Plant Nutrients Essential Elements Plants need at least 17 essential elements: C, H and O from CO 2 and H 2 O; six others are called macronutrients (3 primary, 3 secondary), 8 more are micronutrients. Night Bulletin: CHOPKNS CaFe CuMg cuisine mighty-good (with) ZnMn zinc and manganese ClMo closed Mondays 1
Mechanisms of Nutrient Uptake Prior to absorption, nutrients reach the root by 3 mechanisms: Mass flow movement with the water flow. Most prominent. Diffusion movement in response to a concentration gradient. Slow. Root interception root extension. Very important to find new nutrient sources. 2
Absorption into roots. Passive Uptake: Some ions such as nitrate, can move passively through the outer membrane of the root surface along with water in the transpiration stream. Active Uptake: Not well understood, but many nutrients (e.g., K + and H 2 PO 4- ) must somehow bond with an ion-specific carrier Maintaining an Electrical Balance: As cations are absorbed H + is excreted or organic anions are produced. As anions are absorbed HCO 3 - is excreted or compensating cations are absorbed. 3
Absorption through leaves Stomatal Absorption: Rapid absorption of soluble ions from nutrient enriched water. Used mostly for the immediate correction of critical nutrient deficiencies. Most efficient for the micronutrients. Does not build soil fertility. Danger of phytotoxic effects if over applied. 4
Soil Nitrogen Gains and Transformations N is unique in several ways: No mineral source (usually); SOM stores nearly all N (~90%). Atmosphere is main reserve, but unavailable; must be fixed. Very low soil available pools relative to uptake. Volatile phases (NH 3, N 2 O, N 2 ). Can be taken up as cation (NH 4+ ) or anion (NO 3- ) form. Assimilating NH 4 + costs 2-5% of plant energy, NO 3 - costs 15%; forms proteins & amino acids. Deposition can be major input in polluted areas. Nitrogen Cycling in soils: Biologically controlled N 2 fixation Atmospheric Deposition Plant N N 2, N 2 O Litterfall Root turnover Uptake Uptake Denitrification Mineralization Organic N Clay-fixed NH 4 + NH + 4 NO - 3 Nitrification Immobilization* Leaching Immobilization* *Includes mostly microbial (biotic) but also some abiotic immobilization 5
Nitrogen Fixation Requires great energy Mechanism may be strictly symbiotic, non-symbiotic (free living), or associative symbiotic. 12 g organic carbon per g N. Amounts vary enormously: as low as 1-2 kg ha -1 yr -1 for lichens in Douglas-fir canopies to over 300 kg ha -1 yr -1 for alders (not just 170 kg ha -1 yr -1, as in book) Non-symbiotic N fixations is usually not important, except possibly in desert crusts but no measurements of the latter are available. Nitrogen Mineralization Release from SOM by the conversion of organic N into inorganic N: terminal group aminization, deaminization, ammonification Highly dependent on C:N ratio (occurs at values < 20-30) Approximately 1-5% of the total organic N pool per year Mineralization is critical to N cycling and plant growth because it is the point at which N is converted from organic to NH 4 + form, the latter of which is available to plants and nitrifiers 6
Nitrification of ammonium. Nitrification: Conversion of NH 4 + to NO 3 - + H + (review) Two-stage process Other kinds of fixation Immobilization: Organic tie-up of NH 4 + and/or NO 3 - by microbes Opposite of mineralization Highly dependent on C:N ratio (occurs at values >20-30) Can have abiotic immobilization (chemical reactions between NH 4 + and/or NO 3 - and soil organic matter) in some cases Ammonium Fixation: Adsorption and collapse within the crystal lattice structure (e.g., illites) 7
Nitrogen Losses Leaching: Only the NO 3 - form is important; NH 4 + adsorbs strongly Nitrification is key to facilitating leaching Pollutes water, wastes N, and acidifies soil Nitrification inhibitors are sometimes used to prevent it after fertilization Great need to get fertilizer timing and amounts correctly to minimize this effect. Gaseous Losses: Denitrification: Microbial conversion of NO 3 - to N 2 O and N 2. Anaerobic - may occur in microsites in aerobic soils Requires energy (organic matter) Usually less important loss than leaching in aerobic soil Ammonium volatilization: Chemical, not biological process Occurs only at high ph (8 or above): NH 4 + + OH - NH 3 + H 2 O Usually not important in mesic soils because ph is too low; Exception may be after urea fertilizer, which creates high ph and NH 4 + 8
Materials Supplying N 9
Anhydrous ammonia Highest %N (82%) Injected with chisels (ag use only) Dangerous Urea Cheapest solid N fertilzer because highest %N (45-46%) Volatilization losses of ammonium can be high In forests, abiotic immobilization can be high Ammonium sulfate Relatively expensive (21%N) Ammonium nitrate Relatively cheap (33.5%N) Explosive (mixed with diesel to make bombs - Oklahoma City was this) Nitrate in it can leach readily Slow-release fertilizers Idea is not to flood soil with NH 4 + immediately 10
