• Why is it important to me as a farmer?
• Why do we need potassium?
• How to recognise the need for potassium in the paddock

• Managing the application of potassium

• A large store of potassium is available in most soils
• Potassium (K) deficiency usually arises when plant removal is high
• Hay and silage production removes large amounts of potassium from the soil
• Potassium is relocated around the farm in dung and urine and should be replaced in areas of depletion

• If overused, chloride forms of potassium fertiliser can contribute to high salt levels in the soil
• There are animal health implications from potassium excess, especially in cattle in late pregnancy and early lactation
• Potash is often applied to soils in pasture systems, but there is limited information suggesting that its application to crops is economic

• A greater proportion of potassium (K) accumulates in the vegetative parts of mature plants. Potassium is highly mobile in the soil and is readily leached from stubble and during the breakdown of organic matter - unless crops are cut for hay, or the stubble is burnt or harvested for feed prior to K leaching. This is particularly critical within the first month after harvest

• Where soil potassium levels are high, potassium inputs can be reduced or deleted from the fertiliser regime. High potassium levels are potentially a concern for farmers, as they are implicated in the occurrence of grass tetany (cattle) and grass staggers (sheep)

• Potassium (K) is a macronutrient required by plants in large amounts for photosynthesis, enzyme activity, fruiting and the regulation of water within the plant. It also promotes plant health and disease resistance by influencing biochemical processes and cell structure
• Potassium is highly mobile in plant tissues and is readily leached from stubble into the soil
• Depending on the soil type, as well as rainfall amount and distribution, K may also be leached from sandy soil into groundwater, but is much less mobile in clay-textured soil profiles (e.g. vertosols)

For detailed information about regional soils, refer to: Soils of the Corangamite Region online

• Soils in much of south west Victoria had adequate levels of potassium in the 1800�s. However since that time, regular cutting of pasture for hay and silage, and high levels of nutrient export in stock products and transfer to stock camps have resulted in pasture responses to potassium fertiliser becoming increasingly common
• Improved pasture species allow a much higher stock-carrying capacity; but to maintain this productivity, they require a higher level of soil fertility than do native pasture species.
• Fertiliser applications are required to overcome the soil�s inherent nutrient deficiencies and to replace the nutrients that are lost or removed from the soil by pasture growth, fodder cropping or conservation, and animal products, such as milk or meat
• Nutrient redistribution around the farm and the inherent ability of soils to �retain� applied nutrients are other reasons for fertiliser applications
• Use soil tests to measure potassium levels in different parts of the farm. Potassium tends to accumulate in areas close to the centre of operations e.g. night and springer paddocks in dairies and feedout areas
• Use soil tests to measure potassium levels in different parts of the farm
• Potassium tends to accumulate in areas close to the centre of operations e.g. night and springer paddocks in dairies and feedout areas
• Large potassium losses due to hay and silage removal need to be replaced. Nutrient budgets can be used to predict the amount of fertiliser needed

• Examples:
  1. If you are producing 50,000 L of milk per year, then you are exporting 100 kg of potassium(K) off the farm in the milk. (50,000 L milk x 0.2% K = 100 L of K, which weighs 100 kg)
  2. If you buy in 100 tonnes of hay (80% DM), you are buying in 2000 kg K. If you sell 6 tonnes of silage (40% DM), then you are also selling 60 kg K
  3. If you have 100 cows who excrete 10 times per day, then the 365,000 excretions, each weighing 1 kg fresh weight (or 0.1 kg dry matter), would involve the cycling of 292 kg K. If 10% of this is deposited in the laneways and dairy, then 29.2 kg K are lost from the system and 263 kg of K are recycled through the pasture

• Note that in this section we are only talking about the plant nutrients that end up in various pasture and animal components. A large quantity of nutrients is also required to �drive� the system along, in other words, to produce plant growth and to cover leaching, nutrient transfer, soil fixation, and other parts of the nutrient cycles

• Note the high losses of potassium in the fodders. If the same paddocks are regularly cut for fodder and the fodder is not fed back in those same paddocks in the following season, then these paddocks will be quickly depleted, or �mined�, of their fertility, particularly potassium
• Conversely, if the fodder is continually fed back in the same small area (for example, in a sacrifice paddock situation), nutrient build up is greatly enhanced from both the fodder and the return of dung and urine

• Potassium deficiency symptoms include:
  • Reduced growth (possibly up to a 50% drop in yield of some crops before deficiency symptoms appear)
  • Whitish spots along the outer margin of clover (and lucerne) leaves, which subsequently develop a necrosis, or deadening, of the outer leaf margins
  • In grass, a pale-green colour, which may be followed by a pronounced yellowing to browning off, beginning with the tips of older leaves (called chlorosis, or tip burn). These symptoms are not sufficiently different from nitrogen deficiency or frost effect to allow them to be used to identify a K deficiency in grass
  • Excess salinity may also cause brown, necrotic leaf margins, but this occurs mostly in the younger leaves

