are a measure of the heat useful for the growth and development of potatoes. In the potato production areas of the Prairies, the highest average accumulation of P-Days, 850-950, occurs south of Lake Manitoba to the U.S. border and the lowest accumulation, 750-900, occurs in the seed production areas south of Edmonton and in Central Saskatchewan (Plate 1). The varieties currently grown on the Prairies require anywhere from 800-1000 P-Days to reach full maturity. An early-maturing variety such as Norland requires 800 P-Days, and later maturing varieties such as Russet Burbank require 1000 P-Days. Growing a potato variety in an area with insufficient P-Days will reduce yield and affect tuber quality factors such as accumulation of solids and fry colour (See 1.5 Tuber Quality). The average accumulation of P-Days is insufficient in many parts of the Prairies to produce late season varieties where the crop must be fully mature before harvest. These areas may be suitable for seed or table production, where the crop is killed or harvested before full maturity. Moisture Potatoes require a continuous supply of soil water along with adequate soil aeration. Yields are greatest when soil moisture is maintained above 65% of the available soil water (ASW) capacity Tuber set is particularly sensitive to moisture stress. There are generally fewer tubers set when available soil moisture is maintained below 65% of the available soil water (ASW) capacity. The amount of water needed by potatoes varies with the soil type, temperature, humidity, air movement, plant and stem