LA Course #GCE-1-1404
Fertilizer Myths, Probiotics & Soil pH
Edited by Len Phillips & Richard W Gibney RLA/ISA, updated October 2018
Sections Go directly to the section by clicking on the title below
Fertilizer Myths, Probiotics & Soil pH
Edited by Len Phillips & Richard W Gibney RLA/ISA, updated October 2018
Sections Go directly to the section by clicking on the title below
Fertilizer Myths
A lot has been written about fertilizer, but some of the information is misleading.
Myth: Commercial fertilizers are chemicals that are harmful to people and animals.
Commercial fertilizers are made of natural nutrients that trees can absorb. They are the same minerals as those in the food that people eat.
Myth: Organic fertilizer is better for trees than chemical fertilizers.
Trees can only use nitrogen in the nitrate form. Organic nitrogen requires the nitrification conversion process by microorganisms to become nitrate. Inorganic fertilizer can provide the nitrate form immediately. Nitrate
itself, is the same whether it is from an organic or an inorganic source.
Myth: Organic matter and organic fertilizer are the same thing.
This is not quite true. Organic matter is composed of organic compounds that have come from the remains of once-living organisms such as plants and animals and their waste products in the environment. It is
digested by bacteria and other organisms.
Organic fertilizers contain insoluble nitrogen and act as a slow-release fertilizer. In other words, an organic fertilizer refers to a soil amendment derived from natural sources that increases organic matter in the soil.
Myth: More fertilizer means faster growing trees.
The practical fact of the matter is that soil can only hold on to a certain amount of nutrients at one time. Once the soil is saturated with nutrients, the surplus nutrients just leach out into the groundwater.
Myth: If nutrients are in the soil, why add more in the form of fertilizer?
Trees use available nutrients in the soil so they can grow. Adding fertilizer is actually just replacing what the tree used.
Myth: Lots of phosphorus makes lots of flowers.
Phosphorus doesn't stimulate flower production or root growth, and has no apparent relation to winter hardiness. Overloading phosphorus in the soil actually stretches inter-nodes making the trees leggy and
weak. It can tie up root intake sites for several other nutrients including nitrogen. Once it reaches soil capacity, it leaches readily and has become one of our major water pollutants. It is true that a cold soil can cause plants to show phosphorus deficiency such as red or purplish discoloration. But this is not because phosphorus is not present; the roots simply can not absorb it when the soil is cold.
When fertilizing, read and follow label instructions. Apply these products sparingly and over the root zone. Large amounts may slow the growth of the tree and cause deficiency symptoms of other nutrients.
Myth: Potassium and magnesium will increase cold hardiness of trees.
Studies have shown that neither mineral affects cold hardiness of plants. Common sources of these minerals are fireplace ashes and Epsom salts. Wood ashes are not beneficial but Epsom salts do make leaves greener and reduce the soil pH briefly, so they might be helpful here. Epsom salts will also convert certain nutrients from an insoluble to a soluble state so they can be taken up by the trees roots
Myth: Controlled-release fertilizers release the product at the wrong time.
This myth is partially correct. All water soluble fertilizers are encapsulated with some form of polymer coating. The release of nutrients and predictability of performance is dependent upon the polymer used in the coating process and each manufacturer has its own formula.
The coating is activated by moisture and the release of nutrients is associated with temperature. The higher the temperature, the faster will be the release. When it is applied in the spring, the release increases with the rising temperatures and this is good. However, if it is applied in the fall, a sudden rise in soil temperature such as a long-term January thaw, the fertilizer is released and the tree thinks it is spring. The result is major damage to the tree and this is bad.
Myth: Fall fertilization is bad because it makes wood too soft to tolerate the cold weather.
Trees that are starved for nutrition as they go into their winter dormancy period will have a greater risk for winter injury than those that have a reserve of nutrients to carry them through the winter. In addition, when spring arrives and the trees start their natural growth cycle, those with some nutrient reserves react quickly and make that all important first spurt of growth.
On the other hand, trees that have excess nutrients in the fall are unable to slow down and take advantage of their natural rest period. When the first real cold snap occurs, it freezes the active vascular systems causing the tree major harm. Some people say this is because the wood is soft.
