Do I lose yield if my soil test isn't built up to high levels?

Thursday, January 2, 2014

The answer appears to be no, but if you have soils with very low soil-test levels, there is likely a benefit to building those soils to medium levels

by KEITH REID


An argument that comes up periodically among agronomists is whether a high soil test is needed to maximize crop yields, or if the same yields can be achieved at a lower soil test with added fertilizer. This seems like it should be a straightforward question, but there is no easy answer. The data that are available do not show any clear advantage to building up soil test values above medium levels.

Why might there be differences? Crop roots absorb nutrients from the soil solution that have either been released from the soil or dissolved from added fertilizer. The crop really doesn't care where nutrients come from, as long as there is an adequate supply in the soil solution when needed.
The two main approaches to meeting a crop's nutrient requirements are to build up soil fertility so there is adequate supply without adding fertilizer each year (build-up approach), or to add just enough fertilizer each year to meet the crop requirements that the soil is not supplying (sufficiency approach).  

In theory, either approach should be equally effective, but there will always be cases where theories don't work in the real world.  If the soil is very infertile, it may not be practical to add enough fertilizer to meet all the possible needs of the crop. Alternatively, the total amount of added nutrients might be adequate but not placed where the crop can reach them. For example, plant roots may not be able to access surface broadcast fertilizer if the soil surface dries out. In these cases, a larger reserve of nutrients in the soil would be beneficial.

There are other cases where added fertilizer gives an incremental yield benefit, even though the soil test is high enough that we would not expect this. Weather and soil conditions are often the drivers for these types of responses, so if compacted soils or extreme wetness limit root growth (and therefore the volume of soil they can absorb nutrients from), added fertilizer can make up for the shortfall in the plant.

Occasionally, there will be responses to elements in the fertilizer other than the main nutrient on the label. An example is the response to potassium chloride on high potassium soils in the prairies – the crop is actually benefiting from the chloride.

What do the data show? Interpreting results from long-term trials is challenging, because nutrient levels in the soil are a moving target. As fertilizer is added and nutrients are removed with crop harvest, soil test values will change, so what starts out with a high soil test may end up much lower in the check treatments. Conversely, the plots starting with a low soil test may end up with high values where more nutrients are being added than removed. In a field situation, unlike research plots, the farmer would adjust fertilizer applications to account for these changes.

Despite these challenges, I did find a couple of studies which included a range of data that allowed comparisons of fertilizer response at different soil test levels. Neither is from Ontario, but the soil and climate conditions are similar enough that the results have some relevance.

One from Turkey (initial soil test L-M) showed a slight increase in maximum corn yield where phosphorus (P) was applied to build up the soil test, but only for the first increment of P build-up and only for the first couple of years, after which there was no difference between treatments.

A second study from Iowa included 25 years of yield data from two different sites (initial soil tests M), with three different rates of initial P application to create differences in soil tests along with different rates of P added each year. Both sites showed greater response to the annual P applications where soil tests were not built up, but no increase in maximum yields where the soil tests had been built-up.  

These results suggest that there is no benefit to building soil test values, at least for P, above a medium range, as long as adequate fertilizer is applied each year to meet annual crop requirements. There is always the possibility, of course, that individual years with exceptional weather (e.g., extremely cool and wet weather) may show increased yields with higher soil-test values. But you need to balance this possibility against the cost of maintaining these high values for the majority of years when there is no benefit.

Conclusions. There does not appear to be any greater benefit to building soil tests to high levels than there is in maintaining a medium soil test level and applying fertilizer every year. The data I have presented relate to phosphorus, but I would expect the situation to be similar for potassium.

If you have soils with very low soil-test levels, however, there is likely a benefit to building those soils to medium levels, with the greatest differences showing up when the weather conditions are unfavourable for crop growth. Applying fertilizer to meet crop requirements at these soil-test levels will eventually increase the soil-test values as well. BF

Keith Reid is Soil Scientist, Agriculture and Agri-Food Canada, Guelph.