Fertilizers and Soil Amendments: It’s Tricky Business

Sept. 1, 2000

How we manage our soil determines its health. Apply too much nitrogen and the plant roots will burn and the soil ecosystem will be thrown off balance. If soil health is out of balance, plant growth suffers. In order for plants to take up nutrients to grow and protect themselves from pathogens, the soil needs to have a broad base of organisms performing the functions necessary to change elements into usable forms for plants.

Plant roots are the structural engineers of the soil and therefore vital components in erosion control projects. With strong root systems, plants are the best bet for preventing erosion and keeping the soil where it belongs. It is imperative to understand how soil functions because, according to Peter McRae, president of Quattro Environmental in Coronado, CA, “There is no such thing as every site being equal, and there is no such thing as a panacea for all ills. If you want success you must pay attention to what I call the ‘weak-link principle’—you are as good as your weakest link.”

The choice of fertilizer “really depends on the climate, the site conditions, and the end results you are trying to achieve,” states Eric McCrea, project manager for Wildlands Inc. in Richland, WA. “Here in eastern Washington we have a very arid climate, so a lot of our fall application of native grasses does not require fertilization due to the fact that by the time the seeds germinate, some up to two years later, the fertilizer has leached out of the seeding zone. On some projects, Wildlands and the Washington Department of Transportation did not apply fertilizer at the time of seeding but waited a full growing season and then applied a liquid fertilizer.”

Functioning Soils

Technically, anything added to the soil is an amendment. Fertilizers are amendments that provide specific nutrients, while mulches or soil builders provide the foundation for complex functions in the soil. The goal is to assess the soil and – if there is a limitation or deficiency – to correct that deficiency, enabling the soil to function. Here’s what a functioning soil should do:

  • decompose organic matter toward humus,
  • glue soil particles together in aggregates for good structure,
  • protect roots from diseases and pathogens,
  • retain nutrients,
  • transport retained nutrients to the plant,
  • produce hormones that help plants grow,
  • filter pollutants.
Functioning soil is an extensive ecosystem consisting of weathered rock, minerals, bacteria, fungi, protozoa, nematodes, arthropods, earthworms, and gophers all interacting, each performing a vital function in healthy soil. A functioning soil is able to transport nutrients to the plant, protect roots from diseases and parasites, produce hormones that help plants grow, break down pollutants, retain and improve good structure, and decompose organic matter.

Bacteria and Fungi

Bacteria, kept in balance, are beneficial to soil. They act as decomposers, consuming pathogens and organic matter and producing sticky, mucus-like substances that act as glue to hold the soil together. These aggregates provide good soil structure, allowing air and water to move through.

Millions of species of fungi exist, but only a relatively small number have been identified. Some fungi are decomposers consuming simple sugars, while another group – known as mycorrhizal fungi – are beneficial to plants and, for some plant species, are vital to their survival. Mycorrhizal fungi work in a symbiotic relationship with plants roots, drawing their energy from roots and providing the host plant with increased nutrients, particularly phosphorus and potassium. Mycorrhizal fungi are catalysts in the microbe-producing process. Fungi produce enzymes that further break down tissues, making nutrients such as nitrogen available to plants. Forming vast networks of tiny filaments, fungi contribute to soil structure and the general health of the soil.

Nematodes, Protozoa, Arthropods, Earthworms, and Other Creatures

If beneficial nematodes are in the soil, the root-eating ones are kept in check. Nematodes are tiny roundworms, barely visible to the naked eye. Some types eat other nematodes and some eat bacteria, releasing nitrogen and phosphorus into the soil. Other nematodes consume disease-causing fungi.

Tiny, one-celled protozoa perform valuable functions in the soil by preying on bacteria and fungi, thus keeping those populations in check and turning nitrogen into a usable form for plant uptake. Ciliates, a type of protozoa that feeds on anaerobic bacteria, are an indication of limited oxygen in the soil.

Arthropods consist of all sorts of creatures; the smallest are invisible to the naked eye while the largest include ants, beetles, and centipedes. They eat an array of things, and generally their function is to break up debris into small bits, aerating the soil as they forage.

The predators of the soil are earthworms. They eat everything that crosses their path, leaving nutrient-rich castings behind. If you have earthworms, your soil is on the right track.

Gophers, the rototillers of the earth, move soil, debris, and humus up and down the strata of the soil. They aerate and maintain soil structure. A study done by O.J. Reichman, professor of biological ecology at the National Center for Ecological Analysis and Syntheses at the University of California, Santa Barbara, indicates that in the long run, gophers enhance the plant diversity of an area.

Every creature in the soil fills a niche, and in a healthy, well-balanced system with adequate diversity, the checks and balances are in place so pathogens are kept to a minimum.

