It has been hypothesized that substances that accumulate during the mushroom cropping period are responsible for decreased yields as a mushroom crop ages. The objective of this study was to determine whether chelators added to compost substrate at spawning would affect the later break yield of Agaricus bisporus. Different types of chelators were used to determine if specific ions were involved. Supplemented compost receiving hypnum peat, cation exchange resin, ethyleneglycol tetra-acetic, and citric acid significantly increased later break yield. However, ethylenediamine tetra-acetic and sphagnum peat moss added to supplemented compost at spawning did not significantly increase yield in later breaks. These results suggest the accumulation of calcium or another cation may inhibit later break yields. Calcium oxalate concentrations on mycelial surfaces were measured using electron scanning microscopy. The results of these studies showed a reduction of calcium, as calcium oxalate crystals with the addition of hypnum peat, at the surface of the mycelium during later breaks. This adsorption of Ca2+ by resin, hypnum peat and other chelators might substitute for at least, part of the detoxification function of the calcium oxalate crystals. The effect of calcium chelators on Ca2+ could explain a more efficient use of the nutrients in later breaks, improve substrate utilization and reduce the quantity of spent mushroom substrate at the source.