Nitrogen Fertilization of Peach Trees

Plants rarely respond to applications of phosphorus, potassium, or other elements unless soils are low in those elements. Peaches usually require higher rates of fertilization than apple because peach trees remove more nutrients from the soil than apple. This is because peach fruits are higher in nutrient content than apple fruits and because in the fall peach trees translocate fewer nutrients out of the leaves to the tree than apple. Peaches remove about 0.27 lbs. of N per 100 lbs. of fruit. Trees fertilized with appropriate amounts of N produce good size fruit with good skin color and flavor.

Growers often ask if the source of N is important because claims are made about some forms of N being more readily available and taken up by trees. In addition to organic forms, there are two types of nitrogenous fertilizers – ammoniacal and nitrate sources. Common ammoniacal compounds include ammonium nitrate (33% N), ammonium sulfate (20.5% N), monoammonium phosphate (MAP, 11% N and 21% P), and urea (45% N). Due to regulatory issues, ammonium nitrate is not readily available.

Urea is not an ammonium fertilizer, but when added to soil the enzyme urease in the soil hydrolyzes it to ammonium carbonate very quickly and then it is converted to nitrate by soil microorganisms. It is slightly acid when it reacts with soil. Up to 18% can be lost to volatilization when applied to the soil surface or sod, but incorporation to a depth of ½" or ½" of rain can reduce the losses by half. Losses are greater on warm or alkaline soils. All ammoniacal fertilizers are converted to ammonium and losses of some nitrogen to volatilization are possible if the fertilizer is not incorporated into the soil within several days. Conversion of ammonium to nitrate may be slow in fumigated soils where populations of bacteria responsible for the conversion are low. Continued use of acid-forming fertilizers will lead to a gradual decrease in soil pH unless lime is applied periodically.

Some of the nitrate sources include sodium nitrate (16% N), potassium nitrate (13.8% N & 36.5% K), and calcium nitrate (15.5% N & 19.5% calcium). Nitrate fertilizers are very soluble in water and when added to soil the nitrate ion is readily absorbed by roots. Nitrates are susceptible to leaching by high rainfall, especially in coarse-textured soils. These fertilizers are not acid forming and, and unlike the ammonium fertilizers, annual applications may maintain soil pH at its original level, but probably will not increase soil pH.

Claims that nitrate forms of N are available and taken up by roots more quickly than ammonium forms are questionable because the availability of N from fertilizers depends on many variables, such as soil moisture and temperature, populations of soil microorganisms, soil type, and soil pH. Davidson and Shive (1934) applied ammonium- or nitrate-N to peach trees in sand culture with pH at 4, 6 or 8. At the low pH of 4, nitrate was taken up at higher rates than ammonium. At high pH, ammonium was taken up better than nitrate.

When applied appropriately, losses of ammonium fertilizers to volatilization and leaching of nitrates from nitrate fertilizers are minimized and in most cases the response of fruit trees to different nitrogen sources is similar. If soil or leaf tissue analyses indicate a need for calcium or potassium, then trees may benefit from those nitrates. Therefore, the primary consideration while selecting a nitrogen fertilizer should probably be the cost per unit of actual nitrogen.

During the 1920s and 1930s, several experiments were performed with apples and peaches to evaluate different forms of N and rates of N fertilization. Many of these experiments are difficult to interpret because methods were not explained in detail and data may be bias because treatments were often not randomized. Researchers usually applied urea [(CO(NH₂)₂, 46%N], calcium nitrate (CaNO₃, 16% N), sodium nitrate (NaNO₃, 16% N), and ammonium sulfate [(NH₄)2SO₄, 21%N, 24% S].

