Fundamentals of Nutrient Management Nutrient Cycling in Soils: The Big 3 and Carbon Gurpal Toor Department of Environmental Science & Technology University of Maryland College Park @ToorUMD
N deficiency in Corn Nutrients: Plant food; essential for crop production Nitrogen: N Phosphorus: P Potassium: K P deficiency in Corn K deficiency in Corn Images from Better Crops (1997), Vol. 81. No. 3
What is Nutrient Cycling: Use, movement, and recycling of nutrients Carbon Cycle: Plants use (photosynthesis) carbon dioxide (CO 2 ) to grow. When plants decompose, CO 2 is returned to atmosphere via respiration. Image Credit: USGS
Humans (billions) Haber-Bosch (Tera gram of N) Nitrogen 1 Tera Gram = 1 billion kg or 2.54 billion pounds Most complicated & fascinating Many reactions mediated by soil organisms Wonderful oxidation and reduction reactions N has gaseous forms Atmosphere contains 78% N gas Image Credit: Galloway and Cowling (2002). Ambio.
Nitrogen Cycle
Nitrogen Cycle Inputs: Crop & animal residues Biosolids Compost Fertilizers N fixation
N Fixation Mechanisms N 2 Gas Natural fixation Atmospheric additions electrical discharge of lightning Biological processes symbiotic for example between Rhizobium bacteria and leguminous plants non-symbiotic carried out by free living bacteria and blue-green algae
Soybean Nodules Photo credits: Harold Evans, OSU
Biological N Fixation
Calculating Legume Credits Crop lbs N/Ac Perennial Crops Alfalfa 100 to 150 Red Clover 40 Winter Annual Crops Hairy Vetch 75 to 150 Crimson Clover 50 to 100 Summer Annual Crops Lespedeza 20 Soybeans 15 to 40 Photo credit: Ray Weil
N Fixation Mechanisms Synthetic or industrial processes Industrial fixation for fertilizer manufacturing Haber-Bosch process: N 2 + 3H 2 2NH 3 (g) High temperature fixation internal combustion engines
Transformations of Organic N Proteins Amino sugars Amino acids 2-5% of total soil N NH 4 + NO 3 - NO 2 -
Mineralization (a transformation) Conversion of organic N to inorganic N, ammonium (NH 4+ ) Mediated by bacteria and fungi
Immobilization (a transformation) Conversion of inorganic N (NH 4 + & NO 3- ) to organic N Soil organisms assimilate nutrients into biomass Soil organisms are extremely numerous and very well distributed in the soil Outcompete plants for available nutrients under many circumstances
Nitrification (a transformation) Biological transformation of ammonium (NH ) to nitrate (NO ) 4+ 3- Aerobic conditions and moderate ph suppressed below ph 5.5 Soil bacteria Nitrosomonas and Nitrobacter carry out nitrification
Nitrification: A Two-Step Process Nitrosomonas 2 NH + + 3 O 4 2 2 NO + 2 H 2-2 O + 4 H + + e Ammonium Nitrite Hydrogen Nitrobacter 2 NO 2- + O 2 2 NO 3- + e Nitrite Nitrate
Soil N Cycle: A Biological Phenomenon Influenced by: ph: most bacteria suppressed at low ph Temperature: is the key Moisture: dictates oxidation or reduction oxygen status (aeration)
Effect of Temperature on N Cycling: Water freezes Optimum for nitrification Optimum for mineralization Most microbes killed Biological zero Optimum for growth of corn, cotton, potatoes
Effect of Moisture & Oxygen Status on N Cycling:
Fate of N in Organic Additions C:N ratio is the amount of C relative to the amount of N in a given material A high C:N ratio (>25:1) N immobilized A low C:N ratio (<20:1) N mineralized
a Cornell On-Farm Composting Handbook, Rynk et al, 1992 b The Nature and Properties of Soils, Brady and Weil, 1999
Changes in NO 3- levels when organic additions have wide C/N Adapted from F.J. Stevenson, 1986
Managing C:N Ratio of Inputs Manage cover crops Incorporate cover crops while in vegetative state Leave mature cover crops on surface Monitor soil N and plant growth when incorporating straw, sawdust, and other high C:N materials
Plant-available N in soil solution Ammonium (NH 4+ ) Nitrate (NO 3- ) Small organic molecules present in dissolved organic N (DON) pool
1. Volatilization: hot, dry weather Losses: Nitrate leaching Decades Days/wks Weeks/months Years 2. Erosion/runoff: slopes; heavy rain 3. Denitrification: waterlogged soils 4. Crop uptake 5. Leaching: soil texture; rain amount (J.J. Meisinger, USDA)
1. Leaching (a liquid loss) Primarily as Nitrate (NO 3- ) Moves freely in soil profile Transported by drainage water, especially important in sandy soils can lead to pollution of groundwater An economic loss with environmental consequences
Nitrate leaching Minimal under natural vegetation (forests) Greater under modern row-crop productions A risk if N management is sub-optimal
Reasons for Excessive NO 3- Leaching Inefficient N management Heavy one-time applications Improper timing Over-application of manure/sludge Enhanced by periods of heavy rainfall
Enhancing Synchrony: Minimize Vulnerable Nitrogen If the N is not yet applied, it can not be lost Importance of split applications (Penn State Agronomy 12)
2. Ammonia Volatilization (a gaseous loss) Loss of ammonia-n to the atmosphere Ammonium in the presence of hydroxyl (OH - ) can produce ammonia gas NH 4+ + OH - H 2 O + NH 3 Affects all surface-applied N sources Urea, ammonium nitrate, manure Enhanced by warm, dry atmospheric conditions
Diurnal variations (Pote & Meisinger, 2014. J. Soil & Water Cons.)