It is better to use controlled-release fertilizers that are controlled by temperatures, because they will slow down with the lowering temperatures. Fall fertilization is fine as long as it is done sparingly and at a time after any new growth could occur due to the fertilization. If the controlled-release fertilizer is applied at half or a quarter of the recommend rate, it will carry the tree safely through the winter and still allow it to utilize reserves for growth in the spring.
Sources
* Chalker-Scott, Linda, “The Informed Gardener”, University of Washing Press, 2008.
* Elstrodt, Charles J., “Fall Fertilization and other facts”, Nursery Management & Production, July 2009.
* The Fertilizer Institute, “Frequently Asked Questions on Fertilizer”, 2005.
A lot has been written about fertilizer, but some of the information is misleading.
Myth: Commercial fertilizers are chemicals that are harmful to people and animals.
Commercial fertilizers are made of natural nutrients that trees can absorb. They are the same minerals as those in the food that people eat.
Myth: Organic fertilizer is better for trees than chemical fertilizers.
Trees can only use nitrogen in the nitrate form. Organic nitrogen requires the nitrification conversion process by microorganisms to become nitrate. Inorganic fertilizer can provide the nitrate form immediately. Nitrate
itself, is the same whether it is from an organic or an inorganic source.
Myth: Organic matter and organic fertilizer are the same thing.
This is not quite true. Organic matter is composed of organic compounds that have come from the remains of once-living organisms such as plants and animals and their waste products in the environment. It is
digested by bacteria and other organisms.
Organic fertilizers contain insoluble nitrogen and act as a slow-release fertilizer. In other words, an organic fertilizer refers to a soil amendment derived from natural sources that increases organic matter in the soil.
Myth: More fertilizer means faster growing trees.
The practical fact of the matter is that soil can only hold on to a certain amount of nutrients at one time. Once the soil is saturated with nutrients, the surplus nutrients just leach out into the groundwater.
Myth: If nutrients are in the soil, why add more in the form of fertilizer?
Trees use available nutrients in the soil so they can grow. Adding fertilizer is actually just replacing what the tree used.
Myth: Lots of phosphorus makes lots of flowers.
Phosphorus doesn't stimulate flower production or root growth, and has no apparent relation to winter hardiness. Overloading phosphorus in the soil actually stretches inter-nodes making the trees leggy and
weak. It can tie up root intake sites for several other nutrients including nitrogen. Once it reaches soil capacity, it leaches readily and has become one of our major water pollutants. It is true that a cold soil can cause plants to show phosphorus deficiency such as red or purplish discoloration. But this is not because phosphorus is not present; the roots simply can not absorb it when the soil is cold.
When fertilizing, read and follow label instructions. Apply these products sparingly and over the root zone. Large amounts may slow the growth of the tree and cause deficiency symptoms of other nutrients.
Myth: Potassium and magnesium will increase cold hardiness of trees.
Studies have shown that neither mineral affects cold hardiness of plants. Common sources of these minerals are fireplace ashes and Epsom salts. Wood ashes are not beneficial but Epsom salts do make leaves greener and reduce the soil pH briefly, so they might be helpful here. Epsom salts will also convert certain nutrients from an insoluble to a soluble state so they can be taken up by the trees roots
Myth: Controlled-release fertilizers release the product at the wrong time.
This myth is partially correct. All water soluble fertilizers are encapsulated with some form of polymer coating. The release of nutrients and predictability of performance is dependent upon the polymer used in the coating process and each manufacturer has its own formula.
The coating is activated by moisture and the release of nutrients is associated with temperature. The higher the temperature, the faster will be the release. When it is applied in the spring, the release increases with the rising temperatures and this is good. However, if it is applied in the fall, a sudden rise in soil temperature such as a long-term January thaw, the fertilizer is released and the tree thinks it is spring. The result is major damage to the tree and this is bad.
Myth: Fall fertilization is bad because it makes wood too soft to tolerate the cold weather.
Trees that are starved for nutrition as they go into their winter dormancy period will have a greater risk for winter injury than those that have a reserve of nutrients to carry them through the winter. In addition, when spring arrives and the trees start their natural growth cycle, those with some nutrient reserves react quickly and make that all important first spurt of growth.
On the other hand, trees that have excess nutrients in the fall are unable to slow down and take advantage of their natural rest period. When the first real cold snap occurs, it freezes the active vascular systems causing the tree major harm. Some people say this is because the wood is soft.
