Specialty Gardening: Container Soils - Water Movement and Retention III , 1 by tapla

tapla wrote:
Pine bark is appropriate because it's rich in a lipid called suberin, which is nature's waterproofing for plants. Most hardwood bark lacks the amount of suberin found in conifer bark. As a result, it breaks down much faster. This makes soils made from hardwood bark less structurally stable and makes nitrogen immobilization (tie-up) a significant issue. Because of the rapid decomposition, heat-build up is also a frequent issue when using materials that break down quickly (think 'hot compost pile'). Finally, there is usually a high pH spike associated with the use of hardwood bark/sapwood/heartwood at some stage of the composting process that causes many of the micronutrients to precipitate and become unavailable.

For the gritty mix, uncomposted pine or fir bark is ideal. For the 5:1:1 mix, either partially (that's slightly) composted pine bark or uncomposted bark are suitable with the slightly composted getting the nod. What you see at 3, 6, and 9 in the picture below are all good for the 5:1:1 mix.

Al