It is worth zero.  A newly planted field has no value that can be extracted from it (Digging up the unsprouted seed to plant somewhere else would not be cost effective).  If you are in a livestock area, a half grown crop has some feed value, but it may not be worth the cost to harvest it.  A fully matured crop has zero value if it hails out ahead of the combine or has aflatoxin too severe to allow its use (in which case it may actually have a negative value).  Banks are averse to risk, and a crop that is not in the bin is not a sure thing, and then it isn't a sure thing until it has been delivered and the check cleared.
I am not talking about potential value.  From the bank's point of view, optimism is a liability.