Nutrient Management, Plant Nutrition, and Soil Fertility Flashcards
(59 cards)
1
Q
Essential Elements for Plant Growth
A
- It is generally agreed that 18 elements are essential for plant growth: C, O, H, N, P, K, Ca, Mg, S, Fe, Mn, B, Zn, Cu, Cl, Co, Mo, and Ni.
- Plants do take up some other elements besides these 18, such as Na, Si, I, Fl, Ba, and Sr. While these may enhance plant growth, they do not appear to be universally required for normal growth.
2
Q
Essential Elements for Plant Growth
A
-Plants obtain 3 of these elements primarily from air and water:
Carbon: obtained from CO2
Oxygen: obtained from O2 and H2O
Hydrogen: obtained from H2O
-All other essential elements are obtained primarily from soil by root uptake.
-Plants can take up small amounts of nutrients through their leaves, but this is generally only important with foliar applications of fertilizers, primarily micronutrients
3
Q
Essential Plant Nutrients Obtained Primarily from Soil
Macronutrients
A
Required in relatively large amounts by plants (>0.1% of plant dry matter) Nutrient Form taken up by plants Nitrogen NO3-, NH4+ Phosphorus H2PO4-, HPO42- Potassium K+ Calcium Ca2+ Magnesium Mg2+ Sulfur SO42-
4
Q
Essential Plant Nutrients Obtained Primarily from Soil
Micronutrients
A
Required in relatively small amounts by plants (<0.1% of plant dry matter) Nutrient Form taken up by plants Iron Fe2+ Manganese Mn2+ Boron HBO3 Zinc Zn2+ Copper Cu2+ Chlorine Cl- Cobalt Co2+ Molybdenum MoO42- Nickel Ni2+
5
Q
Behavior of Nitrogen in Soils
A
- N is essential for
- amino acids
- nucleic acids
- chlorophyll
- N stimulates root growth and uptake of other nutrients
- N deficient plants are chlorotic and older leaves are yellowed
- N in excess
- stimulates vegetative growth,
- often lowers fruit quality,
- may cause poor flowering in ornamentals,
- may cause lodging in grain crops,
- increases potential for leaching of nitrates (NO3-)
6
Q
Pools of nitrogen
A
- The vast majority of N is found in the atmosphere
- The atmosphere is 78% N
- About 75,000 Mg of N are present in the atmosphere above 1 ha of soil
- The N content of surface mineral soils is normally in the range of 0.02 – 0.5%.
- A typical value for cultivated soils is 0.15% N.
- One ha of cultivated soil will contain about 3.5 Mg N in the A horizon and possibly another 3.5 Mg in deeper horizons.
- While the soil N content is much less than the atmosphere, it is still 10 to 20 times greater than the amount of N in standing vegetation.
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Q
Behavior of Nitrogen in Soil
A
Nitrogen is extremely dynamic and as a N atom moves through the nitrogen cycle it may appear in many different chemical forms both in the atmosphere and in the soil.
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Q
Nitrogen Fixation
A
- The process of converting atmospheric N2 first to ammonia (NH3) and then may be further transformed into organic N forms.
- Biological N fixation is carried out by several species of bacteria, some actinomycetes, and certain cyanobacteria (blue-green algae).
- Globally about 139 million Mg of N are biologically fixed in terrestrial ecosystems (about twice as much as industrial N fixation)
- The NH3 in turn is combined with organic acids to form amino acids and ultimately, proteins.
- A great deal of energy is required to break the triple bonded N2 molecule
9
Q
Symbiotic N fixation
A
- Mutually beneficial association of legumenous plants and bacteria of the genus Rhizobium and Bradyrhizobium.
- Bacteria form nodules on the roots of the legume
- Bacteria obtain energy (carbohydrates) from the plant
- Plant obtains fixed nitrogen compounds from the bacteria
- Fix 50 – 250 kg N/ha/yr
- Symbiotic N fixation may also occur without nodule formation. Examples include
- Association between floating fern Azolla and bacteria Anabacter
- Loose associations between plants and N fixing bacteria living in the rhizosphere
10
Q
Non-symbiotic N fixation
A
- N fixation conducted by free living bacteria.
- Amount of N fixed is dependent on factors such as soil pH, soil N levels, and soil organic matter.
- Non-symbiotic N fixation may range from 5 – 20 kg N/ha/yr
11
Q
Soil Organic N
A
- Soil organic matter is about 5% N
- 98 – 99% of soil N is in the organic N form, except in soils that have recently received inorganic N fertilizer.
