Many elements act to make soil more acidic: peat moss and bark commonly have a pH of 4.3-4.7 and acidic byproducts are releaseed as a result of their decomposition. The effect of adding lime (Calcium carbonate)depends much on particle size and temperature. Small particles have a much more rapid neutralizing effect but can cause big ‘swings’ in pH values. Particles that are too large may have little or no effect until they break down into a smaller size. When choosing fertilizers their effect on pH must be carefully considererd. Calcium Nitrate for example when used as a fertilizer over a period of more than 5-6 weeks, can cause considerable increases in pH. Bacteria, algae and fungi also have an effect generally making soils more acidic. This is not so relevant when using proprietry composts since they are pretty well sterile. However in the case of outdoor production and where plants are kept in containers over long periods, the effect of micro-organisms can be very considerable. Obviously the pH of irrigation water is an important factor to monitor and control. If the weather is hot, plants require more irrigation which tends to decrease the amount of lime and iron sulphate particles leading to changes in pH. Furthermore, lime particles break down faster at warm temperatures than cold. For these reasons the biggest pH drifts tend to occur in late spring and early autumn. Ohio Florist’s Association Bulletin No. 867 www.ofa.org E-mail ofa@ofa.org