There’s a trend for using less of them to revegetate construction sites and maintain golf courses. Mycorrhiza and other inoculants can help make the fertilizers you do use more effective.

By Janis Keating

Is Fertilizer Necessary?
The Dirt on Soil
Oxygen: The "Forgotten" Essential Element
Fertilizer Under Your Feet
Inoculants: Giving Soil What It Might Lack Most
Sometimes It’s Just a Matter of Giving Back the Soil
There Should Be a Fungus Among Us!
The Fungi That Saved the World?

Fertilizers, especially the manmade "chemical" type, have gotten bad publicity in the past few years. Worries about National Pollutant Discharge Elimination System (NPDES) regulations and chemical runoff getting into water supplies make many people hesitant to use fertilizers, even in situations when it makes sense to use them.

These worries have led some to ask: Does fertilizer really do the best job anyway? If there are any weed seeds or seedlings in the fertilized plot, they’ll receive healthy plant food too, which will eventually negate the whole purpose of installing "good" plants. Also, if not applied correctly, fertilizer can wash out of the area it’s applied to, wasting, at the very least, budget monies and manpower.

Another cost problem: The fertilizer itself is more expensive. Recent skyrocketing natural gas prices have driven up the cost of anhydrous ammonia, a common nitrogen source. Natural gas is used both as a component (its hydrogen is combined with nitrogen from the air to form ammonia) and as an energy source to produce anhydrous ammonia. About 33.5 MMBtu of natural gas are used to create each ton of ammonia, not to mention the energy used in its manufacture.

These factors have led R.G. Hoeft and E.D. Nafziger of the University of Illinois Department of Crop Sciences to project potential spot shortages of nitrogen fertilizers in 2001. According to their report, at the end of 2000, more than one-third of the potential ammonia production capacity was idle (because of high natural-gas prices), and other plants were running at less than capacity. If gas prices remain high, they warn, the ammonia supply could fall 10% short of the previous year’s.

Is Fertilizer Necessary?

In the natural world, the process of adding fertilizer happens automatically: Leaves and other detritus fall to the forest floor, decompose, and return nutrients to the soil. In tilled or disturbed ground, however, this circle is broken, and humans have to make sure the nutrients get back into the soil.

Plants need 16 essential elements as nutrients. Carbon, hydrogen, and oxygen, which plants take from the atmosphere, are considered "primary." The remaining nutrients usually exist in the soil. Nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are considered "secondary" nutrients. Plants also use zinc, iron, manganese, copper, boron, molybdenum, and chlorine in small amounts. Depending on pH and other factors, the soil nutrients might not be readily available to plants. It’s often advised, however, that unless plants show signs of specific deficiencies of the last 10 (calcium through chlorine), fertilizers for those elements should not be applied, as toxic levels can build up in the soil.

Plants use a huge amount of nitrogen. Because it can leach out or evaporate, soil nitrogen often needs to be replenished, sometimes on an annual basis. Phosphorus and potassium help plant and root growth, so these elements are also often applied. Many fertilizers contain only those three elements.

Before considering any fertilizing program, soil testing is a must. This vital test reveals not only what nutrients already exist in the soil, but also the soil’s pH level. Levels above 9 and below 4 are toxic to plant roots; in addition, pH affects how much of the nutrients plants can glean from the soil.

Without the proper soil pH, adding fertilizer is a waste of time and money. Nitrogen needs a minimum pH of 5.5 before it can be "fixed" in the soil, and most plant matter grows best between 6.5 and 7.5. Soil tests, available from commercial labs and many Cooperative Extension offices, will also tell you what soil type a plot contains.

The Dirt on Soil

Soil type can also affect fertilizer need and use. There are three types of soil: sand, silt, and clay. The optimum soil, loam, contains the best qualities of each soil type (and lessens the bad points of each) and consists of about 40% sand, 40% silt, and 20% clay. Unfortunately, few soils match this "magic" ratio.

Along with its chemical bonding charge (+ or -), the size of each individual particle contributes to each soil type’s properties. To illustrate proportions of size, if a sand particle were the size of a dime, a silt particle would be the size of this letter "o," and a clay particle would be the size of the period at the end of this sentence. Just by particle size alone, it’s easy to understand how sandy soils remain "fluid," while it doesn’t take much to compress a clay soil into a near-concrete state.