Required Essential Elements

There are 17 essential elements that plants require. Plants get the first three—carbon, hydrogen, and oxygen—from the air and water. These make up most of a plant’s weight. The three macronutrients that we fertilize for are nitrogen, phosphorus, and potassium, labeled on a bag of fertilizer as NPK. These nutrients are sold by weight percentage, meaning that a bag of fertilizer labeled 15-10-20 contains 15% nitrogen, 10% phosphate (phosphorus), and 20% potash (potassium). So a 100-lb. bag of fertilizer contains 45 lb. of plant nutrients. The 11 micronutrients or trace elements are calcium, magnesium, sulfur, iron, manganese, boron, zinc, molybdenum, chlorine, copper, and cobalt. All 17 elements are equally important to plant health. Along with microorganisms and nutrients soil health requires good soil structure.

Good structure allows water, air, and nutrients to move through the soil; ideal structure holds the soil together without erosion but is loose enough for roots to grow and expand. This soil characteristic is called tilth. Many tiny organisms contribute to good tilth by physically gnawing their way through the soil or by producing a mucus-like substance that glues the bits of soil into aggregates.

Promoting and Maintaining Soil Health

There are various approaches to providing and maintaining good soil health and a huge quantity of products available. Soil testing is done to measure nutrient availability and such factors as pH, humic matter, and exchangeable acidity. Michael Halle, owner of Linwood Supply in Dixon, CA, agrees: “There are many valid approaches to restoring or maintaining soil health. As a rule, testing the soil is a wise and prudent approach, although soil testing might not be needed on quick-fix projects or where someone (is familiar with) an area and is very sure of the soil type. There are two very similar aspects to our industry. One is for quick-fix erosion control, where if we don’t get something on [a piece of land], it is going to slide into the ocean. The other is the long-term solution. On a short-term project we need to get something growing over the winter, and then we can bring back the bulldozers in March. So if all you are looking for is something fast and cheap, then I wouldn’t do a soil test.” For instance, on some of the jobs along California’s Highway 80, the California Department of Transportation had put the soil up, preparing to put an offramp over. But it wasn’t going to be built for about 10 years. “I would not spend a lot of effort on that,” Halle notes. “I would get grass growing because it will be torn out. On the other hand, let’s say they are taking out an old logging road and they want to bring it back to what it was 200 years ago. When you do that, you’d better get a soil test, because basically you are growing a crop for the next century, so you have to plan ahead.

“If we’re looking for something in the fall where all we want is a quick fix, that’s a legitimate management practice-the quick fix.”

Mitigation in North Carolina

“Soil testing is appropriate and necessary on mitigation projects in North Carolina,” states Mickey Henson, principal hydrologist of Appalachian Environmental Services in Webster, NC. “The State of North Carolina says you have to mitigate 1:1 or 1:2 within the county boundaries. We have the only trout waters here in the western part of the state, 25 counties in a world of our own. I love working on trout streams.” Henson completed a stream mitigation in Bryson City, NC, on a year-round stream, a tributary of the Tuckasegee River. “In most cases we go back after three years and do vegetative sampling. When it comes to mitigation, we want to have the best project you can have. I would definitely say that on the stream mitigation project we went all out. We knew what type of plant species we wanted. We knew the soil types and acidity those plants need to have to support the long-term survival of those particular species. We basically matched up what fertilizer and additives we needed to add to the soil to promote the longevity of those plants. We had a few different soil types on the site. The site had historical uses, one being agricultural land on mountain slopes. There were distinctive soil patterns due to the use and how heavily it was tilled over the years. We started in March on the site, which was 500 feet long on both sides of the stream with 25-foot buffers. Sixty native riparian species were used in three planting zones. Immediately along the banks, sedges and riparian plants were put in. Moving away from the stream onto higher ground, silky dogwood was planted while in zone three, red maple and sycamores were placed farthest from the stream.”

On a second project done by Appalachian Environmental Services for the North Carolina Department of Transportation at a rest stop on the Andrews River, Henson remembers, “Six years ago, they had a tree on the opposite side of the river that fell into the water and basically diverted the water onto that side of the bank, taking out the floodplain. It eroded out 20 feet wide over a 400-foot stretch. Basically, on project of that scope, we find 10-20 reference cross-sections. Identify the correct bank-fall elevation and re-create what was there before the flooding took place. And so we rebuilt the shelf and brought in rock and organic matter, so testing wasn’t as stringent as on the other project. The soil was perfect-loam-so we used a fertilizer tablet on each plant. These two projects represent both ends of the spectrum. On the Andrews River Project we started out with a good product as far as the soil was concerned. So we really didn’t have to do a lot as far as amendments and fertilization.” North Carolina has the only deciduous rain forest in eastern North America receiving over 100 in. of rain a year. Says Henson, “If you have your soil ready, plants ready, it’s going to rain, it’s going to take hold, we rarely have a low survival rate of vegetation just because of the climate here. We have a lot of sun and a lot of rain.”

