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Guiding Principals for Managing Pecan Orchard Nitrogen

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1 Guiding Principals for Managing Pecan Orchard Nitrogen
Growers often ask “what is the best source of nitrogen”. The answer might surprise you. B.W. Wood Bruce W. Wood

2 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): 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) There are 4 important points that are often overlooked when developing a nitrogen management strategy for pecan orhcards. #1: We often hear, even from technical experts, that the source of N does not matter. One should simply purchase/apply the one that provides the most N for the least amount of money. This view is too simplistic and leads to a loss of potential revenue. We need to consider N from a tree’s perspective. Not all are the same to the tree. The N-atom is one of the especially unique non-metals, in that it has a lot of different energy states when combined with other elements. We typically apply the least energetic of all forms, nitrate-N, where the trees generally needs ammonium-N. Different N forms possess different chemical energy potentials for plant metabolism and are therefore differentially valued by the tree.

3 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) The tree must convert nitrate-N to ammonium-N before it can be used in most physiological/metabolic processes requiring N. And the conversion costs the tree a great deal of energy, which could otherwise be used in other plant processes if the tree got the relatively high energy form of N to begin with. So, the form of N available to the tree matters a great deal. *Different N-forms possess different chemical energy potentials for plant metabolism.

4 Point #2—Pecan Is an Ammonium-loving Species -Consider Pecan Ecosystems and Their Ecophysiology-
Consider Pecan’s native range of distribution. It is along the floodplains of rivers. Within the planes are different soil environments, possessing different soil-N environments. Because of this, tree species have evolved and adapted to specific N environments. Note that tree species typically are either nitrate or ammonium loving species—2 classes. Pecan tends to be an ammonium loving species; yet, in pecan orchard management, we often treat it as if it were a nitrate loving species. “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.)

5 Pecan Alluvial Lands: ●A Co-dominate “Climax” species in certain forest types, and a “Subclimax” in other forest types. Water Table Within the native habitats, pecan is typically on the second bottom and terrace soil sites. It thrives in these habitats. It loves deep well drained soils where roots can access a water table and feeder roots can access decaying organic matter (that is relatively rich in organic and ammonium-N). While the tree likes nitrate-N, it loves ammonium-N. ►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.

6 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) Lower N:A ratio Within the native habitats, sinker roots are able to access the water table. The water table is relatively rich in nitrate-N, whereas the organic zone of the soil is relatively rich in ammonium-N. Trees need both forms of N in order to be happy. So, what is the best balance of nitrate to ammonium-N? It depends! [N(3-)H4]+1 N(5+)O3 Higher N:A ratio [N(5+)O3]-1

7 [N(3-)H4]+1 [N(5+)O3]-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) Lower N:A ratio “Nitrate-loving”: continuous shoot growth species (Pioneer species) Nitrate loving species are typically those that grow as “pioneer” species in new or disturbed sites as the early-stage successional species. They do not become the climax-dominate species. Conversely, ammonium loving species tend to be dominating climax species and are found during the mid and late stages of forest succession. Pecan is not a pioneer species. It is a mid/late stage successional species. [N(3-)H4]+1 Higher N:A ratio [N(5+)O3]-1 Water Table

8 Nitrate-N loving species z Ammonium-N loving species y
Characteristic Nitrate-N loving species z Ammonium-N loving species y Pecan Pioneer species in forest succession Yes No Heavy reliance on subsurface water table Diffuse- or semi-diffuse porous cambial growth Ring- or semi-ring porous cambial growth Indeterminate shoot growth pattern Determinate shoot growth pattern Early budbreak in spring Delayed budbreak in spring Produces small seeds Produces medium to large seeds Shade tolerant Shade intolerant Relatively small N storage pool in dormancy Relatively large N storage pool in dormancy Highly tolerant to anaerobic soils Alternate bearing and seed masting This table is a list of traits that are typically exhibited by tree species that are either “nitrate” or “ammonium” loving. Note that in every characteristic, that pecan is most closely allied with the “ammonium” loving category! If so, then why do grower and specialists often recommend applying a lot of nitrate? Z There is biological variation in species response to the two N-loving categories

9 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 A 3rd point is that trees react to the soil/tree N-environment in different ways depending of the “nitrate:ammonium” nitrogen ratio. We do not yet know what is the best ratio for producing pecan orchards, but we can make an educated guess. If nitrate is excessively dominating, then the trees will respond by transitioning to partition most of their energy and resources into vegetative growth. High nitrate makes them think they are in a highly competitive shaded forest environment and that they need to grow faster in order to survive. If ammonium is relatively high, then the tree thinks it is safe, with little or no competition from neighboring competitors, and can afford the luxury of partitioning energy into reproduction (seeds) instead of so much shoot growth. This means that the grower can exercise a degree of control over tree growth and reproduction habits by regulating the form of N that the tree is exposed to. *Root uptake of nitrate and ammonium-N is regulated by tree demand and sugar supply in roots