Water movement through the three soil types

Water moves through the three soil types at different rates and diffusion patterns. The differences are caused by particle size and how much each particle bonds to others. Sand allows water to pass through quickly, in an almost straight line from the point of impact. Silt has a certain amount of capillary water movement; moisture radiates out as it moves down. Clay tends to capture the water and hold it close to the surface, as clay’s tiny particle size and chemical charge cause it to compact, prohibiting the water from travelling to lower soil levels.

Whether dry or liquid, fertilizers will be affected by this soil-specific water movement. Too much sand, and the fertilizer might drain right through the soil; too much clay, and the fertilizer might wash off the soil or flood the plant roots, creating toxic levels. Your soil test will reveal the type of soil, allowing you to make necessary amendments.

Oxygen: The "Forgotten" Essential Element

Clay soils also create a problem for the element some people forget–oxygen–which plants need underground. Because of the way clay particles bond together, leaving sparse pore space, there’s little room for O₂ underground. In addition, when rain comes, clay holds onto the water, further squeezing out oxygen–in effect, drowning the plant roots.

The pH level affects nutrient availability. The wider the band, the more the nutrient can be used by plants.

"Ninety-five percent of plants’ problems in construction sites is the lack of oxygen in the soil," says University of Kentucky Cooperative Extension Agent Don Symers. "The ideal soil for roots contains water and O₂ simultaneously." Symers describes a typical new-home site: "The topsoil has been stripped off. What’s left is the too-compacted clay level, which is not well oxygenated–it’s almost an anaerobic environment for roots. To have good results with that soil, the homeowners face a ‘lifelong’ regimen of amendment, adding organic matter and sharp sand and rototilling it all in. The plan reads almost like the back of a shampoo bottle: plow, amend, rototill, repeat."

Because heavy construction equipment compacts all the soil, even existing, undisturbed plantings can suffer. Smyers suggests "vertical mulching" for trees suffering in such conditions. "Drill 2-inch-diameter holes, about 18 to 20 inches deep, a couple feet from the trunk, maybe all around the tree, out to 25 or 30 feet from the tree trunk," he describes. "You’re setting up a grid system. Fill these holes with sand and organic matter, or even good topsoils; the tree roots will grow into this environment, this modified soil zone you’ve created. The tree will not only get good moisture and drainage, but also good O₂ intake."

Fertilizer Under Your Feet

Your plot has enough air and the right pH; now is it time for fertilizer? Maybe, maybe not. Golf courses boast some of the best-looking lawns on the planet, and even they are reducing chemical-fertilizer use. Their reasoning varies. Some want to reduce fertilizer costs, some are concerned about possible runoff into groundwater, and others want to safeguard the health of homeowners around the course.

Ted Fist, golf course superintendent of Wynstone Golf Club in North Barrington, IL, started making changes in 1994. "We’re trying to get farther away from nitrate fertilizers. We’re using more slow-release products, more organics. We also use mulching mowers, leaving those nitrogen-rich clippings on the grass."

Grass clippings on a golf course? With fairway grass just 7/16 in. tall, tees at 3/8 in., and greens 1/8 in., aren’t those clippings evident? "Golfers don’t even notice the clippings," says Fist. "Along with helping the grass, we’ve saved about $15,000 a year in waste removal."

Fist explains his mulching routine. "We try to change our mowing times, mowing less in the mornings and more in the afternoons when the clippings aren’t wet so we don’t have to pick them up. We also drag a hoe in the morning to remove the dew, as most diseases can move over the turf through water. I am saving on herbicides because our turf is dense and healthy, which makes it more resistant to disease."

Wynstone Golf Club is currently surrounded by 380 homes, and the development includes 110 ac. of water. The club’s shoreline restoration reduced a lot of siltation, preventing soil from getting into the development’s waterways. "We’re surrounded by homes, so we’re very concerned about the homeowners’ health, very cognizant of what we put out, making sure it’s nothing harmful," Fist says. "We have done environmental audits to check runoff for over 100 different chemicals. We also test our soil twice a year, as well as do leaf tissue testing on the grass to check its nutrient value."

One chemical Wynstone helped remove from its runoff: sodium. "We use homeowners’ wastewater [gray water] for irrigation after treating it with low levels of chloride," Fist says. "Five wells, containing somewhat hard water, supply the homes; most residents have water softeners, which use salt. The sodium in the wastewater hurt the grass and did some damage to our trees. Three years ago we began supplying homeowners with potassium chloride instead of sodium chloride and recalibrated all their water softeners to accept the change. We’ve significantly reduced sodium levels, and the potassium is better for the plants."