- Organic N is not available to plants.
- Organic N is not readily leached, and is not subject to volatilization.
- Organic N is converted into inorganic forms when organic matter is decomposed by soil microbes in a process called mineralization.
12
Q
Mineralization
A
- Process whereby soil microbes convert organic N to inorganic N.
- The first step of mineralization is termed ammonification which occurs when the amine groups of soil organic matter are converted by soil bacteria into ammonium (NH4+).
- The second step of mineralization is nitrification the bacterial oxidation of N from ammonium to nitrite (NO2-) then to nitrate (NO3-).
- The oxidation of NH4+ to NO2- is accomplished by the bacteria nitrosomonas.
- The oxidation of NO2- to NO3- is accomplished by the bacteria nitrobacter.
- Both oxidation steps provide a significant amount of energy to the microbes.
- The first oxidation step generates acidity.
- Under most soil conditions formation of nitrite is followed immediately by oxidation to nitrate, and nitrite does not accumulate in soil.
13
Q
Immobilization
A
- Immobilization is the reverse of mineralization and refers to the uptake of inorganic N (NO3-, NH4+) by soil microbes and conversion back into organic forms.
- Both mineralization and immobilization occur simultaneously in soil.
- Which process predominates depends primarily on the C:N ratio of the organic materials that are being decomposed.
- If the C:N ratio is high (>30:1) immobilization will predominate.
- If the C:N ratio is low (<20:1) mineralization will predominate.
14
Q
Fate of soil inorganic N
A
- Soil inorganic N may be taken up by plants or soil microbes
- Ammonia volatilization
- Nitrate is readily leached
- Denitrification
15
Q
Ammonia (NH3) volatilization
A
- Occurs when NH4+ is converted to NH3, a gas, and is lost to the atmosphere.
- Favored by high pH, decreasing moisture content, close to soil surface.
- Can be decreased by soil incorporation of manures and fertilizers.
16
Q
Nitrate is readily leached
A
- From soil and is also susceptible to loss in surface runoff water.
- It is an anion and thus not held by negatively charged soil colloids.
- It is very soluble in water so dissolves readily in soil solution.
17
Q
Denitrification
A
- Is the reduction of N from NO3- to gaseous forms of N: NO (nitric oxide), N2O (nitrous oxide), N2 (dinitrogen).
- Occurs primarily by biological means (soil bacteria)
- Occurs under anaerobic soil conditions.
18
Q
Practical N Management
A
- Goals of N management in agricultural soils
- Maintenance of an adequate supply of N in the soil
- Regulate supply of soluble N forms to ensure sufficient availability for optimal plant growth
- Minimize environmentally damaging losses of N from soils
- In agricultural systems need to balance losses of N from harvest removal with inputs of N from fertilizers, manures, and other N sources.
- Approaches to N management include
- Account for N input from all sources and reduce fertilizer inputs accordingly.
- Improve use efficiency of N from all sources
- Avoid overly optimistic yield projections that lead to over-application of N in most years
- Understand crop responses and apply the lowest amount of N likely to produce optimum profits
19
Q
Environmental Impacts of N
A
- The major environmental problem associated with soil N is the leaching of nitrate through drainage waters to groundwater and surface waters.
- NO3- is very soluble and as an anion it is poorly retained in soils
- Potential for contamination of wells
- Problems associated with elevated NO3- include
- Enhanced eutrophication of surface waters and degradation of aquatic habitats. Estuarine and marine systems are particularly susceptible to increased NO3-
- High NO3- in drinking water can potentially lead to health problems in infants (methemoglobinemia). Drinking water standard for NO3- in the US is 10 mg NO3- -N/L.
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Q
Environmental Impacts of N
A
- Quantity of nitrate lost in drainage water depends on
- Concentration of NO3- in soil
- Rate and quantity of water leaching through the soil
- Potential for leaching is greatest in coarse textured soils in areas of high rainfall or irrigation.
- Nitrate leaching can be minimized by matching N application with crop uptake and demand for N.
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Q
Environmental Impacts of N
A
- Gaseous loss of N by denitrification
- Dinitrogen gas is quite inert and environmentally harmless
- Oxides of N are very reactive and have the potential to do serious environmental damage
- NO and N2O can be converted to nitric acid in the atmosphere, one of the principal components of acid rain.
- The N oxides can react with volatile organic pollutants to form ground level ozone, a major pollutant in urban areas
- NO in the upper atmosphere is a greenhouse gas, 300 times more effective than CO2
- When N2O moves up to the stratosphere it participates in reactions that deplete the ozone layer.
