🧫 Tolerance limit in Plant

LECTURE 22

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Tolerance limit in Plant Nutrient for various fertilizers

Category Limit

Straight fertilizers

containing <20% plant

nutrients

Straight fertilizers

containing >20% plant

nutrients

: 0.1 Unit of nutrient

: 0.2 Unit of nutrient

Calcium ammonium nitrate : 0.3 Unit of nutrient

Complex/ Mixed fertilizers : 0.5 unit for each and maximum

of 2.5 % for all nutrients

Fertilizer Movement Control Order

The Fertilizer Movement Order (F.M.O.) was promulgated by

Government of India in April 1973 to ensure an equitable distribution of

fertilizers in various States. According to the fertilizer movement order, no

person or agency can export chemical fertilizers from any State. However,

Food Corporation of India, Warehousing Corporation of India and Indian

Potash Limited; materials like Rock phosphate, bone meal (both raw and

steamed) and zinc sulphate are exempted from the movement restriction.

Agency responsible for Enforcement of F.C.O

The Controller of Fertilizers for India, usually a Joint Secretary to the

Government of India (Ministry of Agriculture) is responsible for the

enforcement of F.C.O. throughout the country.

Electrical Conductivity of the soil saturation extract

Measurement of EC of the soil saturation extract is essential for the

assessment of saline soil for the plant growth.

EC (dS m [-1] )

<2 - Salinity effects mostly negligible

2-4 - Yields of very sensitive crops may be restricted

4-8 - Yields of many crops restricted

8-16 - Only tolerant crops yield satisfactorily

>16 - Only a few tolerant crops yield satisfactorily

Concentration of water soluble boron

The determination of water-soluble boron concentration is also

another criteria for characterization of saline soils. The critical limits of boron

concentration for the plant growth are given below.

Boron

concentration

<0.7 - Crops can grow (safe)

10 (19/26)

0.7-1.5 - Marginal

(ppm) >1.5 - Unsafe

Reclamation of Saline Soils

In saline soils, reclamation consists mainly in removing the excess salts.

This can be done either

• By scraping the salts from the surface (or)

• Washing them down into lower layers beyond the root zone preferably

completely out of the solum (or)

• By growing salt tolerant crops (or) by a combination of two (or) more of

these methods

Scraping helps to remove salts that have formed an encrustation on

the surface, but it is never very helpful in complete reclamation. Substantial

quantities of soluble salts are still present in the soil body and hinder plant

growth.

Salt tolerant crops

High salt tolerant crops - Rice, sugarcane, Sesbania, oats

Medium salt tolerant crops - Castor, cotton, sorghum, cumbu

Low salt tolerant crops - Pulses, pea, sunnhemp, sesamum

The growing of salt tolerant plants with a view to remove salts is also

not a practical proposition. Although these plants remove substantial

quantities of salts from the soil, comparatively larger quantities are still left

behind. Salt formation is a continuous process; hence, the reclamation is

never complete

LEACHING REQUIREMENT (LR) It may be defined as

The fraction of the irrigation water that must be leached through the

LR=

Where

LR - Leaching requirement in percentage

Ddw - Depth of drainage water in inches

Diw - Depth of irrigation water in inches

ECiw - EC of irrigation water (dSm [-1] )

ECdw - EC of drainage water (dSm [-1] )

If the soil is not free draining, artificial drains are opened (or) tile

drains laid underground to help to wash out the salts.

ALKALI SOIL (sodic/ Solonetz )

Alkali (or) sodic soil is defined as a soil having a conductivity of the

saturation extract less than 4 dS m [-1] and an ESP of > 15. The pH is usually

between 8.5 and 10.0. Formerly these soils were called “ black alkali soils ”

Genesis/ origin

It is evident that soil colloids adsorb and retain cations on their

charged surfaces. Cation adsorption occurs due to electrical charges at the

surface of the soil colloids. While, adsorbed cations are combined

chemically with the soil colloids, they may be replaced by other cations that

occur in soil solution. The reaction of cation in solution that replaces an

adsorbed cation is called as cation exchange and is expressed as cmol (p [+] )

kg [-1] .

Calcium and magnesium are the principal cations found in the soil

solution and on the exchange complex of normal soils in arid regions. When 10 (20/26)

excess soluble salts accumulate in these soils, sodium frequently becomes

the dominant cation in the soil solution. In arid regions as the solution

becomes concentrated through evaporation or water absorption by plants,

the Ca [2+] and Mg [2+] are precipitated as CaSO4, CaCO3 and MgCO3 with a

corresponding increase in sodium concentration. When the Na [+]

concentration is more than 15% of the total cations a part of the original

exchangeable Ca [2+] and Mg [2+ ] replaced by sodium resulting in alkali soils.

Na [+]

Clay Ca [2+] + 2 Na [+ ] ↔ Clay + Ca [2+]

Na [+]

Though the reaction is reversible, Ca [2+ ] are removed in drainage water

as soon as they formed. Hence, the reaction proceeds in one direction from

left to right only. The process whereby a normal soil is converted into an

alkali soil is known as “ alkalization ”.

Characteristics various methods are available to characterize

• A direct determination of exchangeable sodium

Exchangeable sodium = Total sodium - Soluble sodium

• The soil pH also gives an indication of soil alkalinity indirectly. An

increase in pH reading of 1.0 or more, with change in moisture content

from a low to high value has itself been found useful in some area for

detecting alakali conditions.

The higher the ESP, the higher is the soil pH.

• Sodium Adsorption Ratio (SAR)

The US Salinity Laboratory

developed the concept of

SAR =

SAR to define the equilibrium between

soluble and exchangeable cations

(Na [+], Ca [2+], Mg [2+] are concentrations in saturation extract in me L [-1] )

The value of SAR can be used for the determination of exchangeable

ESP =

The following regression equation is also used to work out ESP

Y = 0.0673 + 0.035 X

Where Y - indicates ESP and X - indicates SAR

Soils having SAR value greater than 13 are considered as sodic soils.

Impact of soil sodicity

• Dispersion of soil colloids leads to development of compact soil

• Due to compactness of soil, aeration, hydraulic conductivity, drainage

and microbial activity are reduced

• High sodicity caused by Na2CO3 increases soil pH

• High hydroxyl (OH [-] ) ion concentration has direct detrimental effect on

plants.

• Excess of Na [+] induces the deficiencies of Ca [2+] and Mg [2+]

• High pH in alkali soil decreases the availability of many plant nutrients

like P, Ca, N, Mg, Fe, Cu, and Zn.

10 (22/26)

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Summary Cheat Sheet

Key Recall Points

• Tolerance limit in Plant is exam-relevant for SSAC122 and objective questions in soil science.
• Use soil-test based interpretation with focus on pH, CEC, and nutrient availability.
• Apply the 4R principle: right source, right rate, right time, and right method.

Exam Traps

• Do not mix up soil fertility concepts with fertilizer quantity alone.
• Numerical and term-based questions often test definitions, units, and threshold values.
• In problem-solving, interpretation must follow soil reaction, crop stage, and management context.

References

3 sources • [1] [2] [3]

[1]

ICAR e-Course: Soil Chemistry, Soil Fertility and Nutrient Management

Official

[2]

Brady and Weil, The Nature and Properties of Soils

Book

[3]

Indian Council of Agricultural Research

Official