I'm trying to analyze OM content of compost materials. I am using the Walkley-Black method which we use for soil samples which are low on OM. Now, I can't use the same for high-OM samples. How can I adapt the procedure so I can get a reliable data for my samples?
Dear Isabelle,
Rightly said by Mr. Sarkar. You can adopt this. But to make the test more representative you can also increase a) the amount of K2Cr2O7 upto 20mL or as required to oxidise the OC present in sample, b) you can also increase the strength of K2Cr2O7 insted of 1N, you can use 2N, for better oxidization of OC. And then we should modify the (%) OC in compost formula as like
%OC =[ {Vk(1-(Vs/Vb))}*Sk*0.3]/W, where Vk is amount of K2Cr2O7 you used in the procedure, Vs is amount of FAS consumed in sample titration, here compost, Vb is amount of FAS consumed in blank titratio, Sk is strength of K2Cr2O7 you used.....Thanks a lot, and all the best.
Dear Isabelle,
In general, we use 1 g of soil sample for the measurement of organic C by Walkley-Black method. But the compost sample contained much higher organic C (normally >20% approx.) than the soil. As per my knowledge you should decrease the amount of sample taken to analysis. Please try the procedure with lesser amount of compost sample (0.1 g compost sample). If this amount also give higher value of ferrous ammonium sulfate (burette reading), you should simultaneously take much lesser amount of initial compost sample. I think it would give you reliable data.......All the best!!
Dear Isabelle,
Rightly said by Mr. Sarkar. You can adopt this. But to make the test more representative you can also increase a) the amount of K2Cr2O7 upto 20mL or as required to oxidise the OC present in sample, b) you can also increase the strength of K2Cr2O7 insted of 1N, you can use 2N, for better oxidization of OC. And then we should modify the (%) OC in compost formula as like
%OC =[ {Vk(1-(Vs/Vb))}*Sk*0.3]/W, where Vk is amount of K2Cr2O7 you used in the procedure, Vs is amount of FAS consumed in sample titration, here compost, Vb is amount of FAS consumed in blank titratio, Sk is strength of K2Cr2O7 you used.....Thanks a lot, and all the best.
My colleagues have nicely explained modifications for compost. Another method of dilution may be adopted. Compost can be mixed uniformly with sand (having very low organic carbon content, and least interaction with OM) and organic carbon be determined following the usual procedure. So, the organic carbon of compost will be (OC in mixed sample - OC in sand) x dilution factor.
Dr. Abhijit Sarkar has explained nicely the solution. Alternatively you may look for other methods for you test samples and comparison. See the Book by Page et al. . Chemical analysis of soils and the following link.
https://www.google.com.pk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwjYzNKe0qjLAhWDZpoKHT0MCHIQFggkMAE&url=http%3A%2F%2Fwww.agvise.com%2Feducational-articles%2Fsoil-organic-matter-a-choice-of-methods%2F&usg=AFQjCNHhChPAF7B8NiLuG4_ifYRXhYTH0A&sig2=5nJfbjEG3MfWiVhOnSNTKQ&bvm=bv.116274245,d.bGs
If you are the beautiful young woman on the picture which comes with your question, dear Viginia, you should keep away from working with K2Cr2O7! Its well known mutagenous and banned in our laboratories especially for woman who still want to reproduce!
For your purpose a simple method involving weight loss by ashing should be accurate enough and its safe!
Dear Dr.Schnug,it is a news(good or bad ?) to us that K2Cr2O7 is banned in laboratories.The Walkey and Black method served and survived over eight decades.It is convenient and most widely used method for estimating organic carbon in soils.Because of the presence of calcium carbonate in soils ,the ignition method may not serve as alternate to dichromate method.What other oxidizing agent serve as alternative to potassium dichromate?
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