Presentation on theme: "Bruce W. Wood Guiding Principals for Managing Pecan Orchard Nitrogen B.W. Wood."— Presentation transcript:
Bruce W. Wood Guiding Principals for Managing Pecan Orchard Nitrogen B.W. Wood
Point #1--From a Tree’s Perspective: “Not all Forms of Nitrogen (N) are Equal” N 3- (i.e., surplus of 3 electrons): Ammonium N 2+ (i.e., deficient 2 electrons): Hydrazine (rocket fuel) N 3+ (i.e., deficient 3 electrons): Amine (urea, peptides, proteins); ammonia N 4+ (i.e., deficient 4 electrons): Nitrite N 5+ (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 and are therefore differentially valued by the tree. N-atoms have different oxidation states
From a Tree’s Perspective: “Not all Forms of Nitrogen (N) are Equal”* N 3- (i.e., surplus of 3 electrons): Ammonium N 2+ (i.e., deficient 2 electrons): Hydrazine (rocket fuel) N 3+ (i.e., deficient 3 electrons): Amine (urea, peptides, proteins); ammonia N 4+ (i.e., deficient 4 electrons): Nitrite N 5+ (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. Under normal conditions, most nitrate within the plant is eventually converted to ammonium, but this is energetically expensive.
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.)
Pecan ►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. Water Table Alluvial Lands: ●A Co-dominate “Climax” species in certain forest types, and a “Subclimax” in other forest types.
Top soil zone is rich in decaying organic matter [N (3-) H 4 ] +1 Nitrate-N dominates (ammonia intolerant; “nitrate-loving” species) Organic-N and Ammonium-N are common (“ammonia-loving” species) [N (5+) O 3 ] -1 Lower N:A ratio Higher N:A ratio N (5+) O 3
Pecan is exposed to considerable reduced-N (organic and ammonium) in its natural habitats “Nitrate-loving”: continuous shoot growth species (Pioneer species) So, pecan exhibits strong evidence of being an “ammonium-loving” (reduced-N) species!!! (typically shade tolerant mid- late-successional species are ammonium-loving) Water Table Lower N:A ratio Higher N:A ratio [N (5+) O 3 ] -1 [N (3-) H 4 ] +1
CharacteristicNitrate-N loving species z Ammonium-N loving species y Pecan Pioneer species in forest successionYesNo Heavy reliance on subsurface water tableYesNo Diffuse- or semi-diffuse porous cambial growthYesNo Ring- or semi-ring porous cambial growthNoYes Indeterminate shoot growth patternYesNo Determinate shoot growth patternNoYes Early budbreak in springYesNo Delayed budbreak in springNoYes Produces small seedsYesNo Produces medium to large seedsNoYes Shade tolerantNoYes Shade intolerantYesNo Relatively small N storage pool in dormancyYesNo Relatively large N storage pool in dormancyNoYes Highly tolerant to anaerobic soilsYesNo Alternate bearing and seed mastingNoYes 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: ~ :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-) H 4 ] +
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+) O 3 ] -
Commercial Orchards Are Artificial Pecan 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 Do we want orchards to possess the positive traits of their native habitats?
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)
Common Synthetic Nitrogen Sources* FormChemical%NSulfurAmmonium %Amine (Urea) % Nitrate %$/Unit N SolidUrea46No0100%00.49 SolidCalcium Ammonium Nitrate 32No SolidAmmonium Sulfate21Yes LiquidUAN-3232No LiquidUAN-28+S28Yes Liquid S24Yes LiquidUAN-1919No Liquid S15Yes???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 ≈ 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 ( 7 years old)