Traditionally, soil-testing laboratories have used a variety of methods to determine soil organic matter, yet they lack a
practical method to predict potential N mineralization/immobilization from soil organic matter. Soils with high microbial
activity may experience N immobilization (or reduced net N mineralization), and this issue remains unresolved in
how to predict these conditions of net mineralization or net immobilization. Prediction may become possible with the
use of a more sensitive method to determine soil C:N ratios stemming from the water-extractable C and N pools that
can be readily adapted by both commercial and university soil testing labs. Soil microbial activity is highly related to
soil organic C and N, as well as to water-extractable organic C (WEOC) and water-extractable organic N (WEON). The
relationship between soil respiration and WEOC and WEON is stronger than between respiration and soil organic C
(SOC) and total organic N (TON). We explored the relationship between soil organic C:N and water-extractable organic
C:N, as well as their relationship to soil microbial activity as measured by the flush of CO2 following rewetting of dried
soil. In 50 different soils, the relationship between soil microbial activity and water-extractable organic C:N was much
stronger than for soil organic C:N. We concluded that the water-extractable organic C:N was a more sensitive measurement
of the soil substrate which drives soil microbial activity. We also suggest that a water-extractable organic C:N
level > 20 be used as a practical threshold to separate those soils that may have immobilized N with high microbial activity.