soil plant relationships

1. DESIGNED AND DEVELOPED UNDER THE AEGIS OF
NAHEP Component-2 Project “Investments In ICAR Leadership In Agricultural Higher Education”
Division of Computer Applications
ICAR-Indian Agricultural Statistics Research Institute

2. Course Details
Course Name Modern Concepts In Crop Production
Unit IV
Lesson 3 Concept Of Soil-plant Relation-Nutrient Availability
In Soils
Disclaimer : Presentations are intended for educational purposes only and do
not replace independent professional judgement. Statement of fact and
opinions expressed are those of the presenter individually and are not the
opinion or position of ICAR-IASRI. ICAR-IASRI does not endorse or approve,
and assumes no responsibility for the content, accuracy or completeness of
the information presented.
1

3. Name Role University
A.MADHAVI LATA Content Creator
Professor Jayashankar Telangana
State Agricultural University ,
Hyderabad
PROF. (DR.) TAPAS KUMAR DAS Course Reviewer
IARI - Indian Agricultural Research
Institute
Created by
2

4. CONCEPT OF SOIL-PLANT RELATION
-NUTRIENT AVAILABILITY IN SOILS

5. Nutrient Availability in Soils:
1. Chemical analysis
2. Movement of nutrients to the root surface (Interception, Mass flow
and Diffusion)
i. Principles of calculations
ii. Concentration of nutrients in the soil solution
iii. Role of mass flow
iv. Role of diffusion - Soil factors
- Plant factors

6. 3. Role of rooting density
4. Nutrient availability and
distribution of water in the soil
5. Shift in mass flow-Diffusion
supply during ontogenesis
6. Intensity/quantity ratio and its
consequences for soil testing

7. Mass flow & root interception & Diffusion

8. 1. Chemical Analysis of Soil:
The most direct way of determining nutrient availability in soils is to
measure the growth response of plants by means of field plot fertilizer
trials.
This is a time consuming procedure. However, the results are not
easily extrapolated from one location to the other.
 In contrast, chemical soil analysis testing is comparatively rapid and
inexpensive procedure for obtaining information on nutrient
availability in soils as a basis for recommending fertilizer application

9. Indicates only potential capacity of soil to supply nutrients.
It does not sufficiently characterize the mobility of nutrients in the
soil.
 It does not provide any other information on the plant factors-
root growth, root induced changes in the rhizosphere that are of
decisive importance for nutrient uptake under field conditions.
2. Movement of Nutrients to the Root Surfaces:
A. Principles of calculations:
The importance of the mobility of nutrients in soils for nutrient
availability was first stressed by Barber (1962). His concept was
based on 3 components -
• Root interception
• Mass flow and
• Diffusion

10. As roots proliferate through the soil, they move into spaces
previously occupied by soil with available nutrients absorbed to clay
surfaces.
Root surfaces come into close contact with nutrients during
displacement process. This displacement process might contribute to
meeting the total mineral nutrient requirement of plants.
Calculations of root interception are based on:
- The amount of available nutrients in the soil volume occupied
by the roots.
- Root volume as percentage of total soil on an average is 1% of
the top soil volume and
- The proportion of the total soil volume occupied by pores (on an
average -50%)
2nd component is mass flow of water and dissolved nutrients to the
root surface ,driven by transpiration of plants (Transpiration
coefficient)

11. Estimates of the quantity of nutrients supplied to plants by mass flow
are based on the –
B) Nutrient concentration in the soil solution
 The amount of water transpired either per unit weight of shoot tissue
(transpiration coefficient) or per hectare of crop.
The contribution of diffusion to the delivery of nutrients to the root
surfaces can not be measured directly, but must be calculated from
difference between total uptake by plants and the contribution by root
interception + mass flow.

12. The concentrations of mineral nutrients in the soil solution varies over
a wide range depending on such factors as:
• Soil moisture
• pH
• Cation exchange capacity (CEC)
• Redox potential
• Quality of soil OM and microbial activity
• Fertilizer application

13. C) Role of Mass flow:
 Calculations of the mass flow to the nutrient supply of field grown
plants rely on detailed data both on concentration of nutrients in the
soil throughout the season and on the water consumption (availability
of moisture) of the plant in question.
 The contribution of mass flow differs not only among mineral
nutrients but also among plant species
D) Role of diffusion:
 In contrast to mass flow, diffusion is an important factor in ion
mobility only with in the immediate vicinity of the root surface and
this is closely related not only to soil conditions (mechanical
impedance) but also to plant factors such as root growth and root
surface area.

