Not sure - I always thought that the colour changed if the soil acidity changed. So far as I can tell I added nothing to the soil BUT i did start using wash powder to deter the cats from the garden (lot of gravel and stones) - they don't like anything cirtus scented apparently!
Thanks for the 'in' to the site. Its fascinating and I need to spend more time on here.....!
if they are blue the soil is alkiline if they are pink the soil is acidic. If they arewhite they a soil is neutral. To change to read but somethingthat rusts near them.