Soil NH4 + NO3 Normal fertilier Slow-release Time < 1 yr Soil Phosphorus Second most commonly limiting (second on fert bag) Taken up in anion form (H 2 PO 4 - or, at higher ph, HPO 4 2- ) Both soil mineral (apatite) and organic sources are important Deposition is unimportant over the short-term except perhaps the Lake Tahoe Basin Strongly retained by adsorption in acid soils and by precipitation with Ca (forming apatite) in alkaline soils Forms in plants: ADP, ATP (plant energy currency) Most uptake is thought to be by diffusion; root exploration it therefore critical 11
Materials Supplying Phosphorus Often must add bands or dollops or spikes in P- fixing soils Excess P is retained in soil; does not leach like N P fertilization can enhance soil P availability for decades (unlike N) Excess P can fill anion adsorption sites and cause problems with sulfate retention P fertilizers 12
Soil Potassium Second most in terms of plant use. No volatile phases. Mineral sources in soils are important. Organic sources in soils are not important. Taken up as cation (K+ ) Often limiting and often added; third number on fertilizer bag Roles in plants: stomatal control, cell division, translocation of sugars, enzymes Highly soluble Mineral phases (micas and orthoclase feldspar) are highly insoluble Soil total K is often very large K can be "fixed" between 2:1 clays; this form slowly available to plants Exchangeable K + is the major form available to plants and is only a fraction of the total. Smaller than exchangeable Ca 2+ and Mg 2+ But much larger than exchangeable NH 4 + 13
K Management Book discusses crop K fertilization. In forests, K cycling is a major factor allowing long-term responses Also can have long-term increases in soil exchangeable K + K Fertilizers: Table 9-10. Added as ionic K+ form with various anions. Note also that often expressed as K 2 O. This is 39.1*2 + 16 = 94.2; %K in this is 83% 14
Soil Calcium Ca 2 + and Mg 2 + have many similarities in soils: Both divalent Primary minerals are the major source for both Abundant in most soils (except acid soils) Mass flow dominates plant uptake Difference: Ca 2 + is immobile in plants (pectates, etc) Mg 2 + is mobile (chlorophyll) 15
Present in many primary minerals and very abundant in soils Forms secondary minerals (calcite, CaCO 3 ; gypsum, CaSO 4 ) Dominates the exchanger in non-acidic soils Ca 2+ dominates exchange sites and soil solution in non-acidic, non-serpentine soils Ca 2+ also usually dominates the base cation component in acidic soils (where H + and Al 3+ dominate the exchange sites) Ca is a component of cell wall material in plants; very immobile in plants Mass flow is usually adequate for transport to roots Very rarely limiting in nature and when so often indistinguishable from Al toxicity Very large variation in plant demand for Ca In forests we have high Ca uptake trees (oaks, hickories, aspen, cedars) 16
Calcium Deficiencies Rare, because soils low in Ca are usually extremely acidic and have Al 3+ or H + toxicity first Large quantities of Ca 2+ are added in lime Can also add gypsum or CaCl 2 Soil Magnesium Present in many primary minerals Mg 2+ form Adsorbed to exchange sites about equal to Ca 2+ Usually second most abundant in non-acid soils and soil solutions Forms less soluble carbonates and sulfates than calcium does Mass flow dominates uptake mechanism 17
Role in chlorophyll in plants; mobile in plants; less variable in plant uptake than Ca Deficiencies common in acidic soils Fertilizers: dolomitic lime, Mg,K 2 -SO 4, MgSO 4 (epsom salts) Soil Sulfur Many similarities to N: Atmosphere a major source Role in three plant proteins Mobile in plants (translocates) Gaseous phases Few mineral phases (sulfides) C:S and N:S ratio can control mineralization Oxidized and reduced forms 18
Major differences from N: Inorganic form (SO 4 2- ) can be a major form in plants and soils Some soil mineral sources Inorganic soil SO 4 2- pools can be quite large Not limiting as often (air pollution) Required by plants in much lower quantities 19
Excess S in plants present as SO 4 2- (not in book) Some foliar SO 4 2- appears to be necessary Foliar SO 4 2- is a good diagnostic for S deficiency in trees Anionic form in aerobic soils (SO 4 2- ) " easily leached" according to book; not always true Maybe very immobile in acid soils Adsorbed to Fe, Al hydrous oxides Some acid forest soils retain 50-80% of atmospherically-deposited S Elemental S or sulfide S (FeS, PbS) is oxidized by Thiobacillus to sulfuric acid in aerobic soils Sulfate is reduced to sulfide (S 2- ) by S-reducing bacteria in anaerobic soils This further forms either H 2 S (hydrogen sulfide gas, rotten egg smell in swamps) or FeS 20
