Soil water plays an important role in regulating nutrient uptake by plants. The level of soil moisture directly influences how much nutrients are absorbed. Most nutrients are readily taken up by plants when they are dissolved in the soil solution. In submerged soils, certain nutrients like iron and manganese become more available due to reduction, while others like zinc and copper decrease due to precipitation. Overall, sufficient soil moisture is necessary for optimal nutrient availability and absorption by plant roots.
2. Soil water is a great regulator of physical, chemical and biological activities of soil and plays an
important role for nutrients uptake. Soil moisture level directly influences the nutrient
absorption. Inadequate nutrient availability greatly reduces yield loss of crop.
Plant need 17 essential elements for their growth and completion of life cycle.
Classification of essentioal elements
• Basic NUTRIENTS : C,H,O
• MACRO NUTRIENTS : a. PRIMARY NUTRIENTS :: N,P,K
• b. SECONDARY NUTRIENTS :: Ca, Mg, S
• MICRO NUTRIENTS : Fe, Zn , Cu , Mn ,Mo, Cl ,B
• BENEFICIAL NUTRIENTS : Se, Al, Co, Na, Si, Va
• ULTRA MICRO NUTRIENT : Mo
SOIL WATER AND NUTRIENTS
3. Forms of mineral elements absorbed by plants:
i) Absorbed as single nutrient ion
Potassium K+
Calcium Ca2+
Magnesium Mg2+
Iron Fe2+
Manganese Mn2+
Copper Cu2+
Zinc Zn2+
Chlorine Cl-
Silicon Si4+
Cobalt Co2+
Sodium Na+
ii) Absorbed in a combined form
Nitrogen Ammonium (NH4
+) and
Nitrate (NO3
-)
Phosphorus H2PO4
-, HPO4
=
Sulphur SO4
=
Boron H3BO3, H2BO3
-, HBO3
2- , BO3
3-
Molybdenum MoO4
= (Molybdate)
Carbon CO2
Hydrogen H2O
4. Transport of nutrients to root
surfaceTwo important theories explain nutrient availability to the plants.
a. SOIL SOLUTION THEORY
b. CONTACT EXCHANGE THEORY
SOIL SOLUTION THEORY : soil nutrients are dissolved in water and are transported to root
surfaces by both mass flow and diffusion.
MASS FLOW is movement of nutrient ions and salts along with moving water. The
movement of nutrients reaching to root is thus dependent on the rate of water flow and
concentrations of nutrients. In dry soil there is no movement of nutrients .Nutrients move
by mass flow : N , Ca ,Cu ,B ,Mn ,Mg ,S. Nutrients supplied primarily by mass flow are
considered mobile nutrients. e.g. N, S, B.
6. DIFFUSION occurs when there is concentrations
gradient of nutrients between the root surface and
surrounding of soil solution. The ions move from
the region of high concentration to the region of
low concentrations. Diffusion delivers
appreciable amounts of phosphorus, potassium,
zinc, and iron to the root surface.
Nutrients supplied primarily by diffusion are
considered immobile nutrients e.g. P, K
7. CONTACT EXCHANGE THEORY: A close contact between root surfaces and soil
colloids allows a direct exchange of H+ released from the plant roots with cations from
soil colloids i.e root interception. The importance of contact exchange in nutrient
transport is less than with soil solution movement.
The quantity of nutrients intercepted by roots depends on the soil concentration of
nutrients and volume of soil displaced by root system.
Root interception is responsible for an appreciable amount of calcium uptake, and
some amounts of magnesium, zinc and manganese.
8. Nutrients are absorbed by the plants in two ways :
ACTIVE ABSORPTION: When sufficient amount of water is not present
in the soil and soil solution containing low concentration of nutrients
compared to plant sap ,plant absorbs nutrients by expending energy.
PASSIVE ABSORBTION : When sufficient amount of water is present
in the soil, plant absorbs nutrients along with transpiration stream
without the use of energy.
9. Mobility of nutrients in the soils has
considerable influence on availability of
nutrients and presents of moisture in the
soil. Here, two process are important.
