Seed Bed: Avoiding the pitfalls posed by 'cheap' fertilizer

Sunday, January 10, 2010

With last year's fall in fertilizer prices, it's important to guard against overuse that can hurt your fields, your pocketbook and the environment

by KEITH REID

For the first time in my memory, we are dealing with fertilizer prices that are less than half of the previous year's values. The fact that this follows two years of unprecedented increases in fertilizer prices, or that the prices are still higher than they were four or five years ago, is immaterial.

It feels like fertilizer is inexpensive

This could lead to a whole series of pitfalls from the overuse of fertilizer that you will need to guard against. These pitfalls may be economic (if the fertilizer isn't as cheap as you perceive) or agronomic (salt injury or nutrient imbalances). Or it may be environmental (if too much nutrient is applied where it can move off the field). Any one of these could turn your fertilizer savings into a significant expense.

Economic considerations, or how cheap is cheap? While the price per ton of most fertilizer ingredients has dropped significantly, the price of grain has also dropped.

Remember that the real cost of fertilizer is the amount of grain that it takes to purchase a pound of nutrient (the price ratio). With corn trading in the $4 per bushel range, the price of urea needs to be in the $350 per tonne range to justify unreduced N applications.

If the fertilizer price is any higher than this, or the corn price any lower, then the cost of the fertilizer in real terms is higher than the long-term average. This means it is difficult to justify applying higher than normal amounts of fertilizer.

Agronomic considerations, or too much of a good thing? Fertilizer burn (or salt injury) is uncommon when everyone is shaving rates, but it could come back with a vengeance if farmers try to "make up for lost time" by putting on two or three year's worth of fertilizer at once.

Fertilizer burn is most common with banded fertilizer, where too much salt or ammonia is placed in close proximity to the seed. There have, however, been cases of fertilizer injury from high rates of broadcast urea and potash on very sandy soils. Urea application rates on sand or sandy loam soils should not exceed 200 kilograms of N per hectare, or 250 kilograms of N + K2O (potassium oxide). High rates of one nutrient can also interfere with the uptake of another, to the point that yields could actually be reduced. The most common example of this is high potassium applications to low magnesium soils interfering with magnesium uptake by plants, and creating a deficiency. High phosphorus application, particularly where P levels are already adequate, can lead to zinc deficiency.

In northwestern Ontario, where atmospheric sulphur deposition is low, high N rates can induce sulphur deficiency in some crops.

Ironically, the final agronomic consideration is that some fields will receive inadequate nutrients despite higher fertilizer rates. Do not fall into the trap of thinking that inexpensive nutrients require less management than expensive ones. Soil testing continues to be the best tool to identify which fields have suffered from reduced nutrient applications over the past few years, and which nutrients in particular need attention.

Environmental considerations, or the right stuff in the wrong place? Fertilizer application does not automatically equal environmental risk, but that doesn't mean you shouldn't exercise some care. Broadcast applications of phosphate fertilizers are the quickest way to apply large quantities, but they should be incorporated to protect them from runoff.

Nitrogen rates should be matched to crop needs, after accounting for all other sources of N, and applied close to the time of crop uptake. This will minimize the risk of nitrate leaching into groundwater. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture Food and Rural Affairs based in Stratford. keith.reid@ontario.ca