Factors affecting total demand for a commodity when its price falls:

Substitution Effect:
- When the price of a commodity falls, consumers tend to switch from relatively more expensive alternatives to the cheaper commodity. This leads to an increase in demand for the cheaper commodity.

Income Effect:
- As the price of a commodity decreases, consumers' purchasing power increases. This increase in real income leads to a higher demand for the commodity.

Expectations of Future Price:
- If consumers anticipate that the price of the commodity will rise in the future, they may increase their current demand to take advantage of the lower price.

Availability of Credit:
- Lower prices may encourage consumers to take advantage of credit to purchase more of the commodity, further increasing demand.

Advertising and Promotion:
- Companies may increase advertising and promotional activities for the commodity with the reduced price, attracting more consumers and boosting demand.
Overall, when the price of a commodity falls, the total demand for it tends to increase due to a combination of substitution effect, income effect, expectations of future price, availability of credit, and advertising and promotion strategies. This increase in demand can lead to higher sales and potentially increased profitability for the producers of the commodity.