zabanda
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Plants Prevent Nitrate Leaching

Postby zabanda » Sat Aug 09, 2014 1:05 pm

I am doing a science fair project on what types of plants would be the best at stopping nitrates from leaching into water. I am using three different types of plants and one pot with just soil. I am going to fertilize all four pots and then water them for a month and test with nitrate strips to see how much nitrates are in the water at each watering. I am using the same amount of dirt, same size pot, same amount of fertilizer and the pots will be in the same spot outside. I want to test different types of plants like a grass, and a bush. do I have to keep the size of the plants the same because then I couldn't test different plant types. Thanks for your help.

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Re: Plants Prevent Nitrate Leaching

Postby JMP » Wed Aug 13, 2014 8:41 am

I think you are probably fine to test different sizes of plants since in the end, you are just interested in what the best plant is for preventing nitrate leaching, and the size is something specific to that plant. If you want to try to control for this in some way, I would recommend measuring your plants regularly throughout the experiment. Then at the end you can see whether there is any relationship between plant size and nitrate leaching, and you can mention this as a possible explanation for your findings when you present and discuss your results.

I hope this helps! Good luck!
JMP

SciB
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Re: Plants Prevent Nitrate Leaching

Postby SciB » Thu Aug 14, 2014 6:25 am

Hi Zabanda,

That's a great question you asked but not an easy one to answer scientifically. Since you plan to measure how much nitrate leaches out of the soil in the water the size of the plant does matter. A big plant will take up more nitrate than a tiny one. I would weigh each pot with soil before and after you plant your grass, herb or shrub so you have an estimate of the mass of the plant. Then after you measure the nitrate you can express the value as 'weight of nitrate per kilogram of plant'.

I am assuming that you planned to take a number of nitrate readings over time. Your plants will grow and get bigger, so you will need to weigh the plant plus pot and dirt each time. Do this before you water because the weight of the water will change the weight of the dirt. Are you planning to keep the plants under growth lights? If the pots are outside and it rains, that's going to leach out an undetermined amount of nitrate and throw off your subsequent readings.

This is a technically challenging experiment, but it is a good project. Remember to do at least two and preferably three pots for each plant type including the control. in this way you can get averages and standard deviations and do some statistical tests to prove or disprove your hypothesis. Having multiple samples will also tell you if the weighing method works because the amount of nitrate taken up per kilo of plant should be about the same for a single plant type.

One other control that I would suggest is to have three pots (statistical number, n = 3) of dirt only so that you can measure how much nitrate is in the soil itself. This is another problematic area. What were you planning to use as potting soil? Some of the mixes have fertilizer already added, some have slow-release type fertilizers, some have water-absorbing substances added. How were you planning to apply the fertilizer and how much were you going to use per pot? This is very important because if you add too much fertilizer then the amount leaching out will be so much greater than what the plants remove that you won't see any difference in the nitrate measurements.

The amount of nitrate a plant uses depends on how fast it is growing, but I don't know how much that is in terms of actual weight of nitrate. The amount per day is quite small, so your weighing and nitrate measuring will have to be done over an extended period--the longer the better.

I have tried to point out some of the technical details you have to work out to do your project. If you have more questions, please post again and we'll try to help you.

Good luck!

Sybee

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Re: Plants Prevent Nitrate Leaching

Postby zabanda » Sun Aug 24, 2014 7:06 am

Thank you so much for yor help

I was planning to use plain soil without anything added to it. I was then going to use a liquid fertilizer that I would mix up in a separate container so I could measure the nitrates to get reading that didnt fall really high in the ppm . I thought that would prevent a high reading where i would never see the nitrate readings get lower. Then water each plant with the same amount and test. i wss going to use three different plants and one plain soil and then run five tests...so really it would be using five plants for each type

Also...if I weigh the plants before and after planting does that take care of the size of the plant problem? And then I can use different sizes without a problem...

Thanks again!

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Re: Plants Prevent Nitrate Leaching

Postby SciB » Sun Aug 24, 2014 8:19 am

Hi,
It sounds good, but I’m not clear on all the steps. You are going to plant five of each of three types of plant and then do five NO3 readings. Is that right? I would suggest doing at least three soil-only controls with no added fertilizer so that you can get the mean and standard deviation for statistical testing. Also, you will need another soil-only control that you do apply the liquid fertilizer to. Nitrate (NO3) readings on these pots will tell you how much NO3 leaches out of the soil when there are no plants on it.

Where will you keep the plants so that they don’t get rained on but still have enough light to grow quickly? You probably don’t want to apply the fertilizer until the plants are well-established and growing again after the transplanting. This could take a couple of weeks.

In regards to the weights, what you are looking for is the mass of each plant at EACH reading. I would weigh each pot with the soil first and record that as your base weight. The weight of the soil could vary somewhat over time because of changes in its moisture content as you water it and it dries out; but I don’t see any simple way to weigh the soil alone once you have a plant in it.

After you have planted all the pots, but BEFORE you water them, weigh them and record that. Subtract the base weight from it to get the starting weight of each plant.

