Amendments to Soil and Dirt
Some Quick Facts...
This page provides GENERAL information about amendments, but for details about specific products, see the Organic Dirt Amendments link.
What amendments are for:
Many amendments improve only physical properties. The objective of using an amendment is to:
- change soil into DIRT for growing plants better. Yep - that's right, when you put stuff into the ground, don't expect it to be soil anymore. What you have after that, is good 'ole DIRT. Soil is what NATURE made and has not been disturbed by man.
Once the 'A' horizon has been changed - it's not soil anymore according to soil science classification.
Why make that distinction? We'll get to that discussion later, but for now, let's just focus on what amendments are designed to do. Certain amendments can perform one or more of the functions below:
- improve physical properties (texture/tilth/structure)
- increase water holding capacity
- increase nutrient-holding capacity
- improve aeration
- improve water infiltration (percolation)
- improve cation exchange capacity (CEC)
- adjust pH
- add nutrient/mineral values for plant growth
- suppress soil microbes that could harm plants;
- increase the soil microbe community that helps plants grow.
- Wood (high cellulose/lignin) products (wood chips, and/or sawdust from tree trunks/branches) can steal nitrogen from plant roots - but Ramial wood (small twigs and bark) does not.
Ramial? What is that? In French, it is «bois raméal».
1986 (Lemieux) - read about that at Regenerating Soils with Ramial [twig] Wood.
Otherwise, keep wood chips ABOVE GROUND as mulch.
Wood ashes, while it is organic, is not recommended since it is high in both pH and (potentially) salts. It can magnify common plant-growing problems and should not be used as an amendment in any large quantity.
- Sphagnum peat is often superior to other types of peat for certain purposes, but ANY kind of peat is good, when a low pH (acidic) amendment is needed. Mountain peat is not sustainable so don't buy it unless it's the only peat you can get - but be aware that it's the most expensive peat. This author has personal experience with Florida peat, having been the Manager of a peat farm between Avon Park and Sebring, Florida in the early '80's - but it's not sustainable either. Takes thousands of years for nature to make it.
- I do NOT recommend using sewage sludge from the municipal waste water treatment plant to amend soil because sludge is loaded with heavy metals, pharmaceuticals, chemical pollutants and high salt levels - so if you dare to use sludge, ONLY use Grade 1.
The sludge industry made up a term "biosolids" to try and make people think that sludge was an OK waste product. If a package contains any amount of sludge and does not list a grade for the sludge - don't buy it.
And that includes compost. Sludge is now being put into commercial 'compost' as a means of disposing it (now that it cannot be dumped at sea anymore). I was a member of the U.S. Composting Council until I found out they were just a 'front group' for the sludge disposal industry and they don't seem to care if you kill your plants or get a disease from using it.
While sludge CAN be composted - as a cheap filler, but in my opinion - that's a BAD thing especially if you are going to be using the compost for growing human food. I don't mind sludge being TILLED INTO dirt that grows flowers, woody ornamentals and trees - where the microbial community can go to work on trying to neutralize it - but sludge should NOT be used as an above-ground top dressing - anywhere - especially if you have kids who don't wash their hands often enough and will track it into the house. Covered with a 6" thick layer of wood chips is OK - otherwise you'll have it blowing wherever the wind takes it...
The goal of an amendment is to change mineral soil into DIRT with a organic material content, because most soil will contain only a low percentage (2% or less), whereas dirt can contain up to 50% organics and therefore provides assurance of the best growing environment for roots, particularly vegetables, due mainly to the diversity of microbe populations (particularly mycorrhizal strains).
Soil is here to stay. Soil is very important. Soil as Mother Nature made it, is not the same as dirt. But just because it is soil, is no guarantee that it will grow any plants at all (try growing something in the Sahara desert).
When soil is disturbed, it is no longer natural soil, but becomes Dirt. Dirt can have sand, silt or clay in it, that used to be soil - it's just not called soil anymore - in deference to soil science 'purists'.
When no distiction needs to be made between soil or dirt, this author refers to the stable surface we walk on as 'land' or 'earth' or 'the ground'.
The difference in different classes of dirt, is the amount of Organic Matter (OM) in it. A mineral component (sand/silt/clay) should ALWAYS exist in any dirt. It is NOT recommended to plant in straight compost. As a basic standard, a planting mix is usually 33% sand/silt; 33% clay and 33% mature or aged compost.
