Acidification Animation Transcript

Measuring and Interpreting Soil pH | pH and Nutrients | Acidification Animation

Scene one

The opening scene shows the presence of hydrogen ions (H⁺) as green circles in the soil and the presence of hydroxyl ions (OH^-) as purple circles in the soil. A soil pH scale appears on the right-hand side showing the soil to have a neutral pH of 7. A pH of 9 denotes a very strongly alkaline soil, and a pH of 4 denotes a very strongly acidic soil. The accompanying text reads “pH is a measure of how acidic or alkaline a soil is. The more hydrogen ions (H⁺) in the soil the more acidic it is. The more hydroxyl ions (OH^-) in the soil the more alkaline it is.”

Scene two

A tractor enters scene two dispensing ammonium fertiliser onto the soil. A cow appears on screen adding urine to the soil. This scene shows that ammonium fertilisers, urea and the urea in urine produce ammonium (NH₄⁺) in the soil. The soil pH scale on the right-hand side of the screen decreases from 7 to 6.5. The accompanying text reads “Agriculture can change the pH of soil. Ammonium-based fertilisers, urea or the urea from the urine of animals can acidify the soil in the longer-term.”

Scene three

Scene three shows the transformation of ammonium (NH₄⁺) to nitrate (NO₃^-), which releases three hydrogen ions (H⁺) into the soil. The presence of a greater number of H⁺ ions than OH^- ions causes the soil pH to drop from 6.5 to 6, as shown by the soil pH scale on the right-hand side of the screen. The scene also shows unused nitrate (NO₃^-) leaching below the root zone. The accompanying text reads “The transformation of ammonium (NH₄⁺) to nitrite (NO₂^-) to nitrate (NO₃^-) releases H⁺ ions into the soil resulting in the soil becoming more acidic. Nitrate leaching, where NO₃^-moves below the root zone and cannot be used by plants is a significant source of agricultural acidification.”

Scene four

Scene four also shows the transformation of ammonium (NH₄⁺) to nitrate (NO₃^-), which releases three hydrogen ions (H⁺) into the soil. However, in this scene it is the nitrogen fixing bacteria Rhizobia that live in the nodules on the roots of legumes that are producing the ammonium. The release of further H⁺ ions causes the soil pH scale of the right-hand side of the screen to drop further from 6 to 5.5. The accompanying text reads “Soil can also acidify if the ammonium (NH₄⁺) produced by nitrogen fixing bacteria (Rhizobia) living in nodules on legume roots is not all used up by the crop or pasture."

Scene five

Scene five shows the tractor (this time carrying a large bale of hay) and a cow being removed from the agricultural system to signify the removal of alkaline (OH^-) products. As this further imbalances the number of hydrogen (H⁺) to hydroxyl (OH^-) ions in the system, the soil pH scale on the right-hand side of the screen decreases from 5.5 to 5. The accompanying text reads “Agriculture can also accelerate acidification by removing alkaline products such as wool, milk, cereal grain, legumes and hay.”

Scene six

The tractor (carrying three bales of sileage), a cow, and a stockfeeder appear on scene six to signify that alkaline products can also be introduced into the agricultural system. This introduces hydroxyl (OH^-) ions back into the system, so the soil pH scale on the right-hand side of the screen increases from 5 to 5.5. The accompanying text reads “The reverse is also true, where the introduction of manure, decaying animals, silage and stockfeeds can add alkalinity back into the soil.”

Scene seven

The tractor, cow and stockfeeder remain on scene seven and an arrow appears to show, as an example, that if organic matter builds up the breakdown of organic acids (RCOOH) often present in the introduced organic matter can release hydrogen (H⁺) ions into the soil. Thus the soil pH scale on the right-hand side of the screen again shows a decrease from 5.5 to 5. The accompanying text reads “The build-up or this organic matter with its organic acids can acidify the soil as carboxylate function groups (RCOOH) dissociate and release H⁺.”

Scene eight

A tractor re-enters scene eight dispensing lime onto the soil. A series of arrows appear showing the breakdown of lime (CaCO₃) in the soil, where oxygen (O₂^-) is produced and combines with two hydrogen (H⁺) ions to produce water (H₂O). This reaction consumes H⁺ ions thereby decreasing the imbalance of H+ to hydroxyl (OH^-) ions. Subsequently the soil pH scale on the right-hand side of the screen shows soil pH increasing from 5 to 6. The accompanying text reads “One way to combat acidification is to apply lime (CaCO₃) to the soil. Liming soil raises soil pH by consuming H⁺.”

Scene nine

Scene nine zooms into the soil to show the prevalence of soluble aluminium (Al³⁺) in a strongly acidic soil. Arrows demonstrate that Al³⁺ when taken up by the plant can cause stunted root growth. The accompanying text reads “A major constraint to plant production on strongly acid soils can be aluminium toxicity and manganese toxicity. Both are more soluble at low pH, for example, aluminium dissolves into the soil solution as Al³⁺ that is taken up by the plant causing root deformation and stunted plant growth.”

Scene ten

The final scene zooms out to show the stunted plant roots and dead plant material of a strongly acidic soil. The accompanying text reads “If a soil continues to acidify until is becomes very strongly acidic, biological activity, soil structure and nutrient toxicity and deficiency can become significant challenges to productive agriculture.”

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