DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Samples of two components of potting composts. On the left, a coarse leaf mould produced by the bulk decomposition of fallen autumn leaves. On the right, turf loam produced by stacking cut turves. The leaf mould has a high humus content, together with some remaining bulk material ( small twigs ). It has low intrinsic fertility, with available nitrogen often around only 1%. The loam has a humus content of about 5% by bulk, and consists mainly of small mineral particles ranging in size from clays and silts up to sand. To produce a well-balanced free-draining potting compost, the loam and the leaf mould are mixed together, and fertilisers added. It may also be necessary to add coarser mineral material (grit) in order to impart the desired free-draining structure to the compost, together with lime to adjust the pH.
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