To determine NO₃⁻ from agricultural soil. I prepared an extract with KCl, using (1g wet soil : 4 ml KCl). I determined it with HANNA multiparametric, which gives the results in mg/L, because it's used for liquid samples. Also, i used 6 mL because of equipment protocol and a 1:10 dilution (which is 0.6 ml of extract and 5.4 of distilled water), because of lack of disponibility of the extract.
The thing is I don't have the real density for the soil.
Now my logic is this, when already having the percentage of humidity in the sample:
1) Without correcting the dilution, i need to know how much of nitrate is in 0.6 ml of wet soil
2) Then, how much wet soil is in the extract for the measurement
3) Then, deleting the humidity value contained in the quantity of wet soil used for the measurement (Step 2)
4) Subsequently, knowing the weight of water equivalent to the humidity value obtained in Step 3
5) Substract the value of the weight of water in Step 4, to the quantity of wet soil, to obtain the weigth of dry soil used in the measurement
Once here, i wil have obtained the nitrates contained in the dry soil that i used in the measurement.
6) So, the only thing left to do is determining the quantity of nitrates in 1 kg; using the quantity of nitrates obtained in Step1 and the quantity obtained in Step 5.
Is that correct?
@ Mariana, I think you better measure the density of soil extract expressed in kg/l, you can divide your value in mg/l by the density so that (mg/l) / (kg/l) results in a value in mg/kg.
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A few comments.
1) You might consider using air-dried or oven-dried soil for this kind of thing. One reason is that drying stabilizes the form of nitrogen in the soil sample relative to storing the soil in a moist condition. Or as Steven L Larson says, you can dry your soil samples separately to determine the percent moisture.
2) I agree with the approach of Steven L Larson . That is, you can easily determine the mg of NO3-N * in your sample, and then convert this to mg/kg of soil. Just be careful with your dimensional analysis of the units (all the L, ml, g, kg). You would need the soil density only to convert this to mg/ha if that was your goal.
3) I use Hanna instruments all the time, and I'm pretty much a fan. But... I believe I have used the same nitrate method you are using. I've had a lot of problems getting reasonable and reproducible results with this method. I think the process of reduction with cadmium is really inconsistent in trying to do it in a cuvette. I really recommend using a few standards (made in your KCl extract), of known NO3-N concentration.
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* I think the method will report NO3- as N, not as NO3-. Be sure to check the method.
Steven L Larson Salvatore S. Mangiafico
Thank you very much for answering, i really appreciate it
Ive already have the % of moisture, i had already measured it independently from this nitrate determination, and I tried to use it. So, at this point, I used the formula that Steven gave me, but all my tries getting to the final point failed; i'm getting troubles at getting reasonable results, the nitrates are way too high (300 mg/kg, without substracting the % of moisture, and if i do it, the result will increase)
I tried doing it in another way, and i got 0.37 mg/kg, which is too low, but i think that's more possible than the first one. And, also, compared to other results in this same site of study, and even any other study.
For next determination, I will take into account this method, some standards and, of course other methods. And the equipment gives results in NO₃⁻ N and NO₃⁻, both forms
For now, I really need to get something with the values I have. So I've read that that in soil analysis, when using extracts, some people report values in ppm; and that 1 ppm equals to 1 mg/L---- So, as another solution could be using the value that I have (7.5 mg/ L, 1:10), reported as 7.5 ppm, and just substracting the appropiate % of moisture, and that gave me 366.44 ppm, which is still too high; and makes me wonder why the result increases from 75ppm to 366pm or in the other form (300 and something), just by substracting the moisture value
I think the 300 mg/kg is correct for 7.5 mg/L. (Before adjusting for moisture.) Am I doing something wrong? (Image attached).
Salvatore S. Mangiafico I did the same thing... It is just too high !! And adjusting for moisture, it increases much more... I've been thinking if there was a possibility that the soil was already fertilized at the moment we collected the samples, would that situation explain this value? It was fertilized with 150 kg N/ha (Urea) + 40 kg P₂O₅/ha
I guess the first thing to specify if that's 300 kg NO3 / kg or 300 kg NO3-N / kg. If you're starting with NO3, that 300 mg/mg may not be that high (like 70 mg/kg NO3-N ?). But also it seems to me that 300 mg/kg NO3-N wouldn't be impossible. If the soil were recently fertilized, or if conditions were right for a lot of mineralization. I really don't know. Here's a figure I found from a Google searach, "soil no3 mg/kg seasonal variation" : https://media.springernature.com/lw685/springer-static/image/art%3A10.1007%2Fs11368-020-02679-z/MediaObjects/11368_2020_2679_Fig1_HTML.png
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