Answer: they found the charge and trouble very great, and they had little or no crop
Explanation:
An understatement is when a situation is represented lesser than what it actually is. This is done in order to be polite or for an ironic effect.
The oassge simply tells us about how the plowing process was done. In this case, it was expected to be successful but in the end it wasn't which can be seen in the lines that "they found the charge and trouble very great, and they had little or no crop".
It was a disaster and it was an understatement as the severity of the situation wasn't really explained as it was lessened.