It is better to use controlled-release fertilizers that are controlled by temperatures, because they will slow down with the lowering temperatures. Fall fertilization is fine as long as it is done sparingly and at a time after any new growth could occur due to the fertilization. If the controlled-release fertilizer is applied at half or a quarter of the recommend rate, it will carry the tree safely through the winter and still allow it to utilize reserves for growth in the spring.
Sources
* Chalker-Scott, Linda, “The Informed Gardener”, University of Washing Press, 2008.
* Elstrodt, Charles J., “Fall Fertilization and other facts”, Nursery Management & Production, July 2009.
* The Fertilizer Institute, “Frequently Asked Questions on Fertilizer”, 2005.
Probiotics
Probiotics is a new tool to increase yield, improve quality and reduce the cost to maintain your trees. Probiotics are live beneficial microorganisms that, when applied in the correct numbers, break down higher carbon forms and nutrients into more useable forms that trees and other plants can utilize.
Relationship of Trees and Microorganisms
As you know probiotics such as yogurt provides benefits for human health and digestion. Trees also have a digestive system located throughout the soil. The microbes in the soil are as vital or more so for a tree's nutritional needs as microbes are necessary for human digestion. Probiotics are the yogurt for trees, plants, and turf.
Using Probiotics
In many areas, fertilizers are becoming banned or severely restricted in use. This is because surplus fertilizer, especially phosphorus runoff is causing problems with water quality. Soil microbes help to retain, deliver, and cycle existing nutrients in the soil, thus minimizing or negating the need for fertilizer applications. Beneficial organisms fix nutrients into their cell bodies and produce sticky bio-films. This helps retain vital elements and water in the soil and rhizosphere. Microbes also travel through the xylem and phloem and release nutrients as part of their normal life-cycle.
Beneficial microbes also provide and process nitrogen through nitrogen fixation from the air and by cycling higher ammonium nitrogen into nitrates that are useable by trees and other plants. Microbes are also responsible for solubilizing phosphate in the soil. This effort increases the “P” value in soil that is available for plants and trees.
Microbes produce plant growth hormones. This stimulates better root and top growth, better health, yields, and quality.
Probiotics Provide Disease Control
In many areas, pesticide use is being banned on school grounds and other public properties. Once again the use of probiotics can be a tool that also provides disease control. Beneficial microbes produce compounds that can directly kill pathogens so pesticides are not needed. One point to keep in mind with the use of fungicides is that the fungicide will kill all fungi – good as well as bad.
Beneficial microbes will also out-compete pathogens. When two species are competing for the same resources, they will not co-exist if all other ecological factors are constant. One of the two competitors will always overcome the other leading to the extinction of the competitor or the competitor will shift toward a different ecological niche. To illustrate, bacteria and fungi compete for the same resources. Bacteria divide much faster than fungi and will often out-compete them for the limited resources.
Beneficial microbes contain proteins and complex sugars that can not be digested by pests but are very beneficial for creating a healthy plant. Pathogens on the other hand, have evolved by eating dead or weakened plant material with simple amino acids and sugars.
Water Retention
Beneficial microbes help with water retention and drought resistance by producing water as a by-product of their normal cellular metabolism. They also form biofilms that can bind and retain water at the root zone. When they die, their bodies turn into organic material which further helps with water and nutrient retention. By incorporating water as part of their cells and releasing this water to the plants, they provide the plant with water when a drought may be occurring.
Poor Management Practices
The following practices will adversely affect soil microbiology:
Composts
The use of compost is very effective and is a proven practice for encouraging microbial activity in soil. It is more sustainable than other management methods. The wide array of nutrients and natural fertilizer value will greatly enhance soil biology. Plus it is cheap to produce.
On the other hand, composts will not produce consistent results. They can be difficult and labor intensive to apply. If not properly maintained, backyard compost has the potential to grow pathogens or unwanted organisms and should be checked before use with proper testing.
Probiotics Integration
There are several practices and products that stimulate microbes in soil used for growing trees and should be considered by arborists in their tree management practices.
Controlled Biological Inoculants
These products require no preparation, just buy and apply. They are consistent because their sources are known and the microorganisms can be counted and verified. The products are concentrated and are tested for safety and pathogens. The products will be stable for years. However, these products are not as sustainable as compost. Without a microscope test or a known reliable source such as JRM Chemical, the products could be colored water. The diversity of microbes is dependent on the strains used and stabilized. The higher product costs will be offset by the lower labor and application costs.
