Drainage Conditions Complicate the “More Is Better” Paradigm in Soil Health Assessment

Abstract

The concept of soil health is widely adopted to promote sustainable agricultural practices. A common paradigm in soil health assessment, known as the “more is better,” assumes that higher values of soil health indicators are interpreted as improved soil functions and ecosystem services, such as higher yield and greater drought resilience. However, few studies have directly validated this assumption. In addition, it remains unclear whether laboratory‐based indicators align with farmers' perception of productivity. Using bahiagrass pastures as a model system, we evaluated the relationship between soil health indicators and biomass productivity under normal and drought‐stressed conditions in a greenhouse experiment. Soils were collected from paired pastures identified by farmers as the most productive (MP) and least productive (LP) fields ( n = 40). Under normal soil moisture, aboveground biomass was positively correlated with several indicators, including Mehlich‐3 extractable potassium (K), mineralizable carbon (Cmin), and water extractable organic carbon (WEOC), with K being the strongest predictor ( r = 0.64, p < 0.001). However, under simulated drought, no indicators were significantly associated with yield retention. Drainage conditions further complicated the “more is better” assumption: soil organic matter (SOM), K, and WEOC tended to be higher in MP than in LP fields ( p < 0.1) when both shared the same drainage condition; however, LP fields with poor drainage showed higher SOM and WEOC values than their better‐drained MP counterparts. A decision tree analysis further showed that high SOM (> 2.7%) was associated with low productivity in poorly drained soils. Our results demonstrate that drainage conditions confound the monotonic interpretation of soil health indicators for predicting yield and biomass production. Incorporating drainage conditions and farms' insights should be considered for the development of soil health assessment frameworks.