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Methods to study the mineral requirements of plants, Practice problems and FAQs

Each one of us has, at least once, told our teachers that we forgot our homework at home and got to hear in return “Did you forget to have food?”. We never forget to have food. Food is the source of energy and nutrition for every living being be it plants, animals or tiny microorganisms. But what is it inside the food that makes it indispensable for the survival of an organism? It’s the nutrients that it carries.

Nutrition is one of the basic requirements of all living creatures. We get nutrients like carbohydrates, proteins, fats, vitamins and minerals from the food we eat. Animals generally feed on complex food items such as meat, bread, milk, fish, etc which have different nutrients in varying concentrations. Hence, we are suggested to have a balanced diet to have all nutrients in the appropriate concentrations.

But have you ever seen a plant being served with a plate of food? We feed our pets regularly but what about the plants that we have at home? Do we feed them? No, we just water them regularly. Does that mean that plants survive only on water and do not require nutrients? No, that is not the case. Plants generally absorb simple inorganic elements and compounds from the soil and synthesise the complex nutrients like carbohydrates, proteins, lipids for themselves. These inorganic elements are known as minerals and the absorption, distribution and metabolism of these minerals in the plant body is known as mineral nutrition. But how do we determine which are the minerals required for plant growth? Or what is the concentration in which a particular mineral is required by a plant? That is what we are going to discuss in this article.

List of contents:

  • What are minerals?
  • Hydroponics
  • Significance
  • Commercial importance
  • Practice problems
  • FAQs

Essential elements

Most of the minerals that the plants require for growth and development are absorbed from the soil through roots. Around 60 different elements have been found present in different plants and 17 out of these elements are considered to be essential for the growth and development of plants. Not all the elements found in a plant are essential for its growth. Conversely, not all elements essential for a plant’s growth are found in them.

Types of essential elements

Hoagland classified essential nutrient elements into two categories based on their average concentration in the plants -

Macronutrients

Elements required in large quantities, that is, more than 10 mMol mg per kilogram of dry matter, are known as macronutrients. These include carbon, nitrogen, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium. Carbon, hydrogen and nitrogen are mostly obtained in the form of carbon dioxide, water and nitrates and the rest are obtained as minerals from the soil.

Micronutrients

Elements required in small quantities, that is, less than 10 mMol per kilogram of dry weight of a plant, are known as micronutrients. These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Transport of minerals

Minerals are transported with the help of water as they get dissolved in the water from soil.



Fig: Mineral Transport Through Water

Methods to study mineral requirements of plants

Hydroponics

In order to understand the number of elements and quantity of elements required by plants, Julius von Sachs, a German botanist (in 1860), found that plants can be grown in the absence of soil.



Fig: Julius von Sachs

The technique to grow plants in the absence of soil, in a nutrient medium, is known as hydroponics. Since then, a number of improved techniques and different formulations of nutrient media have been developed and implemented in order to understand the mineral requirements of plants.



Fig: Plants produced by hydroponics

Requirements for hydroponics

Mineral nutrient salts

All the essential minerals salts are mixed with water in order to prepare a nutrient solution suitable for plant growth. Chemically pure salts are used for this process. Iron is added along with a chelating agent like EDTA (ethylene diamine tetra acetic acid) or DTPA (diethylene triamine penta acetic acid). This is done to avoid precipitation of the iron salts.

Purified water

To prepare a nutrient solution, it is made sure that purified water is used as contaminated water may hamper the plant growth.

Aerating tube

Aeration is required for nutrient circulation and oxygenation in a nutrient medium.

Sample plant

Seedlings are reared directly in these nutrient solutions. For studying the effect of microelements, cotyledons or other structures which store reserve food are also removed as they would serve as the sources of micronutrients. 



Fig: Hydroponics

Methodology

The sample plant is taken and immersed in the nutrient solution. All the essential nutrients required for growth and development are added, substituted and removed in the varying concentration and growth is analysed. Appropriate amounts of nutrients required for growth are noted and an ideal mineral solution is formulated. 



Fig: Method of hydroponics

Arnon and Hoagland (1940) were the first to develop the complete list of all known mineral salts that are required for preparing a growth culture solution. 

Types of hydroponics

Tank system

In this method, the roots of plants are submerged in the prepared nutrient solution and an aerating tube is used to bubble the air.



Fig: Tank system

Nutrient film system

The plants are grown in a tube or trough which is placed on a slightly inclined platform. The roots of plants are submerged in a thin film of nutrient solution which circulates to and from a reservoir to the elevated end of the tube.



Fig: Nutrient Film System

Aeroponics

In this method, roots of plants float in air and the nutrient solution is sprayed from below in order to provide appropriate nutrition.



