Nitrogen Fertilizer Influences Fruit Quality, Soil Nutrients and Cover Crops, Leaf Color and Nitrogen Content, Biennial Bearing and Cold Hardiness of ‘Golden Delicious’

ABSTRACT

Four rates of ammonium nitrate (NH₄NO₃) (0, 151, 454, and 908 g actual N/tree) were applied each spring for 6 years to ‘Golden Delicious’ (Malus domestica) apple trees. High rates of nitrogen (N) increased N concentration of Orchardgrass (Dactylis glomerata) blades and increased cover-grass growth whereas various legume species were prevalent at the low rates. Leaf N in spur or mid-terminal leaves increased yearly, and was related to leaf color by visual comparison and reflectance. Fruit from the higher N rates had greener peel and lower firmness, soluble solids content and titratable acidity. In vitro freeze tests indicated trees fertilized with lower rates of N were more cold hardy during the fall, winter and spring than those receiving the higher rates. In a similar long-term study on ‘Delicious,’ cold hardiness was related not only to seasonal temperature cycles and shoot dry matter, but to total sugars and sorbitol content in wood or sap.

Keywords:

• quality
• fruit color
• thinning
• growth
• vigor

Notes

^z Means in a column not followed by a common letter are significantly different by Duncan's Multiple Range Test (P ≤ 0.05).

^z Composition of cover crop includes other species not mentioned (alfalfa, red clover and white clover) to equal 100% cover.

^yMeans in a column not followed by a common letter are significantly different by Duncan's Multiple Range Test (P ≤ 0.05).

^zMeans in a column not followed by a common letter are significantly different by Duncan's Multiple Range Test (P ≥ 0.05).

^yEstimated fruit color at harvest using the USDA Fruit Color Chart, 1929 (1 = green; 4 = light yellow).

^zTotal annual increase in yield (on the positive or negative side each year).

^yNet total difference (between total positives and negatives).

^xMeans in a column not followed by a common letter are significantly different by Duncan's Multiple Range Test (Pe ≤ 0.05).

^z Fruit color visually estimated (1 = green; 6 = yellow).

^yMeans in a column, within treatments, not followed by a common letter are significantly different by Duncan's Multiple Range Test (P ≤ 0.05).

^z Cold hardiness in this study is measured by the often-used T₅₀ described by (Levitt, 1956) which is the temperature where 50% of the plant tissue is damaged. The lowest temperature indicates the greatest cold hardiness.

^y Leaf nitrogen (N) concentrations from spur leaves collected in Sept., or Oct. for fall and May, or June for the spring samples over a six-year period.

^x Leaf color measured by reflectance meter where low %R values = green and higher %R values represent shades of yellow.

^w Current shoot growth measured during the latter part of the growing season (fall) and early shoot growth during May or June (spring).

^v Means in a column, within seasonal hardiness, not followed by the same letter are significantly different by Duncan's multiple range test (P ≤ 0.05).

^u Bud stage visually estimated where 8 = full bloom.

^z Total sugars = fructose, glucose, sucrose and sorbitol.

∗∗Significant at 0.01

∗∗∗Significance at 0.001