Measuring Soil Biology
Soil microbiology can be measured visually. The fungal growth is visible on roots as web-like structures. Worms can easily be seen. Trees and plants will visibly increase in growth, health, and quality. Soil microbes can also be measured by extracting the soil liquid and counting them under a microscope. This will not provide information on the diversity of species, but total counts will be possible. Bi-products such as gas and sugars excreted, and enzymes produced can all be measured at soil web labs and DNA labs, but these tests are very expensive and not economically viable for routine tree maintenance.
Source
Probiotics is a new tool to increase yield, improve quality and reduce the cost to maintain your trees. Probiotics are live beneficial microorganisms that, when applied in the correct numbers, break down higher carbon forms and nutrients into more useable forms that trees and other plants can utilize.
Relationship of Trees and Microorganisms
As you know probiotics such as yogurt provides benefits for human health and digestion. Trees also have a digestive system located throughout the soil. The microbes in the soil are as vital or more so for a tree's nutritional needs as microbes are necessary for human digestion. Probiotics are the yogurt for trees, plants, and turf.
Using Probiotics
In many areas, fertilizers are becoming banned or severely restricted in use. This is because surplus fertilizer, especially phosphorus runoff is causing problems with water quality. Soil microbes help to retain, deliver, and cycle existing nutrients in the soil, thus minimizing or negating the need for fertilizer applications. Beneficial organisms fix nutrients into their cell bodies and produce sticky bio-films. This helps retain vital elements and water in the soil and rhizosphere. Microbes also travel through the xylem and phloem and release nutrients as part of their normal life-cycle.
Beneficial microbes also provide and process nitrogen through nitrogen fixation from the air and by cycling higher ammonium nitrogen into nitrates that are useable by trees and other plants. Microbes are also responsible for solubilizing phosphate in the soil. This effort increases the “P” value in soil that is available for plants and trees.
Microbes produce plant growth hormones. This stimulates better root and top growth, better health, yields, and quality.
Probiotics Provide Disease Control
In many areas, pesticide use is being banned on school grounds and other public properties. Once again the use of probiotics can be a tool that also provides disease control. Beneficial microbes produce compounds that can directly kill pathogens so pesticides are not needed. One point to keep in mind with the use of fungicides is that the fungicide will kill all fungi – good as well as bad.
Beneficial microbes will also out-compete pathogens. When two species are competing for the same resources, they will not co-exist if all other ecological factors are constant. One of the two competitors will always overcome the other leading to the extinction of the competitor or the competitor will shift toward a different ecological niche. To illustrate, bacteria and fungi compete for the same resources. Bacteria divide much faster than fungi and will often out-compete them for the limited resources.
Beneficial microbes contain proteins and complex sugars that can not be digested by pests but are very beneficial for creating a healthy plant. Pathogens on the other hand, have evolved by eating dead or weakened plant material with simple amino acids and sugars.
Water Retention
Beneficial microbes help with water retention and drought resistance by producing water as a by-product of their normal cellular metabolism. They also form biofilms that can bind and retain water at the root zone. When they die, their bodies turn into organic material which further helps with water and nutrient retention. By incorporating water as part of their cells and releasing this water to the plants, they provide the plant with water when a drought may be occurring.
Poor Management Practices
The following practices will adversely affect soil microbiology:
- Tilling – destroys the complex organization of the rhizosphere and the top layer of soil.
- Glyphosate – can reduce the beneficial organism populations and cause increases in pathogen growths such as Fusarium.
- Fungicides and antibacterials – will kill fungi and bacterial populations in a non-specific manner.
- Insecticides – can cause secondary effects on beneficial insect populations such as colony collapse disorder. Also extensive use of insecticides often kills the predators of common pests, leading to pest outbreaks.
- Excess fertilization with phosphate – is toxic to beneficial organisms at high concentrations . Excess nutrients will stimulate pathogen growth, and will cause pollution in nearby waterways.
Composts
The use of compost is very effective and is a proven practice for encouraging microbial activity in soil. It is more sustainable than other management methods. The wide array of nutrients and natural fertilizer value will greatly enhance soil biology. Plus it is cheap to produce.
On the other hand, composts will not produce consistent results. They can be difficult and labor intensive to apply. If not properly maintained, backyard compost has the potential to grow pathogens or unwanted organisms and should be checked before use with proper testing.
