Guideline Index

Chapter 15: Nutrient Planning

15.5 Setting target levels

It is important to set targets for the nutrients levels on a dairy farm – in particular phosphorus, potassium and sulphur. Setting targets will assist in determining whether the farm needs only a maintenance application or whether a capital application is required as well. Targets will be dependent on pasture species present, soil type and production levels.

The recommended target levels mentioned throughout this section are discussed in Chapter 9.2.5 to 9.2.7, ‘Interpreting soil and tissue tests’.

15.5.1 The effect of soil nutrient status on different pasture species

Pasture species have a big impact on potential pasture production. An unimproved pasture may have less than half the production of a well fertilised, improved pasture sown to perennial ryegrass and white clover.

Soil nutrient status and grazing management will each affect the botanical composition of a pasture. The application of nutrients alone may not always improve pasture productivity. Applying nutrients to an unimproved pasture will usually give a poorer dry matter response than applying nutrients to an improved pasture.

Figure 15.4 gives an indication of some of the temperate species likely to be present under various rainfall and soil fertility conditions. Higher soil fertility levels and higher rainfall conditions suit the improved species (for example, perennial ryegrass/white clover species). Low soil fertility levels and low rainfall conditions suit the unimproved species (for example, native grasses, sweet vernal, bent grass, and fog grass). Sub clover and paspalum are not included in this diagram because they have a wide range of rainfall and fertility tolerances.

Figure 15.4   Rainfall and soil fertility requirements of a range of temperate weeds and pasture species.  Source: Hill (1993).
Figure 15.4 Rainfall and soil fertility requirements of a range of temperate weeds and pasture species. Source: Hill (1993).

15.5.2 Setting target levels for phosphorus, potassium and sulphur

The Olsen P test indicates the amount of phosphorus that is available to the plant in a soil and can therefore be compared across different soil types. The Colwell P test also measures available soil P as well as P that is less readily available to plants and will vary according to soil type. The optimum level of P when measured by the Colwell test will therefore be soil type dependent (see Chapter 9.2.5 ).

The Olsen P response curve in Figure 15.5 shows diminishing returns in pasture production as soil P level increases. This means there is a greater response to P applications at low Olsen P levels and that the response decreases at higher Olsen P levels.

Figure 15.5   Relationship between soil Olsen P level and milk fat and protein responses to a range of Olsen P levels for soils sampled to a depth of 10cm.  Source: DPI Phosphorus for Dairy Farms project.
Figure 15.5 Relationship between soil Olsen P level and milk fat and protein responses to a range of Olsen P levels for soils sampled to a depth of 10cm. Source: DPI Phosphorus for Dairy Farms project.

The greatest response to applied P (17%) was at an Olsen P below 14, but a sizeable response (5%) still occurred from Olsen P 14. As can be seen from Figure 15.5, the response level above an Olsen P of 22 is only about 2%, and the response decreases further at the higher end of the range.

The ability of a soil to interact and hold on to phosphorus is referred to as a soil phosphorus buffering capacity. A soil with a high phosphorus buffering capacity requires a much greater application of P fertiliser (up to three times more) than a soil with a low phosphorus buffering capacity to give the same production result. The soil buffering capacity is measured on a soil test using the phosphorus buffering index (PBI).

Soils with high iron and aluminium levels, such as red volcanic soils (Ferrosols) and acidic peats, generally have a high PBI. Soils with a course texture, such as sandy loams, tend to have a low PBI. When P fertiliser is applied across a wide range of soil types at the same application rate, phosphorus requirements may not be met on some soils, whereas on others there may be a P loss to the environment due to oversupply.

A vital part of the nutrient planning process is to set P, K and S target levels for each of the FMZs on the farm. The DPI ‘Victorian Better Fertiliser Decisions’ project developed targets for Australian dairy soils – See the soil fertility guidelines for P, K and S in the following sections of Chapter 9:

As mentioned in Chapter 9, ‘Interpreting Soil and Tissue Tests’, 95-98% of pasture performance compared to potential is considered adequate for a dairy farm system based on the latest recommendations from research. However each farm will need to set their own target levels for soil fertility. Farmers operating with levels higher than these suggested targets in their soils may decide to apply only maintenance levels of phosphorous fertiliser. The maintenance level of nutrient applied would be based on the target phosphorus level they are aiming to achieve. Another option is to cut back or to stop applying any fertiliser at all, but when doing this it is essential to monitor soil phosphorus levels annually. Once soil levels have fallen back to target levels, it is important to resume applying maintenance levels of nutrients, or the soil levels will continue to decline. This in turn would negatively affect pasture production and therefore milk production.

Phosphorus experiments undertaken on dairy farms in Western Australia between 2006 and 2010 as a part of the Greener Pastures Project (Bolland et al 2011), have also shown that no phosphorus fertiliser is required when soil test P is above the critical value for that soil. When soil test P is above the critical value for that soil, adding phosphorus fertiliser will have no effect on pasture production. The Green Pastures Project also showed the effect of applying no phosphorus fertiliser (Figure 15.6). The results highlighted that the rate of decline differ considerably depending on PBI of the soil and other factors. The soils with the lower PBI and lower critical Colwell P values tend to fall back quickly below critical values, whereas the higher PBI soils fell more slowly. Again the importance of monitoring fertility status with soil tests is highlighted.

Figure 15.6  Colwell soil phosphorus test for the nil-P treatment of 4 experiments conducted in the dairy region of Western Australia. Critical soil test P values for each site are indicated by dotted lines – the critical value is for 95% of maximum pasture dry matter yield.  Source: Adapted from Bolland et al (Feb 2011).
Figure 15.6 Colwell soil phosphorus test for the nil-P treatment of 4 experiments conducted in the dairy region of Western Australia. Critical soil test P values for each site are indicated by dotted lines – the critical value is for 95% of maximum pasture dry matter yield. Source: Adapted from Bolland et al (Feb 2011).