Wheat gave the most nutritious and palatable bread; barley and oats could be made into an inferior bread, or could be malted and brewed into ale, or, like the peas and beans, be boiled for pottage; oats and beans could also be fed to animals. The cultivators had to be careful to combine their arable with grassland and wood. The best way of maintaining the quality of the soil – or even of creating a decent seedbed in the case of the heavier clay soils – was to keep a good proportion of land as grass or hay meadow, so that animals could be fed and their manure used to spread on the fields. Sheep were especially efficient sources of fertilizer, because they could be fed on pasture land in the day, and penned at night on the land on which crops were to be sown. Not only would they deposit their manure, but their small sharp hoofs would tread the droppings into the surface. Animals were themselves an important source of food – medieval people lived not by bread and pottage alone – and their muscle power was essential for ploughing and hauling.