So great was the profit of cultivating these pastures that landlords who were opposed to having pastures broken up by leaseholders had difficulty in preventing it. Towards the end of the sixteenth century at Hawsted, and in the beginning of the seventeenth, a number of leases contained the express provision that no pastures were to be broken up. In 1620 and the years following, some of the leases permitted cultivation of pasture, on the condition that the land was to be laid to grass again five years before the expiration of the lease.[140] There is no doubt of the fact that much land was being converted from pasture to arable in this period. Evidence of this tendency multiplies as the century advances. In 1656 Joseph Lee gave a list of fifteen towns where arable land hitherto converted to pasture had been plowed up again within thirty years.[141] Barren and insufficiently manured land did not produce good crops merely because other land had been given an opportunity to recover its strength. The conversion of open-field arable to pasture went on unchecked in the seventeenth century because it had not yet had the benefit of the prolonged rest which made agriculture profitable, and without which it had become impossible to make a living from the soil. The lands which have been "heretofore converted from errable to pasture.... have sithence gotten heart, strength and fruitfulnesse," and are therefore being plowed again; but the land which has escaped conversion, and has been tied to the "perpetual bondage and servitude of being ever tilled," is "faint and feeble-hearted," and is being laid to grass, for pasture is the only use for which it is suited. The cause of the conversion of arable fields to pasture is the same as that which caused the same change on other lands at an earlier date--so low a level of productivity that the land was not worth cultivating. Lands whose fertility had been restored were put under cultivation and plowed until they were again in need of rest.