Saturday, January 23, 2010

Soil Chemistry Cookbook

Soil chemistry is the same as cooking. Both use ingredients to get an end result. As a keen pantry chef and soil scientist, I only just noticed. 

Baking a cake is the same as determining the amount of ammonium-nitrogen in soil. When we bake a cake, we gather all our ingredients, certain quantities of each. We then follow some instructions on how to combine the ingredients  and cook them to  ensure that we do it just right, for that perfect cake. If the amount of ingredients, combination or cooking is slightly wrong, we may end up with a cake that falls apart, tastes funny, is burnt, no risen or any other combinations of failure.

Similarly we follow instructions or methods measuring nutrients in the soil or result in similar failures. When we measure the amount of ammonium-nitrogen in soil, we have a list of ingredients or chemicals that we use for determination. We use in a particular quantity of each chemical. We then follow a method for the use of chemicals. For ammonium-nitrogen, we first do an extraction. An extraction is the use of a salt or water with soil to remove the nutrients we are interested in measuring. This extract is then combined with reagents; chemicals which react with the ammonium-nitrogen to produce a colour. This colour can then be measured using a spectrophotometer, which measures different absorbency of light waves.  If one step is missed or not precise, then you can result in an incorrectly measured ammonium-nitrogen, or even worse, a failed batch that can not be measured.

Just like cooking, a failed soil test can be recovered. If we have a failed cake, it is never the end of the world. We can either recreate the cake, and carefully follow the instructions or work out if the recipe is correct and if we need more eggs or flour etc. We can also use some failed cakes in other recipes. For example, a failed cake can be used in tiramisu or triffle

Over the last few months, I have had more than my share of failures in the lab that I have had to 'recover'. Ammonium-nitrogen is one memorable failure. I followed the method precisely. I took some of my soil, extracted it using a salt solution, and then added the reagents to produce colour that I could then measure. However, each time I did this I got white clouds in my solutions. This means I am unable to measure my soil. Where did I go wrong? What part of the method did I not follow? 

Failed Ammonium-Nitrogen: First, third and fourth from left with white clouds, and the second is normal.

Adding too much of one ingredient may result in a different outcome. If you add too much flour to a cake recipe, you may end up with a dry, hard cake. If you add too much milk, it will be too wet and won't bake properly. It is a matter of adjusting the recipe to get it to work correctly: add less flour or less milk. In the case of the ammonium-nitrogen, I knew that I also had to make some adjustments to the method to get it to work correctly.

Calcium-Sulphate, also known as gypsum, has a small solubility tolerance. That means, it is only soluble under particular conditions. If those conditions change, then it becomes insoluble and may be noticed as white clouds in a solution. Therefore, the white clouds in my solution was likely to be calcium-sulphate. What do I need to change to get rid of the white clouds? 

We need to understand what makes calcium-sulphate insoluble, and then how to change the method to keep it soluble. Three factors affect calcium-sulphate solubility: temperature, pH and salts. At 26 degrees Celsius or greater, it starts to become less soluble. When the pH becomes very alkaline (greater than pH 7), it also becomes less soluble. And, with lots of salts in solution, it becomes less soluble. 

I firstly tested temperature. When we add the reagents to the ammonium-nitrogen extracts, we have to do it at 36 degrees Celcius. I changed the temperature and tested at 2, 25 and 30 degrees. The white clouds still formed. We also tested the pH in all of the solutions (photographed above), they all had a pH of 12. This is necessary for the colour to develop. So, we know that changing the temperature and the pH is not possible to stop the white clouds forming. 

When you extract the ammonium-nitrogen, you do it with a salt solution. However, my soils are already filled with salt! Therefore, with the salt in my soil, plus the salt in the extract, I had increased the salt content so much that I reduced the solubility of calcium-sulphate so that it formed white clouds. How do I reduce the salt content to stop the white clouds? DILUTE! 

Ammonia-Nitrogen Win: Diluted extracts + reagents

I had to add one more step to my method. Instead of using my extract with reagents, I had to use some of my extract with water and then add reagents. This would dilute the solution, reduce the salt content and stop the white cloud in my solution.  Success.

So cooking is really just the same as soil chemistry. Both have a list of ingredients and instructions you have to follow. Both can fail if you do something wrong. And you can troubleshoot both to get the outcome you want! 

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