Dawn Fittock, of Duntroon, asks :-
What causes magnetic circles in paddocks where nothing grows on the perimeter?
Bernard Howard, a retired botanist who has investigated crop circles, responded.
I know of two types of circles which may appear in paddocks, but neither have any connection with magnetism, so I am rather puzzled by the question about 'magnetic circles'.
One type is the notorious 'crop circle', first seen in English wheat fields in the 1970's. On first appearance several explanations were offered; mini-tornados or similar meteorological effects, extraterrestials leaving us their 'visiting cards', or messages from the 'Earth Goddess' protesting over environmental damage. It was eventually established that the patterns were man-made, as the two men who had started it all confessed and demonstrated their technique. The patterns were made by flattening the wheat stalks. Patterns of great complexity are still appearing, but few now believe they are other than human artifacts.
The other type of circle, often called a `fairy ring' is entirely natural and is caused by a fungus. The familiar mushroom or toadstool is the reproductive, spore-bearing part; the main vegetative part is underground in the form of a mass of threads called 'mycelium'. When a spore in the soil greminates, the mycelium it produces spreads out like ripples on a pond, taking up nutrients from the soil as it goes. The outer edge of the resulting circle is where the fungus is most active, and so competes with, and supresses the growth of, the pasture. Inside this widening circle the fungus tends to die, and the stored nutrients in it then become available to support increased vigorous growth of the pasture. Some fairy rings have been observed, growing season by season, over many years.