Trends in Fertilizer Use 1981 to 2006

Crop acreages have shifted dramatically in Ontario over the past few decades. This has had a significant impact on fertilizer use, and while there were statistics available on annual fertilizer sales it has been difficult to sort out how much was due to changes in crop acreage, and how much due to underlying trends. I have attempted to address this by "adjusting" the areas of each crop by the amount of nutrient each crop would be expected to use relative to a crop of corn. The starting point for this adjustment is the OMAFRA fertilizer recommendations, modified by some guesses about grower behaviour (forages, on average, probably receive less fertilizer than OMAFRA recommendations, while high value crops probably receive more). The result is the total acres for each year that would use the relative amount of N, P or K if all those acres were planted to corn. This was then used to calculate the average nutrient rate per hectare based on annual sales.

Figure 1. Ontario Nitrogen use since 1981

Aside from the impact of crop acreage, fertilizer rates can vary from year to year based on the availability of materials, weather during planting season, prices for both crops and fertilizer, and the level of optimism among farmers. Nitrogen rates show more year to year variation than either phosphate or potash, but the trend is for a decline in average rates of about 0.7 kilograms of nitrogen per hectare per year. This will reflect more efficient use of nitrogen from organic sources like manure or legumes, as well as better management of nitrogen fertilizers.

Phosphate and potash show more consistent patterns than nitrogen, and also much more significant declines in fertilizer use. Both have been declining by about 1.8 kilograms per hectare per year. While the average decline in N use represents about 13% of the 1981 rate, for phosphate the reduction is more than two-thirds, and potash is almost as large. The reduction in phosphorus rates appears to be leveling out, but this is not evident yet with potash.

Figure 2. Apparent P₂0₅ - Ontario phosphorus use trend for the last 26 years

A large part of the reduction in phosphate and potash use comes from reduced rates of fertilizer on fields where the soil fertility has been built up by past applications of fertilizer. There is also greater value given to the nutrients in manure and biosolids. Ontario farmers are obviously managing nutrients more efficiently, which results in more profitable crop production and reduced environmental impact.

Figure 3. apparent K₂0 Rate, Ontario Potash fom 1981 - 2006

The caution is that, as nutrient application rates fall below crop removal, the risk of crop yield losses from nutrient deficiency increases. This does not imply that fertilizer application rates will need to increase, but it does mean that regular soil testing to monitor nutrient levels in the soil will become even more important.