Sand-bath method is a quick method for determining the water content of a soil sample. It is a field test because we don’t need any specific expensive equipment for this test to perform.
We only need a small vessel or container to take some soil sample and calculations of this method are similar to that of oven drying method.
Firstly we weigh the empty container and say its weight is w1.
Then take some amount of soil into it and weigh it again. We say this weight as w2.
Then we place the container on sand bath. A sand bath is tray type container which contains about 50 mm thick sand layer. This arrangement is heated with stove or burner. The sand layer is used to give the sample a uniform heating.
When sample is placed on sand it becomes dry with in half to 1 hour. The sample is ocassionally stirred with rod or some knife to assist drying.
After drying weight of sample is taken again and noted as w3.
Now we have obtained three weights
W1 = weight of container
W2 = weight of container + moist soil sample
W3 = weight of container + dry soil sample
We can determine weight of water that was present in the sample by removing dry soil weight from moist soil weight
and weight of solids by removing weight of empty container from weight of dry soil + container.
Water content being weight of water divided by weight of solids can be now calculated by substituting these values.
We should note here that this method should not be used for organic soil because in this method temperature is uncontrolled and high heat will oxidise the organic matter.