Selecting Soil Amendments with a Bent Toward Organics

Photo 1: A landscape with acceptable amount of organic material – only need is to monitor for changes. All photos courtesy of the author.

Plant health care, or PHC, is a big and growing division for many tree care businesses, and soil improvement is or should be among services offered. Very seldom will you find “ideal” soils, and PHC technicians often are faced with selecting a soil amendment to correct or improve a site. The question becomes, what is going to work for a given situation? With so many products on the market, making that decision may appear daunting. Let’s see if we can simplify it a bit.

What do we know?

Before you begin your search, determine what the site has on hand. Knowing what you have helps you determine what you need. Basic information might include: pH, percent of organic matter (OM), macro- and micro-nutrient levels, soil texture, presence of soluble salts, plant-available water, drainage rate and bulk density. If you are dealing with multiple sites on a property, be sure to determine this information for each separate site.

Testing is available to help determine these characteristics. Some may be performed on site by your company, or they can be done inexpensively through a lab. This data helps you establish goals and gives you a foundation to determine what characteristics you need – or don’t want – in a soil amendment. Please keep in mind, test results – either the ones you perform or the ones you send to the lab – are snapshots in time. These are not exact and site conditions often change, so you will need to monitor and reevaluate at intervals. That may be once a year while you are making adjustments and less often as the landscape stabilizes.

Goals may include increasing soil organic matter (SOM); improving structure and aggregation, which increases aeration, infiltration and percolation of water; increasing water-holding capacity, microbial activity and the availability of existing plant nutrients; amending heavy soils (high clay or magnesium content); adjusting the pH; or reducing compaction. There might not be a specific crisis to deal with, but simply an overall desire to help the landscape improve over time.

If nutrients are necessary, rather than synthetic fertilizers, I advocate for amendments to supply nutrients in organic forms and/or to stimulate the microbiology to process and release nutrients already in the soil, making them plant available.

If the soil is of good quality with sufficient OM and high fertility, no amendment may be warranted. Approximately 4 to 5 percent OM supplies sufficient nitrogen in most landscape settings.1 Numerous studies and observations of various products have shown that if there is sufficient OM, no significant response is seen by adding products such as humates and biochar.

If that is the case, viable services you may provide are monitoring to determine future needs and providing advice regarding ongoing cultural practices to ensure the best possible care is being given (Photo 1).

On average, OM is decomposed at a rate of 3 percent annually.2 This is a very general estimate and many environmental issues affect it; however, at some point, you probably will want to utilize an amendment to maintain the desired level of OM.

Read the labels

As you start the process of wading through the numerous amendments available, learn to read the labels. What exactly are they telling you? The levels of N-P-K (nitrogen, phosphorus as P2O5 and potassium as K2O), if present, have to be listed per federal guidelines. Individual states vary on what additional information is required and may even have licensure requirements for applying a product listed as a fertilizer. Always double check your local and regional regulations.

Personally, I like to see a lot of detail on a label. It allows me to make an informed decision. You also need to know and understand what function those ingredients perform. It does no good if you have no idea what an ingredient’s purpose is.

Soil amendments come in liquid, soluble or solid forms. The form you choose may depend on desired application methods, convenience, cost and availability.

If using a solid amendment, you need to know the level of decomposition, which is the carbon-to-nitrogen ratio (C:N). This will tell you whether you may incorporate it into the soil or if you should use it as top dressing only. If the product has a C:N ratio greater than 24:1, it should be used as top dressing only (Photo 2). The higher the carbon-to-nitrogen factor, the more important that rule becomes. Why? Because you will tie up nitrogen in the soil, making it unavailable to plants. It is important to understand exactly what is going on so you can make this determination for yourself.

Photo 2: Raw mulch to be used as top dressing only.

The process

Soil organisms have a C:N ratio on average of 8:1. They require three times as much carbon as nitrogen in order to function. Hence the recommendation to only incorporate soil amendments with a ratio of 24:1 or less into the soil (Photo 3). If you incorporate an amendment with more than that, the organisms have to scavenge nitrogen from the mineral soil in order to function. This can deplete nitrogen availability to other organisms, such as plants, until an equilibrium is restored.3

Photo 3: Processed biosolids with a carbon-to-nitrogen ratio of 24:1 or less may be incorporated into the soil.

How can you tell if it is sufficiently decomposed? It should smell good, that often quoted “good Earth smell,” not sour or like rotten eggs. It should be dark brown all the way through and crumbly. There should be no distinguishable characteristics as to its origin (Photo 4).

Photo 4: Well-decomposed compost.

If there is any doubt, use it as top dressing only and let it decompose on site. There is nothing wrong with that, and actually there is a lot of good in doing so.

This is the practice when creating tree islands. Lay down a layer of mulch out to the drip line, or as much as the client will allow. Let it decompose, putting all the nutrients in the mulch back into the soil. Whole tree chips are great for this purpose. Note: Do not use landscape fabric, as it inhibits the biological exchange between the organic mulch and the mineral soil. If more weed suppression is desired than the amount of mulch will achieve, place a layer of cardboard on top of the watered mineral soil, wetting the cardboard thoroughly prior to adding the mulch.

Using a tree with a 36-foot spread as an example, the area under the canopy will be approximately 1,017 square feet. If you want to add top dressing (mulch), it will take approximately 3 cubic yards of product to top dress that area at a level of 1 inch deep (Photo 5).