In the presence of sufficient water -
Movement of nutrients ions to the
absorbing root surface
In the absence of sufficient water- Root
reaching the area where nutrients are
available
Mobility of nutrients in the soil
BASED ON THE MOBILITY IN THE
SOIL
NUTRIENTS GROUPED AS,
MOBILE: NO3
-,SO4
=,BO3
3-,Mn2+,Cl-
LESS MOBILE:
Cu2+,K+,Ca2+,Mg2+,NH4
+
IMMOBILE: H2PO4
-, HPO4
= , Zn2+
11. SOIL WATER INFLUENCES NUTRIENTS AVAILABLITY
Though essential nutrients are present in the soil, the
ABSORPTION OF NUTRIENTS DEPENDS ON MOISTURE
CONTENT OF SOIL.
The nutrients are readily taken by the plant when they are present
in soil solution. Therefore, soil moisture is an important factor
influencing the absorption of nutrient. Water helps in transport of
nutrients to the root surface.
12. N-For availability of any nutrients mineralization is necessary i.c.
Transformation of organic form to inorganic form. The optimum soil
moisture for mineralization is 50-70% water holding capacity of soils. On
mineralization organic matter releases ammonium and nitrate and these
are available to the plants.
Mineralization involved 3 steps, namely
amminization, ammonification, nitrification.
13. Aminization:It is defined as the process by which the hydrolytic decomposition of
proteins from combined nitrogenous compounds as well as release of amines and
amino acids takes place by heterotrophic soil microorganisms.
Proteins R-NH2 + CO2
Ammonification: the reduction of amines to ammoniacal compounds by some other
group of heterotrophic microorganisms is known as ammonification.
R-NH2 + H-OH NH3 + NR-OH + ENERGY
H2O
NH4
+ + OH-
Nitrification: Oxidation of ammonical nitrogen to nitrate form of nitrogen.
NH4
+ NO2
- -N NO3
- -N
nitrosomonas nitrobacter
14. The amount of soil moisture influence soil microorganisms responsible
for carrying out the mineralization process.
The rate of mineralization is highest – at field capacity.
The rate of mineralization is decreased – in water logged condition (
because, this process proceed up to NH4 –N)
Net amount of mineralized in water logged condition is greater.
Between15 bars and air dryness, N mineralization goes on
decreasing.
15. There is some adverse effect of soil moisture which reduce
availability of nitrogen.
Problem : LEACHING LOSS since urea is soluble, leaching loss occur
specially under low land condition.
DENITRIFICATION In waterlogged soil, the rate of denitrification is
increased by impeding the diffusion of O2 to
sites of microbial activity.
VOLATALIZATION This process usually occur in alkaline and
calcareous soil. But it may occur in waterlogged soil.
Solution: mixing urea with soil and incubating for 24-48 hrs. under lowland condition in
acid and neutral soil. Use of slow release N fertilizers, mud balls, S-coated urea, lac
coated urea, nitrification inhibitors etc.
16. P- Phosphorus is present in the soil in organic and inorganic form. P in soil solution is in
the form of primary and secondary orthophosphate (H2PO4
-, HPO4
= ). Organic and inorganic P
is in equilibrium with P in soil solution.
Organic P Solution P Insoluble P
Phosphate is generally less mobile than N,substantial mobility is observed in soil Phosphate.
The solubility of P present in leachates of different soil is averaged to 1.4 mg/L.
The solubility of P present in runoff water from high P soil is 0.11mg/L.
Soil P release to solution is controlled by a combination of Al, Ca, Fe complexes.
Phosphate availability in submerged soil increases within two weeks after submergence.
17. K- It is present as soluble cation in solution. Its concentration depends on type of clay,
WATER CONTENT and the amount of exchangeable K.
Fixed, exchangeable and water soluble K are in dynamic equilibrium in solution.
Fixed K Exchange K Water soluble K
K availability marginally increase in submerge soils. Flooding increases ferrous and
manganous ion which displace K from exchange complex. A portion of displaced K is lost due
to leaching.
18. Ca- the important sources of calcium are dolomite,calcite,apatite and
calcium feldspers.
CaCO3 + CO2 + H2O Ca(HCO3) leaching of Ca2+
takes place
The exchangeable and solution forms of Ca are in dynamic equilibrium as
follows:
soil solutionCa2+ exchangeable Ca2+
The higher concentration of calcium in solution is adsorbed on clay complex.
Exchangeable calcium is released into soil solution when its concentration is
low in soil solution.
Losses of Ca through leaching depend on the amount of Ca present in the soil,
amount of rainfall and texture of the soil.
19. Mg- is available to plants due to weathering of biotite,dolomite,olivine,serpentine.
The released magnesium may be absorbed by plants and microorganisms, lost in drainage or
precipitated as secondary mineral.