Fist also spreads the good word about turf care to the homeowners living around the club. "We do a newsletter every month, which mentions lawncare dos and don’ts. We preach what we practice–plant nutrition, healthy roots, healthy soil. You don’t necessarily need chemicals."

Certified Golf Course Superintendent Dan Dinelli of the nearby Glenview, IL, North Shore Country Club is another proponent of mulching. "We try to mulch-mow wherever we can get away with it; not the putting greens though."

Mulching saves money, according to Dinelli. "Through the season, mulching recycles about a pound of nitrogen. For example, in a 1,000-square-foot area where I would have used 3 pounds of nitrogen, mulch-mowing allows me to only use 2 pounds. I’ve probably saved $3,000 in fertilizer costs each year I’ve mulched."

To make sure his turf is in prime condition, Dinelli conducts yearly soil testing. "Since we have our nitrogen levels pretty well under control, we’re mainly checking for phosphorus and potassium levels," he says. He also uses inoculants to keep the turf and accent plantings healthy. "Mulching is a good biostimulant; it’s good for microbial activity. The [inoculated] microorganisms will eat the carbon [from plant material] and then help build the soil." Characterizing his job as "turfgrass ecology," Dinelli adds, "Twenty-five years ago we were focused on the turfgrass plant; now we focus on how we can keep the whole system healthy. That’s the fun part and the challenge, making the whole system come together."

Inoculants: Giving Soil What It Might Lack Most

Fungus. A lot of time and money is spent in keeping it out of plantings, but certain fungi are crucial to the health of most plants. Mycorrhizal fungi bond with plant roots and help the plants absorb the nutrients in the soil. Mycorrhizal plants (those that need, or at least benefit from, the fungi) find it difficult to survive if the fungi are not present; without the fungi, the plants might be surrounded by nutrients and fertilizers they can’t "reach." Mycorrhizal fungi are usually present in natural soil settings; however, tillage or construction can damage them.

Ted St. John, Ph.D., a California researcher, can’t say enough good things about mycorrhiza. "Even if you have good-quality topsoil, replacing the mycorrhiza could make the difference between [planting] success and failure," he says. Author of several studies on the subject, such as "The Instant Expert Guide to Mycorrhiza" (available at www.mycorrhiza.org), St. John notes that mycorrhiza’s main benefit is improved uptake of soil phosphorus, which spurs plant growth. Mycorrhizal plants are often more drought-tolerant and more resistant to many root diseases. The main benefit of mycorrhizae, however, is that they "network" in the soil, building soil structure, which helps hold the soil together. In this way, mycorrhizal fungi can actually help fight erosion.

Sometimes It’s Just a Matter of Giving Back the Soil

Sometimes an area where soil has been disturbed doesn’t need to be inoculated with mycorrhiza; if the fungus is already present in the topsoil, all one has to do is preserve the topsoil during the construction phase. "It has to be the same good-quality native topsoil that has been fertile before," St. John says. "But you can move that topsoil to one side, stockpile it, and return it to the site, preserving the mycorrhiza."

There’s a limit to the fungi’s life in a stockpile, however. "The best time to collect topsoil is when the mycorrhiza spores are dormant–dry season in the West, cold season in the Midwest. Spores have some energy reserves in them to ‘hang on,’ to wait for roots, but you might want to plant some mycorrhizal plants in the pile to keep the fungi happy," St. John advises.

"Don’t make the stockpile too deep–you will kill the microbes," he cautions. "For sandy soil, the pile should be no more than 6 feet deep; for clay soil, no more than 1 foot deep. For silt soil, the pile can be somewhere in between those two heights. Different fungi components die out at different rates; in wet climates, the stockpile might die out faster. You can stockpile this soil up to a year; after two years, the fungi levels have dropped dramatically. At three years, the important microorganisms have died."

There Should Be a Fungus Among Us!

Arbuscular mycorrhizae (AM) are the dominant variety. AM are endomycorrhizae, which means their fungal hyphae enter the plant's root cells.