14. Soil factors: Under constant conditions, soil moisture affects nutrient
supply via diffusion to the root surface. As the soil water potential
falls, the diffusion coefficient decreases
Role of rooting density:
Although in rooting density long root hairs are important.
 Factors in the uptake of nutrients supplied by diffusion, the
relationship between rooting density and uptake is not linear.
When the rooting density is high, the uptake levels reduces. This is
caused by overlapping of the depletion zones of individual roots and
reflects competition for nutrients.
Nutrient availability and distribution of water in soil:
 Under field conditions, the level of chemically available nutrients is
usually much higher in the top soil than in sub-soil.
Generally rooting density follows a similar pattern and the logarithm of
the rooting density declines linearly with increasing depth.

15. Shift in Mass-Flow – Diffusion supply during Ontogenesis:
The relative proportions of nutrients supplied by mass flow and diffusion
shifts most noticeably for nitrate, the buffering power of which is low in the
soil.
 The concentration is usually high in the top soil early in the growing season
but rapidly declines
Intensity/ quantity ratio and its consequences for soil testing:
The most important soil factors affecting nutrient supply to the plant
roots are-
• Concentration of the soil solution
• Rate of nutrient replenishment
• Amount of readily soluble nutrients in the soil profile.
Nutrient concentration in the soil solution can be characterized by an
intensity factor and the fraction adsorbed to the soil phase or bound to labile
organic compounds by a quantity factor.
• These factors must be considered when chemical analysis is used as a basis
for recommending fertilizer application thereafter as a result of plant uptake.

16. Role of Diffusion:-
Soil Factors:
 At low moisture levels, the soil water potential at the root surface is
much lower than that of the bulk soil.
 When the soil water potential is low, the mechanical impedance of the
soil increases and the root elongation also inhibited which further limits
nutrient supply to the root surface by diffusion.
Plant Factors:
 Root hair formation and root activity is modified by the environmental
factors and also differs typically among plant species.
The spatial availability of nutrients in the soil is not only determined
by the volume of the root hair cylinder but also by the total root length.

17. Ion uptake mechanisms

18. Percentages of nutrient supply to the plant roots from
soil solution
Nutrient Root
interception
Mass flow Diffusion
movement
Percentages in supply
N 1 99 0
P 2 4 94
K 2 20 78
Ca 12 88 0
Mg 27 73 0
S 4 94 2
(Halvin et al ,,2005.)

19. Factors affecting Root Growth and Development
1. Hormonal Control
2. Soil chemical properties
• Mineral nutrient supply
• Calcium/ Total cation ratio and pH
• Aeration
• Low molecular weight organic solutes and ethylene
• Rhizosphere micro organisms
3. Soil physical properties
• Mechanical impedance
• Moisture
• Temperature
• Shoot/root ratio

20. Soil-root interface

21. Hormonal Control
The development of the root system is characterized by a very high adaptability and
involves complex interactions between roots and shoots and roots and their
environment root growth and development are under environmental control.
Extension of main axis and initiation of lateral root formation are regulated primarily
of auxin (IAA) derived from the cytokinin.
 There is increasing evidence that root cap cells are the main sites of ABA synthesis.
ABA has primarily an inhibitory effect on root extension.
Ethylene inhibits extension of main axis and strongly enhances the formation of
lateral
In general root hairs formation is stimulated by a low nutrient supply and /or all
factors which inhibit root extension

22. ii. Soil Chemical Properties
Mineral nutrition Supply:
• The distribution of roots in soils can be modified by the placement of
fertilizers
• Deep placement of fertilizers enhances the plant growth and yield,
particularly, under drought conditions when the water potential of the surface
decreases but ample water is available in the sub-soil.
• Placement of phosphorus fertilizer is a common and effective practice in soils
low in readily soluble P in order to ensure sufficient supply to the roots.
• P deficiency leads to increase in the root/shoot ratio.
• The length of the root hairs and the density of their formation are greatly
affected by N & P.
Calcium/ Total cation Ratio & pH
• The ‘Ca’ requirement for root growth is not a fixed value but rather a function
of both pH and the concentration of other cations, including A1.

