Building Organic Matter Worthy of Growers’ Time and Attention

By Jane Fyksen, Crops Editor
Thursday, November 29, 2007 9:52 AM CST

Most crop producers and agronomists are keenly aware of the need to maintain optimum levels of nitrogen, phosphorus, potassium, calcium, magnesium, sulfur and micronutrients in soil. The one facet of fertility that tends to get overlooked is organic matter (OM).

“Why would we bother? It is difficult to put a dollar value or yield increase on increasing soil organic matter levels,” challenges Sjoerd Duiker, Penn State University soil management specialist. He answers his own question by adding that “we do know that our soils will be more productive when they have higher organic matter contents.”

That was “again confirmed” in a Maryland study, where soils with organic carbon contents of 1.76 percent to a 6-inch depth n or approximately 3.5 percent organic matter n yielded 107 bushels of corn per acre instead of 92 on fields with organic carbon contents of 1.33 percent, or 2.7 percent organic matter. That’s an additional 15 bushels per acre attributed to a 0.8 percent increase in organic matter.

“Humus itself is an important contributor to improved soil physical properties,” says Duiker, such as soil structural stability, porosity, tilth, infiltration and water-holding capacity.

In addition, organic matter also contains nutrients that are released gradually during the growing season depending on weather conditions. For example, the nitrogen contained in organic matter of a soil with 2 percent OM is about 1,667 pounds per acre, whereas it would contain 2,500 pounds of N per acre if the organic matter content were 3 percent.

“Suppose 10 percent would be released in a season, this would be 167 pounds per acre of N in the first, compared with 250 pounds per acre in the latter case,” he explains. “Synchronization of the release of nitrogen from soil organic matter is usually well synchronized with the needs of summer crops such as corn.”

The benefits of OM include: Improved soil structure, resistance to compaction, reduced runoff and erosion, energy and food for soil microbes, increased nutrient holding capacity (cation exchange capacity), and increased water holding capacity.

One proven way to manage and increase OM is no-till. Excessive tillage aerates the soil, mineralizing the OM which then is released as carbon dioxide. This burst of biological activity also results in a flush of nutrients being made available to the crop, but that only occurs for a limited time. In no-till, the release of nutrients is controlled and continues throughout the growing season, thus maintaining, not depleting, soil OM.

As a start, Duiker thinks growers should determine the OM contents in their soils. It’s a relatively inexpensive test and can be requested when sending in a soil sample for fertility analysis. Duiker believes OM should be measured every three to five years. (More frequent testing isn’t justified because OM content changes very slowly.)

The “next question,” Duiker notes, is of course: What can be done to increase soil OM contents?

First, he stresses that OM is a “dynamic soil property,” and diligence is required to maintain or increase its value.

The ultimate source of almost all organic matter in soil is plant material, he states. Through photosynthesis, plants sequester carbon, oxygen and hydrogen in their tissues. The chemical energy stored in the new compounds can now be used by organisms such as bacteria, fungi, protozoa, nematodes, worms, arthropods (mites, springtails, beetles, ants, centipedes, maggots, termites, grubs, spiders and millipedes), even vertebrates like mice.

Historically, in farming, emphasis has often been on the soil organisms that harm our crops, “and the purpose was to eradicate them,” notes Duiker.

“Now we have come to the realization that the vast majority of organisms in soil perform important functions that help soil maintain or improve its suitability for crop production,” he points out. “The question becomes how to feed the soil organisms.” He shares some principles and practices that can help sustain soil life and increase OM content:

- Have living vegetation in the field continuously - Plant roots have their own particular effects on soil quality. Fibrous, fine-root systems stimulate soil aggregation. Taproots help the following crop roots explore subsoil and stimulate water infiltration and aeration of subsoil. Living plants in the soil at all times protect leach-able nutrients against loss to the subsoil.

