Fertiliser 1 - Notes

Plants needs nutrients, and these can be divided into two main categories: Humans need substances that are generally called:
major elements

In both these cases, there are other catogeries.

Diagram Summary


Introduction

1. What is meant by the term "fertiliser?
It is a natural or synthetic chemical substance or mixture used to enrich soil so as to promote plant growth.

2. What are the chemical elements that plants need for proper growth and development are generally referred to as?
plant nutrients

3. What are the three essential nutrients for plants?
C, H and O
Carbon, Hydrogen and Oxygen

4. What are the three primary nutrients for plants?
N, P and K
Nitrogen, Phosphorus and Potassium

Plants

Essential Nutrients C, H and O

1. Why are plants called autotrophic organisms?
They can use simple inorganic Carbon, Hydrogen and Oxygen to synthesize complex organic glucose by photosynthesis
They manufacture their own food

2. What are the sources of these essential nutrients for plants
atmosphere, CO2 and rain

3. By what method does a plant obtain these essential nutrients?
C, H and O are absorbed through leaves and roots

4. What are the functions of C, H and O in plants?
C, H and O are used to synthesize glucose and other organic matter

Humans

Major Nutrients C, H, O and N

1. What are the major nutrients for animals (humans)?
Carbon, Hydrogen, Oxygen and Nitrogen

2. Why are animals called heterotrophs?
They cannot synthesize the own organic compounds.
Animals have to eat plants to obtain Carbon, Hydrogen and Oxygen in the form of organic compounds.

3. What is the source for each of the major elements?
atmosphere
water
animals
plants
(food)

Plants

Primary Nutrients (N, P and K) and Fertilisers

Plants need N, P and K. But as a crop is harvested, the soil is depleted of these nutrients. They need to be replaced, before new crops can be grown. The N, P and K are replaced by adding them through fertilisers.

Fertilisers are natural or synthetic chemical substances or mixtures that are used to enrich soil so as to promote plant growth.
Plants cannot just absorb the nitrogen (N), phosphorous (P) and potassium (K) in their raw form. They are actually absorbed as soluble compounds:

N2 is a very stable molecule and is not soluble in water. Hence it cannot be absorbed by the plants. It has to be changed into a nitrate such as ammonium nitrate.
P (phosphorous) is very reactive, and has to be changed into a safer, soluble compound.
K (potassium) is very reactive in water, actually exploding in water. It has to be changed into a safer, soluble salt in order to be be absorbed.


Nitrogen: in the form of Nitrates
Phosphorus: in the form of Phosphates
Potassium: in the form of Potassium salts

The reason why plants need N and P and K are as follows:
N (nitrogen):
• promotes green leaf growth (good for lettuce farmers)
• forms proteins and chlorophyll
• for rapid growth

P (phosphorus):
• contributes to strong roots, flower and fruit development
(good for fruit farmers, carrots etc)

K (potassium):
• contributes to stem and root growth and the synthesis of proteins
(good for sugar-cane farmers)
• protects against cold and dry weather

So now we know why fertilisers are important. Where do we get fertilisers from?
There are two ways to obtain fertilisers :
Natural or Manufactured

1. Natural Fertilisers (N, P, K)

These are obtained from natural sources.

• N (nitrogen compounds) from guano (bird droppings)
• P (phosphorous compounds) from bone meal (crushed bones from abbertoirs)
• K (potassium salts) from German mines

Nitrogen fixation also plays a role.
Nitrogen fixation is the process where the very stable and insoluble N2 gas molecules are changed into soluble compounds.
This can be done by lightning.
When there is lightning, the immense lightning energy is able to change the very stable N2 molecules in the atmosphere into soluble nitrates. They are washed to the ground by rain, and the plants can then absord these soluble compounds.
Nitrogen fixation also occurs by the action of enzymes that some plants have.

2. Manufactured Fertilisers (N, P, K)

These are synthesized through many chemical processes.
Since they are manufactured, these fertilisers can be tailor-made to suit specific types of plants.
The actual ratio of N:P:K is carefully controlled. As an example, one type of fertiliser may be designed to have more N whilst another can have more P.


They are eventually sold in bags, that are carefully labelled in what is called the N:P:K ratio. This is the ratio of N:P:K in smallest numbers, and these three numbers indicate the relative amounts of N,P and K in the fertiliser in the bag.


The farmer must choose carefully. Remember :

N (nitrogen):
• promotes green leaf growth (good for lettuce farmers)
• forms proteins and chlorophyll
• for rapid growth

P (phosphorus):
• contributes to strong roots, flower and fruit development
(good for fruit farmers, carrots etc)

K (potassium):
• contributes to stem and root growth and the synthesis of proteins
(good for sugar-cane farmers)
• protects against cold and dry weather

Study this bag:


This bag has a lot of potassium. So if the farmer plants sugar cane (stems), he should buy this bag, since potassium promotes stem growth.

What does the (40) mean?
The total mass of this bag is 50kg.
The number in brackets (40) means that 40% of the bag is actually fertiliser. The rest is called filler or ballast.
The actual mass of the fertiliser is :
40% of 50kg
= 40/100 x 50kg
= 20kg

What are the actual industrial steps involved in the manufacture of fertilisers?

• Fractional Distillation of Liquid Air (N2)
• SASOL - from coal and steam / (Chloralkali Industry) (H2)
• Haber Process (NH3)
• Ostwald Process (HNO3)
• Contact Process (H2SO4)

Over - fertilisation

This causes eutrophication.
Eutrophication is the name given to what happens when the farmer uses too much fertiliser.

• The excess fertilizer flows into rivers.
• This promotes algae growth (algal bloom).
• When this extra algae decomposes by the action of bacteria, this process consumes the dissolved oxygen from the water.
• Other water organisms such as fish, then also die as a result of oxygen deprivation.
• Water that has a very low concentration of oxygen is said to be hypoxic.


There would be many problems if the surrounding rivers, dams and lakes are subjected to eutrophication. These water bodies would look very green, and unpleasant.

• Fisherman would have no fish to catch, hence no income
• Holiday resorts with water recreational areas, that are nearby the farm, would not have many guests due to poor and dangerous water conditions (unattractive) - job losses
• Water for drinking, washing and cooking is spoilt, creating a health risk.


How can eutrophication be avoided?
• The farmer must calculate and measure out correct amounts of fertiliser to use.
• Use fertiliser sparingly
• The farmer must use advanced modern, computerised equipment to ensure correct amounts are actually delivered to the ground
• Avoid irrigation just after applying fertiliser
• Do not use aeroplanes to drop fertlisers since this can spread too far
• Do not apply fertiliser when it is very windy
• Ensure that water from crop fields do not run into rivers and dams.
• Redirect water from crop fields into specially made reservoirs, away from rivers and dams.


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