Guideline Index

Chapter 15: Nutrient Planning

15.7. Calculating the maintenance requirement

In this section, we will focus on calculating the maintenance requirement of P for a certain area of the farm using example figures. The major exports and imports on a dairy farm are discussed to demonstrate the nutrient budgeting approach. Nutrient budgeting can also be done automatically by entering the figures into a nutrient budgeting program. This is often done with the assistance of an advisor who will have access to a nutrient budgeting program. It is however useful to know how the budgeted is calculated and the following sections work through how a nutrient budget is calculated.

The end of this chapter also has worksheets which demonstrate how the various components of a nutrient budget are worked out.

15.7.1 Calculating nutrient exports

15.7.1.1 Nutrients exported in milk

Every litre of milk that is exported off the farm removes a certain amount of P, K and S. We know that milk contains close to 0.1% P, so we can work out how many kg of P are exported from the farm. Assuming there is equal production from all areas of the farm (you could break the farm up into different areas of production), you can work out the kg of P exported per hectare.

Example

  • Farm produces 1,000,000 litres of milk from 100 ha in one year
  • 1,000,000 L divided by 100 ha = 10,000 L/ha
  • 10,000 L/ha x 0.1% P = 10 kg P/ha

Therefore, we are removing 10 kg P/ha from the farm in the form of milk.

15.7.1.2 Nutrients lost from dung and urine>

This is a calculation from the ‘Phosphorus for Dairy Farms’ project to include when working out the maintenance fertiliser requirements for the farm. It makes an allowance for the nutrients lost in dung and urine while the cows are walking up the laneways and standing in the dairy yard. These nutrients are not being redistributed around the farm, unlike dung and urine in the paddocks.

You need to know the stocking rate of the farm. This is calculated by dividing the number of cows milked by the milking area of the farm. The P lost from dung and urine in the laneways and dairy yard is estimated by multiplying the stocking rate by 0.8.

Example:

  • Farm milks 200 cows on 100 ha
  • 200 cows divided by 100 ha = 2 cows/ha (stocking rate)
  • 2 cows/ha times 0.8 = 1.6 kg P/ha

Therefore, the loss from dung and urine in the laneways and dairy yard is 1.6 kg P/ha.

15.7.1.3 Soil retention factor

The soil retention factor accounts for losses within the soil structure in the case of P and accounts for leaching from the soil in the case of K and S. The ‘Phosphorus for Dairy Farms’ research illustrated conclusively that the maintenance application must also include a certain amount of P just to maintain the soil Olsen P level.

Each soil type has a different soil retention factor based on its physical and chemical make-up and its origin and weathering. Soils with a high PBI will remove P from the plant available pool more quickly and will have a higher soil retention factor.

A table of recommendations of P required to satisfy the soil retention factor has been developed and is now used to assist in making nutrient decisions (see Table 15.1 for recommendations based on Olsen P soil tests and Table 15.2 for Colwell P soil tests). Both the PBI and the current soil nutrient level affect the amount of P required to satisfy the soil retention factor. Therefore, areas of the farm that have different soil nutrient statuses and PBIs can have different total maintenance fertiliser requirements.

Table 15.1   Approximate amount of phosphorus required (kg/ha/yr) to satisfy the soil retention factor for a range of PBI values and Olsen P levels.  (Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au )
Table 15.1 Approximate amount of phosphorus required (kg/ha/yr) to satisfy the soil retention factor for a range of PBI values and Olsen P levels.
(Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au )

Example:

Table 15.1 shows that a soil with a PBI of 350 and an Olsen P of 16 would require around 26 kg P/ha to satisfy the soil retention factor.

Figure 15.2   Approximate amount of phosphorus required (kg/ha/yr) to satisfy the soil retention factor for a range of PBI values and Colwell P levels  (Source: Gourley and Burkitt pers. comm - Adapted from Burkitt et al. 2002)
Figure 15.2 Approximate amount of phosphorus required (kg/ha/yr) to satisfy the soil retention factor for a range of PBI values and Colwell P levels
(Source: Gourley and Burkitt pers. comm – Adapted from Burkitt et al. 2002)

Note that the amount of P required to satisfy the soil retention factor increases as Olsen and Colwell P increases. This is the opposite of what one might expect in that, as more P is available, one would think that less is needed for maintenance. Why is this so?

