<P> In soil classification systems , topsoil is known as the `` O Horizon or A Horizon , '' therefore , it is the very top layer . </P>
<P> In soil classification systems , topsoil is known as the `` O Horizon or A Horizon , '' therefore , it is the very top layer . </P>
the upper layer of soil called horizon a is where
the upper layer of soil called horizon a is where
the very top layer
the very top layer