Author: Lee Menhenett, IPF Technical Agronomist

Potassium (K) dynamics in farming systems are complex and difficult to track – within the farm, around the farm, and in nutrients leaving the farm.

K enters through purchased feed or fertiliser and is exported when commodities such as fodder or livestock are sold.

Among these, fodder represents the most significant pathway for K loss, with typical concentrations ranging between 2-3%. Table 1 shows the removal of nutrients from two grass pasture paddocks and highlights the extend of K removal.

While farmgate imports and exports of K can be relatively easy to monitor, closing paddock- level K budgets is far more challenging. This is due to seasonal and annual variability in stocking rates and the dynamic nature of fodder conservation and feeding practices.

Given these complexities, effective K management must be underpinned by a robust soil sampling regime and the use of historical soil test data. Understanding K nutrient flow on and around the farm is important to understand, however this needs to be supported with medium to long term nutrient trends that provide evidence for any emerging issues.

Why K matters

K is highly mobile within plants and is vital for many functions, with its primary role being moisture regulation through control of stomatal opening and water transpiration.

Soils contain a large amount of K, however only a small amount of it is available for plant uptake. Once K is within the plant, it is very mobile and is readily transferred from older leaf and root tissue to younger tissue at the growing points. As a result, deficiency symptoms are generally seen in older parts of the plant. Leaf spotting, tipping and margin chlorosis are some of the classic deficiency symptoms, while others include low vigour, weak growing plants, poorly developed root systems and small leaves.

Across all Australian agricultural areas, production figures and fertiliser use have revealed a negative K balance of around 400 kt/year which equates to removing 3.2 times more K than what is replaced (Norton 2017a). While this negative K balance is alarming, the economic benefit in many agricultural situations does not warrant applying K.  This is because soils are often adequately supplied with K (within the soil profile) and adsorbed soil K exchanging to meet K equilibrium. However, when the rate of removal is too rapid and the K equilibrium cannot be maintained, or when soil K levels fall below critical thresholds, agronomic responses to applied K occur.

Plants have varying abilities to forage for K

In pasture systems, once K deficiency symptoms are visible, dry matter yield has already been lost. The most common form of K fertiliser used for pastures is Muriate of Potash (MOP) which is a relatively soluble salt (KCl) typically applied via broadcast. Being surface-applied, effectiveness depends on adequate soil moisture or rainfall to move the nutrient into the soil profile.

Heavier soil types require more rainfall to move MOP into the soil compared to lighter soils, with K movement deeper into the profile occurring on lighter soil types. This is an important consideration, as fibrous root type pasture such as ryegrass, tend to have the greatest activity in the surface soil (0-30cm) rather than the subsoil.

While there is a direct relationship between K update and root density, there is also variations between pasture species on their effectiveness in recovering K. Clovers, for example, have a different root morphology compared to grasses and a lower ability to recover K which is why they often drop out of the sward first in K-deficient conditions.

Soil K critical ranges are typically assessed at the 0-10 cm depth, but deeper reserves may be accessible if the roots can reach them. In intensive fodder removal systems (maize silage or several grass silage/hays/yr) establishing a baseline K availability to root depth is important to monitor soil values overtime.  This can provide important data to ensure the K replacement strategy is correct.

Lighter textured sandy soils have a smaller capacity to hold K (less clay) and may lose K via leaching in high rainfall environments. Lighter soils also have a lower critical range, and with their reduced K-holding and exchange capacity, decisions around K fertilisers applications differ compared to heavier-textured soils.

Surface and shallow applications of K fertilisers and the return of crop residues to the soil surface are enriching the upper level of soils. If a possible K deficiency is detected, then taking a sub soil test (10-30 cm) may provide a better understanding of levels further down the profile. A plant tissue test is also an option to confirm if a soil could be responsive to K.

Optimising K inputs

When deciding what K fertiliser to apply, utilising the 4R’s – Right Rate, Right Time, Right Product, and Right Placement – helps ensure a sound recommendation.

Research conducted by Bell et al (2021) highlighted that cation exchange capacity (CEC) has a large influence on product application technique and product rate. Application guidelines developed based on this research are:

  • Soil with <15 cmol(+)/kg CEC – all application methods are effective but total K rate should be spread out over multiple applications to reduce luxurious uptake or K leaching on soils with <5 cmol(+)/kg CEC.
  • For soils with >15 cmol(+)/kg CEC, banding and collocating K with P may improve plant uptake. Given most K applications are broadcast in pastures the rate of K can be increased on heavier soils (and applied less often).

In a grazing situation, a lot of dry matter produced is unused and returned to the soil, re-supplying K. This contrasts with fodder removal, where large amounts of K are taken off farm.  Where fodder is fed to animals, K in unutilised fodder remains in the paddock, as does some K when grain/fodder is supplementary fed and animals return the paddock post milking. This source of K should not be underestimated.

Animal health issues may arise due to potassium’s interaction with magnesium. High producing lactating cows are the most susceptible. Environmental conditions can favour plants to have greater supply of potassium than calcium (Ca) and magnesium (Mg). To reduce this risk, studies completed by Gourley 1999 and Hosking 1986 have recommended to not exceed 120kg K/ha annually and 60kg K/ha in one application. Pasture sward composition also plays a role in K/(Ca+Mg) ratios, with clovers having a higher ratio of Ca+Mg compared to grasses.

Once pasture K requirements have been established, attention turns to what the best product fit will be. To determine product suitability, several key questions need to be answered: Is nitrogen required? Are soil K levels within the optimum range, or if below, is  capital K needed? What are the K removal levels from grazing and/or fodder conservation? Am I going to have a full or partial replacement strategy, and how will that be split between autumn, winter and/or spring?

K is a complex nutrient, so understanding K flow on farm and at paddock level is important to determine were to focus attention. The greatest priority is to collate sound soil test data (shallow and deep) on a regular basis (every few years) to determine K status and importantly trends over time.  

Figure 1: Forage harvester precision chopping grass silage; chopper harvesting in progress. Source: Incitec Pivot Fertilisers

Figure 2: Windrowed grass pasture for silage conservation. Source: Incitec Pivot Fertilisers

Table 1. Silage nutrient removal (/tDM and /ha) from two pasture paddocks

Further information

For more information on Potassium in pasture, please reach out to IPFs Technical Agronomist, Lee Menhenett on 0412 565 176, or at lee.menhenett@incitecpivot.com.au

References

Gourley CJP (1999) Potassium. In ‘Soil analysis: an interpretation manual’. (Eds KI Peverill, LA Sparrow, DJ Reuter) pp. 229–239. (CSIRO Publishing: Melbourne, Vic.)

Hosking WJ (1986) Potassium for Victorian Pastures—A Review. Department of Agriculture and Rural Affairs, Victoria. Gourley CJP (1999) Potassium. In ‘Soil analysis: an interpretation manual’. (Eds KI Peverill, LA Sparrow, DJ Reuter) pp. 229–239. (CSIRO Publishing: Melbourne, Vic.)

Murrell, RL Mikkelsen, G Sulewski, R Norton, ML Thompson (2021) ‘Improving Potassium Recommendations for Agricultural Crops(eBook)’ (Springer: Switzerland) https://doi.org/10.1007/978-3-030-59197-7