23. Aeration:
• Soil aeration is essential for meeting the respiratory requirements of both roots and
soil microorganisms.
• In a dense crop stand, both O2 consumption, CO2 consumption may be high.
• Lack of aeration in sub-soil restricts root growth in soils with a high water table.
Low molecular weight organic solutes and ethylene:
• At higher concentrations, the low molecular weight fractions such as phenolic acid
and short chain fatty acids which often accumulated in poorly aerated soils or water
logged soils.
• There is decomposition of organic matter which is highly detrimental to root
respiration.
• Under water logging, phytotoxic substances, primarily acetic acid and other volatile
fatty acids may accumulate in phytotoxic concentrations which are detrimental to root
elongation and inhibit root and shoot growth even in plant species adapted to water
logging

24. Rhizosphere Microorganism:
 Rhizosphere microorganisms may stimulate or inhibit root growth
depending on the type of microorganisms and environmental
conditions.
 Inhibition may be caused by production of phytotoxins, stimulation by
the mobilization of mineral nutrients.
3. Soil physical Factors:
Mechanical Impedance:
More bulk density means more compacted soils. Soil moisture or soil
water potential directly proportional to mechanical properties.
Inhibition of root extension in compact soils is not necessarily caused
by mechanical impedance only

25. Temperature:
Root growth is often limited by low or high soil temperature. The
temperature optimum varies among species and tends to be lower
for root growth than shoot growth.
Optimum temperatures are usually in the range of 20-25ºC.
Minimum temperature for the root growth of species native to
warm climate is usually 8-15ºC.
The detrimental effects of high temperature on root growth are
probably related to insufficient carbohydrate supply to the root
meristem.

26. Shoot/Root Ratio:
The ratio of shoot or root growth modified by external factors and also changes
during ontogenesis.
 Competition for photosynthates between shoots and roots play an important role.
When supply of minimum nutrients is insufficient, as a result the roots become
dominant sink for photosynthates
And root growth is favored over shoot growth. With increasing N supply, shoot dry
weight increases faster than root dry weight.
Rhizosphere in relation to mineral nutrition:
Acts as a sink for mineral nutrition by mass flow and diffusion
Release H+ and HCO3
- which changes PH
Consume or release O2 which alters Eh (redox potential)

27. Ion concentration in the Rhizosphere:
 There is depletion of P and K in rhizosphere. This results in deflocculating of fine
particles and accumulation of amorphous minerals.
 Weathering of soil minerals in this zone is enhanced. More absorption of water
leads to ion accumulation
The soil which is about 1-4 mm from root axis is called rhizosphere soil (loosely
adhering soil). The closely adhering soil ( ~ 0-2 mm) is rhizoplane soil. The
accumulation of ions is related to transpiration rate
Eg: at high conc. of Ca2+ and So4-2 in soil solution , precipitation of CaSo4 at root
surface, pedotubules (few mm to cm in diameter) was observed in onion plants.
In saline soils due to Nacl the conc. of salts is increased near root surface and so the
osmotic pressure is high- leading to low water absorption. Immediately after
irrigation, the conc. of salts is 3-4 times more than bulk soils.

28. Rhizosphere pH and Eh:-
 Changes in rhizosphere pH are brought about by net excretion of H+ or HCO-3 from
roots which are relatively immobile into bulk soil.
NO3- supply is correlated with a high rate of HCO3- excretion than H+, while NH4+
supply has opposite effect.
The change in pH near roots is up to 1.9 units.
Ex: Lime induced Fe chlorosis can be alleviated by ammoniacal fertilization but
becomes more severe while NO3- fertilizers are applied.
Genotype differences are also seen in the same soil. As the CEC of the plant
increases, the ratio of cation/anion uptake increase.
Hence, the utilization of P from rock phosphate, Fe deficiency or P availability is
related to genotypes.
Ex: In seeding stage, anions are more absorbed so, less concentration in
rhizosphere, because more of HCO3- are excreted. As the plant grow old, the pH of
rhizosphere decreases and cation/anion absorption ratio is increased.

