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hard to determine the plastic limit

hard to determine the plastic limit

(OP)
Hi all,

today I tried to determine the plastic limit of an agricultural soil and it was not possible, the thread slipped over the glass plate rather than roll. It was the first time to me that an agricultural soil is non plastic. Is it common? I read that a sandy soil has this kind of behaviour. Tomorrow, I will sieve a soil sample to get the soil texture classification. What is the sand percentage threshold over which the soil is non plastic?

RE: hard to determine the plastic limit

Did you try initially to roll the soil between your fingers before rolling on the glass plate?

Generally, soils with less than about 15 percent fines will be non-plastic, therefore, soils with 80 to 85 percent sand will be non-plastic. Depending on the character of the fines (type of clay, etc.) the percentage of fines can go higher without being plastic.

RE: hard to determine the plastic limit

(OP)
Hi Ron,

thanks for your reply. I tried initially to roll it between my palms and it rolls much better than on the glass plate. The thread breaks into many pieces when it is 1cm of diameter and I feel it happens because it slips over the glass plate. I noticed this soil is not sticky when it is wet and it makes the whole palm brown. Is it a sign of something? It is an agricultural soil, so it will have a higher percentage of fines, I suppose, right?

RE: hard to determine the plastic limit

From you description it sounds like a non-plastic silt.

And agricultural soil can be non-plastic.

Mike Lambert

RE: hard to determine the plastic limit

Agree with GPT....also, the brown staining is likely from organic material in the soil.

RE: hard to determine the plastic limit

(OP)
Thank you so much. I believed an agricultural soil is usually plastic because a high clay content is helpful for crops. I'm waiting for the results of liquid limit and of the sieve analysis. I'll let you know.

RE: hard to determine the plastic limit

Agricultural soils in my area are almost never plastic. That will depend on the area you're in.

RE: hard to determine the plastic limit

(OP)
I calculated the liquid limit and the result is 29%. May it be a reasonable value for a non plastic soil? On the computation of the liquid limit fitting line is there any lower threshold on the resulting R^2 to assess if the experiment is properly carried out?

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