Active fraction: lends structural stability, enhanced infiltration, resistance to erosion and easeof tillage (relatively high C/N)

0 40 80

0 .0

0 .4

0 .8

1 .2

0 2 0 4 0 6 0

0

50 0 0

1 00 0 0

1 50 0 0

0 2 0 4 0 6 0

1 0

3 0

0

2 0

4 0

m.eq. p.p.m. p.p.m.

POTASSIUM NITROGEN PHOSPHORUS

0 20 40 60

0

4

8CALCIUM

0 20 40 60

0

4

8

12BORON ALUMINIUM

0 1 0 2 0 3 0 4 0 5 00

1

2

3

4

5

m.eq. p.p.m. m.eq.

Depth (cm) Depth (cm) Depth (cm)

Depth (cm) Depth (cm) Depth (cm)

1st/late 2nd.Early 2nd.Deforested

Figure 5.14 Average soil potassium, nitrogen, phosphorus, calcium, boron and aluminium levels at Centro de Estudios Ambientales Tambito. Note the higher nutrient concentrations in primary/late secondary forest.

Soil Nutrient Concentrations vs. Successional Stage (Tambito, Cauca, Colombia)

Alkaline and Saline Soils

•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 pH>8.5,there are higher concentrations of carbonate and bicarbonateanions (due to dissolution of certain minerals)

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

Nutrient deficiencies in saline soils

•Fe deficiency common because its 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 acid soils).

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)

•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., rapeseed 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

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

Nitrogen Forms

Reduced

NH4+ N2 N2O NO

(Ammonia) (molecular N) (nitrous oxide) (nitric oxide)

Oxidized

2NO2- NO2- NO3

-

(nitrite) (nitrogen

dioxide)(nitrate)

Nitrogen•An essential component of amino acids, and therefore all proteins.

•An essential component of nucleic acids, and therefore needed for all cell division and reproduction.

•Enzymes are specialized proteins, and serve to lower energy requirements to perform many tasks inside plants.

Nitrogen is contained in all enzymes essential for all plant functions.

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).

Nitrification

•Ammonia can be taken up directly by plants, but most of the ammonia produced by decay is converted into nitrates.

•Nitrifying bacteria•Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites

(NO2−).

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

−).

•Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification — converting some of their organic nitrogen to nitrites and nitrates.

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.

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

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.

Nitrogen

• Most nutrient problems in plants are caused by only three elements: N, P and K

• More time and money are spent on the management of Nitrogen than any other element.\

• Nitrogen is an essential component of protein and due to its relative scarcity is sought after by most mammals.

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 3% of the N stored is used on an annual basis.

• Over long time frames N is stable as the losses don’t tend to exceed the additions.

Nitrogen Storage in Soils

• Soils with high levels of OM usually contain the highest levels of N.

• Requires conditions where the accumulation of plant residue is very high, while limiting plant decay.

• With the exception of swampy areas, these conditions are found in the wet regions and in relatively wet semi-aridregions (eg. in some Mollisols)

Phosphorous and Potassium

• In order of importance N => P => K.

• So phosphorous comes next.

• Why is phosphorus so important?– Nothing grows without it (plants or animals.).– Essential component of ATP (adnosine

triphosphate).

NPK Fertilizer Changes.

Figure 14.1

Phosphorous• ATP is synthesized through respiration and

photosynthesis.• Drives most energy-requiring biochemical

reactions.• Aside from photosynthesis, phosphorus is

essential for nitrogen fixation, flowering, fruiting (seed production), maturation, root growth, and structural tissue.

Phosphorus in Soil Fertility• Major limiting factor.• Leads of a variety of problems.• 3 major issues

1. Due to relative importance, levels are usually low.

2. Most phosphorous found in soils is unavailable.

3. When it’s added, it often gets fixed.

Phosphorous Fixation• Only a small proportion ever gets used (10-

15%).

• Rest gets fixed by the solid fraction of the soil or is lost.

• Not a lot is found in plant material.

• Careful management is required as losses are environmentally detrimental.

EUTROPHICATION

Figure 14.22

Figure 14.14

Potassium

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.

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.

Manganese

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

Copper

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

Iron

Iron 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.