MODERN CONCEPT OF NUTRIENT AVAILABILITY

The modern concept of nutrient availability is a multifaceted process influenced by various internal and external factors. In the context of specific nutrients, nitrogen exists in complex organic forms in the soil, eventually breaking down into ammonium compounds. These compounds can replace basic cations in the soil, and plants predominantly absorb nitrogen in the form of ammonium and nitrate ions. However, factors such as excess minerals in the soil or leaching can impact nitrogen availability. Phosphorus, on the other hand, originates from both inorganic and organic sources and becomes accessible to plants in the form of orthophosphate ions after microorganism-mediated decomposition. Soil pH, temperature, and interactions with other elements affect phosphorus availability. Excessive potassium levels can be lost through leaching, and certain clays can fix potassium ions, influencing their availability. Calcium and magnesium are released from primary minerals in the soil, affecting soil acidity as they exchange with hydrogen ions. Sulphur primarily exists in organic forms and is converted into sulphides and then sulphates by microorganisms, making them available for plant uptake. Micronutrients are found in primary minerals and can form various compounds in the soil; their solubility and availability vary with soil pH. External factors include soil composition, pH levels, cation exchange capacity, light, temperature, nutrient interactions, and excess minerals, while internal factors encompass plant growth, aging, mycorrhizal associations, and root system development. Understanding these complex interactions is crucial for optimizing nutrient availability in agriculture, ensuring healthy plant growth, and ultimately contributing to human nutrition and food security.

1. es !
E
LCOM

y

2. PRESENTATION TITLE ©

MODERN CONCEPT OF
NUTRIENT AVAILABILITY

Presented and Submitted by
Mr. ANIKET SUNIL GAIKWAD
(Reg. No.: Ph.D. 2021/22)

3. COURSE TITLE
ADVANCES IN SOIL FERTILITY

Presented and Submitted by
Mr. ANIKET SUNIL GAIKWAD
(Reg. No.: Ph.D. 2021/22)

Submitted to
Department of Soil Science and Agricultural Chemistry,
Post Graduate Institute, M. P. K. V., Rahuri

4. MODERN CONCEPT OF
NUTRIENT AVAILABILITY

5. > Nitrogen basically occurs in the soil in complex organic forms like proteins which are
decomposed by proteinase enzymes produced by micro-organisms to amino acids that
are further decomposed to ammonium compounds. Ammonium ions may replace basic
cations from the clay and humic micelle.

> Ammonium compounds are oxidized to nitrites by nitrosamines bacteria, which are
further oxidized to nitrates by nitrobacteria. Plants take up nitrogen from the soil in the
form of ammonium and nitrate ions. Ammonia is volatilized from hot, alkaline
waterlogged soil when nitrates are reduced to nitrites, which are further reduced to

oxides of nitrogen and nitrogen gas. Nitrates are leached down if added to paddy fields.

‘Anka Sen Gaia

6. O
ll

[| PHOSPHOROUS | aie
H H
O10

> Phosphorus originates in the soil, both in the inorganic form i.e. apatite

mineral, as well as in the organic form ie. phytin, inositol, nucleic acids etc.

> Microorganisms decompose these organic phosphorus compounds to
release the phosphorus in the simple inorganic form i.e. orthophosphate,
HPO, ions. Carbon dioxide is produced from the decomposition of organic

matter and respiration of roots and micro-organisms.

‘Anka Sen Gaia

7. > It reacts with water to form carbonic acid, the reacts with tricalcium phosphate to
form water soluble mono-calcium phosphate:

CaPO), + 4

— CalH,PO), + Ca(HCO),
Monocalcium Phosphate

> Water soluble mono-calcium phosphate reacts with aluminum and ferric ions in
strongly acidic soil to form water insoluble aluminum and ferric hydroxyl phosphate.

2A1"="+ Ca(H,PO), + 2H.OH === 241 — OH + Car + 2H
Monocalcium Phosphate Son
Aluminium Hydroxy Phosphate

_
Are" + CalH,PO), + 2HOH ==> 2Fe CT OH + Carr + 2H
Monocalcium Phosphate NH,

> Water soluble mono-calcium phosphate reacts with calcium ions and calcium
carbonate to form insoluble tricalcium phosphate in strongly alkaline soils.

Ca(HPO), + CaCO, === Caio,
Monocalcium Phosphate 1

8. > Potassium basically occurs in primary minerals like orthoclase, mica etc. in the soil.
These mineral decompose to release the potassium contained them in the water

soluble form.

> When excessive amounts of potassium ions are present in the soil solution, they may
be lost by leaching. Crops assimilate potassium in excessive amounts but the crop
yield does not increase in proportion to the amount of potassium absorbed by the
root. This is known as luxury consumption of potassium. Potassium ions are fixed by

certain clays like illite. ‘Aka Sei Gad

9. > Calcium and Magnesium occur in primary minerals like calcite, dolomite, hornblende etc.

Which are decomposed by organic acids to release calcium and magnesium in the form of ions.

> Usually calcium and magnesium ions are held by clay and humic micelle is exchangeable

cations which are replaced by hydrogen ions as shown below:

c

Ca” Mg" Ca

MICELLE ] My

i
+CAHCO)) + MECO),
Soluble ia

> These soluble basic bicarbonates are washed down by high rainfall; in humid region the above

reversible reaction proceeds in the forward direction, which means hydrogen ions gradually

replace calcium and magnesium ions form the clay and humic micelle. Ultimately soils become

acidic in reaction.

10. > Sulphur mainly occurs in the organic form i.e. Sulphur containing proteins
which are decomposed by micro-organisms to sulphides, that are further

oxidized to sulphate.

