AFP (Air-filled porosity) This is a measure of the degree of aeration in a growing medium and is defined as that portion of the media volume which contains air after saturation and drainage. Peats with higher AFP values are better drained, have a lower water-holding capacity and are better oxygenated than other peats. They are most appropriate for plants with a higher demand for oxygen, for longer-term crops, for growing systems where there is frequent irrigation (e.g. ebb and flood) and when the compost naturally stays wet for long periods – as in winter.