Nitrogen Rates

An early report by Hooker (1923) showed that adequate N nutrition reduced peach June drop. Autcher (1923), at Cornell, indicated that when N was applied to one side of the tree the N in the leaves on that side was higher than on the nonfertilized side. So, the recommendation was to apply N as a band around the tree. Lott (1932) in Mississippi, applied 0, 3, 6, 9, or 12 lbs of sodium nitrate per tree a week before bloom and reported that the higher rates delayed harvest by 3 to 6 days, but had little effect on red color or postharvest fruit softening. The number and length of shoots per tree increased as N rate increased. Most of these experiments used nitrogen-deficient trees that were fruiting and had not been fertilized for a year or more. Ritter (1956) conducted one of the more detailed experiments, but it was not randomized, and he did not report tree spacing, so the N rates cannot be converted to per acre rates. He planted trees at the University Park research orchard and applied 0, 0.125, 0.25, or 0.5 lbs of ammonium nitrate per year of tree age. Leaf N concentration increased with increasing N application. Potassium increased when N was applied, but it was not related to N rate. Manganese increased with increasing N rate. Trunk diameter, and leaf dry weight and area increased with increasing N. All three N rates had higher yields than the nonfertilized CK, but the 0.125 treatment was similar to the 0.5 lb treatment.

Dr. Norman Childers sponsored a peach symposium at Rutgers University in 1975, and the papers that were presented were published in a book entitled "The Peach: Varieties, Culture, Pests, Marketing, Storage". He asked pomologists from five different regions of the country to discuss peach fertilization practices in their states. Jim Beutel said that California growers applied different amounts of N depending on the season of ripening and weather they were grown for processing or fresh fruit. Growers generally wanted about 3' of new growth each year, so early fresh cultivars received 60 to 80 lbs. of N/yr, mostly after harvest to avoid delayed maturity. Early processing cultivars received 75 to 150 lbs. per year. Mid-season cultivars received 100 lbs and late-season cultivars received up to 150 lbs. per acre, mostly in winter. Young trees up to 6 years received 0.15 lbs. per year of tree age. Roy Larson, in Michigan, said that growers usually fertilized in late fall or early spring, about a month before bloom. They applied 50 to 100 lbs. of N/acre and Kenworthy found that trees responded similarly to anhydrous ammonia, urea, ammonium sulfate, sodium nitrate or ammonium nitrate applied in the fall or spring. Edsel Phillips, in Virginia, indicated that growers in the eastern part of the state with sandy soil used split applications of 10-10-10 or 10-6-4 applied a month before bloom and in the fall. The rate was usually 0.05 lbs. per year of tree age up to 1.0 lbs. per tree. Roy Ferree, in South Carolina, said growers adjusted N rates depending on pruning severity. Growers typically applied 50 to 60 lbs. of N per acre 4 to 6 weeks before bloom, but on late-season cultivars they often used split applications. Roy Flannery, in New Jersey, reported that growers often applied 0.05 lbs. of N per year of tree age up to 0.5 lbs. and supplemental N was applied a few weeks after growth starts for trees carrying a heavy crop. Mature trees usually need 0.5 to 1.0 lbs. of N applied a month before bloom. Young trees on sandy soil often received supplemental N during May or early June at 0.02 to 0.03 N of N per year of tree age, not to exceed 0.5 lbs. of N.

My experience is that soil and crop load, which is influenced by pruning severity, are major factors to consider when applying fertilizer. One year when I was at Rutgers, we lost the crop due to winter injury. I fertilized some trees with a half rate of N and I did not fertilize some other trees. Both sets of trees had moderate growth and looked good for the entire season. But the next season, the trees had a very heavy fruit set, and by the time we thinned the trees, those that had not been fertilized the previous year had light green foliage and shorter shoots than trees that received half the normal rate. So, mature trees with no crop should receive about half the N that trees with a good crop receive. The soil at Cream Ridge, N.J., was very fertile, and we usually applied just 0.15 lbs. of N in the spring and tree vigor was greater than ideal. When I went to Virginia, we had a moderately fertile soil and I applied 30 lbs of N/acre a month before bloom and another 30 lbs. at shuck split and shoot growth was ideal. When we moved to a new farm, the soil was non-fertile, and it took me about 7 years to learn that I needed to apply 60 lbs. of N a month before bloom and another 60 lbs. at shuck split. If we lost the crop to frost, I did not apply N at shuck split.

