Potassium Fertilizers. 45

High Potassium Fertilizers.

We discuss the role of Potassium in grass. How to add Potassium to your lawn? What Potassium plant fertilizers are available? How to address Potassium deficiencies? What is the role of Potassium in grass? and some cool things about Potassium Sulphate fertilizer.

We also question if high rates of potassium plant fertilizers lead to harder wearing turf?

The word “potash” often refers to potassium chloride, but it applies to all K fertilizers.

Sadly, most people simply look at the number relating to K content on the side of a bag or drum. The larger the number the better!

That isn’t always true. You should always consider:

• The form it is in. Is it slow release, coated or ‘up front’.
• What happens to it once you add it to a spray tank.
• How does it behave on the leaf and
• What happens to it if you wash it into the soil.

K fertilizer salts vary widely in their properties and all of the above effect the results you get.

The Role of Potassium in the Plant.

After N, Potassium (K) is the second most important nutrient for turf growth.

It has several roles in turf:

• K is important in protein synthesis.
• It helps to control when stomata open and close.
• Low levels can stunt plant growth and reduce growth.

Other roles of K in turf include:

• It Increases root growth and improves the drought tolerance of turf grass.
• It attracts water and reduces water loss and wilting. This is important in the Summer, when turf needs moisture to transpire.
• K helps in photosynthesis.
• K reduces respiration, and prevents energy loss.
• It improves the movement of sugars and starch.
• It helps counter disease.

What Potassium Fertilizers Don’t Do?

K seems to give mixed results when it comes to an increase in the wear tolerance of turf.

• Shearman and Beard (1975) reported increases in K improves creeping bentgrass wear tolerance.

Other studies, show contrasting results on other turf grass species and creeping bentgrass.

• A 1991 study by Carroll and Petrovic. Potassium does not affect wear tolerance or recovery in creeping bentgrass or kentucky bluegrass.
• In 2001 Trenholm found no increase in wear tolerance in seashore paspalum on a high sand- based green. On a clay soil, there were significant increases in wear tolerance.
• Hoffman et al in 2010 found tissue and soil K are not correlated with wear tolerance in perennial ryegrass.
• A 2017 study on the use of K in the Autumn. Potassium does not improve creeping bentgrass winter traffic tolerance.
• Work on seashore paspalum, creeping bentgrass and couch. In four out of five studies, the addition of K does not improve wear tolerance.

How Much Potassium is in grass?

The concentration of K in the soil is from 0.1 to 3%, and it makes up 2% of the leaf dry matter. This means that the right K source is important in any fertilizer strategy to produce high performance turf grass.

Potassium Uptake.

When turf uses K, it replaces what it uses. It does this in two ways:

• Via the roots from exchange sites on clay particles and the soil organic matter.
• By K physically moving from the soil to the roots.

In sandy soils, K rapidly moves out of the soil profile. This happens for two reasons:

• Because sands cannot hold onto nutrients.
• Potassium has a single positive charge (it is monovalent).
• Divalent cations have two positive charges and include Calcium and Magnesium. Divalent cations are more strongly attracted to soil exchange sites and so are not so susceptible to leaching.

Root interception of K only accounts for a small fraction of what the turf needs. Most of the K moves from the soil to the roots.

This transport of K ions occurs by mass flow in the soil solution, and by diffusion.

• Mass flow happens as water that contains K moves to the plant root.
• Diffusion plays a lesser role, and occurs along a concentration gradient, as a result of plant roots absorbing K.

As turf roots remove K from next to the roots, the soil solution quickly runs out of nutrients.

To maintain a supply of K to the plant, the rate of K release to the soil solution, and transport to the roots needs to keep pace with the rate of nutrient uptake. This is one reason why you add K to a lawn or turf grass.

Factors effecting Potassium Availability.

• In the soil, Potassium availability is best in neutral to alkaline soils. As the soil pH falls and it becomes more acidic, K availability decreases.
• Soils with a high OM content have few K minerals that contain K and so are very low in Potassium.
• Poor soil aeration affects Potassium uptake more than other primary nutrient. Air is vital for root respiration and K uptake.
• In waterlogged soils, soil Oxygen levels are very low, and so K uptake decreases.
• Higher soil moisture usually means greater K availability. As soil moisture increases, the movement of K to plant roots increases. This explains why you get a better responses to K fertilizer in dry years.
• As soil temperatures increase root activity increases. This causes an increase in the take up of K.
• The ideal soil temperature for this to occur is 16 to 27°C.
• At low soil temperatures K uptake reduces.