Organic material that contains a diversity of microbe populations - is essential.
So as not to confuse, when this author talks about growing plants in the ground, the medium being discussed is dirt - not soil. Because generally speaking, soil does not grow plants as well as dirt. In either case, soil is the ground one walks on, capable of supporting the weight of a human being on the surface - that has not been disturbed by man. If it is disturbed by nature, that's OK. As long as such disturbance is not caused artificially, by man.
When the ground contains at least one percent (1.0%) of organic matter, most vegetable plants are able to at least survive in it, given sufficient sunlight for photosynthisis and moisture to sustain plant and microbial life, but will probably require supplemental nutrition. Which means that in just plain dirt, vegetable plants will need some nutritional help, to produce much of a harvest for human food.
There are three classes of dirt referred to in this work:
- 1. Plain Dirt - containing up to 1.99% of organic matter;
2. Skinny Dirt - containing 2% to 9.99% organic matter;
3. Fat Dirt - containing 10.0%+ organic matter
ORGANIC MATERIAL vs ORGANIC MATTER
OK - now it's time to make another distinction, just so readers don't get confused. Organic MATERIAL is one thing, and Organic MATTER (OM) is something different.
Actually, the origin of both is the same - raw organics.
So think of it this way: Organic Matter (OM) is simply well-aged organic raw material. Got it? Composting begins with raw organic material, but what is created by the microbial and physical decomposers involved in the process, becomes organic MATTER (OM). It is MICROBES over TIME with MOISTURE and AIR that turns organic material, into organic MATTER.
Until organic material becomes organic matter, microbes are not able to produce plant nutrition from it. How can one tell? Easy. When you can no longer recognize that what you hold in your hand was leaves, or hay, or manure, or kitchen scraps, or whatever it was - then it has become Organic Matter (OM) from organic material. And usually as organic matter, it will be dark, crumbly, and smell 'fresh' like fertile earth. This is compost - but there is NOT much humus in it. Yet.
Organic material amendments to change soil into dirt include raw organics such as twigs, leaves, hay, straw and grass clippings, yard/garden prunings/wastes, uprooted weeds, kitchen/table scraps, etc., etc.
Organic materials that are naturally aged just by weather does not make them 'compost'. Composting is a human management activity. So by that definition can peat be called compost? No - because there was no human management involved in the semi-decomposition process. Peat is only partially-decomposed organic material.
Many raw amendments are mixed into the ground quite often, to rot down in Mother Nature's time, since evidently the people who do that, are not in a hurry for microbes to begin supplying plant nutrition from it. Or perhaps they do that to increase drainage, or porosity?
OK - humans dig a trench or pit and tossed organic material in it and covered it up. Is that composting? No, because although humans initiated the activity, there is no MANAGEMENT of it. What if they watered it regularly? Yes, that is a composting management activity.
In any case, to work best, an amendment should be thoroughly mixed into the ground (but be careful not to destroy porosity by mixing too much). Which is why the activity is called "...being WORKED into..." the ground, instead of of tilling or 'cultivating' - which should be used very sparingly. It IS possible to put too much air into garden bed dirt and stunt root growth of some garden plants.
Paper is considered a 'raw' amendment (especially cardboard and newspaper - even though it is a processed-pulp manufactured item). If paper is merely buried in layers (such as 'lasagna composting'), its effectiveness as an amendment is reduced, and it will interfere with water and air movement. Which is the benefit of lasagna composting to improve very sandy soil. In that case, restricting root growth to areas above the paper is a good thing. At least until the paper is sufficiently decomposed by microbes and larger decomposer 'critters' to have 'filtered down' into lower areas of the sand to help retain moisture and nutrients.
Better to make paper into a slurry in a bucket with water and mix with another amendment (such as compost or dirt) before putting it directly into the ground. However, a lightweight cultivator (such as a Mantis brand), is popular with some gardeners to 'work-in' paper and other raw materials directly - common in northern climates before winter sets in, so decomposition is well on the way before last frost and time to plant. As long as such raw amendments are regularly watered well, and not put down so thick as to create heat from thermophilic microbe reproduction, the practice is fairly safe for immediate vegetable plantings - although it does tend to promote growth of detrimental fungi if the ground does not drain sufficiently.
