No-till, cover crops improve soil

With fertilizer and energy costs at near record levels, many farmers are looking for innovative ways to cut down on both crop inputs.

Cover crops may help with nitrogen use in the long-term, but growers will have difficult management decisions when it comes to short-term return on investment.

Long-term or permanent no-tillage systems are proving to reduce nitrogen use and make crops more drought tolerant. Virginia Tech Agronomist Mark Alley is conducting a series of tests in 2006 to try and determine exactly what effect long-term no-tillage has on nitrogen use.

Virginia grower Randolf Aigner, who grows row crops near Richmond, Va., has farmed no-till systems for over 10 years and rarely uses fertilizer on wheat and little nitrogen on most crops, often nearing 100 bushels per acre of wheat.

Central Virginia grower Jon Black has grown no-tillage crops for nearly a decade and contends the moisture holding quality of his no-till soil is much better than the few acres on which he still uses conventional-tillage.

How long-term no-tillage systems and cover crops work together is yet to be totally unraveled, but the indications are that the combination bodes well for soil quality, with the question being how this equates to farmer profitability.

It's no secret that N is lost in runoff — the question is how much. Nitrogen in nitrate levels in watersheds in which rapidly moving streams run through agricultural land are typically from 4-6 parts per million. When nitrogen levels in runoff get to 6-8 ppm, N loss is equivalent to 20-30 pounds of N per acre.

Years of growing row crops using conventional-tillage systems mine nitrogen from the soil. Reduced-tillage, by comparison, builds N levels. For example, corn stops using soil nitrates in August-September. In the fall soil microbes begin chewing on organic matter and N levels are built back up. Corn is very efficient at using N when it is in its early growing stage, but by July and on into August and until harvest, corn is done with N.

Soybeans are different from corn. Beans don't use much N early in the growing season. When plants set pods, they pull N levels in the soil down, though not as low as corn. Early crop beans pull N levels down earlier and levels build up faster. However, the reverse is true of late season, double-cropped beans.

Some agronomists contend soybeans is a direct cause of loss of soil quality in the Southeast. While there is some evidence that late-planted beans may be a direct cause of soil nutrient loss, the jury is still out as to the culpability of full season soybeans.

Speaking at a recent field day in Virginia, University of Maryland researcher Ken Staver says from a raindrops view, three things can happen when it hits a piece of row crop land: It can be lost in runoff and be gone in an instant, it can infiltrate the top layer of soil and be tied up in the soil for days or weeks, or it can be absorbed into the soil profile and be there for months or years.

Cover crops, Staver explains, unlock N from the soil and make it available to plants. Conventional-tillage mines nitrogen from the soil, while reduced-tillage and no-tillage systems build N in the soil. No-till and cover crops are related in that both can build soil N, but that relationship is difficult to quantify.

Staver says that no-till and year-round crops much more closely mimics natural systems than conventional-tillage.

With conventional-tillage, when crops are harvested, the soil is essentially bare until another crop is planted. Bare soil does not do an efficient job of building N, but is unfortunately very efficient in losing it, Staver explains.

Residual nitrogen, he stresses, is fertilizer the farmer paid for, but the plant didn't use. With today's cost of fertilizer, that's a cost every farmer should look at closely.

Most farmers look at weeds as a bad thing — a cost of production. However, late weeds, in corn for example, serve much the same positive purpose as cover crops.

“Weeds are in effect a cover crop. After the crop is harvested, late weeds use N and help manage N levels when no crops are on the field. The soil continues to let N go, but late in the season, weeds are often the only thing growing in the field to utilize the N,” Staver notes.

The key to getting maximum benefit out of cover crops is not so much which plant is used, rather how early the cover crop is planted. The cover crop builds organic matter in the soil and adds to overall soil quality — the longer it is on the ground, the more benefit it offers.

Cover crops used in conjunction with no-tillage systems is the ideal scenario for soil quality building because both practices are more in tune with natural vegetative growth and the soil's ability to generate usable N and other nutrients.

In no-tillage systems, Staver says, there is more runoff early in the growing season. However, later in the growing season, when the plant actually needs the moisture, there is less runoff in no-tillage systems than in conventional tillage systems.

No tillage systems, he says, also reduces direct evaporation from the soil, which reduces the amount of CO2 released into the atmosphere, and in theory, reducing the greenhouse effect.

Whether no-till systems and/or cover crops is economically worthwhile is best determined by the individual farmer. From a soil quality standpoint, these practices do build soil quality and have positive environmental effects.

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