What does it mean that the land cannot be hardened?

Farmable land cannot be hardened.

Strictly protect cultivated land, especially the permanent basic farmland protection zone. It is strictly prohibited to abandon the land, destroy the cultivated layer, occupy cultivated land to build houses, plant trees, develop forestry and fruit industry, and dig ponds to raise fish. Now the permanent basic farmland is non-grain. We must resolutely put an end to all acts of destroying cultivated land, effectively protect cultivated land, and protect the interests of farmers. Those who occupy or destroy cultivated land should be dealt with seriously.