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.

  • 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 Fertilizers.

 

As we discuss earlier Potassium plant fertilizers provide several benefits to turf. These include better drought, heat, cold and wear tolerance.

So, the temptation is to apply high amounts of K at certain times of the year. In fact, its common for the amount of potassium people apply, to often exceed the amount of nitrogen.

This can in some instances actually be detrimental to your turf.

When you apply too much of a potassium plant fertilizer it takes this K up. This is at the expense of other minerals such as Mg and is called is ‘luxury consumption’.

 As a result of ‘luxury consumption’, if you apply too much K, it causes a Mg deficiency. This is because turf takes up K in preference to Mg.

 

 

 

Research into Potassium Fertilizers in Turfgrass.

 

K content.

 

Snyder and Cisar (2000) examined K/N ratios of potassium plant fertilizer on tifgreen couch grass over a three-year period. Various K and N were applied in ratios of 0 (no potassium), 0.5, 1.0 and 2.0 (twice as much potassium as nitrogen).

The result was that K has a significant positive effect. This is only with the plots fertilised with potassium in comparison to those having none at all.

In contrast, the rate of K had little effect beyond a K: N ratio of 0.5 to 1.0.

These results were the same with leaf tissue potassium in grass. If you apply K it gives higher tissue levels in comparison to no K at all.

At higher rates of potassium plant fertilizer, it only result in a slight increase in tissue K levels.

Potassium Fertilizers and Anthracnose.

 

Murphy and Schmid 2018.

  • Low soil levels of K increase anthracnose severity.
  • The Mehlich 3 soil test K level for winter grass is 43 mg/Kg.
  • The critical leaf K concentration for wintergrass is 20 g/Kg.
  • At K levels above these there is no further reduction in disease severity.

 

 

Potassium Fertilizer options.

 

The following are common potassium plant fertilizers for adding potassium to grass in Australia.

The most common potassium plant fertilizers used on turf are

  • Potassium Sulphate fertilizer;
  • Potassium Nitrate;
  • Potassium Chloride;
  • Potassium mangesium sulphate;
  • Potassium Halite;
  • Potassium Formate
  • Potassium Carbonate;
  • Potassium Thiosulphate;
  • Potassium acetate and
  • Mono-potassium phosphate (MKP).
 
 

How to Choose a Potassium Fertilizer.

 

The following are seldom considered when it comes to how to add a Potassium fertilizer to a lawn or sports ground.

 

Water Quality.

 

 

If you are using poor quality water and salt is an issue your choice of K fertiliser becomes important. If irrigation water is high in chloride salts these can accumulate. Potassium sulphate provides K and S, and so is ideal where soils are deficient in both potassium and sulphur.

 

Rootzone characteristics.

 

 

How long the potassium says in the soil is also worth consideration. Unless it’s a coated potassium all the potassium in ‘slow release’ fertilizers is ‘up front’. This means it is prone to leaching in sandy soils and so goes to waste.

 
 

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.

 

Fernland   pH
Liquid K. 0-0-21 12.6
K Builder. 7-1-17 11-12
Nitro + K. 22-0-13 10.5
Stoller    
Liquid K. 0-0-33 11-12
Nuturf    
Vigor. 0-0-30 > 7

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 chloride (50% potassium), is also known as muriate of potash, and is a common potassium plant fertilizer to add potassium to a lawn or sports turf.
 
 
It can tend to form lumps in damp conditions, and is the least friendly K source to use in poor water water quality areas exist or if high temperatures occur.
 
 
 
Granular potassium is either a pink angular crystal or a white prill. Due to its high potential to burn, you should water this into the soil as soon as possible after use.
 
 
 
The safest option if you can’t water this in is potassium sulphate fertilizer for straight potassium or else MKP if phosphorus is not an issue.
 
 
If you apply 1Kg of potassium chloride, it adds almost 500g of chloride to the soil.
 

 

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

 
 
Potassium sulphate fertilizer contains 41.5% potassium, and is also known as sulphate of potash. Use of this as a potassium plant fertilizer to increase potassium in grass is common, and it is the second most popular K source. This common use of potassium sulphate fertilizer is due to concerns about the levels of chloride in potassium chloride.
 
 
The K in potassium sulphate fertilizer, is no different from other common potash fertilizers, but it also supplies S, which turf needs for protein synthesis and enzyme function.
 
 
It does not set in damp conditions.
 
 
If longevity is an issue then potassium sulphate fertilizer is your best bet on a freely draining sandy rootzone, as it remains longer in the soil than potassium chloride.
 

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 magnesium sulphate is also known as sulphate of potash magnesia and langbeinite. It is a natural mineral salt, and also contains 11% magnesium and 22% sulphur. It is used for both direct application and in bulk blending, particularly where magnesium is needed.
 
Potassium magnesium sulphate has a similar burn potential to KCl and should not be applied to turfgrass without watering it in.

 

 

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

 
Potassium nitrate (38% potassium), also known as nitrate of potash, is common in the turf industry. This supplies K and N.
 

 

 

Mono potassium phosphate. (MKP). 22.7% P and 28.5% K.

 
Mono-potassium phosphate (MKP) (28% potassium), this is a much neglected potassium plant fertilizer. It has some unique and extremely beneficial properties which should really result in its increased usage.
 
The first of these is it is highly soluble and the second is it has a very low salt index.
 

 

 

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.

jerry spencer senior turf agronomist
Senior Turf Agronomist at Gilba Solutions Pty Ltd | 0499975819 | Website

Graduated from Newcastle University with an Hons Degree in Soil Science in 1988, Jerry then worked for the Sports Turf Research Institute (STRI) as a turf agronomist before emigrating to Australia in 1993.

He followed this by gaining a Grad Dip in Business Management from UTS. He has worked in a number of management roles for companies as diverse as Samsung Australia, Arthur Yates and Paton Fertilizers.

He has always had a strong affinity with the Australian sports turf industry and as a result he established Gilba Solutions as an independent sports turf consultancy in 1993. Jerry has written over 100 articles and two books on a wide range of topics such as Turf Pesticides and Nutrition which have been published in Australia and overseas.