Fertilizer Timing

The value of post-harvest N applications in the mid-Atlantic region has been debated for years. Several experiments were performed around the country with different N fertilizers and timings, and results varied slightly depending on soils, tree age, and N status of the trees before the experiment was initiated. Weinerger and Cullinan (1934), in Northern Virginia, applied sodium nitrate, calcium cyanamid, or ammonium nitrate in April, October, or split. Compared to the nonfertilized CK, all N treatments increased vegetative growth, but terminal growth was greatest for sodium nitrate. Spring applications induced the greatest terminal growth. In Maryland, Havis (1956) applied ammonium nitrate to 3 cultivars in early October, early March or split over three timings (early October, March, and June). After 6 years results were similar for all 3 cultivars and when averaged over the cultivars, average yield in bushels per tree was 15.3, 18.3, and 17.2 for fall, split, and spring applications. I have been concerned that late-season applications might delay the development of cold hardiness in the fall, but McMunn and Dorsey (1934), in Illinois, reported no difference in live and dead buds after 7 years of applying sodium nitrate or ammonium sulfate before bloom or in early August. Based on all these early studies it seems that split applications in the spring and fall are at least as good as spring application alone.

Nitrogen Uptake, Storage and Cycling

There are two major sources of N contributing to vegetative growth and reproduction: root N uptake and internal N recycling. A series of papers from Australia (Taylor, 1967a&b; May and Taylor, 1967; Taylor and Van den Ende,1969), and two from California (Niederholzer et al., 2001; Rufat and DeJong, 2001)) reported on the uptake, storage, and movement of N in container- and field-grown trees.

The important points are summarized below:

1. After being taken up, N is translocated, mostly in the form of amino acids, through xylem and allocated to different parts of the tree. There are seasonal changes in amino acid composition of xylem sap that can be used by researchers to distinguish between recently root-absorbed N and the N deriving from winter storage. Most of the N in dormant peach trees is stored as soluble organic N and as the amino acid arginine, and to a lesser extent, two other amino acids (asparagine and glutamine). Amino acids are building blocks of proteins and are involved in many plant functions, including structural, metabolic, and transport. These amino acids can move down in the phloem tissues and up in the xylem tissues of trees. The concentration of arginine in the roots is about twice as sensitive as leaf analysis as an indicator of tree N status, but the test is too expensive for monitoring N in commercial orchards.
2. The pattern of N movement is not affected by time of N application (spring vs. summer). Woody tissues accumulate N in the late summer with the cessation of shoot extension. In late winter, N stored in woody tissues during the fall declines in bark tissues as it moves into buds and flowers. Leaf N increases early in the season, then declines in older leaves as N migrates to fruits and woody tissues in the late summer and early fall.
3. Trees that are low in N respond to N applications more than trees with adequate amounts of N, and this has been reported in other field experiments in the U.S. N level in flowers and young fruitlets is not related to the level of storage N, probably because stored N is used preferentially for reproductive rather than for vegetative growth. But the growth of new shoots and the N content of leaves is proportional to the level of storage N in dormant trees before growth begins.
4. Tree growth during the first half of the season is largely at the expense of N which accumulated in woody tissues of the tree during the previous year. About 65% of the N in young trees during the winter is stored in roots and is exported to new shoots during the first half of the growing season.
5. Late summer N application is an effective means of increasing tree N because N applied in the summer increased storage N in woody tissues but did not influence the accumulation of carbohydrates or microelements.
6. In California, more than 75% of the fall-applied N remained in the soil and was taken up the following spring and summer. There was little effect of fertilizer timing on tree growth, yield, or fruit quality.

Foliar Applied Nitrogen

Urea can be absorbed rapidly by leaves of most plants, but reports in the 1940s and 1950s indicated that urea was poorly absorbed by peach leaves. However, in the 1990s, studies showed that 48 to 69% of a fall urea application was absorbed and translocated to the roots. Urea contains a byproduct called biuret which is toxic to plant tissues. Johnson et al. (2001), in California, compared a soil application of ammonium nitrate in early September to a foliar application of low biuret urea in early October. Foliar application provided adequate N to all tree organs, but fruit size was lower than for trees receiving soil applications. When they applied half the N to the ground in the spring and the other half as a fall foliar treatment, shoot growth, yield, and fruit size was similar to soil applications.

