Rapid method of determining factors limiting bacterial growth in soil

A technique to determine which nutrients limit bacterial growth in soil was developed, The method was based on measuring the thymidine incorporation r ate of bacteria after the addition of C, N, and P in different combinations to soil samples. First, the thymidine incorporation method was tested in t wo different soils: an agricultural soil and a forest humus soil. Carbon (a s glucose) was found to be the limiting substance for bacterial growth in b oth of these soils. The effect of adding different amounts of nutrients was studied, and tests were performed to determine whether the additions affec ted the soil pH and subsequent bacterial activity. The incubation time requ ired to detect bacterial growth after adding substrate to the soil was also evaluated. Second, the method was used in experiments in which three diffe rent size fractions of straw (1 to 2, 0.25 to 1, and <0.25 mm) were mixed i nto the agricultural soil in order to induce N limitation for bacterial gro wth. When the straw fraction was small enough (<0.25 mm), N became the limi ting nutrient for bacterial growth after about 3 weeks. After the addition of the larger straw fractions (1 to 2 and 0.25 to 1 mm), the soil bacteria were C limited throughout the incubation period (10 weeks), although an inc rease in the thymidine incorporation rate after the addition of C and N tog ether compared with adding them separately was seen in the sample containin g the size fraction from 0.25 to 1 mm. Third, soils from high-pH, limestone -rich areas were examined. P limitation was observed in one of these soils, while tendencies toward P limitation were seen in some of the other soils.