Guideline Index

Chapter 12: Nitrogen and Nitrogen Fertilisers

12.1 Introduction

In this Chapter:

Nitrogen (N) forms the largest component (78%) of air that we breathe and is a key component of proteins and amino acids, DNA and nucleic acids in plants.

Pastures require N in large amounts, therefore N is generally the major limiting nutrient in terms of plant growth.

Nitrogen is a very changeable and mobile nutrient, existing in many forms in the soil, water and air. It is important to understand the basics of how N cycles around a grazed pasture system to make the best use of this valuable nutrient.

The national Accounting for Nutrients project looked at how N cycled around 41 dairy farms in Australia using a nutrient budgeting approach. They calculated the N use efficiency of each farm by calculating the amount of N exported off the farm (as milk, animals and feed sold etc.) divided by the amount of N imported on to the farm (in fertiliser, feed and N fixation by legumes etc.). The project found that Australian dairy farms had a wide range of N use efficiencies ranging from 14-50%. These results suggest that some farms are using N very efficiently, whereas others are not. Although the drivers of N use efficiency are complex, these data show that there is an opportunity to improve N management in Australian dairy systems. This chapter discusses how to manage both existing N in the soil and fertiliser N to maximise the value of this valuable resource.

Losses of N to the surrounding environment are an increasing concern and this chapter also discusses how N is lost from dairy pasture systems through denitrification, volatilisation, surface runoff and leaching. This chapter highlights the important aspects of N management and how farm management can optimise the economic use of N, whilst minimising losses to the environment.

12.1.1 Summarised version of the N cycle

Legumes (e.g. clover and lucerne) are unique in their ability to use N from the atmosphere as a source of N for growth. Legumes form a relationship with bacteria that live in their roots, and which can ‘fix’ N from the atmosphere and convert it into ammonia.

Pastures can only take up N in the form of ammonium and nitrate.

The N in pastures is eaten by grazing cows and most is returned to the soil in urine and manure deposition – see Figure 12.1. The urea in urine or urea fertilisers is converted to ammonium and can be lost via volatilisation (as ammonia gas) or converted to nitrate and then lost via leaching. Nitrate can also be lost through denitrification which produces the greenhouse gas nitrous oxide and di-nitrogen gas. Ammonium and nitrate in the surface soil or from fertiliser can also be lost when water moves over the surface as runoff.

Figure 12.1  Summarised version of a nitrogen cycle in a dairy pasture system.  Red labels represent N is lost to the environment.
Figure 12.1 Summarised version of a nitrogen cycle in a dairy pasture system. Red labels represent N is lost to the environment.

12.1.2 Nitrogen in soil

Almost all (98%) of the N in soil is in the organic form. However, plants can only take up N as inorganic N forms (nitrate or ammonium), so organic forms of N need to be mineralised by soil microbes before they can be taken up by pasture. It is important to understand that although N fertiliser and legume fixation add N to the soil, changes in the large organic N content in the soil can have a large impact on the amount of N pasture can take up and the loss of N to the environment.

For example, when soils are warm and moist, soil bacteria and fungi break down the soil organic N to the ammonium and nitrate forms, causing a flush of these nutrients which can be taken up by pasture. This process is called mineralisation. When soils are cultivated to sow fodder crops or sow new pasture, soil organic N is often mineralised causing a flush of plant available N. Ammonium and nitrate can also be immobilised back to the organic form when decaying plant material breaks down. It is important to consider these processes as there may be times of the year when adequate N is available for pasture uptake and N fertilisation is not required.

The positively charged ammonium ion is held in soil by the negative charges on clay particles and soil organic matter, in a similar fashion to how potassium is held in soil. In comparison, nitrate is not held by the soil and is easily lost via leaching as water drains through the soil or when water moves over the surface of the soil. When soils are warm and moist, much of the ammonium is converted to nitrate by soil bacteria and fungi in a process called nitrification. An important message is that nitrate leaching can increase when soils are warm and moist, due to this rapid conversion of ammonium to nitrate. However if soils are water logged and there is a lack of oxygen, nitrate undergoes denitrification by soil bacteria and fungi, converting it to nitrous oxide and di-nitrogen gas.