“In addition, many soil organisms live in the ‘twilight zone’ between root and soil (called the rhizosphere), where they ‘graze’ on the root surface. What do they eat? Duiker lists: Root exudates, secretions and sloughed-off root cells. There is now research, he says, to suggest that the root systems of plants contribute twice as much organic material to the soil during the growing season as what remains in the root system at the end of the growing season. All this organic matter feeds soil organisms.

- Design crop rotations for crop residue return - Leading no-till farmers in South America design their crop rotations based on crop residue return, Duiker reports. They strive for five tons per acre per year of crop residue (dry matter). Because of colder winters, he notes it’s not necessary to return that much in either the Upper Midwest or his state of Pennsylvania. “Perhaps a laudable goal would be three tons per acre per year,” he remarks, cautioning that number is “pulled from the air and needs confirmation through scientific research.”

Average values of crop residue return with grain production in Pennsylvania would be three tons per acre for grain corn and also for rye (for grain), two tons for wheat and barley, and one ton for oats and soybeans. All residue should be left in the field.

“This simple example reveals the challenges we face, and the need for growing more than one crop per year. Our highly extractive practices where we harvest all biomass - straw, hay, silage, energy production - may need revision in the light of a residue return goal. Some farmers in South America even sacrifice one cutting of hay to feed the soil.”

“It also becomes important to get high amounts of biomass out of our cover crops to meet crop residue goals,” he adds.

- Eliminate tillage n It’s firmly established that no-till works on many soils, considering that farmers are successful with the practice.

Duiker compares tillage to “stoking the fire.” “It burns up organic matter,” he stresses. “Eliminating it is important if we want to increase organic matter content.”

Unfortunately, one year of tillage can erase soil improvement achieved through many years of no tillage. It’s therefore important to practice no-till continuously.

In long-term no-till soils, microbial activity is higher than in tilled soils. Fungi are more prevalent in no-till soils than in tilled soils. The fungal hyphae (i.e. hair-like structures) are an important explanation of improved soil tilth at the surface of no-till soils. Crop residue at the surface of no-till soil is essential earthworm habitat and feedstock.

- Add organic matter inputs - The primary sources of organic residues in crop production are manure and compost. Bedded manure contains more organic material and will lead to greater gains in organic matter content than liquid manure. Thus, greater potential to build OM in soils is one of the sometimes-overlooked side benefits of the new compost dairy barns. All liquid manure is not equal, either, Duiker adds; swine manure, for instance, contains fewer solids than dairy manure and can be expected to contribute to small gains in OM.

Duiker says growers sometimes grapple with poor planter penetration in no-till soils. The problem is due to hard soil. What causes a soil to become hard or soft? Water content is a logical first reason, but Duiker says soil tilth is an “important second.” Although Mother Nature typically governs water content, there are things a producer can do to keep moisture content high in the absence of rain. One factor is to have mulch cover. “Here is where many farmers are negligent. Mulch cover is often too low for optimal no-till production. After low-residue producing crops such as corn silage or soybeans, farmers should have a cover crop planted to provide the mulch cover for next year,” Duiker contends.

“What about soil tilth?” he queries. “A farmer can improve soil tilth by increasing soil organic matter content, adding manure, by having living roots growing in the soil as many days of the year as possible, and by favoring activity of microbes and small soil animals such as earthworms.” The best way to achieve it is continuous no-till, including cover cropping during times when the soil would otherwise be fallow.

“This is another area where much improvement can be achieved. Many farmers do not use no-tillage continuously, but periodically plow the fields with moldboard, chisel, or disk plows. The result is that the surface organic matter content and tilth never get up to par, hence planter penetration problems,” Duiker reports.

In a long-term tillage trial at Penn State, aggregate stability - a measure of soil tilth - in the top 2 inches of the soil was 81 percent in long-term (25-plus years) of no-till, 65 percent in short term (about five years), 50 percent in chisel/disked, and 40 percent in moldboard plowed soil.

