Guideline Index

Chapter 15: Nutrient Planning

15.8. Capital nutrient applications

When nutrients are applied at a greater rate than that needed for maintenance, the soil fertility status of that particular nutrient rises. This is referred to as a capital application.

Soil type and PBI has a strong effect on the amount of applied nutrient required to lift soil fertility levels. The degree of fixation can vary. Nutrients may be strongly fixed, forming insoluble compounds, or may form more soluble compounds that can be slowly released back into the soil solution over time.

New Zealand experience has shown that it can take three to four years on some soil types before pastures fully respond to a high capital P application. Similar experience at Ellinbank on the Ferrosol soil indicates that up to two years may be required before the effects of the capital dressings are evident in terms of increased pasture growth and quality.

High P fixing soils have a high PBI, so they require large capital applications of P to raise the soil P level. Soils which contain large amounts of iron (Ferrosols) fix much of the P applied.

Sandy soil types are low P fixing and have a low PBI. In these soils, the majority of the applied P remains in the soil solution, and the remainder is held to the clay or organic matter. Sandy soils low in clay or organic matter are more susceptible to P leaching, however.

An increase in the soil nutrient status using a high capital application will occur more quickly on lighter soils, such as sands or loams, than on heavier-textured soils.

For capital P applications to be economical, pasture utilisation must be maximised. If the extra pasture grown as a result of higher P levels is not utilised, the investment in the capital application is wasted

15.8.1 The amount of capital Phosphorus required to raise the soil level by one unit

The amount of capital P required to raise the soil Olsen or Colwell P test by one unit (1 mg/kg) is variable and largely depends on the phosphorous buffering index (PBI) of the soil (see Table 15.4). Remember that this rate is in addition to the P applied to meet maintenance requirements. Soil texture can also be used as a guide for determining the amount of capital P required to raise the soil Olsen or Colwell P by one unit (where possible use the PBI figure which is more accurate).

Table 15.4   The approximate amount of capital P (kg/ha) required to raise soil Olsen or Colwell P by one unit (1 mg/kg) based on the soil type or PBI
Table 15.4 The approximate amount of capital P (kg/ha) required to raise soil Olsen or Colwell P by one unit (1 mg/kg) based on the soil type or PBI
(Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au )

This means that a soil with a PBI of 133 would require an application rate of 9 kg of P/ha to raise the soil Olsen P by one unit. The amount of capital P (kg P/ha) required to lift the Olsen or Colwell P to your target Olsen or Colwell P level on a particular soil can be calculated using the following formula:

Example: Your soil has a PBI value of 133 and an Olsen P of 12, and you would like to raise the

KM 15.8

soil fertility for P to an Olsen P of 20. You would calculate the estimated amount of capital P to apply as follows:

Example: Your soil has a PBI value of 133 and a Colwell P of 20, and you would like to raise the soil fertility for P to an Colwell P of 34. You would calculate the estimated amount of capital P to

KM 15.9

apply as follows:

KM 15.10

Remember, these are capital applications. Therefore, they only become effective after the total maintenance requirement of the farming system and the soil has been satisfied. The decision would need to be made around how quickly this would be added (e.g. over one, two or three years)

15.8.2 The amount of capital Potassium and Sulphur required

Soil levels of potassium (K) and sulphur (S) can vary enormously due to factors other than soil type.

Single superphosphate contains sulphur. Consequently, if single superphosphate is being applied regularly, sulphur deficiencies are less likely (although can occur on lighter soils). However, if high analysis fertilisers (e.g. triple super) or blends low in sulphur are used regularly, then extra sulphur may be required.

Potassium is fairly mobile in most soils. It is leached readily, particularly from lighter soil types and in extremely wet conditions. It is also removed in large amounts in hay, silage and fodder crops.

Table 15.5   The estimated amount of capital potassium (kg/ha) required to raise the soil level by one unit (1 mg/kg)   (Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au, and Cameron Gourley, DPI, Ellinbank, Pers. Com. 2013)
Table 15.5 The estimated amount of capital potassium (kg/ha) required to raise the soil level by one unit (1 mg/kg)
(Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au, and Cameron Gourley, DPI, Ellinbank, Pers. Com. 2013)

For example, to raise the level of potassium (K) by 30 units in a clay loam soil, approximately 60 kg/ha of K above that required for maintenance must be applied.

Research into yield responses to K have not been as extensive as for P, however the more commonly used K tests have a greater degree of field calibration. Because of the K buffering capacity of soils and many other influences on K concentration in the soil, K levels can vary throughout the year, and substantially from year to year. It is therefore important to monitor K regularly – See Chapter 9.2.6.1 , ‘Tests for Available Potassium’.

Table 15.6   Estimated amount of capital sulphur (kg/ha) required based on soil test result  (Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au, and Cameron Gourley, DPI, Ellinbank, Pers. Com. 2013)
Table 15.6 Estimated amount of capital sulphur (kg/ha) required based on soil test result
(Adapted from Accounting for Nutrients Fertiliser Budgeting tool www.accounting4nutrients.com.au, and Cameron Gourley, DPI, Ellinbank, Pers. Com. 2013)

For example, if soil testing indicates that the sulphur level is 5 mg/kg, then 15 kg/ha of sulphur should be applied in addition to maintenance.

Field-calibrated trials for sulphur are limited and there is insufficient research to quantify the amount of sulphur required to raise the sulphur levels by one unit. In addition to this, sulphur becomes less available in cold, wet conditions, and in these conditions responses to applied nitrogen are sometimes improved if some sulphur is applied at the same time (M. Bolland, Pers. Com. April 2013) – See Chapter 9.2.7 .