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The peasants in this world are already aware of the need for crop rotation, which they perform by leaving the parts of the field idle for a time. The Demon King's improved plan involves constant use of the fields and is supposed to work in a four year cycle. I expect that it'll take more than four years for the peasants to see themselves that the Demon King's method of field rotation does preserve the soil quality and thus come to rely on that method.
And then the Demon King is only teaching the method to one rather backward village on the frontier. It will take even longer for people in other regions to notice that this village has a better method of crop rotation and to copy the method, which further delays how long the improved results can be used to intensify war efforts. (This paragraph assumes she's not going to go around teaching all the frontier villages her new method of rotation but rather leaving the spreading to the current village)
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