Chemistry

Key unit competency:

 To be able to analyze the components of quality Fertilizers and their benefits, effects of misuse and dangers associated with the substandard fertilizers. 

Learning objectives 

At the end of this unit , students will be able to:

 • State the major constituents of fertilizers; 

• Identify the characteristics of good fertilizer; 

• Briefly describe the manufacture of fertilizers; 

• State the advantages and disadvantages of  using fertilizers; 

• Interpret the labels on the fertilizer containers; 

• Classify the fertilizers in terms of composition.

Introductory Activity

A plot of land has been divided into two parts and in both irish potatoes have been cultivated by two cultivators. 

One of them harvested 2000 kg of irish potatoes of big size and the other harvested 50kg of irish potatoes of small size. 

Given that on both plots of  land, the following work has been done at the same time 

• Cultivation,
• planting
• Hoeing (or weeding)
• Harvesting 

Suggest reason(s) which caused the difference in the harvest.

The total population in Rwanda was estimated at 11.3 million people in 2016, according to the latest census figures. Looking back, in the year of 1960, Rwanda had a population of 2.9 million people. Rwanda’s population will shoot to 18.2 million people by 2050 at an average growth rate of 2.3 %, the United Nations Population Fund (UNFPA) has projected. These statistics show that the population of Rwanda is going on increasing but as we know the area of Rwanda is not increasing. That is why Fertilizers and other agricultural techniques are needed for the population of Rwanda to be capable of feeding itself and even feed some other population in the region. 

3.1. Types of Fertilizers

Activity 3.1 

1.  a. What is the role of fertilizers

b. Name any examples of Fertilizers you have ever heard 

2. Using this book or any other book or internet, read and analyse the content about the types of fertilizers and make a summary to be presented to the class. 

A fertilizer is any material, organic or inorganic, that is used to supply nutrients to the soil. 

There exist types of Fertilizers: 

1. Natural Fertilizers(or organic Fertilizers) 

2. Artificial Fertilizers (or chemical Fertilizers) 

3.1.1. Natural Fertilizers 

The name organic fertilizer refers to materials used as fertilizer that occur regularly in nature, usually as a by-product or end product of a naturally occurring process. They are made from remains of dead plants, wastes from animals or they can be minerals. Examples include manures and minerals. Manure is an organic material that is used to fertilize land. 

1. Farmyard manure: animal manure that consists of feces 

2. Green manure: is a term used to describe specific plant or crop varieties that are grown and turned into the soil to improve its overall quality. 

3. Compost manure: is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. 

4. Minerals: Mineral mined powdered limestone, rock phosphate and sodium nitrate, are inorganic compounds which are energetically intensive to harvest and are approved for usage in organic agriculture in minimal amount. 

3.1.2. Artificial Fertilizers 

They are fertilizers which are chemically synthesized which contain one or more of the major elements required by plants for good growth.

Examples: Urea, N.P.K, ammonium dihydrogen phosphate, NH4 (H2PO4),… 

Checking up 3.1 

Give the two main types of Fertilizers and discuss the pros and cons of using one or another  type of fertilizer.

3.2. Components of a fertilizer

Activity 3.2 

1. Name any nutrients you know that plants need in order to grow 

2. Using this book, any other books or internet do a research and find out 

a. The types of nutrients and give any three nutrients in each category that plants need for their growth and classify the nutrients depending on how plants need them

b. Any two roles for each nutrients for the plant growth.

First it is important to understand that all industrial Fertilizers, by convention, regardless of type and specific use, have something called a NPK ratio.The NPK ratio will be prominently labeled on the package and indicates the percentage of major (or primary) nutrients the fertilizer contains. Example: Urea is a fertilizer with an NPKratio of 46-00-00. 

The nutrients of plants are classified into three types namely: 

• Major nutrients

 • Secondary nutrients 

• Micronutrients 

3.2.1. The major nutrients 

The major nutrients for soil are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients usually are lacking or insuffiscient in the soil because plants consume these nutrients in large amounts for their growth and survival.

The letter N represents the actual nitrogen content in the fertilizer by percentage mass while P and K represent the amount of oxide in the form of phosphorus (V) oxide (P2O5) and potassium oxide (K2O) respectively.

3.2.2. Secondary nutrients 

Now, in the category of secondary nutrients, are calcium (Ca), magnesium (Mg), and Sulphur (S). As, these nutrients are generally enough in the soil, so fertilization is not always needed. Also, large amounts of Calcium are added when lime is applied to acidic soils. In fact, Sulphur is usually found in sufficient amounts from the slow decomposition of soil. 

