Several reasons determine the limits between which cultivated plants will grow and thrive. These in general are similar to those which regulate the distribution of all plants, but they apply in a more marked degree. For the cultivated cereals, and most of the cornfield weeds, are essentially southern plants, hence they will not grow where northern plants are quite at home.

Soil, altitude, and climate are the three essential factors for plant growth; the first is especially important in determining the distribution of cereals. The best soil is a loam or clay, with some proportion of silica or sand. Calcareous soils and marl, which contains up to 25 per cent of lime, are also suitable. Sandy and siliceous soils are in general too dry or close, or barren in alumina. Rocky districts, such as those of Scotland, and high hills in England and Wales, are better suited for oats, but even here this crop does not flourish beyond a certain altitude. The limestone massifs of the Pennine Chain are also too bare and lacking in deep soil for cereals.

Altitude, however, governs the distribution of cereals mostly, for with increase in altitude there is increase in rainfall, and this is deleterious. Watson established a zone, the Agrarian zone, up to 1000 ft., above which cereals do not usually grow well. Climate, again, has the same effect as altitude, and for this reason the south and east of England and Scotland are best suited for cereals. The N. and W., and the whole of Ireland, are too moist and wet as a whole.