due date. You want the plant mass to be large enough to utilize as much NO3 as possible, but you want to have some time to water weekly and take several readings and prepare your presentation. When you talk to the ag extension people try to find out how much nitrate a grass like wheat would be expected to use over a certain period of time. Since you are applying a liquid fertilizer rather than a delayed release fertilizer, the NO3 may leach out fairly quickly. That's why I told you to try an experiment first with just soil to see how quickly the NO3 disappears from the soil with successive waterings. The NO3 may be mostly all gone after three waterings. I don't know. Use a volume of liquid fertilizer that is large enough to give the desired dose of NO3 but not so much that it runs out of the pot. You want all the fertilizer to stay in the soil until you water the plants one week later.

To make soil with 10% biochar you need to keep the TOTAL volume at 12 cups, so you must mix 1.2 cups of biochar with 10.8 cups of soil. The best way to do this would be to put soil and biochar into a big metal or plastic tub and make enough soil mix for all the pots at the same time. Be sure to stir the soil and biochar together very well to get a homogeneous mixture. This way you can be sure your mix is the same for each pot.

After the wheat has sprouted, make sure all the pots get the same amount of sunlight and NO rain. Protect the plants from rabbits, deer, squirrels, birds or any other critters that might want to munch on the plants or dig in the pots. Keeping them in a sunny screened porch away from pets would probably be the safest bet.

Let us know how it is going and if you have any questions.

Good luck!

Sybee

Hi Sybee...
I am growing the wheat now. I had been told that biochar helps retain water so I water every four days and measuring the leachate. I was trying to show that biochar would have the least leachate but I am finding that it leaches more water because the soil is still a little wet when I water. The soil only dries out very quickly but if I wait until the wheat/biochar pots are dry the soil only will die from needing water. Do you have any suggestions how to show that yes the biochar is holding water longer because the soil is still moist and can't hold as much as the soil only pots which suck up the water because they are extremely dry in a day or two. Thanks again!

Krys

Hi Krys,

Thanks for letting us know how your experiments are progressing. Sounds good!

I have noticed the same thing as you in watering plants. When I use soil that has a lot of peat moss added, I don't have to water as often because the peat moss holds water in the soil. The BioChar seems to act the same way, so when you pour a measured volume of water onto the soil there's more coming out in the BioChar pots than the regular dirt pots because the soil already has almost as much as it can hold. You know the volumes of the leachates, so when you determine the nitrate concentration in the water that comes out of the pots you can convert the readings to an amount--total nitrate per leachate. This will compensate for the difference in volume.

I think to show that the soil + BioChar pots hold more water than soil only, you will have to do a separate experiment without plants. Put equal amounts of the soil only and the soil + BioChar into each of three pots (3 pots for EACH of the 2 soil types), weigh them dry then add the same volume of water to each of them. Weigh them again after 4 days and subtract the initial weight. This will give you the weight of water held by the two types of soil and will show how much extra water the BioChar holds. The higher the percentage of BioChar, the more water the soil should hold.

Did you add the liquid fertilizer yet or are you waiting for the plants to get bigger? If they are too small they won't be using much nitrate. Let us know what happens when you start to take nitrate readings. Also, be sure to take lots of photos of the plants as they grow. Put a ruler or meter-stick next to them so that it shows their height.

Good luck!

Sybee

Thanks again for your help! I have not added fertilizer yet...they are really starting to grow now but I thought a little bigger would be better. Since they grow in colder weather i figure I still have a lot of time so I think I will be adding the fertilizer in about two weeks. I will let you know!

Krys

Hi Krys,

Sounds like a plan. Don't forget to try the Biochar in soil experiment without plants to see how much water it retains compared to soil without Biochar.

Good luck!

Sybee