Effect of zero tillage on bulk density

In zero tillage with surface residue cover, bulk density initially increases (due to lack of tillage) but later decreases (due to better porosity and aggregate formation under decomposed surface residue).

However, under continuous zero-tillage without any surface residue cover, bulk density increases due to compaction and lack soil turning.

Note: The correct answer of the question no. 10 (page no. 276, An Insight into Agronomy book) is “a”.