you can plant turnips s ever yr if that's what you want, you just have to add the right soil nutrients for it to grow well, a simple soil test will tell you what is needed?
there is not need to leave a field dormant or rotate to a different crop, unless your trying to regain correct soil conditions by using another crop to help solve things , as some crops add different things back into the soil, if you follow here!
SO< if you wanted to plant turnips every yr, you can, as long a s the soil is right to support it!