3.2.3. Micronutrients 

In fact, micronutrients are those elements essential for plant growth which are needed but in only very small (micro) quantities. These elements are even called minor elements or trace elements. The common micro nutrients are boron (B), copper (Cu), iron (Fe), chlorine (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). In fact, recycling organic matter such as grass clippings and tree leaves is an excellent way of providing micro nutrients to growing plants.

3.3. The manufacture of Fertilizers

Activity 3.3 

1. Write reactions for the formation of the following compounds

a. Ammonium sulphate

b. Potassium sulphate

c. Ammonium nitrate

d. Urea 

e. Ammonium phosphate 

2. Using this book or any other book or internet, read and analyse the content about the manufacture of the following Fertilizers and make a summary to be presented to the class; 

Ammonium sulphate, potassium sulphate, ammonium nitrate, urea, and phosphates 

3. Rwanda has a resources that can be used to produce an industrial fertilizer; name that resource.

supply phosphorus to the plants. These minerals are, therefore, converted into soluble materials, by reacting them with sulphuric acid, or phosphoric acid or nitric acid. 

Characteristics of a good fertilizer

A good fertilizer should have the following characteristics:

It should contain the required nutrients, in such a form that they can be assimilated by the plants. It should be cheap. 

It should be soluble in water. 

It should be stable, so that it may be available for a long time for the growing plant. 

It should not be injurious to the plants. 

It should be able to correct the acidity of the soil.

Not pollutant

3.4. Disadvantages of the use of organic and inorganic Fertilizers

Activity 3.4 

“The use of fertilizers is a harm to humanity”, yes or not. Explain

3.4.1. Organic Fertilizers 

The use of organic fertilizer may have many advantages but also it may have some disadvantages 

a.  Advantages 

1. The manures add organic matter (called humus) to the soil which restores the soil texture for better retention of water and for aeration of soil. For example, organic matter present in the manures increases the water holding capacity in sandy soils and drainage in clay soil.

 2. The organic matter of manures provides food for the soil organisms (decomposers such as bacteria, fungi, etc.) which help in making nutrients available to plants. 

3. Nutrient release: slow and consistent at a natural rate that plants are able to use. No danger of over concentration of any element, since microbes must break down the material. 

4. Trace minerals: typically present in a broad range, providing more balanced nutrition to the plant. 

5. They will not burn: safe for all plants with no danger of burning due to salt concentration. 

6. Long lasting: does not leach out since the organic matter binds to the soil particles where the roots have access to it. 

7. Fewer applications required: once a healthy soil condition is reached, it is easier to maintain that level with less work

8. Controlled growth: does not over-stimulate to exceptional growth which can cause problems and more work. 

b. Disadvantages

 1. Many organic products produce inconsistent results. 

2. The level of nutrients present in organic fertilizer is often low. 

3. The time of their preparation is too long. 

4. Eutrophication 

3.4.2. Inorganic Fertilizers 

The use of inorganic fertilizers may have many advantages but also it may have some disadvantages

a. Advantages 

1. Chemical Fertilizers are made with synthetic ingredients designed to stimulate plant growth. 

2. Commercial chemical Fertilizers have the advantage of predictability and reliability 

3. Formulations are blended with accuracy and you can buy different blends for different types of plants; commercial formulated Fertilizers allow you to know exactly which nutrients you’re giving your plants, rather than guessing at the composition of organic formulas 

b. Disadvantages 

1. They can burn plants 

2. They require a specific timetable of application and watering because of fast release of nutrients 

3. Groundwater 

• Increased nitrate levels increase the risks of blue baby syndrome, a rare form of anaemia which affects babies below 6 months of age. The cause is the oxidation by nitrite ions of Fe2+ in haemoglobin to Fe3+. The oxidized hemoglobin cannot bind oxygen, and the baby turns blue from lack of oxygen. Conditions in the digestive tracks of young children are more favourable to the bacteria which reduce nitrates to nitrites than those in adults. 

• Another hazard of chemical Fertilizers is that carcinogenic nitrosoamines (yellow oil substance) may be formed in the human digestive track by the conversion of nitrate into nitrite. The nitrite produced in the stomach it combines with HCl to produce nitrous acid. Nitrous acid can react with any secondary amine in foods to form nitrosoamines and the reaction of nitrite with amino acids. 

4. Repeated use or excess use of the same fertilizer producing acidic ions (NH₄+). Example of such a fertilizer is (NH4)2SO₄.

5. Repeated use or excess use of the same fertilizer producing basic ions. Example of such a fertilizer is CaCO_3. 

_{6. Warm temperat}ures and high rain fall: Cations such as Ca⁺⁺, Mg⁺⁺, K⁺ which are essential to living organisms, are leached (dissolved) from the soil profile, leaving behind more stable materials rich in Fe and Al oxides. This natural weathering process makes soils acid.