Those ignorant of mycorrhizal fungi are doomed to repeat planting failures or limited successes, St. John warns. "They think they’re doing fine with what they’re doing. Of the reclamation plants chosen, maybe only four work, so that’s what they use from then on. Here in the West, wheatgrasses are used over and over, chosen merely because they work on a bare dirt pile.

"If you plant native plants [without mycorrhiza], you might keep them going by ‘brute force’–weeding, fertilizing, lots of water–but that can cost up to $60,000 per acre to restore coastal areas. Using mycorrhiza, or saving topsoil, is much cheaper. If you stockpile topsoil, the cost might be $3,000 to $6,000 per acre. In one recent project, inoculation brought the costs down to just $1,500 an acre."

Mycorrhizal fungi inoculations, especially those using the "wonder fungus" Glomus intraradices, are growing in popularity; the California Department of Transportation has been experimenting with an inoculation/hydroseeding process. "One major seed dealer doesn’t want his seeds used without inoculation because some users blame his seed for failing, when really the problem is the soil is mycorrhiza-deficient," St. John states.

St. John, in conjunction with the firm Bionet LLC, offers an inoculant named Endonet. "Ours comes in granular calcined clay–like kitty litter, but smaller granules–making it easier to handle with machinery. Just get it into the ground, fairly shallow, between 1 and 6 inches down, although the application method might differ in different soils."

The Fungi That Saved the World?

Prairie soils are a major storehouse of organic matter.

While the bulk of St. John’s studies have taken place in the Southwest (California and Arizona), Michael Miller, Ph.D., has been busy with similar studies in the Midwest. Miller, the senior soil ecologist at Argonne National Laboratory in Argonne, IL, along with former colleagues Jim Bever and Peggy Schultz, discovered previously unknown mycorrhizal fungi.

"We came up with about 35 species here; most had never been described. It makes you wonder: How many are there?" he says. "There seems to be a direct relationship between the number of mycorrhizal fungal species and atop-ground species–lots of diversity."

Miller believes certain mycorrhizal fungi evolved within different soil types to help certain species of plants. Seeking to restore some of the laboratory’s land, Miller and his colleague Julie Jastrow, Ph.D., are searching for mycorrhizae that work best with Illinois’ "native habitat"–prairie grasses. "We’re getting there, but we need to do more work," Miller says. "We’re trying to develop the right kinds of isolates for our needs."

Growing healthier plants is not Miller’s only goal; he believes mycorrhizal fungi might even be a key factor in arresting the greenhouse effect. "When people think of the greenhouse effect, they think about losing the rainforests," Miller notes. "That’s a factor, but a large percentage of CO₂ in the atmosphere likely came from plowing the North American prairies, causing them to lose their organic matter. Prairie soils are a major storehouse of organic matter.

"Mycorrhizal fungi may be a major mechanism in the sequestering of carbon in soils," he continues. "Plants can take CO₂ from the atmosphere, and these fungi can get it into the soil as organic matter. In essence, mycorrhizal fungi can help clean up the atmosphere while also allowing better soil and crop yields."

There’s one problem in this equation though: The bulk of plant matter in the United States, agricultural cash crops, is essentially nonmycorrhizal. "Corn and wheat, for example, have been bred to resist detrimental fungi," Miller says. "In the process, the plants now resist even mycorrhizal fungi. I’ve talked to some seed producers, trying to convince them to put those mycorrhizal genes back into their products–that’s not a high priority for them. However, because mycorrhizal fungi may increase pathogen resistance, plants possessing them could ward off ‘bad’ fungi. If the roots are occupied with ‘good guys,’ there’s no room for the ‘bad guys.’"

While Miller admits that, because the process is manpower-intensive, mycorrhizal inoculation can be expensive, he also points out possible cost savings elsewhere. "What if we decreased the amount of fertilizer needed by increasing dependence on mycorrhizal fungi? There might be a slight reduction in yield, but that might be offset by lower fertilizer costs. People need to think of mycorrhizal fungi as ‘biological fertilizer.’

"What many people don’t realize is that our cheap phosphorus sources are gone," he continues. "Although our soils are blessed with phosphorus, most of the world’s soils are deficient. Mycorrhizal fungus is a biological way of bringing phosphorus to the plants.

"It all comes down to better land-use practice," Miller concludes. "Managing our soils better, storing carbon. We must remember: Mycorrhizal fungus is not the silver bullet; it’s just one part of the system."

Janis Keating is a frequent contributor to horticultural magazines.

EC
July/August 2001

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