Guiding Principals for
Managing Pecan
Orchard Nitrogen
B.W. Wood
Bruce W. Wood
Point #1--From a Tree’s Perspective:
“Not all Forms of Nitrogen (N) are Equal”
N-atoms have different oxidation states
 N3- (i.e., surplus of 3 electrons):
Ammonium
 N2+ (i.e., deficient 2 electrons):
Increasing availability
Decreasing availability
 Hydrazine (rocket fuel)
of chemical energy
of chemical energy
 N3+ (i.e., deficient 3 electrons):
(gain or acceptance of
(loss or donation of
 Amine (urea, peptides, proteins);
electrons; reduction;
electrons; oxidation;
ammonia
gain of energy)
release of energy)
 N4+ (i.e., deficient 4 electrons):
 Nitrite
 N5+ (i.e., deficient 5 electrons):
 Nitrate
Different N forms possess different chemical energy potentials for plant metabolism and are
therefore differentially valued by the tree.

From a Tree’s Perspective:
“Not all Forms of Nitrogen (N) are Equal”*
Under normal conditions, most nitrate within the plant is eventually
converted to ammonium, but this is energetically expensive.
 N3- (i.e., surplus of 3 electrons):
Ammonium
N2+ (i.e., deficient 2 electrons):
 Hydrazine (rocket fuel)
N3+ (i.e., deficient 3 electrons):
 Amine (urea, peptides, proteins);
ammonia
N4+ (i.e., deficient 4 electrons):
 Nitrite
N5+ (i.e., deficient 5 electrons):
 Nitrate

Increasing availability
of chemical energy
(gain or acceptance of
electrons; reduction;
gain of energy)




Decreasing availability
of chemical energy
(loss or donation of
electrons; oxidation;
release of energy)
*Different N-forms possess different chemical energy potentials for plant metabolism.
Point #2—Pecan Is an Ammonium-loving Species
-Consider Pecan Ecosystems and Their Ecophysiology-
“Ammonium-loving” vs. Ammonium intolerant (“nitrate-loving”) vs. N-Intermediate species.
(Ammonium-loving= higher tolerance and greater physiological preference for ammonium
relative to that of nitrate-loving species.)
Alluvial Lands:
Pecan
●A Co-dominate “Climax” species in certain forest types,
and a “Subclimax” in other forest types.
Water Table
http://nrs.fs.fed.us/fmg/nfmg/bl_hardwood/eco/spechar/index.html
►Pecan is also native to deep well drained
pockets on “upland soils”; and near small rivers/creeks
on ridges of well drained “first bottom”, or where the
“second bottom” nearly intersects the watercourse.
Top soil zone is rich in decaying organic matter
Organic-N and Ammonium-N are common
(“ammonia-loving” species)
Nitrate-N dominates
(ammonia intolerant; “nitrate-loving” species)
Higher
N:A O
ratio
N(5+)
3
[N(5+)O3]-1
Lower N:A ratio
[N(3-)H4]+1
Pecan is exposed to considerable reduced-N
(organic and ammonium) in its natural habitats
So, pecan exhibits strong evidence of being an “ammonium-loving” (reduced-N) species!!!
(typically shade tolerant mid- late-successional species are ammonium-loving)
“Nitrate-loving”: continuous shoot growth species
(Pioneer species)
Higher N:A ratio
-1
[N(5+)O3]Water
Table
Lower N:A ratio
[N(3-)H4]+1
Characteristic
Nitrate-N loving
species z
Ammonium-N loving
species y
Pecan
Pioneer species in forest succession
Yes
No
No
Heavy reliance on subsurface water table
Yes
No
No
Diffuse- or semi-diffuse porous cambial growth
Yes
No
No
Ring- or semi-ring porous cambial growth
No
Yes
Yes
Indeterminate shoot growth pattern
Yes
No
No
Determinate shoot growth pattern
No
Yes
Yes
Early budbreak in spring
Yes
No
No
Delayed budbreak in spring
No
Yes
Yes
Produces small seeds
Yes
No
No
Produces medium to large seeds
No
Yes
Yes
Shade tolerant
No
Yes
Yes
Shade intolerant
Yes
No
No
Relatively small N storage pool in dormancy
Yes
No
No
Relatively large N storage pool in dormancy
No
Yes
Yes
Highly tolerant to anaerobic soils
Yes
No
No
Alternate bearing and seed masting
No
Yes
Yes
Z
There is biological variation in species response to the two N-loving categories
Point #3—Trees Can Respond Differently to Different Soil
Solution Nitrate:Ammonium Ratios
 “Vegetative growth” is favored by nitrate-N* usage [Nit./Amm. = 5-10:1 (?)]:
 Tree nitrate can act as a quasi-hormone to trigger downstream hormonal changes that
result in enhanced vegetative growth


Nitrate-N triggers “switching” of tree resource partitioning to favor vegetative growth structures
 Less fruiting
 More likely to trigger deficiencies of essential and beneficial nutrient elements
Nitrate-N is best if trying to get trees to grow fast; potential issues with low P and S
 Note: Urea-N is converted to ammonium and then nitrate very rapidly in most soils, with little
or none absorbed as urea by roots; however, foliar applied urea-N is rapidly absorbed and
used by the plant as a preferred high energy N-form.
 “Reproductive growth” is favored by ammonium-N* usage [N:A = 2- 3:1 (?)]:
 Tree ammonium-N enhances synthesis of carbohydrates, amino acids, peptides, proteins
(and enzymes), and nucleic acids, giving enhanced reproduction.

