Guideline Index

Chapter 12: Nitrogen and Nitrogen Fertilisers

12.2 Sources of nitrogen in pasture systems

12.2.1 Fixation by legumes

The amount of N fixed by legumes (eg. clovers) in dairy pastures is generally low in Australia due to the low content of legumes in pastures. Legume contents of pastures tend to be < 30% of the total pasture dry matter, so legumes may only contribute 50 kg N/ha/year or less to soil N (Unkovich 2012). A pasture producing 12 t DM/ha/year requires approximately 350 to 550 kg N/ha/year. It is clear that N fixation by legumes will not provide enough N to maximise pasture production, so additional N is required on most productive dairy farms.

12.2.2 Brought in feed

Common feed supplements such as lupins, palm kernel and pasture silage have higher concentrations of N compared to barley and pasture hay (Table 12.1). The amount of supplement brought in to Australian dairy farms continues to increase, so it is important that these sources of N are accounted for when considering N fertiliser requirements. For example, feeding out 250 t DM of round bale pasture silage across a 100 ha farm will add about 50 kg N/ha to soil, as approximately 75-80% of N eaten by dairy cows is deposited back on to soil as urine and manure. Although the average N contents of common feeds are presented in Table 12.1, it is important to note that these concentrations can vary depending on the quality or source of the feed, so feed testing is recommended if an accurate figure is required.

Table 12.1 Average nitrogen concentration of common feeds brought in to Australian dairy farm systems.
Table 12.1 Average nitrogen concentration of common feeds brought in to Australian dairy farm systems.
Source: Accounting for Nutrients project (Gourley et al. 2010).

12.2.3 Recycling of N

Cattle grazing high-quality pastures return about 75-80% of the N they consume in both dung and urine, with the majority (80%) of excreted N coming from urine. Experimental work has found that dairy cows stocked at 3 cows/ha consumed 377 kg N/ha/year. Of that amount, 50 kg was used for milk and weight gain, 87 kg was excreted as dung, and most (240 kg N) was excreted as urine. Most of the N is recycled back into localised dung and urine patches and non-productive areas, such as around water troughs and gateways and in laneways and the dairy yard. Due to the small area over which urine is deposited, research has found that a single urine patch can apply up to 1000 kg N/ha. Of this returned N, only about 33% is used for pasture growth. The rest is lost through leaching, denitrification and volatilisation (see Section 12.3).

12.2.4 Dairy effluent

Dairy cows urinate between 30-120 kg N/cow/year and defecate between 20-70 kg N/cow/year, which means that effluent collected from the dairy or feed pad can have high concentrations of N. The liquid component of dairy effluent often contains the more N rich urine. Dairy effluent is a valuable nutrient source and it is important that effluent is spread evenly back on to pasture soils, to grow more pasture. However, there are some important aspects to consider when applying effluent on farms. The area of the farm receiving effluent needs to be large enough to ensure that the rate of N being applied per year is not too high. In New Zealand, many regional councils recommended that the annual N loading from dairy effluent should not exceed 150 kg N/ha (DairyNZ 2013). If excessive rates of N are being applied in effluent, this increases the risk of N loss to the environment through surface water runoff or leaching (see Section 12.3). The amount of potassium being applied per ha is also important to consider when applying effluent.

12.2.5 Other organic sources of N

Organic sources of nutrients such as poultry manure and organic waste products, supply a range of N, P, K, and S and some trace elements, although their analysis and moisture content can be variable. The unit cost of N can be high compared to other N fertiliser products, and they are not commonly used on dairy pastures.

However, spreading them is a useful method of disposal and can slowly increase the organic matter content and improve soil structure. The nutrient content of animal manures is usually quoted on a dry matter basis.


If poultry manure is 50% DM, then 100 kg of fresh, or wet, poultry manure with an analysis of 3.4% N (dry matter basis) provides only 1.7 kg N. Therefore, to apply 30 kg N/ha, approximately 1750 kg/ha of wet poultry manure would need to be applied.

12.2.6 Fertiliser

Table 12.2 contains a summary of analyses and recommendations for use of the more commonly used N fertilisers. Straight nitrogenous fertilisers

Straight nitrogenous fertilisers supply N only and should be used where the soil is not deficient in other nutrients (e.g. P, K, S, Mo). Examples include urea and calcium ammonium nitrate (CAN). Nitrogen blends

NPKS blends are designed to supply various proportions of nutrients (N:P:K:S) and were developed for various times of the year when the nutrients are in highest demand or are likely to be removed in fodder conservation. When using blended fertilisers, it is important to consider the environmental risks of the other nutrients in the fertiliser product. For example, DAP contains both N and P and it is important to avoid the risk of surface P runoff when applying this fertiliser.

Blended N and P fertilisers should be used where the P levels are known to be lacking. Examples include di-ammonium phosphate (DAP), mono-ammonium phosphate (MAP), and urea blends.

Blended N and S fertilisers should be used where the S levels are known to be lacking. Examples include sulphate of ammonia.

Table 12.2 Nutrient analysis and recommended usage of some common nitrogenous fertilisers
Table 12.2 Nutrient analysis and recommended usage of some common nitrogenous fertilisers

12.2.7 Nitrogen sources - summary

The choice of fertiliser should be made on the cost per unit of N, taking account of the cost of additional nutrients if using a blend. However, where there are conditions that will adversely affect the performance of a particular fertiliser after application, these should also be taken into account. For example, applying a nitrate based fertiliser (e.g. CAN) to pasture that frequently becomes waterlogged may result in lower than expected benefits, as large losses of N by denitrification and/or leaching can occur. In this situation, other fertilisers may be more appropriate, even at a higher cost.