NRES 201 Lecture 40 (Fall 2014): Micronutrients1NRES 201Micronutrients12Today’s lecture topics- Basic aspects of micronutrients- Micronutrients in plants- Micronutrients in soils- Losses of soil micronutrients- Factors affecting micronutrient availability- Micronutrient deficiencies- Micronutrient fertilizationMicronutrientsNRES 201 Lecture 40 (Fall 2014): Micronutrients2Elements considered micronutrientsEssential for all plants3Essential or beneficial for some plants4NRES 201 Lecture 40 (Fall 2014): Micronutrients3Essential for animals but not plants5Micronutrient similaritiesPlant demand- Essential, but- Required in only traceamounts by mostplants- From mg/kg(parts per million)-To g/kg(parts per billion)Potential for toxicity- Excessive amountsare harmful6Source: http://www.thejuryexpert.comFor part per billion (ppb), multiply the right-hand commodity times 1,000.NRES 201 Lecture 40 (Fall 2014): Micronutrients4Micronutrient differencesNatural abundance- Extreme range- From the fourth mostabundant element in the earth’s crust (Fe)- To a very scarce metal (Mo)Chemical properties- Six exist as cations(Co, Cu, Fe, Mn, Ni, Zn)- Two exist as oxyanions(B, Mo)7Source: http://www.periodictable.comReasons for increased emphasis on micronutrientsHigher crop yields- Due to:- Improved crop varieties- Higher plant populations- More effective pest control- Increased macronutrient fertilization- Increased demand for micronutrients8Source: http://www.montgomerycountymd.govNRES 201 Lecture 40 (Fall 2014): Micronutrients5Use of high-analysis fertilizers- Lower content of micronutrients as impuritiesEscalating production pressure- Any yield limitation cuts profitsIncreased awareness of deficiencies- Micronutrient deficienciesmore likely to be diagnoseddue to:- Fewer macronutrientdeficiencies- More sensitive methodsof analysis9Source: http://www.deere.comMicronutrient functions in plantsKnown functionsCommon role in enzymes- As a constituent- As an activatorSource: Brady and Weil (2002)10NRES 201 Lecture 40 (Fall 2014): Micronutrients6Micronutrient content of soilsHighly variable- Among micronutrients- And sometimes for a single micronutrient11Depends on- Micronutrient content of parent material- Stage of soil development, as affected by:- Climate- Topography- Vegetation-Time- Cultivation, drainage, and synthetic N fertilization- Leads to a decrease in organic-matter content- Organic matter binds many micronutrients12NRES 201 Lecture 40 (Fall 2014): Micronutrients7Forms of micronutrient cationsWater-soluble and exchangeable13Organically bound- Chelated by soil organic matter- Metal ions in solution are surrounded byoriented water molecules- Chelation involves:- Replacement of two or more water molecules- By electron-rich organic groups- Chelation decreases in the order:Fe3+> Cu2+> Zn2+> Fe2+> Mn2+- Chelation usually increases metal solubility14NRES 201 Lecture 40 (Fall 2014): Micronutrients8Minerals- The major soil form- Secondary precipitates formed by weathering- Such as carbonates in calcareous soils15Forms of micronutrient anionsWater-soluble16NRES 201 Lecture 40 (Fall 2014): Micronutrients9Specifically adsorbed- Some retention of Mo under acidic conditions- Similar to P- B reacts with exposed OH groups on clayminerals and oxides- And becomes bound through ligand exchange- Believed to be a major mechanism for Bretention in Oxisols17Organically bound- Important for B but not Mo- B reacts with exposed OH groups on soil organicmatter- Believed to be a major form of soil B18NRES 201 Lecture 40 (Fall 2014): Micronutrients10Minerals- The major soil form- Secondary precipitates formed by weathering- B adsorbed on CaCO3in calcareous soils- Mo reacts like P to form:• Fe and Al molybdates in acidic soils• Ca molybdates in calcareous soils19Losses of micronutrients from soilCrop removal- Generally small compared to total soil content20NRES 201 Lecture 40 (Fall 2014): Micronutrients11Leaching- Negligible, exceptfor sandy soils- All eight micro-nutrients retainedby the solid phaseErosion- Losses couldbe significantVolatilization- Does not occur21Source: http://www.ctic.orgSource: http://www.nrcs.govPlant availability of micronutrient formsAvailable- Water-soluble- ExchangeableSlowly available- Specifically adsorbed- Organically boundUnavailable- Minerals- Precipitates22Source: https://www.behance.netNRES 201 Lecture 40 (Fall 2014): Micronutrients12Factors affecting micronutrient availabilityTotal soil content- Determined by:- Micronutrient content ofparent material- Stage of soil development- Deficiencies may be due tolow soil content- Co in New ZealandSoil texture- Deficiencies of micronutrient cations more likelywith sandy soils due to:- Low CEC- Greater leaching losses23Source: http://hurricanecattlecare.ieOrganic matter content- A source of micronutrients because organicmatter:- Contributes CEC- Chelates micronutrient cations• Especially Cu- Binds B in a stable form- May decrease the availability of micronutrientcations- By forming insoluble metal chelates- Micronutrient deficiencies more common withorganic soils (Histosols) than with mineral soils- Especially for Cu, Zn, and Mn24NRES 201 Lecture 40 (Fall 2014): Micronutrients13Soil pH- Availability of Fe, Mn, Zn, Cu, Co, Ni, and Bdecreased by higher pH due to:- A change in oxidation state• For Fe and MnFe(OH)3 + H++ e- Fe(OH)2+ H2OMnO2 + 4H++ 2e- Mn2++ 2H2O• The oxidized form is much less soluble ()- Precipitation• As insoluble carbonates- Increased microbial activity• Promotes Fe and Mn oxidation• Produces CO2and HCO3-- Greater uptake of Ca, Mg, and K25- Mo and soil pH- Availability increased by liming acidic soilsdue to:• Reduced fixation• Dissolution of Fe and Al molybdatesFe2(MoO4)3 + 6OH- 2Fe(OH)3 + 3MoO42-Al2(MoO4)3 + 6OH- 2Al(OH)3 + 3MoO42-- Ca molybdates decrease Mo availability athigh pH- Analogous to P26NRES 201 Lecture 40 (Fall 2014): Micronutrients14Soil aeration- Fe and Mn much more soluble in flooded soils- Present as Fe2+and Mn2+- Fe and Mn toxicities can develop- Promoted by acidityNutrient interactions- Most are antagonisticSource: Brady and Weil (2002)27Micronutrient deficienciesSource: Brady and Weil (2008)28NRES 201 Lecture 40 (Fall 2014): Micronutrients15Micronutrient fertilizationDifferences compared to N, P, and K- Apply micronutrients only if deficient- Foliar applications common- To maximize availability (esp. for Fe and Mn)- Low rates avoid salt