Another popular way to use heavy layered paper and cardboard is when constructing an in-ground bed to breed earthworms in a sandy soil - using several layers for the bottom and sides, poking sharpened sticks (bamboo works quite well) through the paper at corner edges deep into the sand behind the layers. Layering the paper this way assists in retaining water and in restricting movement of worms below the bottom of the excavated area. Then fill the open cavity with layers of Stable compost and aged manure from grazing animals - watering each successive layer until water begins to puddle
Worms will eventually eat the paper, but will leave a thick layer of castings to assist water retention, and if the worm bedding material is 'fed' sufficient organic materials buried into the bedding at least once weekly, the worms will not desire to move into the drier sandy soil.
For harvesting the worms and their castings from such an in-ground enclosure, follow directions found under the Breeding Earthworms link.
Amending a soil is not the same thing as mulching, although many types of mulch (such as hay, leaves, small twigs and grass clippings) are often used as in-ground raw or composted amendments.
A mulch is placed on TOP of the soil surface for the purposes of reducing evaporation, keeping soil/dirt cool, aiding water runoff without erosion, inhibiting weed growth, and creating an attractive uniform appearance among other reasons.
Mulches also moderate (mediate) soil temperature, helping to warm soils in the spring and cool them in the summer. However, mulches should be incorporated into the dirt as amendments ONLY after they have decomposed to the point that they no longer serve their purpose as a mulch (such as incorporating well rotted wood chips). Both softwood and hardwood mulches (such as mesquite and oak, make good mulches, but an additional organic nitrogen source should be added (such as a heavy spray of fish hydrolysate or blood meal/feather meal/bone meal BEFORE the material is incorporated as an amendment unless the cellulose/lignin is very well decomposed or processed through a shredder to decrease the particle size). If a wood-chip mulch is intended for later use as an amendment, it is recommended that a soft-wood be used such as pine or pine bark, since hardwood mulches tend to decompose less when incorporated into the ground. Ramial wood is always best.
ORGANIC vs. INORGANIC Amendments
There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that was once alive. Inorganic amendments, on the other hand, are either mined minerals or are synthetic (man-made).
Amending soil/dirt with inorganic gardening materials should be avoided such as rocks, gravel, fine sand and chunks of old tires which will not decompose when incorporated underground.
Especially don't add fine sand to heavy clay soils – as it can create a dirt structure similar to cement or mortar. Very coarse (river) sand is OK to add to most types of clay in small quantities, but is not useful except to permit percolation and water movement toward drainage channels since clay does not percolate. If the dirt under clay that has been amended with sand is thick, heavy clay, such effort will not be a worthwhile investment and will become a soggy mess (bog) after heavy rainfall (unless the area drains well).
Inorganic amendments recommended for potting (greenhouse/container) media include vermiculite and perlite among many others - but these products have no nutrient value for either plants or microbes and are used only to retain air/moisture.
All organic amendments increase material/matter content over time, due to microbial activity, and offer many benefits. Organic matter improves aeration, water infiltration and holding capacity of both water and nutrients.
Many organic amendments (such as aged/cured compost) contain significant levels of plant nutrient capacity and act as slow-release organic fertilizers.
Fluid drenches can also qualify as amendments, such as compost leachate, compost tea and liquid compost (called LMC which either stands for Liquid Microbial Concentrate or more commonly Liquid Mineralized Compost when combined with pulverized humate and rock phosphate, both of which add valuable humic and fulvic acid materials in microscopic form.
Even semi-decomposed organic matter ('stable' but unfinished compost) is a very important food source for bacteria, fungi, small 'decomposer' organisms and earthworms over time that (should) be abundant in the dirt when sufficient semi-decomposed organic material exists in it. Note that there is a difference between 'stable' (will no longer heat) and 'mature' compost (which has a high degree of organic matter - usually 50%+).
Carbon-source amendments provide ‘energy food’ for microbes, and nitrogen-source amendment materials are essential (proteins) for microorganism reproduction and building their physical structure. The main difference between soil and dirt - is the presence of a DIVERSE set of microbe populations that can only exist when sufficiently high organic matter content, air and water is available.