22
Q
Sulfur
A
- Vital for plant nutrition
- Constituent of several amino acids, enzymes and vitamins
- Associated with air, water and soil pollution
- Acid rain, acid mine drainage, acid sulfate soils
23
Q
Behavior of Sulfur in Soil
A
- Natural sources of S
- Organic matter (90-98% of total soil S)
- Soil minerals
- S gasses in atmosphere
- Fate of S in soil
- Mineralization
- Immobilization
- Oxidation/Reduction
- Leaching
- Volatilization
24
Q
Soil Phosphorus
A
- P is the “energy” currency of living cells (ATP)
- Essential component of DNA and RNA
- Adequate P enhances photosynthesis, N-fixation, flowering, fruiting, maturation, root development
- P problems in soil fertility
- Total P in soil is low (much lower than N or K)
- Most P in soil is unavailable to plants
- Soluble P added to soil is often transformed to insoluble forms and rendered unavailable to plants
- Both excess P and inadequate P in soil can lead to environmental degradation
- Soil degradation
- Eutrophication
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P in Soil Solution
- Of the total amount of P in soil, generally less than 0.01% is in the soil solution and available to plants.
- Compared to other macronutrients, concentrations of P in soil solution are very low.
- Concentrations range from 0.001 mg/L in very infertile soil to 1.0 mg/L in heavily fertilized soil.
- Inorganic forms of P predominate with very small amounts of soluble organic P
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Uptake of P by plant roots and mycorrhizae
- P uptake by plant roots is often limited by the slow movement of P anions in soil solution to the root surface.
- Can be overcome by extension of roots to zones where P ions are held.
- Mycorrhizal fungi increase P uptake by roots by
- Fungal hyphal threads greatly expand the volume of soil from which P can be drawn.
- Hyphae absorb P ions as they are released into soil solution and then transport them to plant roots.
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Organic P
- Most organic P compounds in soil are believed to result from microbial synthesis.
- Significant amounts of organic P can also be added with manure applications.
- P in soil can undergo mineralization and immobilization reactions similar to soil N
- Mineralization is the conversion of organic P to H2PO4- and is likely to occur when the C/P ratio in soil is 300:1
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Inorganic P forms in soil
- Inorganic P in soils is very immobile due to reactions that remove P ions from soil solution greatly reduce the solubility of P.
- There are two general processes whereby soluble P is retained in soil
- Precipitation
- Adsorption
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Inorganic P compounds in high pH soil (≥8)
- Soluble P (HPO42-) reacts with calcium to form calcium phosphate minerals with decreasing solubility
- Monocalcium phosphate [Ca(H2PO4)2·H2O], (most soluble)
- Dicalcium phosphate [CaHPO4·2H2O]
- Tricalcium phoshate [Ca3(PO4)2], (least soluble)
- Tricalcium phosphates may undergo further reaction to form even less soluble minerals such as the apatites which are thousands of times less soluble
- [3Ca3(PO4)2]·CaF2, (fluorapatite)
- [3Ca3(PO4)2]·CaCO3, (carbonate apatite)
- Rock phosphate consists mainly of apatites and thus is only effective as a P fertilizer in acidic soils and only if ground very fine
-The Ca phosphates are least soluble at high pH, solubility increases as pH decreases
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Inorganic P compounds in low pH soil (≤7)
- Reaction of H2PO4- with dissolved Al3+, Fe3+, or Mn3+ to form hydroxy phosphate precipitates.
- These hydroxy phosphates are initially slightly soluble, but become increasingly insoluble as they age. May eventually be converted to extremely insoluble forms (strengite, variscite)
- Least soluble at low pH, solubility increases as pH increases
31
Inorganic P compounds in low pH soil (≤7)
- Adsorption reactions
- Phosphate may be attracted to positive charges that develop at lower pH and at the edges of 1:1 layer silicate clays. Such P is available via anion exchange.
- Phosphate may react chemically (much more strongly held)
- With hydroxylated surface of 1:1 layer silicate clays (kaolinite)
- With iron and aluminum oxide clays
- With oxides and hydrous oxide coatings on layer silicate clays or other soil particles.
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P replacement
- Phosphate may replace a structural hydroxide and then has very low availability. Over time a second hydroxide may be replaced by the phosphate which then becomes part of the oxide or hydroxide structure and further decreases its availability.
- Over time precipitation of additional oxide may cover the phosphate which is now said to be occluded, and is the least soluble form of P in acid soils.
