Effects of Concentrated Humus on Plant Respiration Note: John & Bob's Soil Optimizer consists of the concentrated humus used for this study along with three other organic ingredients to maximize benefits by blending calcium, iron, and soluble concentrated humus. This experiment by Sladky on tomatoes in 1959 shows the effects of concentrated humus on chlorophyll density and respiration rates. Increased chlorophyll density and deeper green color is one of the most widely reported benefits of concentrated humus reported in the literature. Researchers attribute the enhanced chlorophyll to better iron availability.