S Management S is supplied from: Natural sources Soil organic matter (major reservoir in nonacid soils) Atmospheric deposition Minor except in volcanic and polluted areas But may be major source over the very long term - no S fixation Some soluble minerals (FeS in acid soils, CaSO 4 in arid soils) S is supplied from: Anthropogenic sources: Air pollution - atmospheric deposition (acid rain, SO 2 ) P Fertilizers Pesticides 21
S deficiencies are becoming more common in crops because of: reduced S emissions, reduced use of S in P fertilizer and pesticides S deficiencies in forests are very rare (unpolluted areas like Australia, PNW US) S fertilizers: ammonium sulfate, potassium sulfate, ammonium thiosulfate [(NH 4 ) 2 S 2 O 3 ], gypsum, elemental S 22
The Micronutrients B, Fe, Mn, Zn, Cu, Cl, Mo All essential for plant growth, but taken up in very small quantities Except for Cl, the dominant role of micros is enzyme activation Role of B not well understood; seems related to shoot growth and water Most come from soil primary minerals (but not usually Cl) Soil Boron Essential for growth of new cells Not mobile in the plant Forms a weak acid in soils (boric acid, H 3 BO 3 ) At ph>8.5, forms borate [B(OH) 4- ]; thus mostly in un-dissociated form and easily leached in soils H 3 BO 3 + H 2 O B(OH) - 4 + H + 23
Non-metal; sources include: Primary minerals (as trace element); SOM; Adsorbed to Fe, Al hydrous oxides Exists as H 3 BO 4 or B(OH 4 ) - in soil solution; Commonly deficient in forests of PNW, Australia, Scandinavia Fertilizer: Borax (sodium tetraborate, Na 2 BO 4 O 7. 5H 2 O; 14% B) Problems with leaching away too fast; Must use great care not to over-fertilize: toxic (we have this at Steamboat area) Soil Chloride Exists as Cl - Highly soluble at all ph's Cycles rapidly Poorly adsorbed to soils; often used as a tracer for water Believed to have a role in osmosis About the same concentrations as S (0.2%) Salt tolerant plants may contain 10% No known deficiencies, but may be disease related 24
Soil Copper Deficiencies: Organic amendments and organic soils Sandy soils (low total Cu contents) Calcareous soils Common in forest nurseries Can be toxic (boat bottoms) Add very little - 1.2-2.5 kg ha -1 can supply plants for many years Types of fertilizer Plant enzymes Exists at cupric (Cu 2+ ) and less as cuprous (Cu + ) ions Plants absorb Cu 2+, but CuOH + is common in less acid soils Cu(OH) 2 at neutral to alkaline ph's Most common copper mineral is CuFeS 2 (chalcopyrite) Strongly bonds with OM 25
Soil Iron In soils: Least soluble in high ph and aerobic soils (mainly Fe 3+, as FeOH 3 ) For that reason, liming often causes iron chlorosis Most heavily weathered soils are rich in Fe(OH) 3 Problem is not supply of iron, but how to keep it soluble for plant uptake In anaerobic soils, Fe 2+ is more soluble and can leach 26
Chelates and solubility Soil solution ph is important for Fe solubility Fe is soluble enough for plant needs at ph 3; however, this is too low for most plants and many other nutrients Solubility decreases 1000 X per unit ph rise Chelation is what keeps iron available: organic "claw" around Fe atom, keeping it in solution Ligand: the organic molecule Chelate: Fe + organic Some plants can produce these chelates when Fe deficient (siderophores, iron chelate reductases) Some plants produce too much siderophores (mutant pea) and creates Fe toxicity Chelation potential: Fe 3+ >Al 3+ >Cu 2+ >Co 2+ >Zn 2+ >Fe 2+ >Mn 2+ > Ca 2+ +Mg 2+ 27
Deficiencies, Fe fertilization High ph: calcareous soils, arid soils Rare in forests except nurseries Some plants have high Fe demand Visual: general extreme yellowing, even whitening (iron chlorosis) Fertilize with chelates commonly; also foliar sprays (but these do not last long - 3-4 times/yr) 28
Soil Manganese Many enzyme reactions in plants and electron transport Occurs as Mn 2+ in soil solution In aerobic soils, precipitates as MnO 2 (Mn 4+ ) OM decomposition furnished electrons to reduce Mn 4+ to Mn 2+ Deficiencies: Sandy soils, organic soils, high ph soils Interveinal chlorosis of younger leaves Mn toxicity can be a problem in acidic soils; liming can control this 29
Soil Molybdenum Occurs as molybdate anion (MoO 4- ) Many of the same reactions as phosphate - ads to Fe and Al hydrous oxides More available as ph increases because of this May be toxic to grazing animals Very low amounts Very low amounts needed: 0.04 to 0.4 kg ha -1 30
Soil Zinc Essential for enzyme systems No oxidation reduction reactions as for Fe Occurs as Zn 2+, and above ph 7.7, as Zn(OH) + Does not precipitate to Zn(OH) 2 until ph 9.1 ZnS is only major insoluble form in soils Less soluble in anaerobic than in aerobic soils Deficiencies: Basic soils (limed, naturally high ph) High Zn plants (corn, onions, fruit trees) Commonly deficient in forest nurseries and occurs in plantation forests Interveinal chlorosis in both younger and older leaves Solubility increases 100 X with each unit ph decrease Zn fertilizers Foliar sprays also used, but not efficient 31
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