As a welcome relief from spawn issues, casing and casing growth has been a recent topic for discussion.  Note that any comment below a)  is provided as personal opinion only, for what little it is worth b) hopefully does not intrude upon other supplier's territory and c) growers that I have been talking to should not assume that comment relates specifically to them.
 
Issues have included excess and/or uneven quality casing growth, early crops, limited water holding capacity, low water reserve after flushing and difficulty in replenishing water content.  When asked to comment upon these issues I examined casing conditions (only as part of the investigation) and did note some commonality.

Some casings were lighter than usual and seemed to have a higher brown peat content than normal.  If true, this would readily explain lower water holding capacity and limited post-flush reserve.  This situation would be worsened by less than optimum casing depth which would be obvious by the degree of shrinkage if checked later in cropping.

Other casings seemed quite "slushy" but without any obvious anaerobism present.  Lump size was OK but the individual lumps did seem to feel "gritty" as if again there was a high brown peat content.  Confusingly, there was little correlation between the materials seen on the bed and the specification  assigned by the suppliers.

It is important to now indicate a personal preference for a casing that contains ample long "strings" of fibre found in the "black" content of the casing mix.  The ratio of black to brown content is dependent upon individual grower preference for various reasons, more often than not, principally due to manageability.  In general, I believe one should use the heaviest material that one can handle and  maximize water input. I find that the most successful growers in terms of quality and yield do this and adopt the principle for hybrid strains: "firstly drown the little beggars and then evaporate at just the right amount".

If one is to maximize water input, one needs to be sure that the casing material remains open and will "breathe" i.e. will allow oxygen to be available within the casing layer (all plants require oxygen).  At the same time, the casing should also encourage the development of limited numbers of strong individual strands of mycelia i.e. "large drinking straws for the mushrooms to suck their nutrients through".  Thus casing should be dense, there should be enough of it but the density should not be so high that the casing is unable to breathe.  This may be achieved by having a sufficient proportion of brown content to keep the casing open, but in excess, this will reduce water holding capacity.  A predominant content of black peat is preferred and I am strongly of the view that this should contain ample "strings" of black fibre.

The fibre in question.

If heavier casing materials are overworked i.e. "pudding-ed up" by excess water addition and/or mechanical treatment then individual lumps may become over-dense and air spaces may be excessively reduced.  Later in the crop, after drying, these lumps may turn into "bricks" that are difficult to re-wet.  Rather than guarding against this via the addition of extra brown content to keep lumps "open" and enabling re-wetting, I prefer to see fibrous strings within the lumps that provide a route for water back into these pieces of casing.  In general, a percentage of brown peat provides safety regards retaining openness but at some trade-off in terms of water holding capacity and if one is to use a blacker material the quality of the black component is important, it should be fibrous, not sludgy, to remain "open".

The above preferred material, unfortunately, if chosen, might not yield the desired benefits if the characteristics of the material were lost in the mixing of raw materials, or, even when this has been successful, in the process of getting the prepared mix onto the beds.  Regarding recent observations, I was puzzled by the number of occasions where there seemed to be a distinct lack of black string fibre present.  Possibly the recent extremes of weather was a factor which would be understandable with the supply and management of such a natural product.  (This is not restricted to casing alone.  Last seasons straw was a concern and forecasts for future wheat supplies may soon be an issue.)

 I was left wondering why the long black fibre content seemed low?  Was this a material selection, supply, or farm management issue?  Appreciating that this was politically beyond my remit I did not attempt to volunteer an unqualified opinion.  I did however discuss the situation with one casing supplier and left further investigation at their discretion should they merit it relevant and worthwhile.

In summary, the key point to note and be aware of, is that it is often very difficult to accurately assess a cultural issue especially when there can be wide (or even just subtle, but significant enough) differences between different farms.  Likewise if a grower perceives an issue, he should not look only to his supplier but also take into account the possible differences that may exist within his own system.  Whatever, I remain preferring to see plenty of fibrous strings (that survive management) in grower's casing materials!

Finally, as an aside, I noted that a few compost materials of late were drier and shorter than previous.  Such material used as casing spawn would tend to provide a higher number of inoculum points and rates used should perhaps be reduced where excess casing growth was a problem.

CKB October 2007.