A quick mention about carbon, which is covered in other areas of this work. Both organic and inorganic carbon exists in many forms. Sugars are almost pure organic carbon which is microbe energy food. On the other hand, another form of pure inorganic carbon is a diamond - not going to be eaten by anything. And, there are forms of organic carbon that are difficult for microbes to ingest such as peat, due to it's acidic content, and also amorphous humus, which microbes can no further decompose, being high in humic and fulvic acids.
Test your soil to determine its organic content before you begin changing it into dirt. If the soil analysis shows less than 3 percent organic matter, then incorporate one cubic yard (a cu.yd. is 36” x 36” x 36”) of your chosen organic amendment per 1,000 square feet (50’ x 20’ or any combination of measurement). To avoid issues from organic salts, do not apply more than this amount at one time. Be sure to water-in the amendment, then let it set for at least 3 months after working it into the dirt.
After an adequate 'settling' time, retest the amended dirt again (for salinity and percentage of organic matter), before deciding whether to add more amendment. Make that decision based on percent of organic matter analysis shown on the actual analysis report AND how plants in the area are visually responding.
Using a conductivity (umho/cm) measurement from a Routine Soil Analysis Report can be confusing – but acceptable, IF you ask the soil lab HOW to ‘translate’ that measurement into soil saline content.
Also be aware that soil analysis is NOT conducted the same as an analysis of dirt or an amendment such as compost or manure, and don't expect your land grant university that tests agricultural soil, to know the difference.
Different types of tests are used by different labs. If you ask for a soil analysis, and the sample you send is not labeled as DIRT or COMPOST with a high organic matter content, the university or private laboratory is likely to use the wrong type of analysis and provide misleading information. Using a private soil laboratory costs a bit more, but is well worth the expense because usually more care is taken with samples, and much more information is avaiable on the reports.
Soil analysis by land grant university laboratories is geared toward AGRICULTURAL soil, not garden dirt. Just be aware that there IS a difference. This topic is covered in more detail in other chapters of this work.
At least 3% organic material in soil makes it Skinny Dirt. 5%+ organic material makes it Fat Dirt and more than 10% organic material makes dirt very fertile if it retains good porosity for air and water.
Compost is usually 100% organic material, unless sand, silt or clay was part of the feedstock mixture, but remember that compost is usually not 100% organic matter (well-aged organic material). More about that later in the composting section.
Wood products from any kind of tree trunk (even thick woody shrub) incorporated into soil tends to tie up a percentage of the nitrogen in the soil because the microbes that 'eat' the cellulose/lignin require a LOT of nitrogen to reproduce (several times an hour) and can cause nitrogen deficiency (chlorosis – yellowing/dying of leaves) in some types of plants.
Remember that nitrogen is volatile (can be leached-out in water and can evaporate in air). As the internal wood chips decompose, nitrogen depletion is reduced, so nitrogen may again becomes available to plants, but some woody products may take up to several years to break down). This hazard is greatest with sawdust used as either a mulch or amendment, because it has a greater surface area than wood chips, and therefore ‘tighter’ contact with the soil. Also be aware that tree bark does not require as much nitrogen for microbes to decompose it.
If woody products are used as walkways, foot traffic will press the wood into the soil – thereby somewhat depleting nitrogen to surface 'feeder' roots that extend underneath the walkway.
If you plan to apply wood chips or sawdust as mulch, you should apply organic nitrogen fertilizer just prior to, or when the woody mulch is spread – and any time that non-aged mulch (such as dry leaves from trees) is incorporated into the soil as amendment - to avoid nitrogen deficiency – particularly in vegetable gardens with young plants.
Sphagnum Peat vs. Mountain and Florida Peat
Sphagnum peat is an excellent soil amendment (but quite expensive), especially for sandy soils, which will retain more water after application – but freshly-finished compost is better (in this author’s opinion) because compost adds a microbe population into the soil along with organic material - and no kind of peat does that.
Sphagnum peat is generally acid (i.e., low pH in the 5.5 to 6.0 range) and can help Gardeners grow plants that require a more acidic soil. Sphagnum peat is harvested from bogs in Canada and the northern United States. The bogs can be revegetated after peat harvest and grow back relatively quickly in such a moist environment.
