Flood water from the Snoqualmie River and Cherry Creek, laden with fine silt, colloidal clay, and dissolved ions, stands quietly part of each winter season over land underlain by an old peat bog near Duvall, Washington. This water, instead of leaching and abstracting inorganic plant nutrients as is the usual role of surface water and soil, donates to the soil dissolved ions, colloidal material, and fine-grained, partially decomposed rock fragments (naturally pulverized agstone). Analyses are presented which indicate the annual contribution of the alluviation. In the chemical weathering of rocks to soil the sequence of energy effects begins first with the disruption of relatively strong crystal bonds between ions, thereby making the ions vulnerable to pick up by plants by way of the exchange bonding energy of rootlets. As weathering advances, however, and abstraction of ions from the rocks by leaching continues, the concentration (activities) of available nutrient ions in the soil decreases, and their energies of exchange from soil clay rises above the energy of the rootlets to pull them away. Thus, weathering at this later stage is destructive to soil. Agriculture, therefore, operates most efficiently on soil in which the rocks are in a stage of partial or intermediate decomposition. Economic geologists interested in the sources, use, and production of agstone may apply in practice the principles which may be deduced from this example in nature.