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Nitrogen (N) and Carbon (C) are popular indicators of soil fertility; however, they are not soil fertility itself. In fact, they may be seen as just two aspects of the one entity. Soil microbial biomass (SMB) is also one of soil fertility indicators; furthermore, recent study of co-evolution between plants and microorganisms raises an idea that SMB might be the entity of fertility. The correlation between SMB and crop yield has been found in some studies but not in others. Those studies were conducted from the standpoint of N stock balance; therefore, the correlation between soil properties before planting and plant yields were analyzed. Here, we show—in our analysis of harvest-time soil properties and crop yields—that SMB correlates more strongly than inorganic N, total N, or total C with average crop yield under a wide range of cultivation conditions. From the viewpoint of co-evolution, plant biomass is a part of the plant and soil microorganism system; therefore, increasing SMB will balance by increasing plant biomass. In addition, the SMB could increase independently from the plant growth by artificial organic matter input. This concept will break through the yield limitation of conventional farming.
I moved some paragraph to right positions. Conclusions section was added. Unnecessary sections were deleted. The latter part of discussion was revised.