> The soil solution contain HSO; , SO;, SO, ion.

‘Anka Sen Gaia

11. > The micronutrients occur in primary minerals which decompose during the formation
of soils resulting in the formation of oxide and sulphides and of iron, manganese and
zinc.

> Boron, molybdenum and chlorine occur as borate and borosilicate, molybdate and
chloride respectively. Some of the micronutrients also occur in the organic form in the
surface soil. Micronutrients usually occur in the soil solution in the ionic form.

> Micronutrients except molybdenum are extremely soluble in a strongly acidic soil, and
thus become toxic to crops. Boron has been leached form strongly acidic sandy soil.

> Iron, Manganese, zinc and copper combine with organic compounds to form chelates
which protect them from certain harmful reactions, e.g. precipitation of water soluble
phosphate by ferric ions. At the same time, they remain available for the growth of the

crop. ‘Anka Sen Gaia

12. FACTORS AFFECTING AVAILABILITY OF NUTRIENTS

Availability of mineral nutrients is affected by both endogenous and exogenous factors.
“ EXTERNAL FACTORS INCLUDE :

L

=

7

nun

Soil

Soil hydrogen ion concentration (pH)

Type of the soil and cation exchange capacity
Light

Temperature

Interaction of elements

Excess minerals in the soil

“ INTERNAL FACTORS INCLUDE :

1:

2.
3;
4

Growth and morpho-physiological status
Aging
Mycorrhiza

Root system ‘Aka Sei Gad

13. 1. Soil

Soil is a complex substrate- physically, chemically and biologically. The
size of soil particles and the cation exchange capacity of soil provides a
reservoir for water and nutrients. Soil pH has also large influence on
availability of mineral elements to plants.

2. Hydrogen ion concentration (pH)

Change in hydrogen ion concentration (pH) of the soil solution affects
the availability of nutrients to plants. In general decrease in pH of soil
solution decreases the absorption of anions for example: Boron is taken as
an undissociated acid, H3BO3 ions. It is absorbed at lower pH.

14. 3. Hydrogen ion concentration (pH)

> Organic soil originates from dead plants, animals and micro-
organisms that soil microbes have decomposed to various degrees.

> The negative surface charges of organic particles result from the
dissociation of H-ions from carboxylic and phenolic acid groups
present in this component of soil.

> Mineral cations such as NH,* and K* are adsorbed to the negative
surfaces of soil. This cation adsorbed is an important factor in soil
fertility as these are not easily leached when soil is in filtered by water
and thus provide a nutrient reserve available to plant roots.

4. Light

It has no direct affect but indirectly by photosynthesis and
transpiration, influences salt absorption. aa

15. 5. Oxygen

The active salt absorption is inhibited by the absence of oxygen.

6. Temperature

Absorption of mineral salt is affected by change in temperature. In
general, an increase in temperature results increase in the absorption of salts
up to a certain optimum level. At very high temperature the absorption is
considerably inhibited. The inhibition might be due to denaturation of
proteins which are directly or indirectly involved in mineral salt absorption. _

7. Interaction of nutrients with each other

The absorption of one ion is affected by the presence of other ions in the
medium. For e.g., absorption of monovalent is affected by the absorption of
divalent or polyvalent cations. There is competition for common binding
sites on carriers between several ions (K+, Rb etc.).

‘Kak Seal Galva

16. 1. Growth

Active cell division, elongation and developmental processes promote
the absorption of mineral salts. The size of soil particles and the cation
exchange capacity of soil provides a reservoir for water and nutrients.

2. Aging

As the root matures it increases the surface area which is favorable for
salt absorption, but due to heavy suberization the mineral salt uptake is
greatly reduced.

17. 3. Mycorrhizae symbiosis

Mycorrhizae symbiosis facilitate nutrient uptake by roots Mycorrhizae
(singular mycorrhiza, from the Greek words for “fungus” and “root”) are
not unusual; in fact, they are widespread under natural conditions. Much of
the world’s vegetation appears to have roots associated with mycorrhizal
fungi: 83% of dicots, 79% of monocots, and all gymnosperms regularly form
mycorrhizal associations.

>There are two major classes of mycorrhizal fungi:
1. Ectotrophic mycorrhizae
2. Vesicular-arbuscular mycorrhizae

18. 1. Ectotrophic mycorrhiza fungi

Ectotrophic mycorrhizal fungi typically show a thick sheath, or
“mantle,” of fungal mycelium around the roots, and some of the mycelium
penetrates between the cortical cells. The cortical cells themselves are not
penetrated by the fungal hyphae but instead are surrounded by a network of
hyphae called the Hartig net.

2. Vesicular-arbuscular mycorrhizae fungi

Unlike the ectotrophic mycorrhizal fungi, vesicular arbuscular
mycorrhizal fungi do not produce a compact mantle of fungal mycelium
around the root. Instead, the hyphae grow in a less dense arrangement, both
within the root itself and extending outward from the root into the
surrounding soil.

19. 4. Root system

The ability of plants to obtain both water and mineral nutrients from
the soil is related to their capacity to develop an extensive root system. The
annual production of roots in natural ecosystems may easily surpass that of
shoots, so in many respects, the aboveground portions of a plant represent
only “the tip of an iceberg.” Plant roots may grow continuously throughout
the year.

Their proliferation, however, depends on the availability of water and
minerals in the immediate microenvironment surrounding the root, the so-
called rhizosphere. If the rhizosphere is poor in nutrients or too dry, root
growth is slow. As rhizosphere conditions improve, root growth increases.

‘Anka Sen Gaia

20. THANK YOU

‘Ane Sel