Soil organic N can be an important source of N for plant uptake, so it is important to keep this is mind when determining N fertiliser requirements. Soil testing for N

Due to the changeable nature of N, there is currently no reliable measure to test soil N availability, as in the time it takes to collect the sample and have it analysed by a laboratory, plant N availability could have changed dramatically. Soil N levels also change widely across a paddock, so it is difficult to get a reliable result. It is possible to measure the total amount of N in a soil and the amounts of ammonium and nitrate, however from the discussion above, it is clear that the plant available forms of N (ammonium and nitrate) can change quickly depending on the temperature and moisture of the soil and from where the sample is taken, whether or not the soil has been cultivated, the amount of N the pasture is taking up and any losses to the environment.

Where soil N testing may be more useful is to assess the potential availability of soil N for a future crop.

For example, a 20 t DM/ha crop of maize may be able to access some of the 200-240 kg N/ha/year required for production from organic N mineralised when the soil is cultivated prior to planting in Spring (see Section

For more information of how to take soil samples see Chapter 8.3.

12.1.3 Nitrogen in plants

Plants contain N in a number of forms, including nitrates, amino acids and proteins. Nitrogen in plant herbage (total leaf material, available to grazing animals) is generally measured/estimated in a laboratory using either the Kjeldahl digestion method or near infrared (NIR) and reported as ‘crude protein’, which reflects the total N concentration multiplied by 6.25. Nearly all of the N in plants is present as amino acids in proteins and the average N content of proteins is 16%, therefore 1/(16/100) = 6.25. Crude protein is then used as a standard measure of how much N is available to ruminants for a given type of feed, including pastures, forage crops, and concentrates. It is important to be aware that N measured using the Kjeldahl method measures ammonia and organic N forms, whereas ‘total’ N measures nitrate, nitrite, ammonia and organic N forms.

The uptake of most nutrients is closely controlled in plants, which means that large increases in soil nutrient concentrations only result in a small or negligible change in plant nutrient concentration. Nitrogen and potassium are two nutrients that plants can take up in amounts greater than what’s immediately required for growth. This is sometimes called ‘luxury uptake’ and can lead to a range of issues as discussed in Section 12.1.4. As an example, N levels in plant herbage can vary from 1.5% of dry matter (= 9% crude protein) up to 5.5% of dry matter (= 34% crude protein). This range represents levels that are, on the one hand, marginal for sustaining animal production, to levels that are far in excess of what’s required by high-producing animals, and might even contain a high percentage of nitrates, which are toxic to ruminants (see Section

Crude protein (% DM) = total N concentration (%) x 6.25

Nitrogen is taken up from the soil N pool by the plant root system, and moves from the roots to the tiller bases (stubble) and then to the youngest growing leaf, which is the site of greatest demand. Some of this N is incorporated into plant structure, and as more leaves grow, the remainder of the soluble N is redistributed within the plant to these new leaves. Therefore as the pasture regrows after grazing, the concentration of N in the herbage is initially high and then decreases.

This can be seen in Figure 12.2, where crude protein concentration (% of leaf dry matter) decreases with leaf regrowth in a range of grass pasture types.

Figure 12.2 Crude protein in herbage of grass-based pastures (averaged across a range of studies investigating perennial ryegrass (Fulkerson et al. 1998), tall fescue (Donaghy et al. 2008), cocksfoot (Rawnsley et al. 2002), prairie grass (Turner et al. 2006) and kikuyu (Reeves et al. 1996). Crude protein content was measured in the herbage of plants cut at each leaf stage to 3 leaves (ryegrass), 5 leaves (tall fescue), or 6 leaves (cocksfoot, prairie grass and kikuyu), in both field and glasshouse experiments.
Figure 12.2 Crude protein in herbage of grass-based pastures (averaged across a range of studies investigating perennial ryegrass (Fulkerson et al. 1998), tall fescue (Donaghy et al. 2008), cocksfoot (Rawnsley et al. 2002), prairie grass (Turner et al. 2006) and kikuyu (Reeves et al. 1996). Crude protein content was measured in the herbage of plants cut at each leaf stage to 3 leaves (ryegrass), 5 leaves (tall fescue), or 6 leaves (cocksfoot, prairie grass and kikuyu), in both field and glasshouse experiments.

