From Dr Sidney Alford Sir: Great importance is currently attributed to the handing over of the weapons held by the IRA as a guarantee that their most welcome ceasefire will be sustained. This important act of de-arming will, however, have real meaning only if concomitant steps are taken to ensure that rapid rearming is not possible.

The main weapons of Irish terrorism are the pistol and the large bomb. Small arms are, alas, easily concealed, as are the means of initiating large bombs, such as detonators, detonating cord and plastic explosive (typically Semtex).

What is not readily concealed is the main and the essential component of large bombs, ammonium nitrate, which is in very widespread - and, indeed, some would say excessive - use as a nitrogenous fertilizer, and which was used to cause such expensive havoc in London.

Now is surely the time to reclassify ammonium nitrate more appropriately, so that it is controlled with the same rigour as are other explosives of very much less interest to terrorists. The resultant inconvenience to landowners and quarries (which use ammonium nitrate for most of their blasting) would cost the present government some votes and contributions to party funds, but I suggest that this is not entirely without potential advantage.

Yours faithfully, SIDNEY ALFORD Corsham, Wiltshire 11 December