Seed Bed: Do cover crops really improve soil quality?

Monday, March 2, 2009

The answer depends on the soil conditions, the crop mix and the weather in a given year

by KEITH REID

When it comes to cover crops, I am often faced with proponents on one side and sceptics on the other. The proponents emphasize the value of cover crops for improving soil quality, while the skeptics point to the lack of immediate benefits to pay for the cover crop establishment. Does this mean that one group is right and the other wrong?     

Life just isn't that simple. The reality is that the payback on cover crops will vary with the soil conditions, the crop mix and the weather in a given year, and can probably never be predicted with absolute certainty. We are learning enough, however, that we can start to pick the situations where a response to cover crops is most likely.

Cover crops and nitrogen. Two areas where we have confidence in cover crops is their ability to absorb excess N from the soil in the fall, and the ability of legume cover crops to fix N out of the air.

The amount of N taken up by cover crops is proportional to the amount of growth, so there is greater opportunity for a crop of oilseed radish to tie up manure nitrogen following a winter wheat crop than after soybeans, simply because of the time available for the crop to grow. Similarly, the amount of N fixation is proportional to the amount of clover growth.

Less certain is the transfer of nitrogen from cover crops to the following crop, and this is one of the challenges that sceptics frequently bring up. Cover crops vary in the rate of residue breakdown, which has a huge influence on the time when nitrogen is released into the soil. Red clover keeps most of the N in the organic form until the weather warms up in the spring, so N release from red clover usually matches the time when corn needs it. We have good confidence in the nitrogen credits following red clover. Oilseed radish or oats, on the other hand, break down fairly quickly in the fall, so there is a much greater chance that the N will be lost over the winter, depending on the weather from the first killing frost until the time of crop uptake the following spring. Further work is needed to develop systems that will increase the reliability of "N relay" from one cropping year to the next.

Cover crops and soil quality. I have often expressed misgivings about the build-up and maintenance approach to soil fertility, where the goal is to build up the soil to the point where soil fertility does not limit crop productivity, and then continue to apply fertilizer to keep the soil in that range. It has never appealed to me to spend money on an input that may not give any return for several years in the future, if at all.

Soil quality requires a different approach than soil fertility. At the risk of appearing inconsistent, I am going to suggest that for soil quality, we must be operating on the principal of building up soil quality where it is deficient, and working to maintain it in areas where it is adequate.

My reasoning for this is two-fold. First, the difficulty in correcting deficiency in soil quality is much greater than in correcting a nutrient deficiency. Second, we have much less certainty about the "critical level" for soil quality, both because of a lack of reliable measures and because the response to soil quality is so weather-dependent. Unfortunately, just like the maintenance applications of fertilizer which don't show any pay-back in the year of application, it's easy for the sceptics to "prove" that cover crops aren't effective because we don't see a response most years.

That being said, there is a growing body of evidence from horticultural crops showing that cover crops are having a positive impact. The reasons they are showing here rather than in field crops may be a combination of greater soil degradation from past management, greater sensitivity to soil quality because of the nature of the crop root systems, and a smaller yield increase needed to justify cover crops because of the higher value of the crops.

Other benefits – tangible and intangible. Effective cover crops are always treated as part of a system, rather than operating in isolation. This means that they can suppress weed growth between cropping seasons without becoming a weed problem themselves. They can also provide habitat for predatory insects to control crop pests, although a poorly chosen or improperly managed cover crop may provide a habitat for the pests instead.

In any case, the biomass and ground cover provided by a cover crop will contribute to increased soil organic matter levels and reduced risk of soil erosion. These are impossible to measure in the short term, but will contribute to the long-term productivity of the soil. BF

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