Probiotics Integration
There are several practices and products that stimulate microbes in soil used for growing trees and should be considered by arborists in their tree management practices.
- Use organic fertilizers such as humates, fish fertilizer, manures, and kelp.
- Use probiotics alone or in combination with biological stimulants. Although manures are excellent fertilizers, the odor should be considered before using it.
- Although molasses and sugars induce the growth of all forms of soil microbes, they do not necessarily encourage the growth of beneficial microbes.
Controlled Biological Inoculants
These products require no preparation, just buy and apply. They are consistent because their sources are known and the microorganisms can be counted and verified. The products are concentrated and are tested for safety and pathogens. The products will be stable for years. However, these products are not as sustainable as compost. Without a microscope test or a known reliable source such as JRM Chemical, the products could be colored water. The diversity of microbes is dependent on the strains used and stabilized. The higher product costs will be offset by the lower labor and application costs.
Measuring Soil Biology
Soil microbiology can be measured visually. The fungal growth is visible on roots as web-like structures. Worms can easily be seen. Trees and plants will visibly increase in growth, health, and quality. Soil microbes can also be measured by extracting the soil liquid and counting them under a microscope. This will not provide information on the diversity of species, but total counts will be possible. Bi-products such as gas and sugars excreted, and enzymes produced can all be measured at soil web labs and DNA labs, but these tests are very expensive and not economically viable for routine tree maintenance.
Source
- Magazzi, Joe, “Probiotics: A New Tool to Increase Yield, Improve Quality, and
Reduce Cost”, lecture at New England Grows, February 7, 2013.
Soil pH
Soil pH is a measure of acidity or alkalinity in soil, represented by a number on a scale on which 1 is very acidic, 7 is neutral, and 14 is extremely alkaline. For optimum plant growth, efforts should focus on maintaining a nearly neutral soil pH. "pH" means the power of hydrogen and is a measurement of hydrogen atoms in the soil. Acidic soil contains many H+ ions and alkaline soil contains many hydrogen oxide or hydroxide (OH–) ions.
The most accurate method of determining soil pH is by a pH meter. Secondary methods, which are simple and easy but less accurate than using a pH meter, consists of using certain indicator paper strips or indicator dyes and matching the color to a known pH level.
Soil pH Formation
Under conditions in which rainfall exceeds leaching, the basic soil cations (Ca, Mg, K) are gradually depleted and replaced with cations held in colloidal soil reserves, leading to soil acidity. The woodland floor is carpeted in needles of conifers, leaves of hardwood trees, and other dead plant matter, most of which increase soil acidity as they decompose. Unless this woodland is on top of a huge deposit of alkaline material such as limestone or serpentine, the soil will tend to be slightly acidic.
Importance
Soil pH is important because it influences plant growth, beneficial bacteria growth, nutrient availability, toxic elements, and soil structure:
The pH is not an indication of fertility, but it affects the solubility and availability of nutrients to be taken up by microorganisms and plant roots. A soil may contain adequate nutrients, yet growth may be limited by a very unfavorable pH. Likewise, builder's sand, which is virtually devoid of nutrients, may have an optimum pH for certain plant growth.Descriptive terms commonly associated with certain ranges in soil pH and the pH ranges of common products are:
Soils tend to become acidic as a result of:
How to Correct pH
Normally, limestone (calcium carbonate), dolomitic limestone (calcium carbonate and magnesium carbonate), burnt lime (calcium oxide), or slaked lime (calcium hydroxide) are used to increase the pH, or "sweeten" the soil. Limestone and dolomitic limestone are less likely to "burn" plant roots, while burnt and slaked lime are not recommended around plants. The amount of these materials necessary to change the pH will depend on the soil type. The greater the amount of organic matter or clay in a soil, the more limestone or dolomitic limestone will be required to raise the pH. Table 1 shows the amounts of limestone needed to raise the pH to a pH level of 6.5.
Table 1
Pounds of limestone per 100 sq. ft.
Sandy loam Loam Clay
From pH 4.5 to 6.5 12.6 25.3 34.8
From pH 5.0 to 6.5 10.6 21.2 29.0
From pH 5.5 to 6.5 4.2 8.4 11.6
From pH 6.0 to 6.5 1.7 3.3 4.5
If a soil is tested as too alkaline, determine if this is due to recent application of limestone or whether it is due to an inherent characteristic of the soil. It is quite difficult, if not impossible, to appreciably change the pH of naturally alkaline soil by use of acid-forming materials. If a high pH is due to applied limestone or other alkaline additives, ammonium sulfate, sulfur, or similar acid-forming materials can be applied. Table 2 shows the amounts of sulfur needed to lower the pH.