On the equipment end, there are also some things that can be done to get soil penetration. Having sufficient weight on the planter is very critical. In dry soil conditions, you need about 500 pounds per row; a six-row corn planter needs to weigh at least 3,000 pounds. Adequate down-pressure springs are critical, especially if you have unit-mounted coulters. You can add at least 250 pounds needed for each extra coulter. This can become an issue when using the Rawson Zone-Till system, he remarks. With two extra coulters per row you need also 500 pounds of extra weight, or basically double the weight of the planter.

If you have insufficient weight on the planter, it is possible to lift the whole planter up to the point where the wheel that drives the metering system doesn’t have soil contact. A more common result is, however, that the seed gets planted 1/2 or 1-inch deep instead of 1 1/2 to 2 inches deep. Herbicide damage can then play havoc on your stands.

Long-term benefits of no-till, organic eyed

Duiker says several certified crop advisors (CCAs) have contacted him recently about a study comparing no-till and organic systems done at the USDA Agricultural Research Service (ARS) Sustainable Agricultural Systems Lab in Beltsville, Md. He provides some highlights from the full report of this study, which was published in the official publication of the American Society of Agronomy.

In the study, four cropping systems were compared: No-till (NT), no-till with cover crop (CC), no-till with a crown vetch living mulch system (CV) and a reduced tillage organic cropping system (OR). The study began in 1993. In the NT system, corn was followed by wheat and double-cropped soybeans. In the CC system, hairy vetch preceded corn and rye preceded soybeans. The CV crop rotation was similar to NT until 1998, after which soybeans were dropped from the rotation to allow the crown vetch time to develop. In the CV system, a living mulch of crown vetch grew below the economic crops. The OR system followed the corn/wheat/double-cropped soybean rotation with an overseeded crimson clover cover crop in soybeans until 1998 when the rotation was expanded to a three-year corn/soybean/wheat rotation to allow earlier establishment of crimson clover after wheat and the opportunity to control problem weeds.

Chemical fertilizer was applied in all rotations except the OR system, which received dairy manure to supply crop nutrients. Weeds were controlled with herbicides in all rotations except in the OR rotation, which included chisel/disking, rotary hoeing and sweep cultivation until 1999 and no-till with high-residue cultivator from 1999 to 2002. The whole trial was converted to the NT system (with inputs such as herbicides and fertilizers) from 2003 to 2005, Duiker describes.

Corn yields were highest in the NT and CC system (which had similar yields), with the exception of one year with exceptionally good rainfall distribution, in which the CV system had the highest corn yields. On average, the OR system had 28 percent lower corn yields than NT (due primarily to poor weed control), and CV had 12 percent lower corn yields than NT (due primarily to crown vetch competition for soil moisture).

He says soybean yields are difficult to compare because of differences in planting dates between systems. Wheat yields were higher in the CV system than in NT and OR, which had similar wheat yields.

Organic carbon concentrations were higher to a depth of 12 inches in the OR system than in the other three systems. The CC system had higher organic carbon concentrations than NT and CV to a depth of 6 inches. The organic carbon concentration was similar in the CV and NT systems.

No-till corn yields grown from 2003 to 2005 were higher when following the CV and OR systems (which were similar) than when following the NT and CC systems (which were similar).

Duiker stresses that this study does not show that tillage leads to carbon sequestration or that crop yields were higher in the organic system.

“The study does show that soil carbon losses caused by tillage can be compensated for by increasing organic matter inputs,” he reports.

Over the entire length of the crop rotation, the OR system received approximately twice as much organic dry matter (16.5 tons per acre in manure and crop residue) than the other rotations. Organic DMs in the other systems were: 6, 9 and 7 tons per acre, respectively, in NT, CC and CV systems. “It is likely, however, that greater carbon sequestration could be achieved if these inputs were included in a no-till system,” he notes.

“Secondly, the study shows that especially on drought-prone soils, higher yields can be achieved if soil organic matter contents can be increased,” he mentions.

“The mechanisms to increase organic matter contents are to reduce losses by tillage and soil erosion, as well as burning and harvesting of crop residues, and to increase inputs of organic materials in manure, composts, crop residues, and cover crops,” he sums up, stressing that “these principles are well-known, and we should continue to adhere to them.”

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