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Practical control of P in soils
- Pattern P fertilizer rates to fit soil P status
- Placement of P
- Combine ammonium and phosphorus fertilizers
- Cycling of organic matter
- Control of soil pH
- Enhance mycorrhizal symbiosis.
- Choose P efficient plants
- Reduce sediment and runoff losses
- Capture excess P before it enters mainstream channels
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Pattern P fertilizer rates to fit soil P status
Practical control of P in soils
In low-P soils applications well above plant uptake may be needed. In high-P soils matching P application to plant uptake will be adequate to meet plant needs.
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Placement of P
Practical control of P in soils
Band placement of P fertilizer minimizes reaction with bulk soil and increases availability of added P.
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Combine ammonium and phosphorus fertilizers
Practical control of P in soils
In alkaline soils, acid generated by ammonium oxidation increases P solubility.
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Cycling of organic matter
Practical control of P in soils
Materials such as manures, composts, and green manures are a slow release form of P. Organic chelates in these materials help increase solubility of Al, Fe, Mn phosphates.
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Control of soil pH
Practical control of P in soils
P solubility is maximized at near neutral pH.
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Enhance mycorrhizal symbiosis.
Practical control of P in soils
Fostered by crop rotation, application of organic materials, and minimum tillage.
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# Choose P efficient plants
Practical control of P in soils
Species vary widely in their ability to forage for and extract soil P.
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Reduce sediment and runoff losses
Practical control of P in soils
Do not apply manure or fertilizer to frozen land since soluble P can be washed into surface water. Use conservation tillage to minimize runoff and erosion
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Capture excess P before it enters mainstream channels
Practical control of P in soils
Grass waterways and riparian buffers help to adsorb dissolved P and to capture P rich sediments before they enter surface water systems.
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Environmental impacts of P
| Land
- Land degradation often results from too little P
- Highly weathered soils in warm, humid, and subhumid regions have very limited capacity to supply P for plant growth
- Partly due to loss of P during extensive and intensive weathering
- Partly due to low availability of P in these soils high in Al and Fe oxides
- Undisturbed natural ecosystems with such soils usually contain sufficient P in biomass and organic matter that is tightly cycled within the ecosystem
- If land is cleared for agriculture P is lost via erosion and biomass removal
- Within a relatively few years much of the formerly cycled P is lost and vegetative growth declines
- There are an estimated 1 – 2 billion ha of land in the world where P deficiency limits growth of both crops and native vegetation.
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Environmental impacts of P
| Water
- Water quality degradation results from excessive P
- P is often the limiting nutrient in freshwater systems, so addition of P stimulates growth of algae and aquatic weeds, a process known as eutrophication
- Water becomes turbid, limiting light penetration
- Decomposition of excessive growth consumes oxygen and can lead to fish kills
- Water becomes foul smelling, bad-tasting, and possibly toxic.
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Environmental impacts of P
| Erosion
- Primary route of P transport to surface waters is from surface runoff and erosion.
- Excessive P fertilization enriches surface soil with P
- This increases soluble P that can dissolve in surface runoff and move to surface waters
- Erosion of P rich soil particles from the soil surface carries additional P into drainage ways. Sediments deposited in drainage ways, streams, rivers, ponds, and lakes can release P into the water.
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Potassium in Plant and Animal Nutrition
- K serves primarily as an enzyme activator in plant and animal cells in many important biochemical pathways
- Plays a critical role in lowering plant osmotic potential
- K is essential for photosynthesis, protein synthesis, N-fixation, starch formation, sugar translocation
- Helps plants adapt to environmental stresses (drought, winter hardiness, resistance to some fungal diseases)
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K in Soil
- The amount of K in most soils is much greater than any other plant nutrient, but the amount readily available to plants (in soil solution and easily exchangeable) is very small.
- The vast majority of soil K is held in relatively unavailable forms, primarily in feldspar and mica minerals. These minerals are resistant to weathering and only very slowly release K. Plant uptake of K depletes solution K at the edges of these minerals and favors their dissolution
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Pools of K in Soil
- Soil solution
- Readily exchangeable K
- Slowly available K, or fixed K
- K held in primary minerals
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Soil solution
| Pools of K in Soil
- Contains only 0.1 to 0.2% of total soil K. The K is present is solution primarily as the K+ cation
- Plant roots take up K from the soil solution
- This K is also susceptible to leaching and runoff loss.
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Readily exchangeable K
| Pools of K in Soil
-Is held on cation exchange sites on clays and organic matter and is in rapid equilibrium with solution K.