29. 2. N- Fixation & Rhizosphere pH:
 Leguminous plants naturally fixes symbiotically atmospheric N2. The effect is NH4-
nutrition. The cation/anion uptake ratio is effected.
 The rhizosphere is more acidic even by pH 2-3 units less than non-leguminous plants
rhizosphere.
 Due to that P, Fe, Mn absorption is made by leguminous plants. Because of acidity it
requires lime addition. The legumes producing 10t/ha dry matter require 600 kg lime .
 Hence, leguminous pastures and forests requires more liming especially under humid
ecosystem.
3. Redox Potential(Eh)
i. Eh +ve values – Oxidation
ii. Eh –ve values- Reduced state
• Even in well aerated soils most of anaerobic microsites are existing in the rhizosphere due
to microbial and root respiration.
• When higher rates of N2 fixation occur, as in the anaerobic sites, denitrification is taking
place and there will be abundance of microsites

30. • As the soil water content increases, the Eh decreases in submerged conditions, the
values are on –ve side.
• In –ve Eh conditions, many nutrients are being locked up (Fe, Mn & even P).
• Certain plants like rice adapted to water logged conditions by transport of O2 from
shoots to roots and the release of O2 into the rhizosphere.
• The oxidation power of rice roots is due to release of free O2 and enzymatic action on
root surface.

32. Soil total organic carbon (TOC), total nitrogen (TN), total
phosphorus (TP) and pH in bulk and rhizosphere soils of
three subtropical plantations
Tree species Soil TOC (g kg-1) TN (g kg-1) TP (g kg-1) pH
Chinese fir Rhizosphere 42 2.92 0.27 3.83
Bulk 11.7 1.15 0.18 4.17
Masson Pine Rhizosphere 18.27 1.64 0.22 4.12
Bulk 9.69 1.09 0.19 4.14
Chinese
gugertree
Rhizosphere 33.46 2.53 0.25 4.10
Bulk 10.38 1.12 0.19 4.29
X Guan, 2016
Journal of Tropical Forest Science

33. Root Exudates: (Characters and Amount)
Three major components are present in the
root exudates of growing plants
(Rhizodeposition)-
i. Low molecular weight organic compounds
(Free exudates)
ii. High molecular weight organic compounds
iii. Sloughed off cells and tissues
The amount of carbon ranges from 1-30%
Factors affecting the root exudates:
 Age of the plant (more in young plants)
Various forms of stress viz., nutrient
deficiency, mechanical impedance
Microbial activity also enhances the release of
exudates

34. Low molecular organic solutes:
The exudates contain sugars, organic acids, amino acids (dominant),
phenolic compounds depending upon plant species and nutritional status and
microbial activity.
How they affect mobilization and nutritional uptake?
 Solubilization & chelation with metal cations Ex: Fe, Mn
In certain dicots, when Fe is deficient, roots exudates phenolics which
mobilize Fe and Mn.
Solubilization of insoluble organic phosphates (2- keto gluconic acid) to
soluble rhizosphere P
Citrates, maleates make coating on sesquioxides and avoid fixation of P.
Increasing the density of root hairs or of lateral roots mobilize more
compounds.

35. Functions of mucilage:
Protecting roots from desiccation
Acts as lubricant
Improving root-soil contact thereby increasing solubilization of
micronutrients and P
Mainly P solubilization is due to contact exchange or two phase
effects/rhizosphere effect.
Polygalacturonic acid destroys P from insoluble P compounds and
makes P available
Protects meristem from A1 toxicity.
.

36. Some of the plant species are adapted to infertile acid mineral soils
because of cluster of finely divided, highly branched section of roots
like a mat near the soil surface just below the canopy. These roots are
called Proteoid roots.
The contact of root system with the soil is less.
 The rhizosphere of Proteoid roots is more acidic, contain more
reductants and chelating compounds.
 Fe, Mn absorption is more but, P is reduced.
This is compensated by higher rate of uptake of rapidly soluble
fertilizer phosphorous

37. Organic Carbon Supply and Mineral Activity in the Rhizosphere
The number of bacteria in the rhizosphere is relatively more than in
bulk soil depending on the age.
 This invasion is advantageous in associative N fixation.
0.75% carbon in the rhizosphere population is translocated to the roots
and exuded from roots (respiration, exudation and root decomposition).
There may be net decrease in organic carbon content in soil due to
increase in rhizosphere microbial activity.

