犁底层又称“亚表土层”，是位于耕作层以下较为紧实的土层，由于长期耕作经常受到犁的挤压和降水时黏粒随水沉积所致。一般离地表12—18厘米，厚约5—7厘米，最厚可达到20cm。
“平面型”犁底层剖面构造有利于改善土壤结构。
The plow pan profile constitution of the flat type helps to improve soil structure.
黑土耕层各复合体中有机碳含量均明显减少，犁底层亦表现出降低的趋势。
The organic carbon content in each organic-mineral complex of tillage layer is decreasing obviously with tillage period increasing, and that also has a decreasing trend in below plough layer.
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