Magnesium in soil solution and exchangeable form are in dynamic equilibrium, similar to calcium.
Mn- primary minerals of manganese: oxides & to some extent carbonates and silicates
Secondary minersals: pyrolusite & manganite
Manganese in the soil is generally considered to exist in divalent (Mn2+), trivalent(Mn2O3 ), and
tetravalent(MnO2) forms in equilibrium.
Exch. Mn2+ MnO2
Mn2O3
It is available more in submerged soil due to reduction of insoluble manganic compound to soluble
manganous compound.
20. S-Due to presence of oxidized zone in rhizosphere of rice
roots ,S is available to rice. Sulphates are reduced to
hydrogen sulphide under reduced condition which is toxic to
plant and microorganism.
Hydrogen sulphide reacts with heavy metal Fe,Zn,Cu and
forms insoluble sulphides.
21. Zn- availability of zinc is reduced under flooded condition due
to precipitation as Zn(OH)2 or as ZnCO3.
Cu-Availability of Cu is also reduced due to formation of
soluble copper sulphide.
Fe-its availability is higher in submerged soils due to
reduction of insoluble ferric compounds to soluble ferrous
compounds. organic matter native or added, improves
availability of Fe. Iron deficiency occurs in calcareous soil and
soil with high P.
22. B- Tourmaline ,a fluorite bromosilicate is the important boron
containing mineral. Its resistant to weathering, hence only small fraction
of boron is available to plants from this source.
Mineralization of organic matter releases boron and irrigation water
supply it.
More than 72 % of applied water soluble boron is converted into water
insoluble at pH above 7.5.
Mo- Mo is present in the soil MoO4
=
Higher soil moisture increases molybdenum availability due to decrease
in ferric ion which is responsible for adsorption.
Cl- Cl is present in soil as chloride (Cl-),
It is highly mobile in soil.
23. WATER–NUTRIENT INTERACTION IN SUBMERGED
SOILThe physical, chemical and biological properties of soil are altered
due to submergence.
Rice, paragrass are grown in waterlogged soil.
24. The availability of water to plants is more in submerged
soil.
Oxygen entry is slow trough water. Diffusion of molecular
oxygen occur.
CO2 released during decomposition of organic matter is
dissolved in water to form carbonic acid. Aerobic and
anaerobic layer developed.
PHYSICAL ENVIRONMENT
25. Fe-availability is higher due to reduction in ferric to ferrous compound.
Mn-higher availability,same as Fe
N-supply and availability is more
P-availability increases due to hydrolysis of Fe & Al phosphates ,increase in pH.
K-marginally increases, Fe2+ and Mn2+ displace K+
and that ion leached out.
Zn- availability is reduced due to precipitation
Cu- availability is reduced due to formation of insoluble copper sulphide.
S- Sulphates are reduced to hydrogen sulphide under reduced condition that is toxic to
plants and microorganisms. presence of oxidized zone in rhizosphere , it is available.
CHEMICAL ENVIRONMENT
26. Azolla, blue green algae, denitrifying bacteria and
rhizospere bacteria are important organism.
Actinomycetes and fungi are absent in submerged
soil.
Non-symbiotic bacteria present in anaerobic soil and
those in the rhizosphere of rice roots fix atmospheric
nitrogen
BIOLOGICAL ENVIRONMENT
27. CONCLUSION
PLANTS REQUIRE 17 ELEMENTS FOR GROWTH AND COMPLETION OF LIFE CYCLE.
SOIL WATER HAS MAJOR INFLUENCE ON ABSORPTION AND TRANSPORTATION OF
NUTRIENTS TO ROOT SURFACE.
BIOLOGICAL ACTIVITIES INCLUDING MINERALISATION INCREASES. MOST OF THE
NUTRIENTS ARE ABSORPED BY PLANTS.
IN THE PREESENCE OF WATER IN THEIR AVAILABLE FORM LIKE NO3
-,SO4
=,BO3
3-
,Mn2+,Cl-
IN PRESENCE OF EXCESS WATER NUTRIENTS LOSS ALSO OCCUR BY LEACHING
LOSS,DENITRIFICATION ETC.
IN SUBMERGED CONDITION SOME NUTRIENTS BECOME TOXIC LIKE SULPHUR
REDUCES TO H2S WHICH IS TOXIC TO PLANTS .
