The role of clover

Clover grows well in New Zealand pasture and has long been used to get nitrogen into the soil. It was introduced in the 1800s from England along with most of our pasture plants.

Introducing pasture plants

In the early years, New Zealand bush was burned and removed to make way for pasture for animal grazing. Native plants had a poor growth rate and were low in nutrition. Farmers believed that introduced plants were needed – especially clover – if farming was to be successful.

Clover for nitrogen

Once the initial supply of nutrients in the soil – from ash and organic matter – was depleted, the introduced plants needed fertiliser to grow well. Superphosphate supplied the much- needed phosphorus, and the addition of potash supplied potassium. However, a shortage of nitrogen is usually the main factor (apart from a lack of water) that limits pasture growth. In the early years, the only source of nitrogen was from the cycling of soil nitrogen (dead to living plants) as there was no nitrogen fertiliser. Even at that time, farmers knew that more nitrogen could be obtained by planting legumes – particularly clover.

White clover (Trifolium repens) is New Zealand’s most important pasture plant because of its ability to fix nitrogen.

Making nitrogen usable

If you read about the nitrogen cycle, you will see how important nitrogen is to all of life. You will also see that, although there is an abundance of nitrogen gas in the atmosphere, most plants are unable to use that nitrogen as a nutrient. Nitrogen has to be ‘fixed’ in a way that enables plants to use it. The most common way of fixing nitrogen gas is for it to be converted into different forms of nitrogen by bacteria.

Rhizobia

This is where legumes, particularly clover, are very useful. These plants have small swellings on their roots called nodules. Soil bacteria known as rhizobia live in these nodules. You may be able to dig up some clover roots and see the nodules. When these bacteria are actively fixing nitrogen, the nodules are reddish-pink inside.

Rhizobia take nitrogen gas (N₂) from the air and convert it through a series of steps into forms of nitrogen (such as ammonium and nitrate) that can be used by these plants to make them grow well. The nitrogen compounds help the plants produce protein for their growth. Animals obtain the nitrogen compounds by eating the plants. When the plants and animals die and decay, the nitrogen compounds become part of the soil’s organic matter.

More microbes needed

Nitrogen compounds in decomposed matter in the soil are often tightly bound and need to be transformed (broken down) by microbes (bacteria) before the roots of other plants can take them up.

A cheap source of nitrogen

Clover and other legumes are a cheap way to increase the essential element nitrogen in soil, plants and animals. This reduces the need for expensive nitrogen fertiliser.