Polyhydroxyalkanoates (PHAs) are increasingly proposed for agricultural applications due to their biobased origin and microbial degradability. They are often assumed to serve as environmentally compatible substitutes for conventional plastics. However, PHA behavior in soil is controlled by the interactions between polymer composition, structure, environmental transport conditions, and microbial activity. PHA degradation proceeds primarily via depolymerase-driven surface-controlled erosion, in which initial mass loss is governed by surface wetting, crystallinity, morphology, exposed surface area, and enzymatic access to ester bonds, which determine the initial rate of breakdown. As degradation progresses, surface disruption allows water, oxygen, and microorganisms to penetrate deeper into the polymer, coupling material breakdown with local microbial activity and nutrient demand. This coupling is central to understanding soil and plant responses. At field-relevant exposure levels, PHA degradation has generally been reported to cause limited or neutral effects on crop performance and bulk soil properties. However, when PHA inputs are high or spatially concentrated, rapid microbial use of PHA-derived carbon can increase demand for nitrogen, phosphorus, and sulfur in localized soil zones. Under these conditions, nutrients may become temporarily immobilized in microbial biomass, reducing plant access and creating short-term plant–microbe competition. Thus, reported growth inhibition is better understood as an indirect soil-mediated response rather than evidence of direct polymer toxicity. This review critically examines laboratories, mesocosms, and field studies to identify the abiotic and biotic factors that control PHA degradation in soils. It evaluates how polymer structure, formulation, processing history, soil environment, and microbial community interact to produce variability in degradation outcomes. The review further identifies the conditions under which PHA degradation may influence soil nutrient dynamics and plant performance. These insights provide a framework for more realistic assessment and responsible design of PHA materials for agricultural systems.