However, when temperature records are kept and heat units are calculated, a better estimate is obtained. Grower s experience may be needed for this index because a grower must know the variety or apple strain. Some varieties may be fully colored several weeks before optimum harvest time. Excess nitrogen fertilizer could cause poor fruit color. Potassium applications can improve fruit color only when this nutrient is deficient.
Iron deficiency can also cause poor fruit color.
Additional variation may be caused by climatic conditions, irrigation, soil type and other cultural practices. Grade for Red Delicious apples in the United States is based primarily on the percentage of red skin color. For yellow apples like Golden Delicious there are charts available that contain various degrees of coloring, depending on the degree of apple maturity. Apples in the field are compared against colors on the chart to learn about apple maturity. Color of Golden Delicious apples progresses from dark green to yellow.