Apple Tree Nutrition As Related To Cover Crop And
Fertilization Management
Roland D. Meyer and Paul M. Vossen
Two experiments were conducted on apple orchards in Sonoma County. Site one was initiated in 1990 with two system treatments, an organic and conventionally managed orchard. Cover crops in the two systems consisted of resident vegetation in the conventional while a bell bean-vetch mix was planted each fall in the organically managed plots. Each treatment was replicated four times. Two permanent nine-tree subplots were established from which soil and leaf tissue samples were taken. Leaf samples were collected on a monthly basis from May through October during 1990 and 1991 and three times a year (May, July, and September) in 1992 and 1993. Nutrient concentrations were determined for N, P, K, Ca, Mg, S, Zn, Fe, Mn, Cu and B in 1990 and 1991, while in 1992 and 1993 only N, Ca, Zn and B were determined. Soil samples consisting of 12 cores from each of the same two-9 tree subplots were collected from the 0-6", 6-12" and 12-18" depths in April and May 1991, April, May, and June 1992, and April 1993. Initial total N as well as NH4-N, NO3-N was determined.
Site two consisted of three permanent cover crop treatments; subterranean clover, 'Blando' brome and 'Zorro' fescue along with a resident vegetation-annually spring tilled treatment that had been established several years earlier. Leaf samples were collected from two subplots (5 trees each) of three replications of the three cover crop treatment according to the schedule described for site one.
The conventional treated plots had significantly higher N, P, Zn, Fe, Mn, Cu, and B, in July 1990 leaf samples but both organic and conventional leaf analysis were in or above the adequate nutrition range. Monthly leaf samples taken April through October in 1990 indicated higher N, Ca, Zn, Cu and B concentrations much or part of the year for the conventional treated plots. Only P, Zn and Cu were significantly higher in July 1991 but monthly samples indicated slightly higher (some significant) levels of N, P, Ca, Zn, Cu and lower S concentrations for the conventional treated plots. Both organic and conventional nutrient concentrations were in or above the adequate range during 1991. Leaf tissue levels in 1992 and 1993 were similar for N, Ca, and B with only Zn having a higher concentration in the conventional treated plots. It is difficult to tell if any benefits were achieved with the organic foliar sprays or the foliar applications of nutrients in the conventional system. Spray residues could have been on the leaf surface and not taken up by the leaves but still reflected in the total tissue concentrations.
Soil total N, NH4-N and NO3-N concentrations were essentially the same for the organic and conventional treated plots. If a trend were to be indicated, it might be that during 1992 the organic treated plots had slightly higher NH4-N and slightly lower NO3-N concentrations than the conventionally treated plots.
No significant differences in leaf nutrient levels in apple leaf samples were observed at site two between the three cover crop treatments; subterranean clover, 'Blando' brome and 'Zorro' fescue.
Fertility maintenance is fairly easy to achieve in both organically
grown and conventionally grown apples since the annual nutrient
removal by the fruit is fairly low. Nutrient removal by the fruit
in a 20 ton crop was 29, 4 and 56 pounds per acre of nitrogen,
phosphorous (P2O5) and potassium (K2O)
respectively based on the laboratory analyses performed in apple
samples. Soil levels of phosphorous and potassium were adequate
to supply the above mentioned nutrient removals plus that necessary
for tree maintenance requirements. In the organic plots, compost
additions of total nitrogen were 52, 35, 39, and 30 lbs/A for
1990, 1991, 1992, and 1993 respectively while total phosphorous
as P2O5 was 47, 66, 53, and 48 lbs/A and
potassium as K2O was 53, 34, 43, and 43 lbs/A. Additions
of total nitrogen from the bell bean vetch legume planting based
on above ground biomass measurements, were from 15 to 30 lbs/A
greater than the conventional plots having resident vegetation.
Attempts to quantify potential volatilization losses of nitrogen
if the compost and plant residues were left on the soil surface
indicated perhaps as much as 10 to 20% of the nitrogen might be
lost. Other researchers have reported volatilization losses up
to 50-75% of the nitrogen in composts and plant residues if left
on the soil surface and not incorporated. In this study all vegetation
was disked into the soil about mid-April of each year, thus reducing
the potential for most volatilization losses of nitrogen. Since
leaf nitrogen levels for both the organic and conventional systems
remained in or above the adequate range, both nutrient management
schemes were adequately meeting crop needs.
Cover Crop Research and Education Summaries