Do high yields demand higher rates of fertilizer?
Sunday, April 3, 2011
The answer depends on what factors led to the high yields and whether you are talking short or long term
by KEITH REID
Farmers in Ontario grew record or near-record harvests of corn, soybeans and wheat in 2010, and many farmers had higher yields than they had ever achieved before. This has led many to ask whether we should be using higher rates of fertilizer.
The answer is no, and yes, because it is really two different questions – first, whether you need higher rates of fertilizer each year to support higher yields; and, second, whether you will need to add more fertilizer in the long run to replace what the higher yielding crops are removing.
When you consider how much added fertilizer your crop will need in any year, it is important to remember the factors that lead to high crop yields: good soil conditions and good weather. Good soil conditions mean that there are few impediments to root growth, so the above-ground portions of the plants are supported by extensive root systems. This, in turn, means efficient uptake of both water and nutrients from the reserves in the soil.
Good weather conditions for crop growth are also optimum necessary conditions for the release of nutrients from organic materials in the soil and the transport of those nutrients to the roots. Good weather also means fewer incidents of nutrient loss through leaching, denitrification or runoff, so the supply of nutrients is retained in the soil more effectively.
The net impact of these factors is that the crop requirement for immobile nutrients like P and K will be no more than in a normal year, and may actually be less than the crop would require in a poor growing year. In some soils, the crop may have responded to a little more N than normal, but this is more likely in soils with low organic matter that simply don't have the reserves of organic N to release when conditions are right.
As for the long term, it is obvious that increased removal of nutrients from a field will eventually deplete the supply of nutrients in the soil.
Table 1 shows the impact of different levels of crop yield on nutrient removal over a three-year crop rotation of corn, soybeans and wheat. If the left column represents pretty good yields from 10 years ago, and the right column represents what many farmers actually achieved in 2010, you can see that removal of some nutrients has close to doubled.
Since the natural supply of these available nutrients needs to come from dissolving soil minerals, or deposition from external sources (atmospheric deposition of sulphur or deposition of fresh topsoil in flood plains), it is easy to see how the supply of P and K in the soil could be depleted if not replaced by adequate fertilizer or manure. The rate of this depletion will depend on how large the soil reserves are, both in easily dissolved forms and in more resistant compounds.
Many soils can meet the needs of high-yielding crops for many decades before you will see appreciable diminishing of soil fertility, while others will show problems within a few years. Regular soil testing is key to monitoring the impacts of crop removal of nutrients on fertility levels. BF
Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture Food and Rural Affairs based in Stratford. Email: keith.reid@ontario.ca
