Researchers conducted a greenhouse experiment to measure available sulfur in soil by growing cotton plants with different levels of sulfate-sulfur. They used a technique involving radioactive sulfur to determine available sulfur levels. The study found a strong correlation between available sulfur levels and plant growth responses to added sulfate-sulfur. The total uptake of sulfur by plants closely matched the total available sulfur in the soil. Increasing the rate of sulfur addition by sixteen times led to a 20% increase in available sulfur levels.