The old party trick of rubbing a comb or a pen with flannel, or a stick of sealing wax with silk, and making small pieces of paper leap up to attach themselves. This is caused by static electricity.
It doesn’t take much to realize that ‘brute force and ignorance’ methods of trying to alter the nature of clay – which is what sand and grit amount to – are a waste of time. This may kill a few sacred cows, but there it is – sand and grit cannot change the nature of clay; the cause is very, very much more involved than that.
You may have heard of the use of ‘Nottingham marl’ in the preparation of cricket pitches, and the excellence of Nottingham’s Trent Bridge ground. Marl is a dense clay that does not settle into a wet, soggy mess like ordinary clay. Some of the finest rose growers in the world are to be found around Nottingham, and you would be right in thinking that there must be a connection between the two facts.
Over the years, in addition to sand and grit, there have been some ingenious ideas and products put on the market. None of them have lasted very long, and have proved to be more effective at getting money out of gardeners’ pockets than they have at getting the wet stickiness out of clay!
The maximum rate that a clay can assimilate gypsum, is 8 ounces (230g) per square yard (square metre), per annum, and this is best interpreted as 2 ounces (50g) per square yard (square metre) every 3 months – simply spread it on the surface and let the weather take it in from the mulch. The process may take a couple of years to show a marked improvement on heavy clays, and you will have to be both patient and persistent, hence the need for your understanding.
The above cultivations will gradually flocculate the clay, get air into it and make it more responsive to organic input, feeding and mulching.