Comparison of Some Methods for Measuring Primary Soil Particle Size Distribution and Introducing Appropriate Times for the Four-Reading Method for Determining Soil Texture

Different methods are used for determination of the primary particle size distribution (PSD) in Iranian soils laboratories, which in many situations makes it difficult to compare results and their precision. This research was conducted to achieve an easy and inexpensive strategy with acceptable accuracy. First, 112 soil samples from 16 different provinces of Iran with a wide range of soils were taken from different climates and land uses. Soil PSD was measured by hydrometer with and without cement removal. The results showed that the common reading method of 40 seconds and 2 hours (Bouyoucos, 1962) measures the clay fraction with an average absolute error of 9.5% as compared to the complete record method. This error was more than 10% in 40.5% of the proposed soils and more than 5% in 94.2% of the soils. The deviation of this method from the actual value is increased as the soil becomes finer. Two four-reading methods with the classification of International Soil Science Association (ISSS) including: a) 2 and 10 minute readings (to measure silt fraction) and 6 and 8 hours (to measure clay fraction), and b) readings of 2 and 10 minutes (to measure silt fraction) and 6 and 24 hours (to measure clay fraction) were suggested to determine the soil texture. The two later methods show the soil texture correctly in 97.1 and 94.2% of the soils, respectively. The error of these two methods for determination of clay fraction is less than % in more than 90 and 81% of the proposed soils, respectively.