Potassium Fertilizers.

• Potassium plant fertilizers provide several benefits to turf. These include better drought, heat, cold and (possibly) wear tolerance.
• The temptation is to apply high amounts of K at certain times of the year. In fact, its common for the amount of Potassium that people use, to exceed the amount of N. In some instances this is detrimental to your turf.
• When you apply a lot of a Potassium plant fertilizer plants will take this up. They do this at the expense of other minerals such as Mg. This is called is ‘luxury consumption’.
• ‘Luxury consumption’, can cause an Mg deficiency. This is because turf takes up K in preference to Mg.

Potassium Fertilizer options.

Dissolution Rate.

The dissolution rate is how fast a K salt dissolves in water. All K salts don’t dissolve at the same rate. Differences even exist between for example Potassium Sulphate fertilizers. The analysis on the bags may say the same thing but they may not behave the same in water.

Effect on Tank water pH.

This is an important if you use a K salt in conjunction with certain chemicals.

• Some Potassium Sulphate fertilizers lower the tank water pH.
• Some liquid K fertilizers will increase the pH of your spray tank water and the leaf pH.

A 1% solution of Solupotasse Potassium Sulphate fertilizer has a pH of 2.9. Thats fine in many situations. But if you tank mix this with a sulfonylurea herbicide then this will have a negative impact on the results you get.

Effect on Leaf pH.

To further complicate things, you can use some Potassium fertilizers to increase the leaf pH and so potentially suppress dollar spot.

There are a range of Potassium plant fertilizers that will increase Potassium in grass. We next discuss how each of these helps add Potassium to a lawn or to sports turf.

K Fertilizers.

Potassium Chloride. (KCl). 49.8% K and 45% Cl.

• Potassium Chloride (50% potassium). This is also known as muriate of potash, and is a common Potassium plant fertilizer to add potassium to a lawn or sports turf.
• It tends to form lumps in damp conditions.
• Not the best K source to use in poor water water quality areas or in high temperatures.
• Due to its high potential to burn, you should water this into the soil immediately after use.
• If you apply 1Kg of Potassium Chloride, it adds almost 500g of chloride to the soil.

Potassium Magnesium Sulphate. 18% K, 22% S and 10.8% Mg

Potassium Fertilizers based on Potassium nitrate. (KNO3). 13% N and 36% K.

Mono Potassium Phosphate. (MKP). 22.7% P and 28.5% K.

Potassium Halite. (K₂SO₄·MgSO₄·2CaSO₄·2H₂O). 11.6% K, 3.6% Mg, 12.14% Ca, 19.2% S.

• Potassium Halite occurs naturally, and supplies K, S, M, and Ca.
• It acts as a slow-release source of K.

Potassium Fertilizers based on Potassium Formate. (KHCO₂).

• The only commercial product we are aware of in Australia that contains potassium formate is YaraVita Safe N.

Potassium Acetate. (CH3CO2K). 27% K.

• Potassium acetate has a very low salt index
• Its foliar uptake is up to 5x greater than traditional K sources.
• It has the  lowest POD of all potassium fertilizers. 
• The lower the salt index for a fertiliser the less likely it will burn turf.
• The lower the point of deliquescence (POD) is, the more likely it will stay in solution and so remain available to the turf plant.

Potassium Carbonate. (KCO3). 56.78% K.

• Potassium carbonate is the next potassium fertilizer to add potassium to a lawn, and is also known as Pearl Ash.
• It has unique physical and chemical properties that make it a fast-acting, alkaline (high pH) fertilizer.
• It is a colourless or white crystal or granular powder, that is soluble in water (12 g/100mL at 25°C).
• When you mix this with water it gives a violent reaction which is why it is sold in a pre-mixed liquid form.
• It is alkaline and caustic, so care must be taken to prevent the powder or liquid from splashing onto the skin or into the eyes

In a 1996 article reference is made to the control of fairy rings with Potassium carbonate. You use this at a rate of 0.9 kg of material/100m2 for two applications spaced two weeks apart. The potassium carbonate seems to work best when you do this in the Spring or the early Summer when you first see the rings.

Best results occur if you water this in straight after you use it to prevent turf damage due to its high salt index. This also moves the chemical into the soil  to where the fungae are.

Potassium Thiosulphate. (K₂S₂O₃). 30% K (w/v), 25% S (w/v).

The final common Potassium plant fertilizer to add Potassium to a lawn or sports turf is Potassium Thiosulphate. This has potential to act as a nitrification inhibitor, but its effectiveness as an inhibitor depends on the soil type and climatic conditions.