It is not only in the seventeenth century that this double conversion movement took place. In the second half of the fourteenth century pastures were being plowed up. At Holway, 1376-1377, three plots of land which had been pasture were converted to arable.[35] In this period much land was withdrawn from cultivation. The explanation usually advanced by historians for the conversion of arable to pasture at this time is that the scarcity of labor since the Black Death (a quarter of a century before) made it impossible to cultivate the land as extensively as when wages were low, or when serf labor was available. If this is the whole case, it is difficult to account for the conversion to arable of land already pasture. Other factors than the supposed scarcity of labor were involved; land in good condition, such as the plots of pasture at Holway, repaid cultivation, but the yield was too low on land exhausted by centuries of cultivation to make tillage profitable. In the sixteenth century, also, the restoration of cultivation on land which had formerly been converted from arable to pasture was going on. Fitzherbert devotes several chapters of his treatise on surveying to a discussion of the methods of amending "ley grounde, the whiche hath ben errable lande of late," (ch. 27) and "bushy ground and mossy that hath ben errable lande of olde time" (ch. 28). This land should be plowed and sown, and it will produce much grain, "with littell dongynge, and sow it no lengar tha it will beare plentye of corne, withoute donge", and then lay it down to grass again. Tusser also describes this use of land alternately as pasture and arable.[36] A farmer on one of the manors of William, First Earl of Pembroke, had an enclosed field in 1567, which afforded pasture for 900 sheep as well as an unspecified number of cattle, "_qui aliquando seminatur, aliquando iacet ad pasturam_."[37] The motives of this alternating use of the land would be clear enough, even though they were not explicitly stated by contemporaries; arable land which would produce only scant crops unless heavily manured made good pasture, and after a longer or shorter period under grass, was so improved by the manure of the sheep pasturing on it and by the heavy sod which formed that it could be tilled profitably, and was therefore restored to tillage.