Table 2
Pounds of sulfur per 100 square feet needed to lower the pH
To pH 6.5 To pH 6.0 To pH 5.5 To pH 5.0
From pH 8.0 3.0 4.0 5.5 7.0
From pH 7.5 2.0 3.5 5.0 6.5
From pH 7.0 1.0 2.0 3.5 5.0
From pH 6.5 None 1.0 2.5 4.0
Not more than 1 pound (0.46 kg) of sulfur per 100 sq. ft. (9 sq. m.) should be applied in one treatment. If the soil is clay loam, heavier applications of sulfur will be necessary. Repeated applications of sulfur should not be made more often than once every 8 weeks. Sulfur oxidizes in the soil and mixes with water to form a strong acid that can burn the roots of plants and should be used with caution. It is easier to raise the pH of soil than it is to lower it.
Organic Matter Effect
As organic matter decomposes, minerals are slowly converted to salts that dissolve in water and become available for plant roots to absorb. Using overly acidic compost won't usually do any long-term damage to the soil, but using one that's too alkaline might. Regular applications of good-quality compost help maintain neutral soil pH. High-pH composts often contain carbonates, usually in the form of limestone (calcium carbonate). In naturally alkaline soil (most common in drier regions), avoid using high-pH compost because other nutrients, such as phosphorus and zinc, will become unavailable.
Tree Growth
Trees grow within a limited range of pH values. It is difficult to change the pH in a landscape situation as compared to an agricultural situation where the soil is turned frequently and soil amendments are easily added. In the landscape situation, limestone or sulfur can be conveniently added only during the planting process. Therefore, it is better to plant trees that tolerate the existing pH rather than trying to change it after planting, unless the entire root zone is replaced with soil having the desired pH. To do otherwise may result in nutrient deficiencies that would affect plant growth and survival.
Trees Planted in Alkaline Soil
Trees that are tolerant of alkalinity can get their needed nutrients through a process that acidifies the soil around their roots. At planting time include well-decomposed organic matter and soil sulfur in the planting mix of an over-sized planting hole. The organic matter naturally tends to moderate the alkaline conditions, especially over time.
Sulfur is only available in pellet form. When intact, the pellet has a limited effect and is overly concentrated in that spot. To counter this condition, add the sulfur to the backfill piles and turn the pile once. The pile must be at least somewhat moist. Allow the pile to sit for a few hours so the soil moistens the sulfur pellets and the sulfur becomes softer and prone to disintegration. When the soil is turned a second time and again in the planting hole, the pellets break apart and disintegrate. This process of allowing the sulfur to soften greatly enhances its effectiveness.
The final step in the process of creating and maintaining better soil pH levels in our environment is in the use of organic mulch. The moderating effects of this soil treatment are slow and gradual, but are profound. Even if we properly prepare soils, with alkaline irrigation water any moderating effects are eventually lost and pH levels begin to climb again. The use of the organic (usually wood-chip) mulches counters this natural and inevitable climb in pH levels and helps to build and maintain soils that are nutrient rich, dark, pliable and of course, of lower pH.
Instead of trying to correct the soil, it is much more cost effective and sustainable to grow trees that will tolerate alkaline soil such as those on the second list below. One recent development for growing trees in alkaline soils is the introduction of Redpointe Maple Acer rubrum ‘Frank Jr.’ PP 16769. This tree was selected for its beauty and size as well as its extreme resistance to chlorosis in high pH soils. Expect to see similar alkaline tolerant trees to be introduced in the future.
