• Soil testing is a very effective means of determining potassium requirements.
• Potassium export in crops, milk or meat must be accounted for when calculating nutrient budgets.
• Potassium removal by crops can be similar to or exceed removal rates of nitrogen.
• Potassium deficiency symptoms first occur in the older leaves of plants and can be mistaken for disease infections.
• Dairy shed effluent applied to paddocks can result in excess soil potassium and potential animal health problems.

Potassium (K) is one of the essential nutrients in plants and is one of three nutrients (including nitrogen and phosphorus) that are commonly to the soil to ensure optimum productivity on many soil types in Tasmania (Cotching and Sparrow 2012). Compared to other nutrients, plants need relatively large amounts of K and over the years many soils have been depleted of K because of K removal in farm produce. Potassium has many functions including the regulation of the opening and closing of stomata which are the breathing holes in plant leaves that regulate moisture loss from the plant. For this reason, K is known as the poor man’s irrigation because it can help crops tolerate dry spells. Potassium in the soil solution is subject to leaching and is more readily leached than phosphorus, but less so than nitrate nitrogen or sulfur.

Soil and plant analysis give insights into the availability of K in the soil. Research has shown that the concentration of extractable soil K for optimum production of pastures and many crops varies with soil texture (Table 7, Appendix 6). However, no such dependency has yet been shown in cereals which have a lower soil K need than dicotyledonous plants (Table 7). The test commonly used to determine plant available soil K is known the Colwell test. When the Colwell test is not available on your soil test results, the cation exchange value of K (cmol/kg) can be used, and the conversion commonly used is to multiply the exchange value by 391 to calculate the equivalent Colwell K value.

The availability of K may be affected by soil type and the ability of the plant to acquire sub-soil K (Edwards 1993). Deep-rooted species can make use of such K. Sandy-textured soils (with a low cation exchange capacity) in higher rainfall areas have limited ability to store K and are, therefore, more prone to leaching losses of K. A critical value for 95% maximum yield for potatoes on Ferrosols is 400 mg K/kg soil for Russet Burbank and 350 mg/kg soil for Kennebec (Chapman et al. 1992). For Sodosols and Tenosols the critical value of 200 mg K/kg is used for potatoes in Tasmania. Soil test results can vary within paddocks due to differences in soil type and in pasture paddocks that have hump and hollow drainage installed. Fertiliser K freshly applied to the tops of humps can be washed in surface runoff to the hollows (Cotching 2000). Practices such as burning of windrowed crop stubbles can concentrate K in particular parts of a paddock. For these reasons growers should use soil test results in conjunction with plant tissue testing to determine application rates for paddocks.

Potassium lost through product or biomass/dry matter removal should be replaced. Removal rates for each crop differ, and this must be accounted for when budgeting K requirements for the coming season (Table 8; Appendix 2). When removal rates of K by potato and poppy crops were compared to fertiliser application rates on Tasmanian soils, K fertiliser rates on poppies and potatoes were found to be mostly at, or close to, maintenance K rates (Sparrow et al. 2003). Large amounts of K are applied to potato crops, and potatoes remove much of this.

Most soils in Tasmania are prone to K deficiency, and pasture legumes are particularly susceptible to K deficiency while cereal yields appear to remain unaffected. Unless plant symptoms are recognised, or soil or tissue testing done, the first signs of K deficiency in a paddock may be poor growth and a gradual disappearance of the pasture legume component. Crops tend to react faster to water deficits by wilting, if K is in short supply. The quantity of K within dairy effluent is high and will vary with feed type and quantities fed. Minimal K is likely to be lost from the farm through collection and dispersal of dairy effluent on the farm, and it can be assumed that most K collected will be available for reuse. Transporting of hay and silage around the farm and feeding these out on different paddocks to those from which it has been harvested can result in high soil K levels on some paddocks.

Excessive soil K levels can result in high K uptake by pasture, thus increasing K intake by animals (Hosking 1986, Judson and McFarlane 1998). This can increase the risk of stock health problems, notably calcium deficiency (milk fever or hypocalcaemia) and magnesium deficiency (grass tetany or hypomagnesaemia). The high K concentration in pasture suppresses the uptake of calcium and magnesium by stock, leading to low concentrations of each in the cow’s bloodstream. Dairy cows are most susceptible to high K levels in the diet during the transition period (before calving) and early lactation. Not grazing cows on areas where dairy effluent has been applied during these times, particularly on consecutive days, will minimise the risk of grass tetany. In addition, grazing the pasture when ryegrass has reached the three-leaf stage is recommended, because the concentrations of Ca and Mg will have increased in the plant by that stage. High sodium (Na) and or magnesium (Mg) uptake can depress K uptake in plants leading to a cation imbalance. Increasing the proportion of legume in the pasture will also increase the concentration of Ca and Mg in the animals’ diet. Late winter is a risky time because pastures then tend to be grass dominant. Magnesium oxide can be added to stock feed to reduce the risk of grass tetany. To minimise the risk of grass tetany and milk fever, annual applications of K should be based on representative soil test results plus a whole farm nutrient budget. K is often at different levels within paddocks and across the farm which means that blanket fertiliser recipes may lead to over or undersupply of potassium.

Potassium is highly mobile in the phloem of plants and can be moved to newer leaves if the nutrient is in short supply, with deficiency symptoms appearing first on older leaves. General symptoms initially include a light green to yellow colour of the older leaves especially around leaf margins. Marginal scorch of the edges and tips of these leaves follows, often resulting in senescence. As the severity increases, this condition progresses towards the top of the plant. These characteristic symptoms of K deficiency can often be mistaken for leaf diseases such as yellow spot and Septoria nodorum blotch in wheat or brown leaf spot in lupins. Other symptoms include slow plant growth, weak stems and lodging, high screenings levels in the harvested grain and reduced disease resistance. Aplication of potassium fertiliser tends to diminish the incidence of fungal and bacterial disease as well as insect pests in crops (Amtmann et al. 2008). Given the severe effect of potassium deficiency on plant functions and health, and the difficulty in diagnosing deficiency early, it is not recommended to wait for symptoms to appear before trying to rectify the problem. If plant testing shows that potassium levels are low, foliar application have shown to be effective in crops.

Muriate of potash (MOP - KCl; 50% K) is considered the cheapest form of K. It is applied by top dressing either before seeding or up to 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 (22cm row spacing) affecting germination significantly due to its high chloride (Cl) content. MOP may appear to be ‘cheap’ per unit of K, but each unit of K comes with one unit of Cl. Plants need only very low amount of Cl and high uptake is detrimental. In high rainfall areas and under irrigation in Tasmania, Cl is usually washed out of the rootzone on an annual basis. Sulphate of potash (SOP – K2SO4; 41%K) has a lower salt index than MOP (Appendix 4) and is often preferred to the latter in crops sensitive to chloride or susceptible to fertiliser root burn. SOP is significantly more expensive than MOP per unit of K, but SOP comes with sulfur as a second nutrient that is often neglected in production systems. The cost of SOP must be allocated to both K and S, rather than just K. Potassium nitrate (KNO3 – 38% K) is often used where potassium is to be applied in solution (through irrigation systems and as a foliar spray) as it is more soluble, it contains available nitrogen and doesn’t contain chloride, but it is more expensive.

Potassium can be applied once per year where application rates are low, but at high rates (> 40 kg K/ha) it is usual to apply split applications to avoid luxury uptake by plants and increase K use efficiency. On sandy soils, more splits are often advised in order to minimise leaching losses.