Managing Ammonia Losses Know where and when ammonia loss occurs First day of application Sunny, warm, low humidity, breezy conditions Incorporation is required under many circumstances in Maryland Spread and incorporate manures in the early morning or evening (when dew still present)
3. Denitrification (a gaseous loss) NO 3 - N 2 O N 2 nitrate ions nitrous oxide gas dinitrogen gas Biological reduction of nitrate to N 2 gas NO 3- transformed to gaseous compounds Favored by saturation of the soil (anaerobic conditions)
A Visual on Denitrification in Field Photo credit: www. Agleader.com
Principles of N Management Maintain soil ph appropriate for crop Reduce runoff/erosion losses Apply N fertilizers and manure when plants need it, where they need it Timely incorporation of manures and sewage sludge when practical Use cover crops to scavenge residual N
Practice Question #1 In which form is nitrogen most likely to be lost in the greatest quantity from soils in Maryland? a) dinitrogen gas (N 2 ) b) nitrate (NO 3- ) c) ammonium (NH 4+ ) d) organic N
Phosphorus (P) Cycle Simpler cycle than N No gaseous forms Soluble P can be fixed to less available forms Image Credit: Jaisi and Blake (2014). Advances in Agronomy
Transformation of Organic P INORGANIC PHOSPHORUS POOL ORGANIC PHOSPHORUS POOL HPO 4-2 H 2 PO 4 - P mineralization and immobilization occur simultaneously in the soil
Plant-available P Orthophosphate (water-soluble phosphate) H 2 PO 4 - at ph 5 to 7.2 HPO 4 2- at ph > 7.2 Phosphate moves slowly in soil by diffusion Image Credits: Utah State and University of Minnesota
P Fixation (a transformation) A set of processes through which P is converted to less available forms Precipitation Adsorption
P fixation Adsorption or Sorption In acid soils, P is adsorbed to surfaces of Al/Fe oxides and clay minerals In neutral and calcareous soils, P is adsorbed to surfaces of CaCO 3 and clay minerals
P Fixation Precipitation as secondary P compounds In acid soils, P combines with iron (Fe) and aluminum (Al) to form insoluble compounds In neutral and calcareous soils, P combines with (Ca) to form insoluble compounds
Mn N.C. Brady, 1974
Availability of Fixed P P relatively available when first fixed dissolution from newly formed minerals desorption from sorption sites Availability decreases as time passes
Occluded P Sorbed P tends to become occluded P as time passes Sorbed P covered by coatings of iron, aluminum, or manganese oxides Unavailable
Summary of different P pools in soils Image Credit: Frossard et al. (2011).
P Loss Pathways: 1. Eroded and runoff P: slopes; heavy rain 2. Crop uptake Phosphate leaching (tiles) (ditches) 3. Leaching losses: tile drains; ditches, coarse textured soils; high P fertilization
Principles of P Management Maintain soil ph for desired crop Apply P fertilizers when needed, where most efficiently utilized Band starter fertilizer When practical, incorporate manure Utilize practices that reduce soil erosion and runoff
Practice Question #2 What is the optimum ph range for maximizing plant-available P in soils? a) 5.0-5.5 b) 5.5-6.8 c) 6.5-7.7 d) >7.5
Potassium (K) Cycle Simpler cycle than N no oxidation and reduction no gaseous forms Soluble K can be fixed to less available forms in some soils different mechanism than P minimal water quality issues
PLANT RESIDUES Sources Losses Maryland Cooperative Extension, 2000
Plant-available K K + dissolved in soil solution Exchangeable K + Moves primarily by diffusion along moist soil pores
Soil solution K Only 0.1 0.2% of total K in soils Readily available for plant uptake
Exchangeable K K + at exchange sites on soil colloids in dynamic equilibrium with K + in soil solution replenishes the soil solution Readily available for plant uptake Usually less than 1% of total K in soils
Fixed K Fixed as K + trapped in interlayers of clay minerals Non-exchangeable Minimally available by weathering About 1-10% of total K in soil
N.C. Brady, 1974
Mineral (structural) K K is a structural component of primary minerals such as feldspars and micas 90 98% of total K in soil Relatively unavailable
K Loss Pathways Leaching coarse soils/high rainfall Erosion Crop removal luxury consumption
Luxury consumption of potassium by plants. If excess amounts of potash fertilizers are applied to soil, plants will absorb potassium in quantities exceeding that required for optimum yields. This may be wasteful if crops are completely removed from the soil. N.C. Brady, 1974
Principles of K Management Maintain soil ph for desired crop Utilize practices that reduce soil erosion Split application reduce losses to luxury consumption
Practice Question #3 Potassium is lost from the soils by: a) Microbial transformation to gaseous K + b) Leaching through clayey, fine-textured soils c) Luxury consumption by plants
Summary: Nutrient Cycling in Soils
Questions? Email: gstoor@umd.edu @ToorUMD