Reasons Why Agricultural Land Becomes Less Productive.

This chart and this table present information about the causes of land degradation and this problem in three places in the 1990s.
Over-grazing is by far the biggest reason for less productive land, specifically, at 35%, in contrast to 30% of land degraded by deforestation and 28% caused by over-cultivation. And also, some low-productive lands are influenced by other factors without listing in this chart.
As shown in the table, the percentage of land degraded in Europe was the highest, with 23%, among which deforestation was attributed to nearly 10%, which was the priority reason in this area, compared with other two factors. The total figure for Oceania was much lower, at 13% in the 1990s. The data for over-grazing was distinct, almost taking up 90% of the total number, even though there was no data for over-cultivation in Oceania. Only 5% of land in North America was degraded, but its main cause was over-cultivation, which was different from the other places.
Overall, over-grazing is the primary factor, leading to degradable land while the number of degraded land in Europe was higher than in other regions in the 1990s, and the main contributor to land degradation was different in three regions.