Mountain peat is mined from high-altitude wetlands that will take hundreds of years to rejuvenate, if ever. This mining is extremely disruptive to hydrologic cycles and mountain ecosystems, and therefore is not recommended by this author.
Florida peat, if harvested from an area near the coast, should be tested for salt content before using it and is also not a sustainable supply. Florida is a very sandy peninsula bordered on 3 sides by very salty ocean and Gulf waters. However, central Florida peat usually has a low salt content if from inland bogs due to the freshwater table from consistent rainfall.
Other types of peat are often too fine in texture to effectively aerate soil, but all peat holds about 20+ times its weight in water – and holds nutrients well too.
Is SLUDGE (Biosolids) Safe?
NO. Far from it. Sludge is a byproduct of human feces (sewage) treatment along with untold pollutants that can be harmful to plants and humans both. Sludge can be purchased alone (such as Milorganite), or combined with 'top soil' (which has been disturbed, so it's not actually soil anymore) or in compost or other organic materials (such as aged cattle manure) under many brand names. Does the producer have to state that sludge is part of the mixture? Unfortunately, NO. When a manufacturer does, it's usually stated under the title "biosolids" which is a nice way of saying 'processed human shit'.
The primary concerns about sludge are heavy metal content, potentially high pathogen (human disease microbes) levels and high salts. If you're going to pay money for shit, at least try to avoid excessive levels of heavy metals and to ensure that pathogens have been killed, by choosing only a Grade 1 sludge from a reliable source. While Grade 1 biosolids are supposedly acceptable for some types of food gardens (such as herbs), it is definitely NOT recommended for use on root crops (such as onions, potatoes, carrots, etc.) because they will come in direct contact with the edible portion of the plant. NEVER use sludge rated below Grade 1. If you don’t know the grade – DON'T use it.
Fresh manure can harm plants due to elevated ammonia (NH4) levels and heating due to thermophyllic microbe activity. To avoid such problems, use only well-aged manure (at least six months old). Pathogens could (but not likely) be another potential problem with fresh manure, particularly on vegetable gardens, but in that context, consider the source. Healthy horses and cattle are not likely to have pathogens harmful to humans. Pet manures should be always be well-composted first, before using as an amendment in any garden.
Compost made with fresh manure from ANY animal (including pets) is quite safe if processed for at least two heating cycles at a minimum of 131 degrees F (using a compost thermometer) for at least 3 days. Such heat will kill any and ALL pathogens, that come in contact with that level of temperature - which rarely includes the peripheral material of any compost pile - so the key is to ensure that the source has followed the proper compost management practices.
Why two heating cycles? Because most home compost piles are not actively ‘managed’, the piles do not sustain temperatures at this level on the periphery (on the top or around the outside of the pile). So completely turning the outer parts (top and sides) of a pile (called 'scalping') to the bottom or inside of the new pile – and putting the pile through another heat cycle are needed. If that compost is NOT processed through TWO such heat cycles, the compost should not be used to amend dirt for vegetable plants within 6 months, after the last fresh manure has been added to the pile.
The 'hot' aerobic batch method is recommended for composting fresh animal manures, with no manures being added to a pile after the first turn.
Home-composted products containing manure that do not experience two heat cycles are best used in flower gardens, shrub borders and other non-food producing gardens unless the compost is at least 6 months old. Harmful coliform bacteria such as E.coli strain H157 are not common in properly-managed compost piles. For more information on E.coli, search for Fact Sheets 9.369, Preventing E. coli From Garden to Plate, and 7.212, Composting Yard Waste.
During composting, ammonia gas is lost to air by volitization or leaching by water put through the manure, and microbes consume some of it too. Therefore, nitrogen levels will always be lower in composted manure than in raw manure. On the other hand, the phosphorus and potassium concentrations will be higher in composted manure due to microbe activity and the 'concentrating' effect of solids reduction by decompostition. Modify fertilizer practices accordingly. Salt levels also may be slightly higher in composted manure than in raw manure for the same reason. You should monitor salt levels in dirt used for garden crops.
Other composts are available that are made primarily from leaf or wood products alone or in combination with manures, but consider the C:N ratio of ANY composted product prior to purchase. If no information about the C:N ratio used in the composting process is available, assume the worst - and this author recommends that if full labeling of the product is not presented, it should NOT be purchased.
