Magazine - Good Science Should be Applauded.

By Jerry Spencer in Editorial on 19th Jun 2009 6:30

New products are great and should be encouraged within the industry. Good science should be applauded. However, sadly our industry does suffer from more than its fair share of snake oils or misleading promotional products and when it comes to calcium based amendments there does appear to be a considerable amount of confusion in the marketplace of how these products work and if they actually work at all!!

Let’s focus on calcium as this does appear to be rather topical!! These are sold as either amendments to modify the soil such as calcium carbonate (lime) or calcium sulphate (gypsum) or alternatively to increase calcium tissue levels within the plant.

Calcium Soil amendments

pH

Generally fairway or sports turf soils  largely buffer soil acidity unless they possess a high sand content. This has great implications on nutrient management since buffering capacity determines the amount of resources, such as lime, that must be added to correct soil acidity. Soils that have high buffering capacities require larger amounts of liming resources to raise the pH to a target value than soils with low buffering capacities.

Rule of Thumb

• Finely-textured clay soils tend to have greater buffering capacities than coarse-textured soils

pH is a measure of the hydrogen ion* concentration of a substance ranging from 1 -14. It is not a measure of the calcium/lime content of a material, or soil in this case!

The lower the pH reading, the higher the hydrogen ion concentration [H+], and the more acidic the soil. The higher the pH, the higher the hydroxyl ion concentration [OH-], and the

more alkaline the soil. A soil pH of 7 is considered neutral, ie. [H+] = [OH-]. pH is the negative logarithm of the hydrogen ion concentration, therefore if the pH changes by 1, the ion concentration actually changes by 10. For example, pH 5.8 is ten times more acid than pH 6.8, so that a unit change in soil pH is actually quite dramatic!

In order to change soil pH you basically 'mop up' hydrogen ions! There are a variety of ways to do this which can include lime or calcium hydroxide - but it is certainly not limited to this.

Lime (CaCO3 - calcium carbonate) mops up hydrogen the following way: CaCO3 + 2H+ → Ca2+ + H2O + CO2

Calcium hydroxide is a strong base and rapidly ionizes to CA2+ and OH- ions. The calcium ions replace absorbed H ions on the soil colloid and thereby neutralize soil acidity. The carbonic acid formed (H2CO3) is a weak acid and slowly and partially ionizes to H+ and CO2 -2 ions. Therefore, the net effect is that more Ca than H ions are released in the soil and consequently soil acidity is neutralized.

Hydrogen is a highly reactive/aggressive cation capable of stripping other cations from their bonded anionic partners, e.g. (CO3)2-. Calcium, magnesium and other cations are merely passive passengers in the process of raising pH - it is their anionic partners that actually do the 'mopping up'.

Gypsum (CaSO4) contains Ca in appreciable amounts, but does not reduce soil acidity. The reason it does not is because when gypsum hydrolyzes in the soil, it converts to a strong base and a strong acid as shown in the following equation:

CaSO4 + 2H2O = Ca (OH)2 + H2SO4

These two products formed, Ca(OH2) and H2SO4, neutralize each other and the result is a neutral soil effect. Some unexpected results can occur though. For example, calcium sulphate (CaSO4) found in gypsum can react the following way in the presence of excess hydrogen:

CaSO4 + 2H+ → Ca2+ + H2SO4 + CO2

H2SO4 is sulphuric acid!! Therefore adding sulphate-based inputs (even with a calcium component) to highly acidic soils can plummet pH levels down even further.

The graph below shows how a recently introduced product, namely Calsap© effects soil pH. When it is realised that it has a pH of 12-13.5 (as shown on the manufacturers MSDS) this pH effect can be put down to the anionic partners of the calcium rather than the Ca itself. Calsap© is made on Calcium Hydroxide (Hydrated Lime) that has been reacted with Potassium Hydroxide (Liquid as an acid).

Sodium

In the mid 1980’s a product was released called Sodex© based on calcium chloride and was sold as the be all and end all of sodium displacement. Calsap© is also being sold making the same claims as shown by the diagrams below  taken from both of their respective promotional brochures (similar?!!)

 

 

 

 

But what exactly is the science behind sodium displacement? The most common procedure for correcting sodium affected soils is to amend the soil with materials that replace the exchangeable sodium on the soil colloids.  Remediating sodium damaged soils in a timely manner requires lowering the exchangeable sodium percentage (ESP) with a stronger more favourable cationic sources.  The result is the displacement of the sodium ions in the soil, which are then free to be flushed out of the growth zone in the case of gypsum as sodium sulphate.

Longevity of amendments

Due to the fact that calcium sulphate and carbonate are very insoluble and only release small amounts of calcium into the soil solution, an application of these amendments will produce very few short term, but great long term benefits and this should be given serious consideration when choosing the appropriate amendment  Many turf managers are constantly amazed that their total salt levels can actually rise after applications of gypsum and limestone.

Basic rule: When you are attempting to reclaim either a saline, sodic (sodium saturated) or saline sodic soil, the first thing you need is good drainage or an outlet to send the sodium to when it is displaced. The next thing is a source of calcium (already in the soil), and exchange process, and finally, a source of water to flush the sodium from the system. Remember, a calcitic soil amendment  alone does not solve a high Na problem, you must apply adequate irrigation water to leach the displaced Na out of the root zone.

Jerry works for Endeavour Turf Products. To find out more about E.T.P head to www.etpturf.com.au

Like Jerrys work? Head to the Pitchcare Bookshop to snap up his latest best seller.Nutrition of Sports Turf in Australia www.pitchcare.com.au/shop/product/468/4

Read more articles in Editorial, by Jerry Spencer or from June 2009.