Potassium Fertilizers. 45

Potassium Fertilizers.

We discuss the role of potassium in grass. How do you add potassium to your lawn? What potassium plant fertilizers are available? Does potassium increase the wear tolerance of Turf grass? 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 fertilizer leads 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 it behaves on the leaf and
• What happens to it if you wash it into the soil.

K fertilizer salts vary widely in their properties and the above all 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 grass:

• K is important in protein synthesis.
• It helps to control the opening and closing of leaf stomata, and
• If K levels are low it stunts plant growth and reduces growth.

Other roles of K in turf grass include:

• K Increases root growth and improves the drought tolerance of turf grass.

• It actually 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, and finally

• K helps counter diseases.

What Potassium Fertilizers don’t do?

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

• 1975 wear tolerance increases with  K fertilizer. K fertilizer increases the concentration of K in leaf tissue and improves the load-bearing and leaf tensile strength of creeping bentgrass
• Shearman and Beard (1975) reported increases in K improves creeping bentgrass wear tolerance.

Other studies, show contrasting results on other turf grass species (including creeping bentgrass).

• A 1991 study by Carroll and Petrovic Potassium did not affect the wear tolerance or recovery from wear injury in either 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 K significantly increases wear tolerance of seashore paspalum.

• 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 autumn, potassium does not improve creeping bentgrass winter traffic tolerance.

• 2021 work on seashore paspalum, creeping bentgrass and couch. In four out of five studies, the addition of K does not benefit wear tolerance.

How much Potassium is in grass?

The concentration of K in the soil is from 0.1–3%, and it makes up 2% of turf grass leaf dry matter.

Potassium Uptake.

When turf grass uses K, it replaces what it uses. It does this via the roots from exchange sites on clay particles and organic matter.

As you may be aware in sands or sandy soils, K rapidly moves out of the soil profile and this occurs for two reasons.

1. Because sands cannot keep nutrients and
2. Potassium has a single positive charge (it is monovalent). Divalent cations have two positive charges and include calcium and magnesium. These are more strongly attracted to soil exchange sites and so are not so susceptible to leach.

This root interception of K only accounts for a small fraction of what the turf needs. Instead, the majority of K moves from the soil to the roots.

This transport of K ions occurs by mass flow in the soil solution. This mass flow occurs as water that contains K in solution moves to the plant root. Diffusion also plays a lesser role. This occurs along a concentration gradient, as a result of plant roots absorbing K.

As turf roots remove K from directly 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, availability decreases.

• Soils with a high organic matter content have few K containing minerals and so are very low in potassium.

• Poor soil aeration affects potassium uptake more than other macronutrients, and air is vital for root respiration and K uptake. Root activity and K uptake decrease as soil moisture content increases. This is because in waterlogged soils, soil oxygen levels are very low.

• 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 temperature increases root activity, and physiological processes all increase. This increase in activity causes an increase in the take up of K. The ideal soil temperature for this to occur is 16 to 27°C, and at low soil temperatures K uptake reduces.

Potassium Fertilizer options.

Dissolution Rate.

The dissolution rate is how fast a K salt dissolves in water. All potassium 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 potassium salt in conjunction with certain chemicals.

• Some potassium sulphate fertilizers will actually lower the tank water pH.
• Some liquid K fertilizers will increase the pH of your spray tank water and in turf 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 use it 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 of the pre mentioned 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 sports turf.

Potassium Fertilizers.

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

Potassium Fertilizers based on Potassium Sulphate. (K₂SO₄).  41.5% K and 18% S.

When K is being applied to turf that is already showing signs of stress, potassium sulphate may be a better option than other, high-burn potential K sources.

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, and acts as a slow-release fertilizer.

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 the lowest salt index, the highest K uptake – up to 5x greater than traditional K sources, and the  lowest POD of all potassium fertilizers. So if you want the best possible chance to increase the potassium in grass this is worth a look.

• 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 in common use 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 (12g/100mL at 25°C). When you mix this with water it gives a violent reaction which is why it is sold in a 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 early summer when the rings first appear.

Best results occur if you water this in straight after you apply it to prevent turf damage due to its high salt index. If you water it in this also moves the chemical to greater depths as teh fungae are well below the surface soil.

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

The final common potassium plant fertilizer in use to add potassium to a lawn or sports turf is potassium thiosulphate. This has potential to act as a nitrification inhibitor, but its how effective it is as an inhibitor depends on the soil type and climatic conditions.