Now once the plants are established and growing, I am assuming that you are planning to add a measured amount of liquid fertilizer to each pot as the nitrogen source for leaching. When are you planning to take your first reading after that? Once a week watering is usually enough for potted plants that are not in hot, direct sun so you could do weekly readings—at least at first. I really have no idea how much NO3 a plant uses in a week. A large plant that is rapidly growing and putting on lots of leaves would use the most NO3. That’s why it is important to measure the weight each time before you water. If the plants are not growing well then they won’t be using much nitrate. By comparing the NO3 readings from the controls to those from the plants, you will get an accurate measure of how much of the nitrate has been taken up each time by the plants.

As you can see from all the details that have to be considered, doing good experimental science is a real challenge! But the satisfaction of asking a question and getting a scientifically meaningful answer is very satisfying. Testing something by repeatable experiments is the best way to learn about nature.

Good luck!

Sybee

zabanda
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Re: Plants Prevent Nitrate Leaching

Postby zabanda » Sun Aug 24, 2014 5:24 pm

Hi SciB....

I planned to have three types of plants and one control with just the soil. I would then do five trials at the same time so that would mean five plants of each type and five pans of fertilized soil only. I will now add five pans of soil only to the group as you suggested. I had planned to keep them on a covered porch so that no rain would get in them. I would water once a week and test at each watering. I will measure the plants just like you suggested.

So I would have an herb pot, a flower pot, a grass pot, a soil only pot, and a soil only(never fertilized) pot as my testing independent variable that would be fertilized weekly and nitrate readings taken at each watering and I will do this for as long as I am getting readings since I have plenty of time. I would then do this four more times for a total of five trials, but i would be doing the trials at the same time. I will weigh each pot of soil and then plant the plants and weigh each pot again. then before watering each week I will record the weight of each pot. I hope I have all of the suggestions in this procedure. Thank you so much for all of your help. I dont think I could have done this without you.

zabanda

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Re: Plants Prevent Nitrate Leaching

Postby SciB » Sun Aug 24, 2014 7:10 pm

Hi zabanda and thanks for the kind words!

It makes us happy when we hear that our help has really been useful. Good luck on your experiments and I look forward to hearing about the results. Reducing nitrate runoff into our lakes and rivers is definitely a worthy goal. A zone of nitrate-absorbing plants around a field might help to reduce this problem. If grass works well, then it could serve a dual purpose of nitrate collection and animal feed as hay.

Best wishes,

Sybee

zabanda
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Re: Plants Prevent Nitrate Leaching

Postby zabanda » Tue Aug 26, 2014 7:42 am

I have one last question for now. I filled a gallon milk jug to add fertilizer to. I was trying to get the nitrate reading to read 40ppm since I thought over the next two months the readings would at least go to 20 ppms or close to zero. II couldnt find any information on how long it takes for nitrates to be used by a plant so I was just guessing. The problem is that I had to use four times the fertilizer to get to 40ppm and my mom asked me if I was going to kill the plants from over fertilizing. If I use the recommended amount I get a 20 ppm. We talked about it and thought maybe one time of high fertilizer wouldnt kill it. Do you think using tthe 20 ppm is enough of a science fair project to show a difference? I just thought since I picked plants that will grow through the fall 40ppm would be better. Thanks again!

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Re: Plants Prevent Nitrate Leaching

Postby SciB » Wed Aug 27, 2014 5:55 am

Hi Zabanda,

That’s a very good question and it shows you are thinking scientifically. The planning that goes into an experiment often makes the difference between getting good data and getting useless data.

So, is it ok to use one application of 20 ppm? I don’t know. But first let’s think about this. The 40 or 20 ppm value is the concentration of NO3 in the water and not the amount. The concentration of NO3 in the soil is really what is important. How much liquid did you plan to add to what volume of soil? What was the amount of fertilizer recommended by the manufacturer for one plant? And, lastly, what is the sensitivity of your NO3 test—what is the lowest concentration that you can measure accurately and reproducibly?

I don’t know if 20 ppm would work for your experiment. In the lab, when we are doing a new procedure we always have to do some exploratory experiments first to make sure the conditions are optimal for detecting what we want to see. You need to add enough NO3 but not too much and your assay method needs to be sensitive enough to detect small changes in NO3 concentration. I would try to use the recommended amount of fertilizer. If you add too much then you won't be able top detect small changes.

The volume of liquid fertilizer you add is important and the volumes of water you add later to measure leaching are also important. If you have enough of your potting soil, you might want to set up a pilot experiment first to see how much water the soil can hold before it runs out the bottom of the pot. You need to know this because when you do the leaching you don’t want to add too much water so that the NO3 concentration is too dilute; and you don’t want to add too little water or you won’t get any coming out of the pot.

When you do the actual experiment, I think you should take each ‘leachate’ and measure its volume. You can do this easily using an electronic kitchen scale set to grams. Put a suitably sized container on the scale, zero it, add your leachate and weigh it. One gram is about one milliliter so then you will know how much water came out of the soil relative to how much you poured on. The volumes of the leachates should be about the same, but it will be useful to be able to prove this if someone asks you.

Also, be sure to take some good photos of your plants and their location and pictures of the nitrate assay.

Good luck and keep us posted on your progress.

Sybee


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