Fertiliser

Just as we have essential needs for our growth, plants also have essential needs for their survival. These include:

• sunlight
• air
• water
• nutrients
• appropriate temperature.

As long as plants have water, carbon dioxide and sunlight, they are able to photosynthesise. This means that a chemical reaction takes place within plant leaves to produce glucose (food) and oxygen. The glucose is broken down by enzymes in the plant to provide the energy that causes it to grow. But plants also need nutrients (chemical elements) from the soil to help them grow. These nutrients are often added to the soil in the form of fertilisers.

What are fertilisers?

Fertilisers are chemicals that are added to soil to supply nutrients to make it more fertile. The chemicals in fertilisers contain essential elements required for plant growth.

Essential elements that plants need

Plants need many different elements for their growth. Carbon, hydrogen and oxygen are available from air and water. Nitrogen, phosphorous, potassium, sulfur, calcium, magnesium and sodium are also needed for plant growth. In addition, plants need very small amounts (trace elements) of boron, copperiron, cobalt, manganese, molybdenum, chlorine, iodine, selenium and zinc for healthy growth. Some elements may not be needed for plant growth but may be important for the nutrition of grazing animals.

Nitrogen, phosphorus and potassium are the most important elements because they are:

• building blocks for cells in the plants
• needed in the greatest quantities
• often depleted in New Zealand soils.

Fertilisers replace essential elements

Fertilisers supply plants with the elements that may be missing or in short supply in a form that can be used by the plants for faster growth. Most fertilisers supply nitrogen, phosphorus and potassium. The other elements needed by plants are required in much lower quantities (trace elements) and are generally available in most soils. In nature, nitrogen, phosphorus and potassium often come from the decay of plants that have died.

The early fertiliser industry

Forests were burnt to clear the land. The ash provided a source of nutrients, but once these were depleted, pastures and crops began to fail. Early farmers added chemical elements to the soil by adding compost, animal manure, dried and ground animal blood and bone or ground nutrient-rich rocks mixed with chemicals that allowed the plants to absorb the nutrients.

Superphosphate

Superphosphate is an artificial fertiliser and is the most important fertiliser used in New Zealand. Farmers often shorten its name to ‘super’.

Superphosphate was developed to address the shortage of phosphorus in soil. It is made by reacting finely ground phosphate rock with sulfuric acid. In this form, phosphate is rapidly released into the soil, where it can be used by plants. Superphosphate manufacture began in New Zealand in 1882, and over 3 million tonnes are produced annually. The main nutrients in superphosphate fertiliser are calcium, sulfur and phosphorus.

Potash (potassium chloride) is often combined with superphosphate to provide potassium. The nutrients in superphosphate promote the growth of clover. The clover then converts nitrogen gas in the atmosphere to the essential plant-available nitrogen.

Adding nitrogen

Farming has become increasingly more intensive (more is produced from the same size piece of land) due to the cost and availability of land and demand for produce. Compared to clover, manufactured nitrogen fertiliser was found to be more productive (because it could be applied at just the right time to increase production). Urea CO(NH₂₂) became the main nitrogen fertiliser, particularly on dairy farms. Diammonium phosphate (NH₄₂HPO₄), more commonly known as DAP (invented in the 1960s), contains both nitrogen and phosphorus and is now in common use.

Fertilisers now

Fertiliser companies now often blend superphosphate with potash and add forms of nitrogen such as ammonium sulfate (if needed). Other elements may be added in. Different nutrient combinations are made depending on the needs of the soil and plants.

Effects on the environment

Water quality is a concern and focus of research. Run-off from farms can carry nutrients from fertilisers (and animal effluent) into waterways. The nutrients stimulate plant growth in streams, rivers and lakes, which can upset the balance of natural flora and fauna and affect water quality.

Farmers try to balance their fertiliser use to manage production while minimising nutrient loss through leaching to surface and groundwater.