Fertiliser making your soil acidic?

A common question during on-farm fertiliser discussions is, ‘does nitrogen and superphosphate use acidify the soil?’ Not specifically. In fact, many of the processes contributing to soil acidification are naturally occurring, such as:

»The natural activities of plant growth

»Soil micro-organisms breaking down soil biomass

»Addition of acid compounds to the soil through application of certain fertilisers

»Weathering of soil minerals

»Water draining through the soil profile, and the subsequent loss of soil nutrients.

Most acidity generated in a soil comes from the activities of plant roots and soil micro-organisms. Anything increasing micro-organism activity or the growth rate of the plants, like soil fertility improvements, can increase the natural acidification rate. While a few commonly used fertilisers, that undergo reactions in the soil, may have the potential to increase soil acidity; soils themselves also naturally acidify at different rates dependant on their acid buffering capacity.

Some nitrogen fertilisers can create acidity when applied to soil, particularly for those containing ammonium. However, it can take many years of normal fertiliser application to decrease soil pH by any appreciable amount. Phosphate fertilisers can temporarily acidify small, localised zones in the soil, but with a negligible direct effect on the bulk soil acidity. Therefore, fertilisers like superphosphate do not generate any measurable increase in soil acidity. Fertilisers containing elemental sulphur can markedly decrease pH when compared to ammonium and superphosphate fertiliser. Acidity generated through natural process or addition of particular fertilisers is easily neutralised by a regular maintenance liming programme.

If your soil test reveals a below optimal pH (depending on crop/grass grown), then capital lime is required. A rough rule of thumb is to apply good quality lime (80 percent calcium carbonate) at a rate of one tonne / ha, which will increase soil pH by 0.1 units. This is highly dependent on soil properties, like buffering capacity, and depth of soil testing. Your local PGG Wrightson Technical Field Representative can tailor your lime programme to allow for your soil properties and depth of soil testing.

For more information on reducing soil acidity or tailoring your lime programme, contact your local PGG Wrightson Technical Field Representative today.