This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Reclamation of Saline and Alkaline Lands”.
1. Which among the following salt is also called black alkali?
Explanation: Na2CO3 is present in excess in black alkali soils. It is known as a black alkali because it dissolves some organic constituents of soil which appears black in solution with it. The patches of black stain are generally spotted on the ground.
2. Which is the least harmful salt for agricultural soil?
Explanation: NaCl is found in excess in white-alkali soils. In this, salinity is caused by soluble salts other than alkali salts. The reaction between NaCl and soil (S) produces Na(S) and Cl– ion and the soluble products are leached away from the soil surface by drainage water.
3. The plant roots act as a semi-permeable membrane.
Explanation: The plant roots contain pure water on one side of the membrane and highly concentrated salt solution on the other side. By osmosis, the pure water within the roots will start flowing out of the roots towards the salt solution until the pressure becomes equal.
4. Alkali soils are reclaimed by ___________
a) leaching of soil
b) using limestone as a soil amendment
c) using gypsum as a soil amendment
d) provision of drainage
Explanation: Alkali soils are best reclaimed by cationic exchange i.e. replacement of alkali from soil colloids by calcium ions. Application of gypsum in the soil reduces alkalinity to a great extent and makes the soil fertile. Good drainage leaches away the by-product of the reaction.
5. If the salt concentration of the irrigation water ranges from 0.5 – 2 g/L, then what is the restriction on its use?
a) No restriction on its use
b) It should be used with appropriate water management practices
c) It is not generally advised for use unless consulted with specialists
d) No dependency on the salt concentration
Explanation: If the salt concentration is less than 0.5 g/L, then there is no salinization risk and no restriction on the use of irrigation water. If the concentration is more than 2 g/L, there is high risk and is not generally advised for use. If the concentration is in between there is slight to moderate risk and it should be used with appropriate water management practices.
6. If the salt concentration of the soil water (saturation extract) in milli-mhos/cm is in between 4.5 – 9, then what is the salinity of the soil?
b) Slightly saline
c) Medium saline
d) Highly saline
Explanation: The salt concentration in the water extracted from a saturated soil defines the salinity of the soil.
|S.NO||The salt concentration of soil water||Salinity|
|1.||0 – 4.5 milli-mhos/cm||Non-saline|
|2.||4.5 – 9.0 milli-mhos/cm||Slightly saline|
|3.||9.0 – 18 milli-mhos/cm||Medium saline|
|4.||>18 milli-mhos/cm||Highly saline|
7. Solonchak soil is the other name for ____________
a) alkali Soils
b) sodic Soils
c) saline-alkali soils
d) saline soils
Explanation: Solonchak soil is also called white alkali or saline soils. They have an excessive concentration of natural soluble salts and low exchangeable sodium.
8. A soil sample has an Exchangeable sodium percentage (ESP) of 16%, its electrical conductivity is 3.2 Milli-Mohs/cm and a pH of 9.5. How is the soil classified?
a) Saline soil
b) Saline-alkaline soil
c) Alkaline soil
d) Non-saline soil
Explanation: The three classes of salty soil are based on these three properties pH range, ESP, and conductivity of saturated extract. Alkali soil has pH range in between 8.5 and 10, ESP greater than 15, and conductivity of saturated extract less than 4 milli-mhos/cm at 25°C.
9. What are the three classes of salty soil?
a) Saline soil, Saline-alkali soil, Non-saline soil
b) Saline soil, Alkali soil, Non-saline soil
c) Saline soil, Alkali soil, Saline-alkali soil
d) Alkali soil, Non-saline soil, Saline-alkali soil
Explanation: The three classes are –
i. Saline or white alkali or solonchak soil- High soluble salts and low ESP
ii. Alkali or Sodic or black alkali soil – Low salt content but high ESP
iii. Saline-alkali soils – High salt content and High ESP.
10. What is Solonetz soil?
a) White alkali soil
b) Black alkali soil
c) Highly saline soil
d) Non-saline soil
Explanation: Solonetz soil is another name for sodic soil or alkali soil or black alkali soil. These are formed by excess accumulation of alkalies such as Na, K, etc. Such soils have low salt content but high Exchangeable sodium.
11. Estimate the leaching requirement when electrical conductivity (EC) value of a saturated extract of soil is 10 m mho/cm at 25% reduction in the yield of the crop. The EC of irrigation water is 1.5 m mho/cm.
Explanation: The given values are:
E.C value of saturated soil = 10 milli-mhos/cm and E.C value of irrigation water = 1.2 milli-mhos/cm.
The EC of leaching water is assumed to be twice the EC value of saturation soil extract i.e. = 2 x 10 = 20 milli-mhos/cm
L.R = EC of irrigation water / EC of leaching water = 1.5 / 20 x 100%
L.R = 7.5%.
12. The higher the concentration of sodium present in the irrigation water, the lower is the risk.
Explanation: Saline soil is the land affected by efflorescence and if it continues for a longer period, a base exchange reaction sets up leading to unproductive and ill-aerated land. This also reduces the osmotic activity of plants and the plants will die due to lack of oxygen and water. The higher the salt concentration of the irrigation water, the greater the risk of salinization and higher is the risk.
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