Nitrates/fertilizer link not as solid as supposed

To the Editor:

Further to Wendy Elliott's column (“We're drinking fertilizer”, Kings County Advertiser, May 13, 2008), while it is correct that fertilizers may contain nitrate or nitrogen compounds that can be converted to nitrate in the soil, and also correct that these nitrates may move downward into groundwater, it will be in very exceptional circumstances that most nitrate in groundwater will be directly attributable to applied fertilizers.

For example, considering only agricultural land, nitrate may be produced in soil by mineralization of soil organic matter (conversion of insoluble organic material such as plant parts, animal parts, manures and amorphous organic matter to soluble constituents by action of organisms). And most precipitation contains some nitrogen compounds, predominately nitrate, as I recall.

I cannot readily locate typical annual values of nitrate deposition for this region, but in the early 1990s, when I was helping a local poultry processing plant get set up to measure oxygen in their discharge, I was interested to see that the allowable concentration of nitrate in their discharge was well below typical concentrations in precipitation.

Seepage from manure holding tanks can be substantial, as illustrated by some measurements that I made down-slope of a hog manure cistern over a four-year period (1989-1992). During this period, Reed Canary grass plots captured a total of nearly 2100 kg of nitrogen (N) per hectare in top growth; i.e. an annual average of more than 500 kg N/ha.

By way of contrast, the average annual application of nitrogen (as fertilizer plus mulch) to an apple orchard (Block 64) at the Research Station over 19 years was 31.8 kg N/ha and annual tile discharge was typically less than 2 kg N/ha. And nearly all soils contain at least 6,000 kg N/ha in the upper metre, mostly as insoluble organic matter, and mostly as potential raw material for mineralization.

And it will also be in very exceptional circumstances that most nitrate in groundwater is attributable to agricultural activities. In addition to natural sources, such as Alder swamps and woodland sources such as clear-cuts and burns, bright green (and therefore N-rich) lawns no doubt release substantial amounts of nitrate.

Houses tend to be built on well-drained soils and lawns tend to be watered during dry spells and these conditions set the stage for substantial downward percolation of nitrate during wet periods. Septic fields and often leaky sewage lines are additional sources of nitrate.

Downward movement of a solute, such as a nitrate, is possible only while there is a downward movement of water or other solvent. And because the amount of water per unit area that is available for percolation will be unusually large in septic fields and below leaky sewage lines, one may expect unusually rapid penetration of solutes, including nitrates, to depth at these locations.

Yours truly,

David H. Webster

Kentville