Fertilisers

Fertilisers are substances that are widely used in agriculture to increase crop productivity. There are two types of fertilisers - natural and artificial.

Plants use inorganic mineral ions, especially nitrates, for growth. A lot of agriculture takes place on land used repeatedly. When the plants are removed for consumption, mineral ions are removed too. The plant doesn't die and decay, so the ions aren't returned to the soil. Fertilisers are essential to maintain the maximum yield of crops. Fertilisers add the essential nutrients such as nitrogen, phosphorus and potassium back to the earth to be used again.

Types of fertilisers

Two types of fertilisers can be added to the soil - natural and artificial.

Natural (organic) fertilisers

Natural fertilisers consist of dead and decomposing organic matter of plants and animals, including animal waste such as manure. Unlike artificial fertilisers, organic fertilisers will usually consist of a single ingredient. There are three main types of organic fertilisers.

Plant-based fertilisers

Plant-based fertilisers will break down faster than other types of organic fertilisers, such as manure (animal-based). Plant-based fertilisers are more important for soil conditioning. Soil conditioners will improve the soil's overall fertility, ability to drain water moisture retention.

Examples of plant-based fertilisers include:

• Alfalfa meal - alfalfa meal is a product of fermented alfalfa seeds. Alfalfa is an important fertiliser for blooming plants. It contains trace elements that help the flowering plants bloom faster and for extended periods.
• Compost - made up of organic material that breaks down in the soil. Compost will add essential nutrients and enhance the structure of the soil. Microorganisms and earthworms break down the material.
• Kelp meal - kelp is a marine macroalga. Kelp meal is available in different forms such as powder, liquid (cold-processed) and enzymatically digested liquid forms. Kelp meal is valued for its nutrient contents, including nitrogen, phosphorus, and potassium.

Animal-based fertilisers

Animal-based fertilisers contain dead animal parts and their refuse. Animal-based fertilisers will add large amounts of nitrogen into the soil. Using animal-based fertiliser will allow substantial growth in the early week and is especially important to the development of leafy plants.

Examples of animal-based fertilisers include:

• Fish emulsion - fish emulsion is made of a whole or parts of a fish (you could bury an entire fish under a plant). Fish emulsion will provide a quick nitrogen boost.
• Bone meal - Bone meal is made from ground animal bones, usually from cattle. Bone meal increases the concentration of phosphorous in the soil. Phosphorous will help flowering plants to bloom and grow bigger flowers.
• Urea (urine) - Urea is a component of urine. Urea is produced when the liver breaks down proteins and ammonia. It contains a high concentration of nitrogen.
• Manure - Manure is often mixed with compost to enhance the benefits. Manure will provide large amounts of organic nitrogen. It also helps condition the soil by reducing nutrient runoff and leaching of nutrients.

Animal-based fertilisers contain high amounts of nitrogen, and if too much is added, nitrogen burn can occur. Nitrogen burn describes the scorching of a plant from over-fertilisation. It appears as a brown burn-like tissue on the leaf edges and tips.

Mineral-based fertilisers

Mineral-based fertilisers can add nutrients to the soil and are also important in lowering the pH.

Examples of mineral-based fertilisers:

• Calcium - natural calcium sources include eggshells (usually ground into a powder), calcium-rich seaweed and chamomile. Calcium is essential in the structural growth of the cells. This includes the growth of the cell wall and the membrane.
• Epsom salts (magnesium and sulfur) - magnesium will better take up nitrogen and phosphorus. It is also a building block for making chlorophyll.
• Glacial and basalt rock dust - increases the ability of soil to hold moisture and provides nutrients such as calcium, iron and magnesium, trace elements and micronutrients.

Artificial (inorganic) fertilisers

Artificial fertilisers consist of chemical compounds which mimic natural minerals for optimal plant growth in the soil. Artificial fertilisers will have higher concentrations of compounds and faster release rates than natural fertilisers.

There are three main groups of artificial fertilisers:

1. Fast-release fertilisers are usually dissolved in water, and the fertiliser is watered onto the soil.
2. Slow-release fertilisers - active compounds are coated with a layer that does not immediately dissolve immediately when it contacts water. Slow-release fertilisers are used to avoid chemical burn (including nitrogen burn).
3. Gaseous fertilisers are most commonly used in commercial agriculture. Anhydrous ammonia (water is absent) is the most widely used as it is very nitrogen-rich.

Comparison between the natural and artificial fertilisers

Table 1. The comparison between natural and artificial fertilisers.

The process of fertilisation

Fertilisation in agriculture refers to applying fertilisers to the soil or land. Soil fertility will depend on the cycling of the nutrients between their organic and inorganic forms.

Natural nutrient cycling and fertilisation of the soil

During the natural cycling of nutrients, microorganisms break down organic matter to provide inorganic nutrient forms to plants. Earthworms distribute the organic matter within the soil, and their burrows aerate the soil. Organic matter will become humus.

Microorganisms will break down humus; the plants can then access the nutrients. Humus improves the soil structure and makes it more porous for easier air and water flow. Humus can act as a sponge to retain nutrients that plants can access when they need it.

Chemical fertilisers and soil quality

Chemical minerals are widely used in agriculture to improve plant growth. Naturally, microorganisms would be there to make the nutrients accessible. However, when the compounds are added, microorganisms are not required. Chemical nutrients added to the soil can be accessed when they contact water, i.e., dissolve. Fertilisers containing these minerals will often have adverse effects on the soil's overall health, such as an increase in soil acidity.

Another effect would be a decrease in microorganisms and earthworms, which contribute to the decomposition of organic matter. Due to the increased acidity of the soil, the conditions become unfavourable to some. For example, the enzyme activity of decomposers that work at a neutral soil pH (at around 6.0) will be drastically reduced in acidic soil. In addition, fewer decomposers and reduced activity will lead to the production of less humus, reducing soil fertility.

Natural soil renewal

The soil will naturally contain organic plant matter and ecosystems with microorganisms to create soil humus. There will also be decaying organic matter from the animals, such as earthworms.

Fig. 1 - The fertilisation of soil using organic matter for renewal

Environmental issues with nitrogen-containing fertilisers

Nitrogen-containing fertilisers increase plant growth and crop yield; however, they can also have adverse effects.

Reduced species diversity

Increased nutrient concentration will favour fast-growing plants, which can outcompete slower-growing and smaller species by blocking sunlight.

Leaching

Leaching is the process by which essential nutrients are lost from the upper layer of soil. Artificial fertilisers will have inorganic ions that readily dissolve in the water. Nutrients are often washed by the rain and end up in water bodies. Nutrient runoffs can cause eutrophication.

Eutrophication

Eutrophication is the harmful accumulation of nutrients in water bodies. In rivers and lakes, nitrate and nitrite ions usually occur in low concentrations and limit algae and plant growth. As the nutrient concentration increases, the upper layer of water will become concentrated with algae, known as an algal bloom. Algal bloom limits the light permeating the upper layers of the water and causes plants and algae underneath to die.

Saprobiotic bacteria (decomposers) use the decaying plants and algae as a food source, and their growth increases. Decomposing bacteria will create a higher oxygen demand, and this causes oxygen to become a limiting factor for aerobic organisms in the water. Anaerobic organisms will thrive as they do not require oxygen to survive.