The fate of newly added nutrients is greatly affected by soil chemical and physical properties, the soil solution, soil temperature, soil moisture content, soil pH, amount of organic matter present, current soil nutrient status and other soil related factors, as well as by the amount of new nutrients added. The soil is complex and has ever changing soil reactions and fixation rates.

The increase in the amount of P required to satisfy the soil retention factor as Olsen and Colwell P increases is due to the high level of reactivity of freshly applied P and the substantial capacity of soil minerals to transform this P into less available forms – see Chapter 3.4.2 . For any soil with a particular phosphorus buffering capacity (or PBI value), there appears to be a constant proportion of the applied P that is retained by the soil. Therefore, the greater the amounted added, the greater the amount retained, and therefore the greater the amount that needs to be replaced. A soil with a lower phosphorus buffering capacity (or PBI value) will retain a lower proportion of P than a higher PBI soil.

A table of recommendations for potassium and sulphur to satisfy the soil retention factor has also been developed to assist in making nutrient decisions (See Table 15.3). It is noted that there needs to be more research into this area.

Table 15.3  Approximate amount of potassium and sulphur (kg/ha/yr) required to satisfy the soil retention factor over a range of soil types  Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au, and Cameron Gourley, DPI, Ellinbank, pers. com.)
Table 15.3 Approximate amount of potassium and sulphur (kg/ha/yr) required to satisfy the soil retention factor over a range of soil types
Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au, and Cameron Gourley, DPI, Ellinbank, pers. com.)
15.7.1.4 Other losses

Nutrients are also removed in livestock leaving the farm, nutrient runoff, and fodder conserved and taken from the farm. These nutrient losses can be significant and are discussed further in the developing a nutrient budget worksheets at the end of this chapter.

For the example being worked through, assume that no fodder is removed from the farm, that other stock (for example young stock) are not on the area, and that nutrient runoff is minimised by best management practices.

15.7.2 Calculating nutrient imports

15.7.2.1 Nutrients imported in feed

Grain and fodder brought onto the farm contain a significant amount of nutrients. Both fodder and grain contain 3 kg of P per tonne of dry matter (DM). If we know how many tonnes of DM are fed over a certain area (tonnes DM/ha), we can work out the phosphorous imported onto the farm in brought in feed (see Appendix H , for nutrient contents of other feeds).

Example: 150 tonnes of grain and 100 tonnes of hay were imported and fed to animals on 100 ha

Grain 150 tonnes divided by 100 ha = 1.5 tonne/ha1.5 tonne/ha x 3 kg P/tonne DM = 4.5 kg P/ha
Hay 100 tonnes divided by 100 ha = 1 tonne/ha1 tonne/ha x 3 kg P/tonne DM = 3 kg P/ha

The total phosphorous imported in feed is therefore 7.5 kg P /ha

It should be noted that the example above assumes the feed is spread evenly over the whole farm. In practice, there will be uneven distribution of nutrients over the farm both in terms of where the feeding out occurs and where the animals excrete the nutrients. Consider the issue of nutrient distribution when doing a nutrient budget. Regular soil testing will help identify issues associated with nutrient distribution.

15.7.2.2 Nutrients imported in effluent

The nutrients imported when effluent is applied must also be taken into account when calculating maintenance levels. Effluent is not actually imported to the farm, but is actually imported from the effluent system to the paddock. Depending on the concentration of nutrients and the rate of application, areas receiving effluent can have significant amounts of nutrients applied. If effluent has been applied to an area, the rate of nutrients can be determined using one of the worksheets in nutrient budget worksheets at the end of this chapter. (Also see Chapter 13 , ‘Using dairy effluent’ for more information.)

For this example, assume that effluent isn’t spread on the farm.

15.7.2.3 Calculating maintenance nutrient requirement

As mentioned, the nutrient budgeting approach takes into account the nutrients leaving the farm (exports and losses) and the nutrients that are coming onto the farm (imports). The shortfall between exports/losses and imports is the maintenance requirement, which is illustrated by this equation:

KM 15.5

Using the figures from the examples above, we can work out the phosphorous requirement for this particular area of the farm.

KM 15.6

Therefore, the maintenance requirement of 30 kg P/ha is the amount that is required to maintain the current level of soil phosphorous. Similarly, we could calculate the maintenance requirement of K and S using the same approach.

Maintenance rates will vary between years because of farm management or production changes and climatic influences, such as wet weather (which increases nutrient leaching and runoff) or droughts (which reduce nutrient requirements). As always, monitoring soil tests over time will be important to validate P, K and S fertiliser application rates.