Guideline Index

Chapter 15: Nutrient Planning

15.6. Nutrient budgeting to help determine nutrient application rate

Nutrient budgeting is a technique used to quantify or predict nutrient deficits or surpluses, either at a whole-farm or field scale, in an attempt to determine fertiliser requirements. It can improve nutrient use efficiency and reduce nutrient losses from agriculture. The approach discussed in this chapter is based on research led by Cameron Gourley from the Department of Environment and Primary Industries in Victoria. Gourley et al. 2007b discusses nutrient budgeting as an approach to improving nutrient management on Australian dairy farms.

Nutrients rates are always measured in kilograms of nutrients per hectare. This allows us to convert between kilograms of nutrient required/applied and the rate of fertiliser required/applied as discussed in Chapter 14 . It also allows us to measure the nutrients coming and going, in a nutrient budget, on a per hectare basis.

A farm nutrient budget can be used to estimate the nutrients removed, or accumulated, for any area on the farm and this can be used to determine nutrient and therefore fertiliser requirements.

Nutrient budgets are often done on the effective milking area of the farm as it is easier to calculate the nutrients coming and going from this area. Farms with mixed production systems such as hay making, silage removal and cropping may require nutrient budgets prepared for each farm management zone.

Any imbalance between nutrient inputs (primarily as feed and fertiliser) and nutrient removals (in milk and livestock) can result in significant nutrient accumulation or depletion on dairy farms. Where there is nutrient accumulation this can represent both an opportunity cost and a threat to the environment.

15.6.1 Maintenance nutrient applications

The intent of a maintenance application is to keep the soil nutrient status at a steady level.

To determine the annual nutrient application required to meet the production goals of the farm the balance between the quantity of nutrients leaving the farm and the nutrients that are coming onto the farm must be considered. When more nutrients leave than are brought onto the farm the balance to be replaced is referred to as the maintenance requirement. In situations where more nutrient is coming onto an area of the farm than is being removed an excess of nutrients will develop and there is no maintenance requirement.

This method of calculating maintenance requirement is based on a nutrient budgeting approach and is currently recognised as the standard for calculating nutrient requirements on Australian dairy farms. The nutrient budgeting approach is a more accurate method across different production systems than the rules of thumb used in the past.

Nutrients enter and leave the soil and farm system via several pathways (see Figure 15.7). The question marks in the diagram indicate that the information is not readily or accurately known (such as the amount leached) or is highly variable (such as the distribution of manure and urine).

Figure 15.7   Major nutrient imports and exports in a dairy farming system
Figure 15.7 Major nutrient imports and exports in a dairy farming system

Losses include:

  • Nutrients removed in milk and other animal products
  • Nutrients sold or moved off farm as fodder
  • Nutrients lost in dung and urine deposited on laneways and yards
  • Soil losses, including leaching and nutrients held strongly or ‘locked up’ in the soil and its processes (commonly referred to as the soil retention factor)

Nutrients coming onto the farm include:

  • Nutrients imported in feed (hay, silage, grain etc.)
  • Nutrients returned in applied effluent

Nutrient budgeting allows you to determine the shortfall between losses and imports, which is the maintenance nutrient requirement. A simplified diagram of a nutrient budget is shown in Figure 15.8.

Figure 15.8  The nutrient imports and losses that determine the maintenance requirement
Figure 15.8 The nutrient imports and losses that determine the maintenance requirement

By calculating the amounts of imports and losses shown in Figure 15.8, the maintenance requirement of different areas of the farm can be determined as shown in the following equation :

KM 15.2