Ammonium-N triggers “switching” of tree resource partitioning to favor reproduction
 More fruiting and reduced incidence and severity of nutrient element deficiencies
 Because ammonium competes with K in uptake by roots, be careful to ensure good K
nutrition
*Root uptake of nitrate and ammonium-N is regulated by tree demand and sugar supply in roots
Synthetic Nitrogen Sources: Ammonical (Ammonium)
 Ammonium:
 ~100-1000:1 preference by roots over nitrate
 A high energy N-form (highly reduced N atoms)
 Not as easily lost in soils as nitrate due to leaching or denitrification, as ammonia binds to soil particles
 Uptake is best at pH 7, declines as soil pH drops
 Trees tend to have higher carbohydrate and protein levels than when fertilized with nitrate-N
 Easier to get phytotoxicity using ammonium, due to rapid and great uptake preference by roots, so have
to be careful to not over fertilize (very toxic to plant cells). Better micronutrient nutrition.
 Acidifies soils as it is converted to nitrate in the soil, and therefore affects availability of other elements
(1 molecule releases 3H+ ions into soil solution; low CEC soils (e.g., sands) are more susceptible than
higher buffered high CEC soils (e.g., clays and loams)
 Rapidly converts to nitrate form in soils unless “Nitrification Inhibitors” are use to retard oxidation;
conversion is more rapid in high pH soils.
 Subject to loss when soils are waterlogged and also due to denitrification and mineralization
 Can suppress uptake of Alkali and Alkali-earth metals; e.g., reduces K uptake, so can trigger K deficiency
if tree is low in K (e.g., increased June-drop of fruit). Can also suppress Ca, Mg, and transition metals (Fe,
Mn, Cu, Zn, Ni, Mo) uptake.
 Increases tree uptake of “non metals” (e.g., S, P, Cl, and Si)
[N(3-)H4]+
Synthetic Nitrogen Sources: Nitrate
 Nitrate:









The dominate N-form in most fertilized orchard soils and their soil solutions
Most N absorbed by roots is nitrate, even in ammonium-loving species
A low energy N-form (highly oxidized N atoms)
More easily lost in soils than ammonium due to leaching , as not as much is bound to soil particles
Trees will hyperaccumulate (luxury consumption) nitrate, but the accumulated N is not necessarily
assimilated and used for desired end products‼!
Trees are far more tolerant to high nitrate than to high ammonium, so low chance of phytotoxicity
All synthetic N-forms will convert to nitrate in soil within 2-3 weeks unless they are bound to soil
particles or organic matter
Can suppress uptake of non-metals; so trigger P or S deficiency (also Cl and Si) if tree is low in P or
S and add a lot of nitrate.
Enhances uptake of K, Ca, Mg, and transition metals
[N(5+)O3]-
Commercial Orchards Are Artificial Pecan Habitats
Do we want orchards to possess the positive traits of their native habitats?
►If:
-Relatively little “reduced-N”
-Lots of nitrate-N (“oxidized-N”)
-Lots of water via irrigation
►Then:
-More nutrient deficiencies
-Lots of vegetative growth
-Less reproductive growth
Point #4--N Usage Efficiency by the Tree Depends on Status of Other
Elements, Especially Sulfur (S) and Micronutrients (Fe, Cu, Mo, Ni…)
 The N and S assimilation (integration into needed N or S containing
biochemicals) processes within pecan trees are tightly linked, with a
deficiency of one “repressing” assimilation (conversion into needed
biochemicals) of the other.





Can have high leaf tissue N; yet, have a physiological deficiency of N because of
insufficient sulfur (also applies to Mn, Cu, Zn, Fe, Ni, and Mo)
Mo, Fe, and S are key components of nitrate reductase enzyme; and Cu, Fe and S are
key components of nitrite reductase (to make ammonium in the plant)
Cysteine and Methionine are S containing amino acids, and are also key components
of almost all plant enzymes.
Ni is key component of urease and probably certain other N-associated enzymes
N @ ~2.3-2.8%? dw; S @ 0.20-0.50% dw
 Failure to consider tree S status (same is true for micronutrients) when
applying N can be a costly mistake.
 Spring foliar sprays of a balanced micronutrient mix is potentially very
important for best usage of spring allied N (especially to alkaline soils)
Common Synthetic Nitrogen Sources*
Form
Chemical
%N Sulfur
Ammonium % Amine
(Urea)
%
Nitrate %
$/Unit N
Solid
Urea
46
No
0
100%
0
0.49
Solid
Calcium Ammonium
Nitrate
32
No
25
0
75
0.60
Solid
Ammonium Sulfate
21
Yes
100
0
0
0.61
Liquid
UAN-32
32
No
25
50
25
0.55
Liquid
UAN-28+S
28
Yes
31
46
23
0.56
Liquid
24-0-0-3.5S
24
Yes
33
22
45
0.55
Liquid
UAN-19
19
No
33
0
57
0.39
Liquid
15-0-0-3S
15
Yes
?
?
?
0.47
* Best source of N is usually decomposing organic matter
Other Considerations
 Most pecan orchards are excessively fertilized with N, especially




nitrate-N; needing only ≈70-100 lbs of N/acre/year if done right.
In most cases, we get by with applying less N/acre if ammonium-N
is properly applied.
Excessively shady trees do not require as much N as well sunexposed trees, but will hyperaccumulate nitrate-N and vegetative
growth will be triggered, creating more shade in the orchard
Use primarily ammonium-N or urea-N on “mechanical hedgepruned” trees (us as little nitrate-N as feasible).
Use predominately nitrate-N in young orchards (<5-6 years old), but
predominately ammonium-N or urea-N in bearing orchards (> 7
years old)
Thanks!!!

Image by Dr. Ted Cottrell, USDA-ARS