What temperatures are fertilisers and ameliorants exposed to when stubble is burnt, and for how long?  

This obviously varies with many factors including type and amount of stubble, humidity, wind, ambient temperature and stubble moisture. Ground temperatures for high cut/harvested paddocks are often quoted as 300 – 400°C, usually for only a few seconds. How far these temperatures penetrate fertiliser particles is a bit of a guess.  

For windrows it seems temperatures can be as high as 500°C for wheat and lupin stubbles and 600°C for canola stubbles. This could be for 10 – 20 minutes which some may presume would be long enough to affect fertilisers and other ameliorants. 

The temperature is important because, as discussed below, different fertilisers decompose at different temperatures. A given fertiliser can also decompose at different temperatures depending upon its grade and impurities. So there are a couple of provisos on the information below. Lots of the information comes from CSBP’s Material Safety Data Sheets (MSDS’s) which can be found at https://csbp-fertilisers.com.au/safety-data-sheets

Lime  

Calcium Carbonate (CaCO3) is very safe given its melting point is above 800°C, and even if heating did do something, it would only change CaCO3 to CaO. 

Sulphate of Ammonia  

Could be risky – this is where the temperature of the burn is important. At 230 –280°C ammonium sulphate converts to ammonium bisulphate (which is available to plants). But at about 350°C it decomposes to gases. 

Super Phos 

This is more complicated. Monocalcium phosphate in Super Phos does not decompose, rather it changes to less soluble (and therefore less plant available) forms of phosphate. In theory this starts when the temperature exceeds about 200°C. 

CSBP historical research has shown available phosphorus can decline by around 30% of Super Phos applications when heated to 600°C. More recent work suggests the reduction is even greater. Preliminary results of Dr E Francisco (IPNI Brazil, pers. comm.) indicates if Super Phos is heated for 5 minutes to 300°C, phosphorus solubility declines by 85%, higher times and longer times reduce solubility even further. 

The availability of sulfur in Super Phos is also likely to decrease with heating. The sulfur in Super Phos is in the form of calcium sulphate dihydrate (CaSO4·2H2O) or gypsum, which has a very high decomposition temperature (1400°C). But when gypsum is heated above about 130°C, water is driven off, partially dehydrating (or calcinating) the mineral and reducing the solubility. 

Potash 

Muriate of Potash (MOP) looks very safe across all burns. It doesn’t even melt until about 770°C and won’t boil or decompose until about 1400°C. Sulphate of Potash (SOP) is even safer given a boiling point above 1600°C.

By Brad Smith
- Senior Product Development Chemist

Agronomic insights

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