Alkaline and Saline Soils

Why do some soils become saline?

•Precipitation is less than potential evapotranspiration

•Cations released from mineral weathering accumulatebecause there is not enough leaching to wash them away

•Saline soils occur in soils with pH>8.5

•Ca2+, Mg2+, K+ and Na+ do not produce acid upon reacting with water

The do not produce OH- ions either, but in soils with high concentrations of carbonate and bicarbonate anions, pH>8.5. Hence the association between salinity and pH.

CaCO3 Ca2+ + CO32- or NaCO3 2Na2+ + CO3

2-

CO32- + H2O HCO3

- + OH-

HCO3- + H2O H2CO3 + OH-

H2CO3 H2O + CO2(gas)

•pH rises more for most soluble minerals (eg. NaCO3)•pH rise is limited by the common ion effect

Micronutrient deficiencies in saline soils

•Fe and Zn deficiencies are common because their solubility is extremely low in alkaline conditions

•Addition of inorganic fertilizer may not improvethis deficiency as they quickly become tied up ininsoluble forms

•Chelate compounds are often applied to soils (Feassociated with organic compounds)

• Under high pH, B tightly adsorbs to clays in an irreversible set of reactions. In sandy soils, B content isgenerally low under any pH level, especially in wetenvironments due to leaching (problematic in wet or dryenvironments, but less so in between).

Effect of soil pHon nutrient contentand soil microorganisms

•Phosphorus is often deficient in alkaline soils, because it is tied up in insoluble calcium or magnesium phosphates[eg. (Ca3(PO4)2 and Ca3(PO4)2]

•Some plants excrete organic acids in the immediate vicinity oftheir roots to deal with low P

Other notes of interest:

•Ammonium volatilization is commonly problematic duringnitrogen fertlization on alkaline soils (changes to gas – lost to atmosphere)

•Molybdenum levels are often toxic in alkaline soils of arid regions

Salinization

The process by which salts accumulate in the soil

Soil salinity hinders the growth of crops by lowering the osmotic potential of the soil, thus limiting the ability of roots to take up water (osmotic effect). Plants must accumulate organic and inorganic solutes within their cells.

Specific ion effect: Na+ ions compete with K+

Soil structure breaks down, leading to poor oxygenation andinfiltration & percolation rates

•36% of prairie farmland has 1-15% of its lands affected bysalinization and 2% has more than 15% of its lands affected.

•Most prairie farmland (61% in Manitoba, 59% in Saskatchewan, and 80% in Alberta) has a low chance of increasing salinity under current farming practices.

Conservation farming practices to control soil salinity

•Reducing summerfallow

•Using conservation tillage

•Adding organic matter to the soil

•Planting salt-tolerant crops (eg., canola and cabbage)

Conditions promoting salinization:•the presence of soluble salts in the soil

•a high water table

•ET >> P

These features are commonplace in:

•Prairie depressions and drainage courses

•At the base of hillslopes

•In flat, lowlying areas surrounding sloughs and shallow water bodies.

•In areas receiving regional discharge of groundwater.

Source: Agriculture and Agri-food Canada

Signs of SalinizationA. Irregular crop growth on a solonetz

Whitish crust of salts exposed at the surface (B,C)

Aerial photo of saline deposits at Power, Montana

D. Presence of salt streaks within soils

E. Presence of salt-tolerant native plants, such as Red Sapphire

Human activities can lead to harmful effects of salinization, even in soils of humid regions

(a) (b)

Effect of road salt on Maple leaves

Calcium carbonate accumulationin the lower B horizon

The white, rounded "caps" of the columns are comprised of soil dispersed because of the high sodium saturation

Salinization inresponse to conversion of natural prairieto agriculture

Measuring the electrical conductivity (EC) of a soil sample in a field of wheatgrass to determine the level of salinity.

A portable electromagnetic (EM) soil conductivity sensor used to estimate the electrical conductivity in the soil profile

Effect of salinity on soybean seedlings

Influence of irrigation technique on saltmovement and plant growth in saline soils

The Importance of Soil Nitrogen

Amino acids

Enzymes

Proteins thatcatalyze chemical reactionsin living organisms

Nucleic Acids

A nucleic acid is a complex, high-molecular-weight, biochemical macromolecule composed of nucleotide chains that convey genetic information.

Nitrogen Deficiency

•Pale, yellowish-green colour due to low chlorophyll content

•Older leaves turn yellow first and may senesce prematurely

•Spindly stems or few stems

•Low protein, but high sugar content (not enough N to combinewith carbon chains to produce proteins)

•Low shoot:root ratio

•Rapid maturity

Nitrogen Deficiency

“Chlorosis”

Yellowing of olderfoliage

Restricted growth

Few stems or spindlystems

Nitrogen oversupply

•Lodging with wind or heavy precipitation due to excessive growth

•Delayed maturity

•Susceptibility to fungal diseases

•Reduced flower production

•Poor fruit flavour

•Low vitamin and sugar content of fruits and vegetables

Nitrogen Forms

Reduced

NH4+ N2 N2O NO

ammonia molecular N nitrous oxide nitric oxide

Oxidized

NO2- NO2- NO3

-

nitrite nitrogen dioxide nitrate

Nitrogen Fixation

The nitrogen molecule (N2) is very inert. Energy is required to

break it apart to be combined with other elements/molecules.

