Seed Bed: Soil quality is a long-term investment

Tuesday, February 16, 2010

Even though it's difficult to show that changes in soil quality have affected yield, investing in it does make sense, even if only from the perspective of risk management

by KEITH REID

I've written many times in these pages about the need to improve soil health or soil quality, and I really do feel it is critically important for the long-term productivity of our soils.

The trouble is that it's difficult to show a consistent yield response to practices like cover crops or no-till that should improve soil quality. The challenge, then, is to prove to the skeptics that soil quality is more than "motherhood and apple pie" and can have a real impact on the bottom line.

There are a number of reasons why it is difficult to show that changes in soil quality have affected yield. The first is that it takes a pretty significant loss in productivity to show up against the background of natural variability in the field, particularly with good growing conditions.

Even with poor weather, the soil quality needs to be significantly degraded before a yield loss becomes apparent, as you can see in Figure 1. Prior to that, however, you can be suffering from invisible yield loss that is quietly eroding your bottom line.

This also means that many trials intended to show an impact from soil quality end up on the "good" fields, where the physical condition of the soil is not yet limiting productivity.

The short time period allowed for most trials also makes it difficult to see any differences due to soil quality. A couple of good growing seasons can "hide" the impact of soil degradation, unless the soil is so degraded that you really didn't need a trial. Changes in soil physical characteristics take much longer to appear than changes in fertility, so the shortest term for a trial of this type should be the length of the rotation, and preferably a couple of cycles of the rotation. 

Further complicating the analysis of soil quality trials are the confounding factors of weeds, slugs and insects that can come along with many of the management systems used to improve soil quality. There are also cases where techniques to combat soil degradation have been imported from other areas, but are really not suited to our climate or soils, so the lack of any measurable impact is because there is none.

Should you, therefore, invest in soil quality, with such uncertain returns?

Soil quality has been, and will continue to be, a long-term investment.  Serious losses from soil degradation don't show up until the damage is severe, so there is little feedback to the grower's pocketbook before the situation is critical.

The scientific challenge is to develop better tools to measure soil quality, since most of the current tools are not well calibrated, and are no more sensitive to changes in the soil than crop yield. In the meantime, we can use them as gross measures, and recognize that we cannot manage soil quality with the same level of precision as soil fertility. 

Along with the lack of precision is the difficulty in restoring the quality of a degraded soil. Figure 2 shows that soil degradation happens more quickly than recovery. That is in marked contrast to soil fertility, which declines or builds at about the same rate with the removal or addition of nutrients.

From the perspective of risk management, if nothing else, investing in soil quality makes economic sense. 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