Answer:
The price of farmland is actually very stable, although it has slightly decreased in the last few years. But the decrease is not because farmland is more or less productive, but because the price of crops decreased a lot in the last 4 years. Do to president's Trump trade wars, the demand for US crops fell dramatically. This hurt farmers a lot, not because the price of their land decreased, but because their revenues collapsed. A farm is just like any other business, if the cash flows decrease by 30 or 40%, it is normal that the price of the business will decrease.
Actually the price of farmland didn't decrease as much as their cash flows. Each farmer probably lost at least 30% of their total revenues, but the price of the land only decreased by 0.8% on average. This happens because the supply of land is extremely inelastic. You cannot just make more farmland, it doesn't work that way.
As productivity increases, the price of farmland increases also. But on the long run, as the prices and productivity increases, new technologies increase productivity even more, diminishing the amount of farmland needed to produce the same output. Farmland productivity has increased a lot in the past decades, which has increased total food production. The problem is that as more food is supplied, its price goes down. This results in prices of farmland decreasing over time due to technological break troughs.
This only happens in some countries like the US, Canada, Brazil, Argentina, and a few others where farmland is extremely abundant and the population per square mile is very low. In other places, e.g. south east Asia, where people are packed like sardines, the price of land hasn't stop rising and will continue to rise because the demand is much higher than the supply.