Revegetating the Sierras

Eric McCrea talks about a project that Wildlands designed and implemented for the national parks in the Sierra Mountains of California. Located between El Portal, CA, and the main entrance to Yosemite National Park, the Foresta project is a 45-ft.-high, south-facing, 1:1.5 slope. McCrea describes the site as predominately granitic with little to no nutrients whatsoever. Above the slope is a two-lane county road with no shoulders, and the slope terminates in the Merced River. In 1996 the Merced River reached flood stage and washed out this section of bank, causing the road to collapse into the river. The Federal Highway Department replaced the road and attempted to revegetate the site with no success. In 1998 the National Park Service contacted Wildlands about revegetating the site.

“With no grasses or plant life growing through the turf reinforcement mat, the soil underneath had allowed rill erosion to take place, so most of the topsoil was at the toe of the slope or off-site altogether. Our first task was to remove the matting and regrade the slope. Once the slope was at its original grade, we handplanted yerba santa, live oak, spice bush, and wild grape. Supplemental watering was a main concern to get the plantings established, so we installed an aboveground drip system that is gravity-fed by a remote tank. Once the drip system was completed, we applied a slurry consisting of Ecoaegis, FertilFibers NutriMulch, Kiwi Power Organic Soil Treatment, and native seeds collected by the park service. Ecoaegis is a bonded fiber matrix that, when dried, forms a three-dimensional erosion control mat.”

John Kahl of Terra Technologies in Overland, KS, says he usually specifies Gro-Power or an equivalent primary soil conditioner because he has a strong familiarity with that particular product. “We know that it helps make the projects more successful. We use it when we are trying to establish native vegetation. Most of the inorganic fertilizers don’t break down into nutrient forms that native plants can use with the end result being just feeding the weeds.”

Julie Etra, designer, erosion control and restoration specialist for Western Botanical Services, and co-vice president of the Western Chapter of the International Erosion Control Association, provides design specifications for projects in the Tahoe/Reno area of Nevada. Etra has observations from working in the Sierra Nevadas: “It depends on what your project is. If you’re looking at watershed restoration, then maybe restoration is what you want to do. If you have cut slopes on a busy highway, then you want to keep things from eroding into your roadway, and sometimes natives may not be best for that.” She recalls a pipeline job for the South Tahoe Public Utility District from which the contractor was shut out of in November three years ago. “He couldn’t do the work and it got very muddy, as the pipeline road was also an access road through the forest and part of a trail system. The road froze and became very compacted; it was really a mess when we came back in the fall to do the work. We regraded, ripped everything up from a width of 8-12 feet, and the road was mostly forested – a conifer shrub-dominated plant community – but opened up into some stream environment zones. So we had a couple of different mixes. We chose Kiwi Power because it has wetting agents and surfactants that help break up the soil, which was in bad shape. We used FertilFibers because it is a source of complex carbohydrates for microbes. I don’t use fertilizers at all anymore. The two I use now are Kiwi Power and Bio-Sol because they are soil-building amendments, and I sometimes spec RTI [Reforestation Technologies International] in teabags for container plants when I know it is a severely disturbed site. The bottom line is that I look at every site differently. I’m not going to use a soil amendment when we have fairly good or undisturbed soil, and I’m certainly not going to use soil amendments in stream environment zones and where there might be runoff. There is no standard, there is no boiler plate; every project is different, has different objectives, and has different needs. A road shoulder is different than a logging-road closure: There are all the variables—elevation, slope, soil type, rodent populations, hard-freeze precipitation, who takes care of maintenance, and what kind of budget they have.”

Success Is Site-Specific

Don Wysocki, extension soil scientist at Oregon State University in Pendleton, OR, confirms, “If you looked at the research and body of knowledge out there, there are certain properties and behaviors you would like the soil to have to serve a specific purpose. The correct approach to dealing with any soil problem is understanding what’s going on and trying to correct the situation for the purpose that the soil will serve. We can correct chemical problems, physical problems, and biological problems in the soil if we understand the processes that are going on in the soil and what additives or amendments are needed to correct the situation. If you don’t know what you have, it’s hard to correct the problems.

“It boils down to being site-specific,” Wysocki points out, “and applying agronomic principles that suit the location you are in.” If a road cut has exposed shale, Wysocki says, “those generally have a physical problem, so peat moss is a good amendment that provides better physical pore space. On the other hand, if you have sandy soils, they generally have good porosity, but the problem is infertility and water-holding capacity, so in sandy soil you might provide some organic amendment and a small amount of nutrition, again depending on what species you plant.”

To ensure success, remember that each project is site-specific; know your site, know your soils, get soil tests if needed, and have clear goals and parameters. Soil amendments and fertilizers, if used properly, can restore soil nutrients lost by erosion or construction and make up for sites’ natural deficiencies. The goal is to create a soil environment that is healthy and functioning so that plant roots can do their job of holding the soil in place. 

About the Author

Karen Brooker

Karen Brooker is a restoration horticulturist based in Santa Barbara, CA, and a frequent contributor to horticultural publications.