UK standard visitor visa refused: income in cash

Recently my mother applied for a UK standard visitor visa from India, which was refused. Her circumstances are:

• My mother owns land, which is cultivated, so we mentioned her as self-employed (agriculturist) with yearly income.
• We attached bank statements and certificate of fixed deposit from the bank.
• As all/most of the transactions in rural India are done in cash, there are no regular bank deposits.

Reason for the refusal:

• From refusal letter - "There is no indication that the funds in these accounts have been generated through your stated employment. This causes me to doubt your circumstances are as claimed and I am not satisfied that you have presented an accurate picture of your economic circumstances in India."

I am on a Tier 2 visa. I sponsored my mother's entire travel and stay in the UK, which they acknowledged. My wife and I are expecting a baby, so we wanted my mother to be here with us and stated this in the application as a reason for my mother's visit.

Edit (refusal letter)

Any help would be highly appreciated.

Thanks.