• Potassium is very mobile in the plant (in other words, rapidly transferred around the plant), and deficiency symptoms initially occur in the older leaves. Deficiencies are most obvious at times of peak potassium demand (in other words, spring). Potassium deficiencies may not appear if a combination of nutrient deficiencies, such as phosphorus and potassium together, are limiting growth
• Grasses tend to be more deeply rooted than clovers and therefore can compete more strongly for potassium. A symptom of potassium deficiency is a grass-dominant pasture that often has an abundance of weeds. Older urine patches may show good clover growth if clover is present in the pasture, as 80% to 90% of the potassium in pasture consumed by stock is excreted in urine

• Deficiencies of potassium are most likely to occur on lighter sandy soils and regular �day� paddocks and particularly in paddocks that have been repeatedly cut for hay or silage

Figure 1 - Symptomatic scorching and necrosis indicating potassium deficiency in wheat. � Source: DAFWA

Figure 2 - Discolouration in sub-clover indicating potassium deficiency. � Source: DAFWA

• Soil nutrients need to be managed so that they meet crop demand and maximise profitability without causing off-site pollution. This suggests that replacement should equal nutrients removed through crop/animal products. An understanding of which nutrients are important, nutrient form and availability, and soil testing is required.
• Soil test results are the most useful tool for deciding on the use of potassium fertiliser
• Potassium is measured using the Skene K or Colwell K tests. The results are reasonably similar and are expressed in mg/kg (ppm). The nutrient status of potash differs depending on soil type

• Soils in the �adequate� range will generally achieve 95% of potential growth
• When potassium levels are high, inputs can be reduced or deleted from fertiliser regime as a pasture response is unlikely
• Nutrient budgets can also be used to indicate paddocks that may need fertiliser
• Fertiliser blends which contain potassium are useful if other nutrients are required

• An application of potassium will influence pasture growth for 6-8 years on clay loam soils but only 1-2 years on lighter soils. Typical application rates are:
  • 50-100kg K/ha (usually 100-200 kg/ha of muriate of potash) every 5-10 years on clay soils
  • 20 kg K/ha annually on lighter soils
  • After a hay or silage crop, 15kg K/ha needs to be applied for each tonne per hectare removed

• Muriate of potash (potassium chloride) is the most cost-effective source of potassium and consists of 50% potassium and 50% chloride (Cl). The application of chloride to pastures by using potash should not be of concern. The amount added with potash fertiliser (25-50 kg Cl/ha per application) is less than that in natural rainfall, about 170 kg Cl/ ha.year at the coast and deceasing to 22kg Cl/ha.year in the northern part of the region
• Potassium sulphate, containing 42% potassium, is also available but in much more expensive and generally only recommended in horticulture

• Muriate of potash (MOP�KCl; 49.5% K) is the cheapest form of potassium and is applied by top dressing either before seeding or up 5 weeks after seeding
• However, if it is a recognised deficiency, muriate of potash (MOP�KCl; 49.5% K) is the cheapest form of potassium and is applied by top dressing either before seeding or up 5 weeks after seeding
• Sowing MOP directly with the seed can significantly reduce crop germination and establishment, with rates of MOP higher than 30 kg/ha (22 cm row spacing) affecting germination significantly
• The development of sulphate of potash (SOP) is a less damaging form of potassium and can be drilled with seed. This product is significantly more expensive than MOP per unit of potassium

• What are the optimum nutrient targets for pastures?
• How do I improve soil without using too much fertiliser?

• Potassium - Soil Health Knowledge Bank � Department of Agriculture, Fisheries and Forestry


• Potassium factsheet - Soilquality.org.au


• What Nutrients do Plants Require? - Department of Environment and Primary Industries, Victoria


• Nutrient Planning - Department of Environment and Primary Industries, Victoria

• Soil Assessment Component - HDLN Soil & Water Dairy Action Program - June 2008 - Victorian Resources Online - Department of Environment and Primary Industries, Victoria


• Fertilising Pastures :(Chapter 5)- Greener Pastures for south west Victoria (2006) � Department of Environment and Primary Industries, Victoria


• Fertilisers for pastures - (See page 13) � Department of Primary Industries, NSW

• What Nutrients do Plants Require? - Department of Primary Industries, Victoria
• Nutrient Planning. - Department of Primary Industries, Victoria.
• Potassium. Soil Health Knowledge Bank � Department of Agriculture, Fisheries and Forestry.
• Bluett, C., and Wightman,B. (1996) Cropping in South-West Victoria. Depatment of Primary Industries, Victoria.

• Fertilising Pastures � (Chapter 5).Greener Pastures for south west Victoria (2006) � Department of Primary Industries, Victoria.
• Fertilisers for pastures. � Department of Primary Industries, NSW.
• Soil Assessment Component. HDLN Soil & Water Dairy Action Program - June 2008 - Victorian Resources Online - Department of Primary Industries, Victoria.

Page Updated: September 2013
Produced by AS Miner Geotechnical