38. Role of non infecting rhizosphere
microorganisms
• Growth and morphology of roots
• Physiology and development of plants
• Availability of nutrients
• Nutrient uptake process

39. Mycorhizae:
 There are two types of Mycorhizae viz., ecto
and endo.
 Mycorhizae are wide spread association with
roots of most higher plants.
 The fungus wholly and strongly dependent on
higher plants may be or may not be beneficial
to plants.
 Mycelium strands penetrate the surrounding
soils. Endogroup is both inter and intra
cellular in the root cortex.
 The most common one is VAM. This is an
obligate symbiotic and non-host specific
belonging to genus Glomus.
 In certain families, the fungus is absent in
certain families such as Cruciferae and
Chenopodiaceae.

41. Infection and Energetics:
The rate of infection is effected mainly by soil factors such as-
• Soil pH-slightly acidic pH
• Temperature- 20 to 25ºC
• Low NH4
+supply
• High nitrate supply
• Optimum or medium P
P uptake : P uptake enhances due to VAM infection. But, there is variation among
host plants and VAM species.
Other microbial effects:-
Other than P, root infection with ecto-mycorhiza enhances the uptake of rate of K
and micronutrients viz., Cu and Zn.
VAM infections may influence the carbonization of roots by other symbiotic
microorganisms.
 Increase the tolerance of roots to soil borne pathogens such as nematodes and fungi.

42. Mycorrhizal Dependency:
 In general, stimulation of growth and P uptake by VAM is limited in plant species
(grasses and cereals) which may have extensive, highly branched root systems and
long root hairs.
 In contrast, the responses are high in species with coarse root systems that are not
highly branched.
 In legumes, due to more P supply, the nodulation and N fixation is enhanced
Adaptability of Plants to Adverse Chemical Conditions :
• Soil chemical properties such as soil pH, salinity and nutrient availability determine
the distribution of natural vegetation.
• In many instances, adaption to adverse chemical soil conditions requires tolerance to
excessive levels of mineral elements such as Al and Mg in acid mineral soils, Mn in
water logged soils and sodium chloride in saline soils.
• Thus, multiple stress tolerance is often necessary for adaption.

43. Acid mineral soils- Major Constraints:
 In acid mineral soils, a variety of individual chemical constraints and interactions between
then limit plant growth.
At low pH (<4.0), it is not the low pH per se that limit the growth, but toxicity and / or
deficiency of mineral elements.
The main factors are excessive levels of free and exchangeable Al.
In some instances, an excessive levels of Mn and deficiency of P, Ca, Mg are also
involved.
Less frequently. Levels of S, K and Micronutrients, Mo, Zn and Cu may remain deficient.
The N level is generally low.
Normal fertility of these soils is very low.
In order for plants to adapt to acid mineral soils, both high tolerance to Al and Mn and
highly efficient utilization of the mineral nutrients are required.
Liming the soil up to 30 cm improves the soil, but amelioration below 30 cm is also
important

44. • Mechanisms of adaptation to Acid Mineral Soils:
 Plants adapted to acid mineral soils have a variety of mechanisms to
cope with adverse chemical soil factors.
 Among annual root crop species, Cassava is known for high
tolerance to acid soils. Other acid soil tolerant crops are cowpea,
peanut, potato.
 Non-tolerant species are Maize, Soybean and wheat
Three major mechanisms involved in Al tolerance are-
1. Exclusion from uptake (Excluder plants)
2. Inactivation in the roots (Excluder/ Includer plants)
3. Accumulation in the shoots (include plants)
The third mechanism exists in highly Al tolerant natural vegetation.
The effect is more at reproductive stage than at vegetative stage.