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Slowly available K, or fixed K
| Pools of K in Soil
- Is held in interlayer positions of 2:1 layer clay minerals such as smectite, vermiculite and illite.
- This K is not readily exchangeable, but serves as an important reservoir of K because it is in slow equilibrium with readily exchangeable and solution K.
- As those reserves are depleted, fixed K is slowly released from the interlayer positions as the clays weather and interlayer spacing expands.
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K held in primary minerals
| Pools of K in Soil
- Such as micas and feldspars
- Largest reserve of K in most soils
- Very slowly available
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Cycling of K
- Plant residues, manures and other organic materials returned to soil contain K.
- As soil microbes decompose the organic material, some of the K is released as inorganic K into the soil solution.
- K in soil solution
- may be taken up by plant roots, thus completing the cycle
- may be held on cation exchange sites
- may be converted to less readily available forms in the soil
- may be leached or lost in surface runoff
- Some K taken up by plants can be washed by rainwater from plant foliage and returned to the soil, a process termed throughfall.
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The Potassium Problem in Soil Fertility
- Although there is a large amount of K present in most soils, only a very small fraction is readily available to plants at any given time.
- K is much more readily lost via leaching than is P. Leaching loss of K can be reduced by liming an acid soil. This is because K+ can more readily exchange with divalent Ca2+ or Mg2+ than with trivalent Al3+, thus after liming more K+ will be held on exchange sites.
- Plants take up 5 – 10 times more K than P, and similar amounts of K and N. Thus harvest removal of plant biomass can remove large quantities of K from the soil.
- If soil availability of K is high, plants tend to take up more K than they need for nutrition (luxury consumption), which can lead to even greater K removal.
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Practical aspects of K management
- K availability to plants is usually related to the rate of transformation from nonavailable to available forms rather than to the total amount of K in soils.
- K uptake by vigorously growing crops with a high K demand can deplete soil solution K and readily exchangeable K much more rapidly than they are replenished from less available K forms.
- Frequency of Application. While large, infrequent K fertilizer applications may be convenient, smaller more frequent applications will generally be more efficient
- Small applications result in less leaching loss
- Small applications result in less luxury consumption
- Small applications may result in greater fixation of K (a large application will satisfy fixation capacity and supply extra K that remains in available forms).
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Practical aspects of K management
- Maintain soil pH to minimize K leaching and maximize crop use of K released by weathering of K bearing soil minerals
- Annual crop removal of K can be large, as great as 400 kg/ha for legumes removed for hay production.
- Such removal needs to be compensated by returning manures and residues whenever possible and by addition of K fertilizer.
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Micronutrients in Soil
- Micronutrients are required by plants in very small quantities.
- Relative numbers of atoms of essential elements in alfalfa at bloom stage. Note that there are more than 10 million atoms of H for every atom of Mo, yet normal growth would not occur without Mo.
- Deficiency vs. Toxicity.
- Normal plant growth will not occur if the supply of any essential micronutrient is insufficient (deficiency).
- If excessive amounts of micronutrients are present in soil they may become toxic to plants or plant concentrations may reach levels that are toxic to animals feeding on the plants.
- The sufficiency range for micronutrients is much narrower than for macronutrients
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Factors influencing availability of micronutrients
- Soil pH
- Micronutrient cations (Fe3+, Mn2+, Zn2+, Cu2+, Ni2+) are most soluble and available under acid conditions.
- Under low pH conditions these elements can become toxic (often occurs with Mn).
- As pH increases these metals become less soluble and less available.
- Over liming an acid soil can lead to deficiencies of these elements and is why for most plants a slightly acidic soil pH (6 – 7) is desirable.
- Many micronutrient anions (chlorine, boron, molybdenum, selenium) are more available at neutral to slightly alkaline pH
- Soil organic matter
- Most micronutrient cations are strongly bound by soil organic matter which decreases their availability
- Soluble organic matter may increase micronutrient availability by forming “chelates” with metals and thereby keeping them in solution
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Soil Management and Micronutrients
- Changes in soil acidity. In general, maintaining soil pH between 6 and 7 will provide adequate micronutrient levels in medium textured soils.
- Soil Moisture. Drainage and moisture control influence oxidation status of soils. In general improving drainage will lead to oxidized forms of metals and reduce their availability.
- Fertilizer applications.
- Micronutrient deficiencies are rare in soils to which crop residues are returned, or to which manures or sewage sludge are regularly applied.
- Micronutrient fertilizers can be applied to correct deficiencies.
- Foliar applications and seed treatments are often more effective than soil applications where soil conditions are limiting micronutrient availability.