Trees that Tolerate Soil more Acidic than pH 4
Scientific name Common name
Abies spp. Fir
Betula nigra River birch
Carpinus betulus European hornbeam
Chamaecyparis obtusa Hinoki false cypress
Chionanthus virginicus Fringe tree
Cornus florida Dogwood
Crataegus mexicana Mexican hawthorn
Cupressus lindleyi White cedar
Fagus grandifolia American beech
Franklinia altamaha Franklinia
Gordonia lasianthus Gordonia
Grevillea robusta Silk oak
Halesia tetraptera Carolina Silverbell
Ilex verticillata Winterberry
Jacaranda mimosifolia Jacaranda
Liquidambar styraciflua Sweetgum
Maackia amurensis Maackia
Magnolia virginiana Sweet bay
Oxydendrum arboreum Sourwood
Pinus rigida Pitch pine
Quercus spp. Oak (especially pin oak)
Sassafras albidum Sassafras
Stewartia pseudocamellia Japanese Stewartia
Styrax japonicus Japanese snowbell
Symplocos paniculata Asiatic sweetleaf
Taxodium mucronatum Montezuma baldcypress
Taxus canadensis Canada yew
Tsuga spp. Hemlock
Trees that Tolerate Soil more Alkaline than pH 8
Scientific name Common name
Abies spp. Fir
Acacia longifolia Acacia
Acer spp. Maple
Aesculus spp. Horsechestnut
Amelanchier spp. Serviceberry
Asimina triloba Pawpaw
Betula spp. Birch
Carpinus spp. Hornbeam
Casuarina equisetifolia Australian pine
Celtis australis Mediterranean hackberry
Celtis occidentalis Hackberry
Cercidiphyllum japonicum Katsura
Cercis canadensis American redbud
Chamaecyparis spp. Cypress
Cladrastis lutea Yellowwood
Cornus kousa Kousa dogwood
Davidia involucrate Handkerchief tree
Eucommia ulmoides Hardy rubber tree
Ginkgo biloba Ginkgo
Gleditsia triacanthos Honeylocust
Gymnocladus dioicus Kentucky coffeetree
Hamamelis virginiana Witch hazel
Holodiscus discolor Holodiscus
Kalopanax pictus Castor-aralia
Koelreuteria paniculata Golden rain tree
Laburnum x watereri Goldenchain
Liquidambar styraciflua Sweetgum
Lonicera tatarica Tartarian honeysuckle
Maackia amurensis Maackia
Magnolia spp. Magnolia
Malus spp. Crabapple
Nyssa sylvatica Tupelo
Ostrya virginiana American hop hornbeam
Phellodendron amurense Amur corktree
Pinus spp. Pine
Prunus spp. Cherry, plum
Pseudotsuga menziesii Douglas fir
Pyrus calleryana Callery pear
Quercus macrocarpa Bur oak
Quercus muhlengergii Chinquapin oak
Phoenix canariensis Canary Island date palm
Platanus x acerifolia London plane tree
Platanus occidentalis American sycamore
Robinia pseudoacacia Black locust
Sciadophitys verticillata Umbrella pine
Styphnolobium japonica Scholar tree
Syringa spp. Lilac
Tamarix gallica Tamarix
Taxodium distichum Bald cypress
Tilia spp. Linden
Ulmus spp. Elm
Washingtonia robusta Mexican fan palm
Zelkova serrata Japanese Zelkova
Soil pH is a measure of acidity or alkalinity in soil, represented by a number on a scale on which 1 is very acidic, 7 is neutral, and 14 is extremely alkaline. For optimum plant growth, efforts should focus on maintaining a nearly neutral soil pH. "pH" means the power of hydrogen and is a measurement of hydrogen atoms in the soil. Acidic soil contains many H+ ions and alkaline soil contains many hydrogen oxide or hydroxide (OH–) ions.
The most accurate method of determining soil pH is by a pH meter. Secondary methods, which are simple and easy but less accurate than using a pH meter, consists of using certain indicator paper strips or indicator dyes and matching the color to a known pH level.
Soil pH Formation
Under conditions in which rainfall exceeds leaching, the basic soil cations (Ca, Mg, K) are gradually depleted and replaced with cations held in colloidal soil reserves, leading to soil acidity. The woodland floor is carpeted in needles of conifers, leaves of hardwood trees, and other dead plant matter, most of which increase soil acidity as they decompose. Unless this woodland is on top of a huge deposit of alkaline material such as limestone or serpentine, the soil will tend to be slightly acidic.
Importance
Soil pH is important because it influences plant growth, beneficial bacteria growth, nutrient availability, toxic elements, and soil structure:
- A pH determination (soil test) will tell whether the soil will produce good plant growth or whether it will need to be treated to adjust the pH level. For most plants, the optimum pH range is from 5.5 to 7.0, but certain trees prefer a more acidic soil and others may require a more alkaline level.