All this information can be summarized in a few statements:

• Trees do not respond to fertilizer applications unless the trees are low in one or more nutrients. This can be verified with leaf and soil analyses.
• Trees usually respond similarly to all sources of nitrogen (nitrate vs. ammonium), so choice of N fertilizer should be based on price per unit of N.
• The nitrogen needs of peach trees will vary with many factors, but typically trees should receive 50 to 100 lbs. of N per acre per year, depending on soil, pruning severity, and crop load.
• Stored N is insufficient to maintain commercial crops, so annual N applications are required to maintain tissue N status. Spring N fertilization or split applications of N in spring and after harvest seem to be equally good for supplying the nitrogen needs of the tree. Early-season varieties may benefit from post-harvest N applications. Spring fertilization can be supplemented with a foliar application of low biuret urea in October.

Literature Cited

Auchter, E.C. and A.L. Schrader. 1928. A preliminary report on peach fertilizer experiments in Maryland. Proc. Amer. Soc. Hort. Sci. 25: 263-268.

Davidson, O.W. and J.W. Shive. 1934. Influence of the hydrogen ion concentration of the culture solution upon the absorption and assimilation of nitrate and ammonium N by peach trees grown in sand culture. Soil Sci. 37:357-385.

Havis, L. 1956. Effects of times of applying nitrogen and of disking on bearing Elberta, Halehaven, and Triogem peach trees. Proc. Amer. Soc. Hort. Sci. 48:70-76.

Hooker, H.D. 1923. Peach culture in Missouri. Mo. Agr. Expt. Sta. Bul. 207.

Johnson, R.S., R. Rosecrane, S. Weinbaum, H. Andris, J. Wang. 2001. Can we approach complete dependence on foliar-applied urea nitrogen in an early-maturing peach? J. Amer. Soc. Hort. Sci. 126:364-370.

Lott, R.V. 1932. Some fruiting responses of the peach to applications of nitrate of soda. Proc. Amer. Soc. Hort. Sci. 28:23-27.

May, L.H. and B.K. Taylor. 1967. The nitrogen nutrition of the peach tree III. Metabolism and translocation of L-[guanido-14C] arginine hydrochloride and l-[U-14C] asparagine in young dormant trees. Aust. J. biol. Sci. 20:413-418.

McMunn, R.L. and M.J. Dorsey. 1934. Seven years' results of the hardiness of Elberta fruit buds in a fertilizer experiment. Proc. Amer. Soc. Hort. Sci. 32:239-243.

Niederholzer, F.J.A., T.M. DeJong, J. L. Saenz, T.T. Muraoka, and S.A. Weingaum. 2001. Effectiveness of fall versus spring soil fertilization of field-grown peach trees. J. Amer. Soc. Hort. Sci. 125:644-648.

Ritter, C.M. 1956. Effects of varying rates of nitrogen fertilization on young Elberta peach trees. Proc. Amer. Soc. Hort. Sci. 48:48-55.

Rufat, J. and T.M. DeJong. 2001. Estimating seasonal nitrogen dynamics in peach trees in response to nitrogen availability. Tree Physiology 21:1133-1140.

Taylor, B.K. 1967. The nitrogen nutrition of the peach tree I. seasonal changes in nitrogenous constituents in mature trees. Aust. J. Biol. Sci. 20:379-387.

Taylor, B.K. 1967. The nitrogen nutrition of the peach tree II: storage and mobilization of nitrogen in young trees. Aust. J. Biol. Sci. 20:389-411.

Taylor, B.K. and van den Ende. 1969. The nitrogen nutrition of the peach tree. IV. Storage and mobilization of nitrogen in mature trees. Aust. J. biol. Sci. 20:869-881.

Taylor, B.K. and van den Ende. 1970. The nitrogen nutrition of the peach tree. VI. Influence of autumn nitrogen applications on the accumulation of nitrogen, carbohydrate, and macroelements in 1-year-old peach trees. Australian Journal of Agricultural Research 21:693-698.

Weinberger, J.H. and F.P. Cullinan. 1934. Nitrogen intake and growth response in peach trees following fall and spring fertilizer applications. Proc. Amer. Soc. Hort. Sci. 32:65-69.