The question: "We blame the growing season's weather anomalies for crop problems. But, is it possible there are sizable chunks of it that have to do with fertility or farmer error?"
Obviously, the answer is that these two things likely play a part in it. BUT..
Check this out:
Mulipik Summer Squash by Foot by Planting Succession
The only variable that may play a role here is that succession planting dates do not line up perfectly. Planting 3 was planted too late in 2009 (for example).
The results? I can point to weather or weather events that correspond with the numbers in all cases. Hmmmm.
2007 started with beautiful growing weather - but we got torrential rains in late August and well into September - result ridiculous July numbers, getting progressively poorer.
2008 was disastrously cold through most of the year, but gave us a gloriously long fall. Result, very poor early with surprising September production.
2009 evenly cool, making it difficult for warm season crops, but not impossible. A cool start resulted in poor germination in succession 1, so we start #2 earlier. But, this explains a poor per foot number set in planting 1 - they took the same number of row feet, but fewer plants per row.
2010 very wet early, drying out later with normal temperature ranges. Result is a disastrous early season and a normal appearing August and September (still some picking to do)
What might we expect for a normal year?
Planting 1 would give us 4 per foot through both July and August and be pulled in before September is very old.
Planting 2 would give us 3 per foot in August and 2.5 in September
Planting 3 would give us 1 per foot in both August and September
But, what we really see here is that a crop that is not full season can be reasonably successful even in a poor year if we have succession plantings in an effort to "capture" weather that works for the crop.