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.
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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 (CitationLevitt, 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