Three natural processes liberate nitrogen atoms from its atmospheric form

•Atmospheric fixation by lightning

•Biological fixation by certain microbes — alone or in a symbiotic relationship with plants

•Industrial fixation

Atmospheric fixation by lightning

•Energy of lightning breaks nitrogen molecules.•N atoms combine with oxygen in the air forming nitrogen oxides. •Nitrates form in rain (NO3

-) and are carried to the earth. •5– 8% of the total nitrogen fixed in this way (depends on site)

Industrial Fixation

•Under high pressure and a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) is combined to form ammonia (NH3).

•Ammonia can be used directly as fertilizer, or further processed to urea and ammonium nitrate (NH4NO3).

Biological Fixation

Performed mainly by bacteria living in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa), although some nitrogen-fixing bacteria live free in the soil.

•Biological nitrogen fixation requires a complex set of enzymes and a huge expenditure of ATP.

Although the first stable product of the process is ammonia, this is quickly incorporated into protein and other organic nitrogen compounds.

Carried out by Rhizobium bacteria in a SYMBIOTICrelationship. Host providescarbohydrates for energy; Rhizobium supplies plant withfixed nitrogen.

Nitrogen mineralization

95-99% of N is in organic compounds, unavailable to higher plants, but protected from loss

1. Soil microbes attack these organic molecules, (proteins, nucleic acids, amino sugars, urea), forming amino compounds

2. The amine groups are hydrolyzed, with N released as NH4

+ (ammonium ions; See pg. 548)

3. Oxidation of NH4+ to NO2

- and NO3-

The reverse process (incorporation of NO3- or NH4

+ into soil micro-organisms) is called immobilization

Nitrification

•Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites

(NO2−).

•Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3

−).

Soil Organic Nitrogen

Organic (as opposed to mineralized) nitrogen has variable structure (still poorly understood)

Most SON uptake occurs after mineralization of SON to NO3

- or NH4+

Plants may also take up SON directly, or the N can beassimilated by mychorrizal associations

Ammonium fixation by clay minerals

Occurs more in the subsoil than in the topsoil

Ammonium may become ‘fixed,’ or entrapped within Cavities of the crystal structure of vermiculites, micas and smectites

Ammonia volatilization

NH4+ + OH- H2O + NH3(gas)

Occurs more in soils with high pH, especially when dryingand when temperatures are high

Soil colloids (clay and humus) inhibit ammonia volatilizationthrough adsorption

Nitrate leaching

Negatively-charged nitrate ions are not adsorbed by colloids,so they move freely with drainage water

Result: (i) impoverishment of soil N; (ii) environmental problems (eutrophication)especially in heavily irrigated zones with N-fertilizer application or manure

Denitrification

•Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere.

Performed by bacteria in anaerobic conditions. They use nitrates as an alternative to oxygen for the final electron acceptor in the respiration process.

Nitrogen Storage in Soils

• Current levels of nitrogen in soils reflect the accumulation of N in the organic fraction over long periods of time.

• Only about 1.5-3% of the N stored is used on an annual basis.

• Over long time frames N is stable in natural ecosystems (dynamic equilibrium establishedbetween losses and additions)

Soil Phosphorous and Potassium

Why is phosphorus so important?

1. Essential component of ATP(adenosine triphosphate)

* Notice the 5 N and 3 Pin the ATP molecule

Molecular currency of intercellular energy transfer

Used as energy source during photosynthesis and cellular respiration

Consumed by many enzymes in metabolic reactions and during cell division.

2. Incorporated into nucleic acids

Sugar-phosphate backbone

DNA and RNA

Genetic instructions for the development and functioning of all living organisms

3. Phospholipid bilayer

Cell membranes,composed of a phospholipid bilayer,control what goes into and out of a cell

Active transport acrossthe cell membranerequires ATP

Phospholipid

Phospholipid bilayer

The Importance of Phosphorus

P is involved in: P deficiency:

Photosynthesis StuntingNitrogen fixation Thin stemsFlowering Bluish-green leavesFruiting & fruit quality Delayed maturityMaturation Sparse floweringRoot growth Poor seed qualityTissue strength

•Similarly to nitrogen deficiency, the olderleaves are often first affected

•P deficiency is often difficult to diagnoseas visual changes are subtle

The Phosphorus Problem in Soil Fertility

1. The total P content of soils is low.200-2000 kg/ha in uppermost 15 cm (topsoil)

2. Phosphorus compounds found in soils are oftenhighly insoluble

3. When soluble sources are added (fertilizers and manure)they often become fixed into insoluble compounds

• 10-15% of P added is taken up by crop in year of application

• Overfertilization for decades has led to saturation of the P-fixation capacity (large P reserves in N. American soils)

• In contrast, P deficiency is a serious problem in sub-SaharanAfrica (removal repeatedly has exceeded addition)

