if you have a very light sandy soil you will need to dig in lots of manure and compost on an annual basis to keep the soil fertile nd to prevent it drying out. If you can get to a garden centre, virtually all of them now sell sealed bags of composted stable manure which I am sure will make a difference to the fertility and moisture retention properties of your soil if you dig it in, in sufficient quantity. The other alternative would be to add some fertiliser in the form of Growmore or chicken manure pellets, but it sounds as if your soil perhaps needs some long term pepping up. Do you keep a compost heap. This, when rotten down and returned to your soil will help improve its condition.