Decomposition products of dead plant materials form intimate associations with minerals, making it difficult to isolate and characterize soil organic constituents. 18th century soil chemists successfully used alkaline extraction to isolate a portion of the organic constituents in soil. This led to the theory that a humification process created humic substances; most commonly humic acid, fulvic acid, and humin. However, these humic substances have not been observed in soil.   Although humification theory is unsupported by evidence, "the underlying theory persists in the contemporary literature, including current textbooks." Attempts to redefine humic substances in valid terms have resulted in a proliferation of incompatible definitions, "with far-reaching implications beyond our ability to communicate scientifically accurate soil processes and properties."