Fig: Aeroponics

Sand Culture

In this method, sand is mixed with nutrient solution and plants are allowed to grow in sand.



Fig: Sand Culture

Commercial hydroponics

The technique for growing plants commercially using hydroponics was developed by Gericke (1940). Suitable tanks of concrete or metal are built which have a wire netting to support the plants. A device ensures regular replacement of the nutrient medium and popper aeration is provided. pH is monitored periodically and necessary corrections are made. The composition of nutrient media, as given by Arnon and Hoagland (1940) is widely used for the optimum growth of plants.

Significance

Hydroponics is used in discovery of various deficiency symptoms caused due to lack of any specific nutrient. It is also helpful in identification of essential mineral elements. There is no wastage of water when a plant is grown by hydroponics. This method provides protection from soil borne diseases. Steps involved in conventional methods like ploughing, making beds etc. are not required in hydroponics. Land is not required since plants can be grown on terraces or balconies. Off season cultivation is possible with the help of this method.

Production of various vegetables like tomato, seedless cucumber and lettuce is done using hydroponics.



Fig: Crops developed commercially by hydroponics

Practice problems

Q1. In order to perform hydroponics, a scientist was listing down all the requirements he needed. Which of the following components is not required to perform hydroponics?

A. Mineral nutrient salts
B. Aerating tube
C. Purified water
D. Agar agar

Solution: Mineral nutrient salts, aerating tube, purified water and a sample plant are the essential requirements to perform hydroponics. Agar agar is a solidifying agent that is not required as hydroponics involves growing plants in a nutrient solution.

Hence, the correct answer is d.

Q2. Match the following and choose the correct option.

Type of hydroponics

Description

A- Tank system

  1. Roots of plants float in air and the nutrient solution is sprayed from below.

B- Nutrient film system

  1. Roots of plants are submerged in the prepared nutrient solution and an aerating tube is used to bubble the air

C- Aeroponics

  1. Roots of plants are submerged in a nutrient solution circulating to and fro in a thin film.

D- Sand Culture

  1. Sand is mixed with nutrient solution and plants are allowed to grow in sand.

a. A-1, B-2, C-3, D-4
b. A-2, B-3, C-1, D-4
c. A-3, B-4, C-2, D-1
d. A-4, B-1, C-4, D-3

Solution:

Type of hydroponics

Description

A- Tank system

2. Roots of plants are submerged in the prepared nutrient solution and an aerating tube is used to bubble the air

B- Nutrient film system

3. Roots of plants are submerged in a nutrient solution circulating to and fro in the thin film.

C- Aeroponics

1. Roots of plants are floating in air and the nutrient solution is sprayed from below in order to provide appropriate nutrition.

D- Sand Culture

4. Sand is mixed with nutrient solution and plants are allowed to grow in sand.

Hence, the correct match is option b.

Q3. Explain the significance of hydroponics to farming.
Answer: Hydroponics helps in water conservation as the amount of water used is less compared to conventional farming. It also helps to avoid soil borne infections, uses less land area and allows off season cultivation of crops.

Q4. What are the different pieces of information that we can collect about a plant’s mineral requirement by growing it in nutrient media having varying concentrations of different nutrients?
Answer: Experimenting on the composition of the nutrient media with varying concentrations of different nutrients and monitoring the growth of the plants can tell us - 

  • The essentiality of a mineral element
  • Its role in the plant’s physiology
  • Its role in the plant’s structure 
  • Deficiency symptoms of different elements
  • Toxicity level of minerals

FAQs

Q1. Why are plant produce grown by hydroponics considered safer?
Answer:  Plants grown by hydroponics are safer for consumption as the technique eliminates the use of harmful chemicals like fertilisers, pesticides or weedicides. Chemical salts added in hydroponics are highly refined and leave no residues in the food. The crops grown by hydroponics also have higher concentrations of vitamins and minerals and are considered to remain fresh for a longer period of time.

Q2. Are there downsides to hydroponics?
Answer: The technique of hydroponics is not cost effective as infrastructure is expensive. Also, it requires technical expertise which is difficult to achieve for poor farmers.

Q3. Do vegetables grown by hydroponics taste better?
Answer: As crops grown by hydroponics are grown in a controlled environment in greenhouses, the level of light can be adjusted to make the leafy vegetables softer and more oily with larger leaves. As proper concentration of water and mineral elements is maintained, the produce tastes better.

Q4. Why are PVC hydroponic systems avoided?
Answer: Although PVC hydroponic systems are cost effective and do not clog easily, they should be avoided as they become a source for chlorinated toxins.

Related Topics

Criteria for Essentiality 

Macronutrients 

Micronutrients 

ANTHE 2022

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