Jacqueline Rowarth — Fertiliser Application

Dr. Jacqueline Rowarth, plant physiologist of Lincoln University's Soil, Plant and Ecological Sciences Division, wanted to find the right amount of nitrogen fertiliser to use on ryegrass in order to increase seed yield.

Since modern agricultural practices began, New Zealand farmers have been supplementing the soil's natural nutrient level with fertiliser. Using fertiliser is a key step in making soil fertile and therefore an essential part of New Zealand's agricultural and forestry industries. These industries are a major source of income for many New Zealanders and the country as a whole, primarily through the export of meat, wool, trees and produce to offshore markets.

Only eight percent, or around 6,530 tonnes, of our perennial ryegrass is exported, mainly to Canada, Japan and Australia. Indeed, the value of these exports for New Zealand in the year to the end of March 1999 was $76.6 million. However, applying too much fertiliser can lead to environmental and health problems as well as being a waste of money. Finding the right balance of when and how much fertiliser to put on the land has always been a question for researchers.

One researcher, Dr. Jacqueline Rowarth, plant physiologist of Lincoln University's Soil, Plant and Ecological Sciences Division, wanted to find the right amount of nitrogen fertiliser to use on ryegrass in order to increase seed yield. In the Canterbury region ryegrass is grown for export seed. This seed is then sold to other countries for their farmers to plant. Ryegrass is given to dairy, and beef animals for food, usually as pasture grass. For us to understand the significance of Dr. Rowarth's research, we must first understand how plants grow and how nitrogen is converted into nitrates for plant growth.

Plants, like humans, need proteins, starch, fats and oils to grow and to stay healthy. Plants make sugar by photosynthesis: by combining the sun's light energy, water and carbon dioxide. The chemicals in the sugar and other minerals are then used by plants to make protein, starch, fats and oils. Minerals get into plants by being dissolved in ground water, to be taken up by the roots. The three main minerals entering plants in this way are nitrogen, magnesium and iron.

Nitrogen is found in abundance in the air. In fact over 78% of the air is nitrogen, the rest is mainly oxygen. But before nitrogen can get into the soil, and be used by plants to make proteins and enzymes, it must be first turned into water soluble nitrates. Converting nitrogen gas to nitrates takes a lot of energy. There are four main ways of doing this:

  1. The decomposition of dead plants and animal material
  2. Lightning: the high energy in lightning mixes the oxygen and nitrogen in the air to form nitrates. These dissolve in rain.
  3. Industrial processes: need great pressures and high temperatures to makenitrogen fertiliser
  4. Legumes (peas, beans, clover). At the end of plant's root, there are swelling or lumps where friendly bacteria change atmospheric nitrogen into soil nitrates.

Applying nitrogen fertiliser is a large cost for farmers, but it is a lot faster in restoring the soil's nutrients than the other three methods mentioned above. To use fertiliser wisely, it is a good idea to understand when plants need it and just how much nitrogen they need for healthy growth. Dr. Rowarth began her ryegrass research on the Canterbury Plains in 1990 with Keith Cameron, Professor in Soil Science, and Dr. Prue Williams, of Crop and Food Research.

First, she observed nitrogen and seed yields in ryegrass crops on the Canterbury Plains, then she experimented with running pot and field trials in her university pastures to find out exactly how much nitrogen ryegrass needed to grow optimally.

Dr. Rowarth wanted to find out more about ryegrass. She wanted to know what happened to the nitrogen in the soil that wasn't used by the plant because if the plants were not using all the nitrogen, that meant that these nitrates went into the underground water systems. Large quantities of nitrates in drinking water are of concern. While we need some nitrate to live, too much is not beneficial. 
If people or animals drink water that contains excessive nitrates, it can make them quite sick. Excessive nitrate levels in rivers and streams also helps aquatic plants to grow more prolifically, which in some circumstances can lead to clogged river beds, reduced water flows and even eutrophication.

In order for Dr. Rowarth and her team to study nitrates and nitrogen better they needed a better system. They decided to grow ryegrass in self-contained plugs. These plugs enabled the scientist to measure three things:

  1. Exactly how much nitrogen was put on;
  2. How much nitrogen the plants used, and;
  3. How much leached out the bottom

Her plug results encouraged her to measure nitrogen levels on farms around Canterbury. From this research, Dr. Rowarth was able to help farmers save on nitrogen fertilisers because she knew the quantities and the best time to put nitrogen fertiliser on crops resulting in a minimum of leaching.

This time was in August if the right weather conditions prevailed. She also found that it was better to apply this fertiliser in two sessions: between autumn and winter, and winter and early spring. These are the times when ryegrass is more receptive to fertilisers and leaching can be minimised. 

Dr. Rowarth believes that her research will some day help growers improve their overall yields (or production), especially when she researches irrigation. "Fine tuning the timing and amount of water for a crop such as ryegrass will be the next step in helping the industry achieve better results for the farmers as well as being environmentally more friendly."

Classroom activity - Testing Materials