The CRAFT of potassium for pastures

03.04.2018

In grazing country across southern Australia, the content of potassium (K) in the soil varies depending on parent rock, degree of weathering, leaching, redistribution by plants and animals and fertiliser applications.

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The great majority of potassium in our soils is bound in primary minerals or is present in secondary minerals which largely make up the clay fraction of the soil. High clay content soils are generally high in potassium.

When we test soils for potassium, it shows the extracted or available potassium levels.

Critical K values for shallow soil tests (0-10 cm) are a good source of information for pasture fertiliser decisions. Deeper 10-30 cm soil tests can also add to the information on available potassium, particularly for deep-rooted perennial species such as phalaris and lucerne.

Following soil testing, consider the CRAFT of potassium.

CRAFT is an acronym for Choice of Product, Rate of application, Application Method, Frequency of Application and Timing of Application. 

This principle is used in fertiliser recommendations to ensure there is no ambiguity in what, when, how and how often a product will be applied.

Choice of product

Muriate of Potash is the most commonly used potassium fertiliser in pastures. It is the cheapest form of potassium and is also a good blend partner with many other fertilisers, including SuPerfect®.

In fact, Muriate of Potash is more commonly used in blends than as a straight.

The table below highlights a number of potassium containing blends and their fit in certain pasture situations.

Sulphate of Potash is more expensive and less commonly used, however it does have a lower salt loading than Muriate of Potash.

Potassium fertilisers and blends for pasture

Analysis
Product N (%) P (%) K (%) S (%) Comment Typical rates
Muriate of Potash 50 Typcially used in blends. Applied as a straight where large capital potassium rates are required. 100 kg/ha
SuPerfect Potash 1&1* 4.4 25 5.5 Used where high rates of K are required in relation to P. Low soil test value, large removal previous season. 100-200 kg/ha
SuPerfect Potash 2&1* 5.9 16.7 7.3
SuPerfect Potash 3&1* 6.6 12.7 8.2 Used where a balance of P and K is required. 125-400 kg/ha
SuPerfect Potash 4&1* 7 10 8.8
SuPerfect Potash 5&1* 7.3 8.3 9.2 Used where soils are low in P relative to K. 100-600 kg/ha
* trace elements can be added where required, such as zinc, copper and molybdenum
Cal-Gran 150 18.6 3 12.5 1.4 Nitrate nitrogen for winter/early spring dry matter responses. Partial P and K replacement. 160-270 kg/ha
Cal-Gran 50/50 17.9 15 3.6 Nitrate nitrogen for winter/early spring dry matter responses. Used where soil P levels good, K needed to supplement low soil test value or fodder. 170-280 kg/ha
Cal-Gran Aftergraze 20.6 3 7.5 3.8 Nitrate nitrogen for winter/early spring dry matter responses. Partial P and K replacement. 150-250 kg/ha
PastureBoosta® 23.8 3.7 13 4.1 Spring application. Good for boosting dry matter production and supplementing some P and K. 130-200 kg/ha
FodderBoosta® 11.5 7.6 19.5 6.1 Spring application. Good for boosting dry matter production and supplementing some P and K in lower fertility situations (or high removal). 250 kg/ha
HayBoosta® 11.7 4.7 23.9 4.6 Spring application, for boosting dry matter production and supplementing some P and more K. Pastures may be clover dominant. 200 kg/ha
Source: Incitec Pivot Fertilisers, 2018

Rate of application

Before a rate is considered, there are other, higher priority soil considerations that need to be addressed. 

First, we need to account for the immobilisation of potassium which increases rapidly as pH(w) decreases below 5.5. These acid soils have a depressive action on potassium uptake.    

There can also be toxic levels manganese and aluminium associated with these acid soils, limiting pasture species establishment, production and persistence.

Correcting soil acidity through liming not only allows a greater scope for growing more productive pasture species, but also drives greater potassium availability.

Phosphorus is the next major soil consideration, as it is the driver for improved legume-based pastures. For beef and sheep systems, Olsen P levels should be in the range of 12-15 mg/kg and for dairy 18-24 mg/kg.

Critical ranges for potassium vary depending on soil type. Heavy soil types have higher critical values, as do more intensive/higher production systems. 

Table 1 highlights the critical ranges in topsoil for various soil types and relative pasture performance.

If soil test values for potassium are in or above the critical range, then potassium fertiliser will not give a pasture response. When soil test values are in the marginal or deficient range, applying some potassium fertiliser may increase potential pasture yields.

Where large amounts of potassium are being removed from the system in hay or silage, a full or partial replacement strategy may be warranted.

As a rule of thumb, for every tonne of dry matter removed, 25 kg of nitrogen, 2 kg of phosphorus, 25 kg of potassium and 2 kg of sulphur are removed from the soil if that fodder is not fed back on the paddock. 

Depending on the soil test, the decision may be to mine the soil potassium if values are high, adopt a partial replacement strategy if soil test levels are marginal and there is some potassium being supplied from depth or use a full replacement strategy if soil test values are low to deficient and there is no potassium at depth. These soils are likely to be lighter and be in high rainfall or irrigation environments. They may also have had a long history of fodder conservation.

Annual rates can range from 0 to 200+ kg/ha of potassium.

Maximum single applications of potassium should not exceed 50 kg/ha. There are two main reasons for this. Plants can consume luxury quantities of potassium, which can be wasteful if fodder is being removed from property, and grass tetany to sensitive stock can be a major problem.

Soil Test Guidelines - Department of Primary Industries

Source: Department of Primary Industries, Victoria

Application method

Broadcast application is the best method of applying potassium in pastures.

Being relatively soluble, the potassium fertiliser will move into the soil with rainfall or irrigation. 

Correct spreader setup is important to ensure an even distribution across the swath. If potassium is being applied as part of a SuPerfect blend, it should be applied to the maximum MOP spread width rather than the maximum SuPerfect spread width, otherwise pasture striping and underperformance will be evident. MOP will not spread as far as a well granulated SuPerfect fertiliser.

Rarely is potassium banded with the seed due to the salt index of MOP, which heightens the risk of seed burn.

The combination of nitrogen and potassium in contact with the seed should not exceed 20 kg(N+K)/ha on 15 cm row spacings on loam soils with good moisture. Safe seed rates decrease with wider row spacings, and in lighter dry soils.

Frequency of application

Similar to rate, frequency of potassium applications will vary depending on the enterprise (fodder conservation, grazing), the critical soil test value and the total annual amount of potassium that needs to be applied.

If an application of less than 50 kg/ha of potassium is required, this could be applied once in autumn with SuPerfect or in spring with nitrogen (being mindful of critical K soil test value and class of stock).

If paddocks are to be cut numerous times in spring for pasture silage, hay or lucerne hay, then several smaller applications are better.

Where four to five cuts of lucerne hay are being achieved, consider applying potassium after every second cut.

Timing of application

If soil test critical values are marginal to deficient, it is important to get potassium on early in the major growth periods of the pasture. 

Typically, this is in autumn when it can positively influence clover composition, and spring when it can assist in producing additional dry matter or as a replacement strategy for fodder conservation.

For more information or to discuss potassium in more detail, feel free to call me on 0412 565 176 or email lee.menhenett@incitecpivot.com.au.