10 Synthetic Nitrogen Sources: Ammonical (Ammonium)
~ :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) Tree roots will absorb ammonium-N preferentially over nitrate-N. … like us choosing to pick up $100 bills over that of $1 bills should we find a lot of money lying on the ground. Note that ammonium will quickly convert to nitrate once in the soil solution, usually within a week or two. But during this time, trees will absorb a lot of ammonium, and a lot will be adsorbed to soil particles, especially if the soil is not too sandy. Read off the specific points listed in the slide. [N(3-)H4]+

11 Synthetic Nitrogen Sources: 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 Read off the specific points listed in the slide. Note that most of the N in the soil, regardless of the form applied, will be nitrate within 2-3 weeks of applicaition. This means that if we want to ensure that trees have access to ammonium-N, then ammonium containting N sources should be applied slowly over the relevant parts of the growing season, not all at once (or it will all become nitrate-N within a few days, thus defeating the purpose for applying the more expensive ammonium forms). [N(5+)O3]-

12 Commercial Orchards Are Artificial Pecan Habitats
Do we want orchards to possess the positive traits of their native habitats? We would like to immitate in commercial pecan orchards the positive characteristices of the native habitats in which pecan evolved and is most adapted. Note from the slide what happens if orchards have excessive oxidized, or nitrate-N. The results are not generally what we want. ►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

13 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 ~ %? dw; % 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) #4: It is critical that we understand that regardless of N-source, the tree must possess the appropriate nutritional physiology status for S and the essential trace metals. If these are limiting, then the tree cannot convert the N applied to the desired N end products. Pecan trees are commonly S deficient; thus, such trees cannot convert N to the right end products as it desires, ending up with N being stored as nitrate in vacuoles within cells. The leaf analysis will show a lot of N, but this N is not being used properly by the tree, and the tree is actually N deficient!!!! The same can be true if Fe, Mn, Cu, Mo, Zn, or Ni are limiting!!! When managing orchards for a N, one should also ensure good micronutrient and S management. Applying foliar trace metal sprays during early canopy expansion and applying fertilizer sources containing S are very important.

14 Common Synthetic Nitrogen Sources*
Form Chemical %N Sulfur Ammonium % Amine (Urea) % Nitrate % $/Unit N Solid Urea 46 No 100% 0.49 Calcium Ammonium Nitrate 32 25 75 0.60 Ammonium Sulfate 21 Yes 100 0.61 Liquid UAN-32 50 0.55 UAN-28+S 28 31 23 0.56 S 24 33 22 45 UAN-19 19 57 0.39 S 15 ? 0.47 This table lists the N-sources avaialbe to farmers in Georgia. Note that they differ substantially in ammonium-N and in S. Often, the cheapest source of N is not the best from the standpoint of the tree. Recall that the tree prefers ammonium-N forms, so purchase sources relaivley high in ammonium-N and low in nitrate-N, and apply gradually at proper times over the growing season, not all at once. Note that urea is also a high energy form of N, that converts to both ammonium and nitrate within the soil. Applying urea within irrigation systems is a satisfactory means of getting ammonium to trees. The best source of N is decomposing organic matter, which releases a lot of ammonium N, amino acids, and amino-sugars. * Best source of N is usually decomposing organic matter

15 Other Considerations Most pecan orchards are excessively fertilized with N, especially nitrate-N; needing only ≈ 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 sun-exposed 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 hedge-pruned” 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) Use nitrate N if you want trees to grow fast, but use ammonium N if you want them to produce nuts. Avoid high nitrate sources if hedge pruning trees, as it only pushes trees to grow vegetative, instead of reproductively. Recall that under high nitrate conditions, trees can have very high N levels in foliage >3% yet still be N deficient if tree S, Fe, Mn, Zn, Cu, Mo, or Ni are too low. You can cut the fertilizer needs of trees by using high ammonium N sources, as trees are typically over fertilized with N. Applying relatively high ammonium sources slowly over time can in many cases allow the grower to get better yields, with less N being applied. Orchard N management is complex, and there is still much that needs to be better understood. We need to understand how the tree perceives N, and manage to give the tree what it wants.

16 Thanks!!! I hope there is something here that helps you to better understand what you need to do in your orchard situation to improve orchard yield and quality. Every orchard is different, and a multitude of soil factors can interact to influence the optimal N management strategy. You have been presented with some basic principals that should help you make better decisions. Image by Dr. Ted Cottrell, USDA-ARS


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