What would be better to till into clay : topsoil or course sand for the longest benefits?

Lime is the answer to the clay problem. Breaks it up and stops it clumping. However,for a large area,that can be expensive. If you can wait a year,potatoes are good at cleaning up clays,and will grow well. Also roses do very well. My father had 350 rose bushes in our heavy clay soil in Hertfordshire and exhibited every year at the National Rose Society shows at Olympia, and won many prizes. Their national show grounds are only eight miles from us,on the same type of soil,in St. Albans. We've always grown our own stuff,but he started from scratch with the roses when we moved house. To break it up more quickly,grit and course sand are useful,and as much organic matter as you can find, like clippings,trimmings,gr cuttings,anything. This time of year there are loads of leaves to collect. Sacks of leaves can be dug in,and more left to form leaf-mould for use later. It takes an awful lot of material to be effective,so it's a year-on-year job to convert your clay into a more friable soil. You can buy topsoil from garden centres,but to get enough to be of much use for a clay garden would be very expensive. Sometimes you can find it advertised by builders or others who are clearing a piece of land and you can buy a truck-load of it for maybe £50-£100. Topsoil is the top layer of soil that has had stuff growing in it for ages,and so has the structure and nutrients that the normal growing process gives. Old rotted roots,generations of plants decaying into it,old insect bodies,worms having their go at it,all makes for a friable,nutrient-rich soil.Hedgerow soil is marvelous,if you can get it. All nutrients get leached out of soil if they are not replaced,as rain-water soaks through it. That's why the sea is salty. The salt got washed from rocks and soils into the rivers and into the sea,over millions of years. Clay is almost impervious to water when it's heavy,so the nutrients don't get washed through as easily. Also,clay particles are very small and so have a lot of surface area in relation to their diameter (look up the maths,it's all fun,'onest),and nutrients can cling much better by adsorption (with a 'd',not 'b'....also look up). Clays can therefore be quite nutritious,but have the problem of waterlogging due to them having only small gaps between the particles. Even dry clays have very little aeration and this is one of the reasons why ancient artifacts,some of them thousands of years old, are often found preserved in them with relatively little damage. Not much use for growing most plants,however, hence the advice of adding grit and course sand,to ist in aeration and drainage. Only aerated soils can allow most plants to develop good roots. Roses do well largely because of the style of roots they have,as with potatoes. Also,as plants grow they use the nutrients available in the soil. If you are growing veg and keep taking the veg out without adding nutrients to the soil,the available nutrients will get depleted,so the plants grow less well year after year. That is the idea of crop rotation. Putting different plants into the same bit of soil each year balances what is taken out and what is put in. Some plants actually put nutrients into the soil as they grow. Legumes(peas,beans) are the best-known example. They have root nodules which allow them to put nitrogen into the soil by converting nitrogen from the air into water-soluble nitrates which other plants can use.....The top paragraph here explains it briefly. ... a href="http://www.clima.uwa.edu.au/schools/legumes" rel="nofollow"http://www.clima.uwa.edu.au/schools/legu…/a ...Digging in manure,green manure(clippings,gr cuttings,etc)and compost (rotted vegetable matter),also adds nutrients to the soil,and encourages worms,which do their own bit of soil processing. Chemical fertilisers like potash and bone meal can also be used,but are not considered to be 'organic' by some modernists. Woodland has it's own way of doing it. As leaves fall and rot into the ground they put back many of the nutrients the trees have taken out,apart from those already used for the woody growth of trunks and branches. Other plants growing as brush and soft plants under the trees make up for that,and animal activity also adds to the available nutrients,and together they form a self-sustaining ecological system,called an ecosystem.