One of the most common forms of agricultural pollution is nutrient pollution.

“Hol’up, aren’t nutrients a good thing?” my strawman retorts. Well dear strawman and hopefully reader (in the hope that someone actually reads these), ever heard about the saying ‘too much of a good thing is bad’? Yeah, this is one of those times.

As a consequence of having to feed the planet in and/or in the pursuit of ever greater profit margins, farming practices have intensified over the past few decades. One very simple and efficient way to maximize your yield fast and dirty is to hit your plants with fertiliser, and I’m not talking about some weaksauce organic compost, this ain’t amateur hour. You hit them with the good stuff, chemical fertilisers strong enough to give your plants chemical burns*^{see disclaimer}* to kick things into high gear.

(disclaimer: organic fertilisers may also cause fertiliser ‘burn’, i.e. not a real burn in the traditional sense, it’s actually osmotic damage as a result of the high solute concentration in fertiliser media compared to plant tissue).

While this is great for the plants, they are usually unable to uptake all the nutrients, which are typically in water soluble forms that allow for the plants to take them in through their roots. The solubility of these nutrients also makes them very readily mobilised by further watering or rain and transported to surrounding water bodies in the same catchment. Sometimes, this can be easily observed point sources such as drainage channels or surface runoff. Other times however, the danger is quite literally beneath our feet; such as diffuse sources like crop fields when nutrients are transported into rivers and lakes through subsurface flow contributing to eutrophication, or leach deeper into the earth and percolate, contaminating groundwater (Khan et al., 2018).

Fertilisers aren’t the only source of agricultural nutrient pollution, the livestock industry is a particularly egregious offender, as a percentage of total soil nutrient inputs, manure was reported by Sheldrick et al. (2003) to account for for 14% of Nitrogen, 25% of Phosphrous, and 20% of Potassium. And that’s before taking into account the fertilisers used in production of animal feed.

If the diagram above looks kind of complicated, that’s because it is (at the very least it’s complicated to me). But we’ll learn together as we zoom in further on various downstream impacts of in the next few parts of this series on nutrient pollution. Such as groundwater contamination, drastic changes in soil pH and eutrophication.

But before we get to that, we need to get acquainted with the major players in nutrient pollution, so we understand what these nutrients are and why they are so important that we cannot do without them, despite their negative impacts. From the diagram above (Figure 2.) Khan et al. (2018) had identified the most relevant components of fertiliser-related pollution. Of particular interest to us are Nitrogen (N), Phosphorus (P) and Potassium (K). If you take a look at most fertilizers sold at garden supply shops, they tend to have three numbers on them (e.g. 10-5-5). This is the known as the NPK ratio. This convention is so ubiquitous because they are the most important plant macronutrients. Each of the big three, play crucial roles in plant development and physiology. Nitrogen for example is required for the synthesis of organic compounds, such as amino acids, proteins, nucleic acids, chlorophyll and hormones. Phosphorus is just as essential, forming the backbone of DNA and being used in vital functions such as photosynthesis, respiration, energy storage and transfer (Ingram et al., 2011). Potassium is used for maintaining osmotic balance within the plant, which is especially important given that plants can’t walk around to get a drink.

So we’ve established that nutrients are not inherently bad and why plants crave these scrumptious substances, however, they are not the only ones with a sweet tooth for our dear friends N & P unfortunately. Join me in the next chapter, where we confront our bad HABits (sic) in…

On Nutrient Pollution, Part 2: Bad (HAB)its

Harmful Algal Blooms (HAB) and Agricultural Pollution

 

References

Ingram, D. S., Vince-Prue, D., & Gregory, P. J. (2011). Science and the garden: The scientific basis of horticultural practice. John Wiley & Sons.

Khan, M., Mobin, M., Abbas, Z., & Alamri, S. (2018). Fertilizers and their contaminants in soils, surface and groundwater. Encyclopedia of the Anthropocene, 225-240. https://doi.org/10.1016/b978-0-12-809665-9.09888-8

Sheldrick, W., Syers, J. K., & Lingard, J. (2003). Contribution of livestock excreta to nutrient balances. Nutrient Cycling in Agroecosystems, 66, 119-131.