The following is the method i use. Keep in mind that i am relying on my memory on some of the factors.

What i do is to weigh the wet soil with a gram scale and record that weight. Then i stick the sample into the microwave oven to dry the sample. I use a 800 watt oven and set it on high and run in 2 minute intervals. I do this untill the sample wieght dosen`t change for two times. I then subtract the dry weight from the wet weight and take that number and divide it by the dry weight to come up with the moisture content. Now comes the part where i am relying on my memory. The plant available water depends upon the texture of the soil, what i remember is that sandy loam holds back about 8% of the water that is not available, silt loam about 13% and clay loam about 15%.

So using the core from the corn field here as an example.

The soil core that i took was 38 inches deep. The core weighed 391.1 grams wet, after drying it weighed 320.5 grams.

Subtracting the dry weight from the wet weight i came up with 70.6 grams of water removed. Then i divided the weight of water removed and divided by the dry weight and came up with 22 % water removed. Subtracting the factor for silt loam(13%) i came up with 9% available water. Taking the 9% and multiplying by the core depth(38 inches) i came up with 3.42 inches plant available water.