footer position problem

Check my site (nature.ig4s.net) and compare the footer at home page and contact page. The footer at homepage (and other long content pages) is positioned at bottom of the page and it’s below the sidebar, but the footer at contact page (and other short content pages, e.g., category-announcement) is positioned above the bottom of sidebar and so the right part of footer is covered by sidebar. How do I position the footer below sidebar if the content is not long enough? Or is there any solution to resolve this issue? Thanks for help. (the theme is wp classic)