45. Waterlogged and flooded Soils:
 In waterlogged soils, air is displaced from the
pore space either to different depths of soil or to
the top soil.
 Flooded or submerged soils are permanently
below the water table or at least under water for
several months every year.
 Most species not adapted to water logging
develop injury symptoms sequentially over a
period of several days.
• Wilting
• Epinasty
• Decrease in water permeability in the roots
• Accumulation of ethylene in shoots
• A decrease or cessation of shoot extension
• Senescence of lower leaves.

46. Mechanisms of Adaptation:
 Adaptation can be achieved by avoidance of the stress factor or stress
tolerance.
In general, avoidance is the principal mechanism and the tolerance
plays an additional role in adaptation to an O2 deficit in soil.
The root porosity is important to ODR (oxygen diffusion rate) under
flooded conditions.
Changes in root anatomy is closely related with changes in root
morphology.

47. • After water logging many old roots die. But numerous adventitious with well
developed aerenchyma emerge from the base of the stem and grow to a limited extent
in anaerobic soils.
• Aerenchyma provides low diffusion resistance pathway for O2 transport from
roots
• Diminishes respiratory demand of basal root zones. Ethylene production is partly
responsible to aerenchyma.

48. Metabolic Adaptations:
• Intolerant species suffer from accelerated glycolysis and ethanol
production.
• In contrast, tolerant species avoid this acceleration and also under go
metabolic switch from ethanol to malate production.
Iron Deficiency:
• In Fe inefficient Spp. mobilization of Fe in the rhizosphere can be
brought about by both non-specific and specific mechanisms.
• Root induced decrease in pH
• Release of organic acids by roots
• Release of photosynthates by roots as substrates to microorganisms
(siderophores)
• In the specific mechanisms- use of synthetic Fe- Chelates

49. Mn deficiency is not a major constraint in alkaline soils.
When Fe becomes deficient, Mn is more available. Resistance to lime induced Fe
chlorosis is significantly controlled by genes.
Differences in Zn deficiency can be attributed to genetic variability. High P effect
must always be achieved for calcareous soils but not for sodic soils.
 Sodic soils will have higher extractable P mechanisms of adaptation to salinity.
Plants grown in saline soils face two problems:
• High salt concentration in soil solution
• High concentration of potentially toxic ions such as cl- and Na+ (or) unfavorable
combinations of salt ions (a high Na+/Ca 2+) ratio

50. Osmotic adjustment:
 Due to salinity, the osmotic adjustment is achieved by decrease in tissue
water content.
 In genotypes in which salt inclusion is the predominant strategy, osmotic
adjustment is achieved by the accumulation of salts in vacuoles of leaf cells.
• Compartmentation and compatible osmation:
 Certain enzymes, such as membrane bound adenosine tri phosphates, the
roots are either activated or inhibited in vitro by high salt concentration
depending on the tolerance of intact plants.
 Osmatic adjustment must protect their enzymes in the cytoplasm and in the
chloroplasts. This requires accumulation of compatible organic solutes.
• Ex: Proline in Gramineae

51. Salt excretion and leaf drop:
Salt glands secrete large quantities of salts to
the leaf surface which can be washed off by rain
water or dew.
Salt excretion from leaves-
Ex: Kallar grass (Diplachne Fusca), a forage
grass.
 Even in moderate salt tolerant tomato,
deposition of salts in leaf hairs is another
mechanism of preventing excessive
accumulation in photosynthetically active leaf
cells, providing the opportunity for salt loss by
leaching or by mechanical means.

52. REFERENCES
• Barber SA (1962) A diffusion and mass-flow concept of soil nutrient
availability. Soil Sci 93: 39–49
• Halvin, L., Tisdale, S. L., Beaton, J. D. and Nelson, W. L. (2005). Soil
Fertility and Fertilizers: An Introductory to Nutrient Management, 7th
edn. New Jersey: Pearson Education.
• Robert, E.S and Mary, E.L. 2002. ABA, ethylene and control of shoot
and root growth under water stress.Journal of Experimental
Botany.53(366).Pp:33-37
• Soil Water, and Plant Relationships. Kansas State University
Agricultural Experiment Station and Cooperative Extension Service
• X Guan, SL Wang and WD Zhang. (2016), Availability Of N And P In
The Rhizosphere Of Three Subtropical Species. Journal of Tropical
Forest Science.Vol. pp. 159-166.

53. THANK YOU