Factors to Consider When Choosing an Amendment
There are at least four factors to consider in selecting a soil amendment:
- how long the amendment will last in the soil,
- texture of the dirt before and after adding the amendment,
- salinity and plant sensitivities to salts, and
- pH of the amendment.
Laboratory tests can determine the salt content, pH and organic matter of any organic amendment. The quality of bulk organic amendments for large-scale landscape uses can be determined with a simple test of a properly-gathered sample.
Longevity of the Amendment
The amendment you choose depends on your goals.
- Are you trying to improve the physical properties quickly? Choose an amendment that decomposes rapidly.
- Do you want a long-lasting improvement? Choose an amendment that decomposes slowly.
- Do you want a quick improvement that lasts a long time? Choose a combination of amendments.
|Table 1: Decomposition rate of various amendments.|
|Grass clippings, manures||Rapid decomposition (days to weeks)|
|Composts||Moderate decomposition (about six months)|
|Wood chips (redwood, cedar), hardwood bark, peat||Slow decomposition (possibly several years)|
Dirt texture, or the way a dirt feels, reflects the size of the soil particles. Sandy soils have large particles and feel gritty. Clay soils have small soil particles and feel sticky when wet. Both sandy soils and clay soils are a challenge for Gardeners. Loam soils have the ideal mixture of different size soil particles.
When amending sandy soils, the goal is to increase the soil's ability to hold moisture and store nutrients. To achieve this, use organic amendments that are well decomposed, such as composts or well-aged manures.
With clay soils, the goal is to improve soil aggregation, increase porosity and permeability, and improve aeration and drainage. Fibrous amendments like peat, wood chips, tree bark or straw are most effective in this situation. Use Tables 2 and 3 for more specific recommendations.
Because sandy soils have low water retention, choose an amendment with high water retention, like peat, compost or vermiculite. Clay soils have low permeability, so choose an amendment with high permeability, like wood chips, hardwood bark or perlite. Vermiculite is not a good choice for clay soils because of its high water retention.
|Table 2: Permeability and water retention of various soil types.|
|Soil Texture||Permeability||Water Retention|
|Table 3: Permeability and water retention of various soil amendments.|
| Humus |
Soil Salinity and Plant Sensitivity to Salts
Some forms of compost and manures can be high in potassium salts, depending on what feedstocks were used, and how the pile is managed - information that should be available on the product label. Avoid salty amendments in soils that are already high in salts (above 9 mmhos/cm) or when growing plants that are sensitive to salts such as raspberry, strawberry, bean, carrot, onion, Kentucky bluegrass, maple, pine, viburnum and many other landscape plants are salt sensitive. In such cases, choose sphagnum peat or finely-leaves instead of manures. Live Oak leaves and pine needles are best for this purpose.
Salt Content and pH of the Amendment
Always beware of high salt content in soil amendments. High salt content and high pH are common problems in Colorado soils. Therefore, avoid amendments that are high in salts or that have a high pH. Amendments high in salts and/or pH include wood ash, mountain peat and high-manure compost from which compost leachate has NOT been encouraged by regular pile waterings.
Mineral salts content is compost is quite variable, depending on the feedstock mix and watering practices. Most aged ‘backyard’ compost is well within salt content, but commercial compost purchased in bags can have a very high salt content – check the label. If salt content is not listed, DON'T buy it.
An amendment with up to 9 mmhos/cm total salts is acceptable if well-mixed into low-salt soils (less than 5 mmhos/cm). Amendments with a salt content greater than 10 mmhos/cm are questionable. Choose a low-salt amendment for soils testing high in salts. See the page on soil testing for more information. And remember that if the surface soil has already been disturbed, it is no longer classified as soil, and may not test like an undisturbed soil sample.
Sphagnum peat and compost made from mostly plant sources are low in salts and are good choices for amending every type and texture of soils. Ask for an analysis of the organic amendments that you are considering, and choose your amendments wisely. If no analysis is available, have a small amount of the amendment tested, before purchasing a large quantity. Salts in soil/dirt hinder moisture uptake in plants.
Other soil/dirt amendments to be discussed as time permits inclusion of this information:
- Bat guano
- Cottonseed meal
- Feather meal
- Bone meal
- Blood meal
- Rock phosphate
© Robert C. Moore ~ All Rights Reserved