In practice, crude protein concentrations in herbage will vary with soil fertility levels, fertiliser management, pasture species composition and grazing management. What Figure 12.2 indicates however, is that herbage will always have higher crude protein concentrations with early regrowth (soon after grazing), reducing over time. In other words, there is a ‘dilution’ of N in pasture herbage with regrowth, with levels that can be far in excess of what animals require in an early stage of regrowth, reducing to more reasonable levels at later stages. For example, cows in early lactation require about 22-24% crude protein, and requirements decline later in lactation (see Section 12.1.4). Supplying levels of N (crude protein) in excess of animal requirements can have an adverse effect on animal performance and results in greater N losses to the environment.

High N concentrations in the early stages of pasture regrowth can be detrimental to animal health, production and the environment.

The important message is that there are 2 ways of influencing N levels in the plant – firstly through fertilising, and secondly through grazing management, and these will be further discussed in Section 12.4.

Nitrogen is an important stimulator of growth, and its application results in longer, wider leaves, particularly in the grass component of mixed pastures. It also stimulates tillering in grasses (tillers are the shoots from the base of the plant stem), which is important as tillers only live for about a year, and their replacement – usually in autumn and spring each year – is what drives future production and persistence. Lastly, N may have a role in helping plants survive stress periods such as drought, frost and heat, and can keep grasses in a healthy state that reduces the effects of infestations by rust fungus. Identifying the reasons for applying N fertiliser should be a first step in N management, and is discussed further in Section 12.4.1. Plant tissue testing for N

Plant tissue testing measures the nutrient concentration in a plant tissue and is a good method of measuring how much N has been taken up by pastures. However, the N content of pasture changes with its growth. Figure 12.2 shows how crude protein decreases with pasture leaf stage, so it is important to sample pasture when it is ready for grazing as this will accurately show the N concentration cows are eating. Although measuring pasture N levels are useful to monitor the effect of N fertiliser management, plant tissue analyses for N faces the same issues as those faced when soil testing – see Section Concentrations can change across the paddock and with the time of day. Additionally, in the time it takes to collect the sample and have it analysed by a laboratory, plant N availability could have changed dramatically.

For more information of how to take plant tissue samples see Chapter 8.4.

12.1.4 Nitrogen in animals

Animals require different levels of crude protein in their diet depending on production. For example, high-producing cows (30 kg milk per cow per day or about 2.4 kg milk solids/cow/day) in early lactation require a diet containing between 22-24% crude protein. This falls to a requirement of around 16% crude protein in mid-lactation, to 14% in late lactation, and 12% when cows are dried off or on a maintenance diet.

Cows fed mostly on pasture might be deficient in protein in summer under dryland conditions, or with tropical pastures, which are both naturally lower in N. Under these situations, farmers often supplement the animals’ diet with feed types that are higher in N, for example lucerne hay, lupins, high protein pellets, etc., so that the total crude protein level in the diet increases.

However, a situation more often seen in pastoral systems, especially in spring when growth is fast, or in highly-fertilised pastures, is that the N content of the pasture is well in excess of animal requirements. When cows eat a diet too high in N, excess N is converted to ammonia in the rumen, but this is toxic to the animal, and needs to be quickly converted to urea. This leads to increases in urea levels in the blood, milk and urine as the animal excretes the excess N. The energy used to do this could be more effectively used for milk production, growth or reproduction, and so continually feeding diets that are too high in N can have detrimental effects on animal production and even reproduction.

Under these situations, farmers often supplement the high N pasture with feed sources that are higher in energy and/or lower in N, e.g. cereal grains, maize silage. If the high N levels in pasture are a natural result of high soil N levels and optimal climatic conditions for growth, then this is a sound management decision. However if the high N levels in pasture are a result of other management imposed (e.g. fast grazing rotations, high rates of N fertiliser applied), then there are more basic and cost-effective strategies that could be implemented, and these will be discussed further in Section 12.4.