N, P and K Fertilizer Use in USA

Figure 14.1

Impact of Phosphorus on Environmental Quality

1. P deficiency: Land degradation

Little P is lost in natural ecosystems as P cycles between living biomass and soils

Once cleared for agriculture:(i) Soil erosion loss(ii) Biomass removal• P-supplying capacity decreases, even if total P is sufficient• Nodulation is affected by P-deficiency, thereby promoting

N-deficiency

Most problematic in most highly weathered soils• Warm, moist environments of the tropics• Oxisols & Andisols• Low availability of P when in association with Fe & Al• Lots of P needs to be applied to Andisols (Fig 14.20)

Combined P & N deficiency limits biomass and promotesfurther erosion

Water Quality Degradation due to Excess P (and N)

Point sourcesSewage outflows (phosphates in soaps)Industries

Non-point sourcesRunoff waterEroded sediment from soils in affected watershed

“Too much of a good thing”

The Phosphorus Cycle

Phosphorus in the soil solution•Very low concentrations (0.001 to 1 mg/L)•Roots absorb phosphate ions, HPO4

2- (alkaline soils) and H2PO4

- (acid soils)

Uptake by Roots•Slow diffusion of phosphate ions to root surfaces•Mychorrizal hyphae extend outward several cm from root surface•P can then be incorporated into plant tissues (Fig 14.9)•Soil P replenished by plant residues, leaf litter, and animal waste•Soil microorganisms can temporarily incorporate P into their cells•Some soil P gets tied up in organic matter (storage & future release)

Available P seldom exceeds 0.01% oftotal soil phosphorus

Forms of Soil Phosphorus

Organic phosphorusCalcium-bound phosphorus (alkaline soils)Iron-bound phosphorus (acid soils)Aluminium-bound phosphorus (acid soils)

•Low solubility – not readily available for plant uptake•P is slowly released from each of these types of compounds•Leaching loss is low, but can play a role in eutrophication•Unlike N, P is not generally lost in a gaseous form

Gains and Losses•Losses from plant removal, erosion of P-containing soil particles and dissolved P in surface runoff water•Gains from atmospheric dust are very limited, but a balance isestablished in most natural ecosystems

Figure 14.22

Leaching of P after saturation of fixed pool

Potassium

•The nutrient third most-likely to limit productivity•Present in soils as K+ ion (not in structures of organic compounds)•Soil cation exchange and mineral weathering dominate itsexchange and availability (as opposed to microbiological processes)•Causes no off-site environmental problems

•Igneous rocks are a good source – alkaline soils keep it.

•Activates certain enzymes. •Regulates stomatal opening•Helps achieve a balance between negatively and positively charged ions within plant cells. •Regulates turgor pressure, which helps protect plant cells from disease invasion. •Promotes winter-hardiness and drought-tolerance

Potassium deficiency

•Leaves yellow at tip (chlorosis) and then die (necrosis)The leaves, therefore, appear burnt at the edges and maytear, leaving a ragged edge•White, necrotic spots may appear near leaf edges•Oldest leaves are most affected

The Potassium Cycle

•High concentrations in micas and feldsparsK between 2:1 crystal layers becomes available

•Returned to soil through leaching from leaves and from plant residue decomposition

•Some loss by eroded soil particles and leaching

•Replenishment required in most agroecosystems (1/5 of plant K is typically removed in product). Excess in plants can cause a dietary imbalance in ruminants

Calcium

Vast reserves in calcareous (chalk) soil.

•Calcium is a part of cell walls and regulates cell wall construction.

•Cell walls give plant cells their structural strength.

•Enhances uptake of negatively charged ions such as nitrate, sulfate, borate and molybdate.

•Balances charge from organic anions produced through metabolism by the plant.

•Some enzyme regulation functions.

Magnesium

Reserves in magnesium limestone.

Magnesium is the central element within the chlorophyll molecule. It is an important cofactor the production of ATP, the compound which is the energy transfer tool for the plant.

Sulphur

Found in rocks and organic material.

Sulphur is a part of certain amino acids and all proteins.

It acts as an enzyme activator and coenzyme (compound which is not part of all enzyme, but is needed in close coordination with the enzyme for certain specialized functions to operate correctly).

It is a part of the flavour compounds in mustard and onion family plants.

Boron Boron is important in sugar transport within the plant. It has a role in cell division, and is required for the production of certain amino acids, although it is not a part of any amino acid.

ManganeseManganese is a cofactor in many plant reactions. It is essential for chloroplast production.

CopperSynthesis of some enzymes important in photosynthesis Copper is a component of enzymes involved with photosynthesis.

IronIron is a component of the many enzymes and light energy transferring compounds involved in photosynthesis.

Zinc Zinc is a component of many enzymes. It is essential for plant hormone balance.

Molybdenum

Molybdenum is needed for the reduction of absorbed nitrates into ammonia prior to incorporation into an amino acid.

It performs this function as a part of the enzyme nitrate reductase.

Molybdenum is also essential for nitrogen fixation by nitrogen-fixing bacteria in legumes. Responses of legumes to Molybdenum application are mainly due to the need by these symbiotic bacteria.