Photo 5: Area under canopy to be mulched.

Once you have gathered site information and determined a reasonable goal, you need to determine specific characteristics about amendment options as well. The list is similar: pH, percent OM, macro- and micro-nutrient level and soluble salts. In addition, you will want to know what it is composed of, including whether there are any contaminants (herbicides and insecticides) and its level of decomposition, the carbon-to-nitrogen level (C:N).

You would think if you are purchasing a product, it naturally wouldn’t have anything harmful in it. But unfortunately, that may not be the case. There have been many cases of residual herbicides in top soil and compost. In addition, manures may have insecticides harmful to soil organisms. Insecticides in manure appear to be degraded in a fairly short time period if hot composted; however, some herbicides have been shown to survive the hot-
composting process (See Table 1).

Marketing terms

You will see terms such as soil primers, bio-stimulants and soil conditioners on product propaganda and labels. These are terms used to let you know that the purpose of the product is to stimulate microbial activity in the soil. They don’t tell you what kind or how much activity, just that they will stimulate it.

Common products or ingredients

Many of the types of amendments available have no standards or regulations: humates, compost, biochar, mulch products, etc. On the other hand, biosolids are closely regulated specific to the amount of heavy metals allowed. The quality and characteristics of all these products may vary depending on processing methods.

Humates and humic substances

Humates are mined substances found with near-surface lignite deposits. Humic substances, humic acid and fulvic acid, are constituents of humates. There have been reports stating various levels of success with these products, but this varied success could be attributed to content and amount of humic substances in the humates (which can vary from 10 to 90 percent) or where it was sourced (fresh-water as opposed to salt-water origins), as well as other factors including soluble salts, heavy-metal content and ash count.


There has been a certain amount of confusion created by lack of consistent terminology regarding biochar. Biochar is a soil amendment; it is not charcoal as in cooking fuel. Charcoal (cooking fuel) may have compounds added back into the product to increase ignitability or enhance flavor, and these compounds could be toxic to plants.

Nor is it activated carbon. Activated carbon is made specifically to filter or decontaminate by increasing the sizes and numbers of adsorbing sites. It will adsorb mineral molecules and not release them.

Biochar also has a tremendous number of adsorbing sites that give it many of its beneficial characteristics, a list that reads very similar to organic-matter benefits. However, in its pure state, uncharged, it will act much as activated carbon does, in that it will adsorb minerals until all of the sites are filled. Only then will it start exchanging those elements. This will create a nutrient depletion, with trees and plants being unable to access those minerals until they are released or exchanged. Therefore, you want to be sure the product you are considering has been “charged” or “primed” with something so the exchange process happens immediately. Common products used in the charging or priming process are humates, nutrients and/or biologicals such as mycorrhizae.


Many product lines offer mycorrhizae and other biologicals in their ingredients. It is important to realize organisms are already in the soil. Enhancing the habitat, planting diversity of species and watering appropriately are the best methods for allowing these indigenous species to multiply and flourish.

However, if you have a seriously degraded site, you may want to “jump start” the process to help things get going faster. Just realize that those species you are adding to the soil probably won’t survive the indigenous species once cultural practices have improved the site and the native biota are active.


Compost is a very generic term; in and of itself, it tells you nothing – not what the product is made of or what process was used. Therefore, it is very important to determine the source of materials and the composting method (See Table 1). Basically, anything organic may be composted. Common sources are plant-based, manure-
based and spent mushroom substrate.

Plant-based materials include woody debris, leaves and kitchen scraps. These will have little to no soluble salts and have lower phosphorus levels than the manure-based sources.

Manure-based sources will have soluble salts, no matter the animal they came from. They tend to have higher phosphorus (P) loading, with poultry manure being noted as higher than most. Many areas are very sensitive to P being added to the soil due to the leaching potential and groundwater contamination, and there may be local and regional regulations or restrictions on how much
manure-based compost can be applied.

Spent mushroom substrate is not composted mushrooms but the material used for the mushrooms to grow on. This product can be high in soluble salts, and its pH varies considerably.

Vermicompost, or worm castings, is organic material composted by worms. The process differs from either hot or aged (cold) composting and produces a rather specialized and, generally, high-priced product. You will more often see vermicompost or worm castings listed as an ingredient of another product in fairly small amounts. As it is a nutrient-rich substance, this is a practical application.


We have a tendency to want to make things complicated, but often a simple method is best. Do your homework to determine site and amendment characteristics and work for the best match. Then realize it comes down to what works in the field. One more thing. Working with the natural ecosystem in an environmentally sound manner may not produce the immediate results your clients have been accustomed to with synthetic products. Let them know to be patient, that nature takes time. Give it that time to achieve a landscape that will contain healthier, more resilient plants.


  1. Colorado State University Fact Sheet No. 7.235
  2. Ontario Ministry of Agriculture, Food and Rural Affairs
  3. Weil and Brady, The Nature and Properties of Soils, 15th Ed.

Sylvia McNeill is an ISA Board Certified Master Arborist with McNeill’s Tree Service, an 11-year TCIA member company based in Corvallis, Montana.

This article is based on her presentation on the same subject at TCI EXPO 2018 in Charlotte, North Carolina, this past November. To listen to the audio recording of the entire presentation, go to this page in the digital version of this issue of TCI online, at, under Publications, and click here.

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