• Man-made processes also contribute significantly to soil acidity. For example, Sulphur dioxide (SO₂) and nitrogen oxides (NOx) released primarily by industrial activities react with water to form acid rain, which acidifies soils, particularly forest soils with. 

• Organic acids from plants during decomposition; 

• CO₂ from root respiration and microbial respiration. 

Effects of acid soil

• Major effects of extremes in pH levels include gaps in nutrient availability and the presence of high concentrations of minerals that are harmful to plants. In very alkaline soil, certain micronutrients such as zinc and copper become chemically unavailable to plants. In very acidic soil, macronutrients such as calcium, magnesium and phosphorous are not absorbed while others reach toxic levels, 

• Acid soil, particularly in the subsurface, will also restrict root access to water and nutrients. 

• In addition to affecting how nutrients are dispensed to growing plants, pH levels also influence microorganism activity that contributes to the decomposition of organic materials. A neutral pH is ideal for microbial action that produces chemical changes in soil, making nitrogen, sulfur and phosphorus more available. A pH that is either too high or too low may also interfere with the effectiveness of pesticides by changing their basic composition or weakening their ability to kill unwanted insects. 

Plant growth and most soil processes, including nutrient availability and microbial activity, are favoured by a soil pH range of 5.5 – 8. Example: The optimal pH range for most plants is between 5.5 and 7.0. The optimal pH range for some plants is between is given in the table below.

For soils the pH should be maintained at above 5.5 in the topsoil and 4.8 in the subsurface. 

Eutrophication

The undesirable overgrowth of vegetation caused by high concentration of plants nutrients (Nitrogen and Phosphorous) in bodies of water (lakes, rivers,...)

As consequence, water plants (e.g: water hyacinth: amarebe) grow more vigorously and this prevents the sun light from reaching the water and stops photosynthesis of aquatic plants which provide oxygen in the water to animals needed then animals die, deposits of organic matter on the bottom of the lake build up.

When lake water is enriched with nutrients (e.g.: nitrates and phosphates), algal flourish, and produce an algae bloom, a green scum with an unpleasant smell. When algal die they are decomposed by aerobic bacteria. When the oxygen content falls too low to support aerobic bacteria, anaerobic bacteria take over. They convert the dead matter into unpleasant-smelling decay products and debris which falls to the bottom. Gradually, a layer of dead plant material builds up on the bottom of the lake. The lowering of the oxygen concentration leads to the death of aquatic animals (fish, crabs,…….)

Checking up 3.4 

1. Ammonia itself can be used as a fertilizer but has some disadvantages. Explain the disadvantages of using ammonia as a fertilizer. 

2. Give any two advantages of the use of 

a. Natural Fertilizers 

c. Artificial Fertilizers 

3. Give any two causes of acid soils

3.5. Dangers of the use of substandard Fertilizers

Activity 3.5 

Using books or internet find out the dangers of substandard fertilizers

Sub-standard fertilizer means any fertilizer which does not conform to the required NPK ratio. 

Example: A fertilizer may be labelled 16-00-00, while the real NPK ratio is for example 25-00-00, 10-00-05, etc 

Using these Fertilizers can lead to:

• Soil pollution (basic soil or acidic soil) due to accumulation of ions which are acidic or basic • Poor growth of plants 

• Poor harvest 

• Eutrophication 

• Fertilizer burn: leaf scorch resulting from over-fertilization, usually referring to excess nitrogen salts. Fertilizer burn is the result of desiccation of plant tissues due to osmotic stress, creating a state of hypertonicity. 

In order to reduce the effects of substandard fertilizers different measure can be taken; 

• Standardization of the fertilizer before use

• Production of fertilizers in Rwanda, as this will help us to choose good minerals (where necessary) in producing fertilizers

 • Use of chemical fertilizers with coated pellets so that nutrients are released slowly 

• Regular watering

You provided with the following

1. A Solution prepared by mixing 5.0 g of a sample of ammonium sulphate fertilizer which were warmed with sodium hydroxide and the ammonia evolved was absorbed in 100 cm3 of 0.5moldm-3 sulphuric acid 

2. 1M sodium hydroxide 

Procedure

a. Fill the burette with solution of sodium hydroxide

b. Pipette 20 cm3 of solutions of the prepared solution in (1), in conical flask. Add 2-3 drops of methyl orange indicator.

c. Titrate this solution with sodium hydroxide from the burette until the indicator changes colour (indicator changes from pink to yellow).

d. Record the results in the table.