Bay Journal

It’s time to study ability of no-till soil, cover crops to trap nutrients

  • By Kenneth E Carter on May 01, 2010
One of many fields in the Chesapeake watershed that has been farmed using a minimum tillage system.  (Dave Harp)

Over the last several months, I have enjoyed the scientific dialogue and "banter" from Dr. Lynton Land and Mr. Chase Tanner concerning the fate of nutrients and their impact on the Bay. I believe this dialogue is worthwhile and a great role for the Chesapeake Media Service and the Bay Journal.

In all of Dr. Land's articles, he is correct in stating the traditional scientific view as published in most agronomy and basic soil science text books. His statements are exactly what I heard more than 35 years ago in my basic soils and agronomy classes. But the simple math and basic subtraction does not totally describe what may be happening in the soil profile. Land's position leads the reader to believe that the entire surplus is automatically lost to the environment in the form of groundwater and surface water pollution.

There are a number of forces and management techniques being used that have not been discussed that I believe are critical to this ongoing discussion.

The late newscaster Paul Harvey had a great storytelling technique of presenting "the rest of the story" to fill in interesting gaps. So too, does the total picture need to be explained to your readers about the fate of nutrients in the soil profile.

Of all of gaps that have not been represented or described, I would like to shed some light on two that are critical in trapping nutrients and do negate many of the losses implied.

The first is the use of cover crops or grain crops that have the ability to quickly take up any remaining nutrients in the soil profile.

In Virginia alone, more than 100,000 acres of cover crop and several hundreds of thousand acres of wheat, barley, rye and oats are planted each year. These cereal grains have the ability to send roots quickly and deeply into the soil and scavenge any remaining nutrients.

Many of the fields that Dr. Land cites as receiving high levels of animal manures are cropped in a fashion in which small grain silage crops are routinely planted after waste application. These grains use the applied nutrients and receive no additional fertilization.

In addition, cover crops are often planted for the specific purpose of scavenging residual nutrients. In my own professional experience, I have seen cover crops planted that showed signs of nutrient deficiency because they were able to quickly scavenge what little nutrients remained in the soil profile. These cereal grains are quite effective in a wide variety of uses in capturing and holding the surplus nutrients that Land is rightly concerned about.

The second factor in the fate of these surplus nutrients is the level of organic matter found in the soil profile. I think this is one of the scientific horizons facing Chesapeake Bay cleanup efforts that is receiving very little attention.

Many of the proven agronomic principles and scientific studies that the Bay cleanup effort is operating under are decades old. Much, if not all, of the data were collected under conventional cultivation systems where the fields were annually plowed and cultivated. I suspect that the organic matter content of the soil profiles used in most of this research was less than 1 percent.

Over the last several decades, farmers in the Bay watershed have done an admirable job in adopting the new technologies available and moving into more conservation cropping systems. The adoption of minimum tillage, annual no-till and the more recent continuous no-till and "never till" systems have had a dramatic impact on soils.

The Bay watershed now contains agricultural croplands that have not been tilled in any manner for 10-20 years. With these systems, several tons of organic matter-in the form of crop residues-are allowed to remain on the soil surface and decompose.

The addition of tons of high carbon material is raising the organic matter levels in our cropland soils. It is common to find organic levels under a long-term conservation tillage system in the 2-3 percent range, with some fields approaching a 4 percent level. These high-carbon materials have the ability to bind to free nutrients. With organic levels steadily increasing, it stands to reason, these soils are doing a much better job in sequestering nutrients and preventing their loss to the water column.

But, very little research has been done to determine how these soils react and the total extent of their nutrient-trapping ability. This is the new frontier. Very little agronomic research has been performed on soils under conservation tillage systems running this long. It is my contention that many of the old paradigms and theories we have been operating under for years need to be closely examined, and additional efforts need to be targeted toward finding the answers to these new questions.

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About Kenneth E Carter

Kenneth Carter is a retired Natural Resources Conservation Service-U.S. Department of Agriculture official and has worked with the Chesapeake Bay cleanup effort professionally and as a volunteer since the program began in the 1980s. He practices many agricultural best management practices on his own land and volunteers for several Bay organizations.

Read more articles by Kenneth E Carter

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