Soil management pays off for the long term

Friday, December 5, 2014

Trials held over a sustained period clearly show the benefits of maintaining adequate nutrients in the soil and practicing diverse crop rotations

by KEITH REID

Farming is a series of decisions balancing immediate gains against long-term impacts on the productivity of the resources we use – soil, air, water, equipment and infrastructure.  Some decisions are straightforward. If we don't maintain our equipment regularly, for example, then it could break down at a critical time. Decisions regarding soil management, however, are less clear because short-term benefits may mask long-term detriments. It is only by looking at long-term trial results that we can begin to predict outcomes from today's management decisions.

There are a number of long-term plots in Ontario comparing the effects of various combinations of crop rotation, tillage and fertilizer additions on soil quality and crop yields. Among the longest-running are the Bolton plots at the Eugene F. Whelan Research Station operated by Agriculture and Agri-Food Canada (AAFC) in Essex County. Established in 1959, this study has data from over 50 years of continuous treatments on the same plots. Treatments include continuous bluegrass sod, continuous corn, and a rotation of corn-oats-alfalfa-alfalfa. Each treatment is split, with one half receiving fertilizer and the other half receiving none. There is also a forested area next to the site allowing comparisons with soil conditions that were present before the area was cleared for agriculture.

In the first few years of the study, there were differences in crop yields among the different treatments, but the differences in soil conditions were not large enough to be measurable. After five decades of continuous management, however, the changes in soil conditions are much more evident, and allow us to draw some conclusions about what factors in soil functioning have the biggest impact on crop yields.

Drs. Craig Drury and Dan Reynolds at AAFC Harrow, along with a number of their colleagues, have recently analyzed samples from each of the plots to measure the changes that have occurred.

Their analyses showed that differences in soil conditions were much greater and more consistent between crop rotations than whether the plots received fertilizer or not. Not surprisingly, the soil closest to optimum physical condition was from the woodlot, followed closely by the continuous sod treatments. The poorest soil quality was in the continuous corn treatments, while the rotational corn was intermediate. A higher intensity of cropping had caused lower soil organic matter contents, less plant-available water, lower air capacity (indicating a higher risk of denitrification) and higher bulk density.  

Within each cropping sequence, the addition of fertilizer had a small positive effect in most cases, but no effect at all in others. There were no cases in this study where adding fertilizer made the soil physical quality worse.

Soil quality does not on its own, however, pay the bills, and the picture presented by corn grain yields is somewhat different. Improved soil quality between cropping treatments did improve corn yields, causing almost a doubling of yield between the rotational corn and the continuous corn.

There was an even larger difference between the fertilized and unfertilized treatments. Adding fertilizer to meet the needs of the crops growing in each plot increased the yields of rotational corn by two and a half times, while in continuous corn the fertilized yield was almost four times that of the non-fertilized yield. The smaller yield response to fertilizer in the rotational corn is the result of the alfalfa in the rotation that provides much of the nitrogen needed by the corn crop. To maximize yield, there needed to be both adequate nutrients and adequate soil quality.  

How much is this worth? At current corn prices (about $170/ a tonne), maintaining adequate nutrients added almost $420 an acre in gross income each year, or a net increase of $270 an acre (assuming about $150 an acre in fertilizer costs). Maintaining the soil in good physical condition paid an additional $310 an acre each year.

It is easy to see the long-term value that can come from maintaining diverse crop rotations. Even if it is not practical to include forages in crop rotations on every farm, it does make sense to incorporate cover crops (and particularly legumes) into your system whenever possible. BF

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