- Bacterial activity that releases nitrogen from organic matter and certain fertilizers is particularly affected by soil pH, because bacteria function best in the pH range of 5.5 to 7.0.
- Plant nutrients leach out of soils with a pH below 5.0 much more rapidly than from soils with values between 5.0 and 7.5 and is generally most available to plants in the range of 5.5 to 6.5.
- Aluminum, iron, and manganese may become toxic to plant growth in certain soils with a pH below 5.0.
- The structure of the soil is affected by pH. Clay soils for example, are granular and are easily worked at the optimum pH range (5.5 to 7.0), but if the soil pH is either extremely acid or extremely alkaline, clays
tend to become sticky and hard to cultivate. - Raising the pH will add calcium and reduce the effects of calcium leaching.
- Raising the pH will also raise phosphorus, molybdenum, and magnesium levels.
The pH is not an indication of fertility, but it affects the solubility and availability of nutrients to be taken up by microorganisms and plant roots. A soil may contain adequate nutrients, yet growth may be limited by a very unfavorable pH. Likewise, builder's sand, which is virtually devoid of nutrients, may have an optimum pH for certain plant growth.Descriptive terms commonly associated with certain ranges in soil pH and the pH ranges of common products are:
- Extremely acid - lower than pH 4.5; lemon-2.5; vinegar-3.0; stomach acid-2.0; soda-2.0-4.0
- Very strongly acid - pH 4.5 to 5.0; beer-4.5-5.0; tomatoes-4.5
- Strongly acid - pH 5.1- to 5.5; carrots-5.0; asparagus-5.5; boric acid-5.2; cabbage-5.3
- Moderately acid - pH 5.6 to 6.0; potatoes-5.6
- Slightly acid - pH 6.1 to 6.5; salmon-6.2; cow's milk-6.5
- Neutral - pH 6.6 to 7.3; saliva-6.6-7.3; blood-7.3; shrimp-7.0
- Slightly alkaline - pH 7.4 to 7.8; eggs-7.6-7.8
- Moderately alkaline - pH 7.9 to 8.4; sea water-8.2; sodium bicarbonate-8.4
- Strongly alkaline - pH 8.5 to 9.0; borax-9.0
- Very strongly alkaline - higher than pH 9.1; milk of magnesia-10.5, ammonia-11.1; limestone-12
Soils tend to become acidic as a result of:
- rainwater leaching away basic ions (calcium, magnesium, potassium, and sodium),
- the formation of strong organic and inorganic acids, such as nitric acid and sulfuric acid, from decaying organic matter and oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids.
How to Correct pH
Normally, limestone (calcium carbonate), dolomitic limestone (calcium carbonate and magnesium carbonate), burnt lime (calcium oxide), or slaked lime (calcium hydroxide) are used to increase the pH, or "sweeten" the soil. Limestone and dolomitic limestone are less likely to "burn" plant roots, while burnt and slaked lime are not recommended around plants. The amount of these materials necessary to change the pH will depend on the soil type. The greater the amount of organic matter or clay in a soil, the more limestone or dolomitic limestone will be required to raise the pH. Table 1 shows the amounts of limestone needed to raise the pH to a pH level of 6.5.
Table 1
Pounds of limestone per 100 sq. ft.
Sandy loam Loam Clay
From pH 4.5 to 6.5 12.6 25.3 34.8
From pH 5.0 to 6.5 10.6 21.2 29.0
From pH 5.5 to 6.5 4.2 8.4 11.6
From pH 6.0 to 6.5 1.7 3.3 4.5
If a soil is tested as too alkaline, determine if this is due to recent application of limestone or whether it is due to an inherent characteristic of the soil. It is quite difficult, if not impossible, to appreciably change the pH of naturally alkaline soil by use of acid-forming materials. If a high pH is due to applied limestone or other alkaline additives, ammonium sulfate, sulfur, or similar acid-forming materials can be applied. Table 2 shows the amounts of sulfur needed to lower the pH.
Table 2
Pounds of sulfur per 100 square feet needed to lower the pH
To pH 6.5 To pH 6.0 To pH 5.5 To pH 5.0
From pH 8.0 3.0 4.0 5.5 7.0
From pH 7.5 2.0 3.5 5.0 6.5
From pH 7.0 1.0 2.0 3.5 5.0
From pH 6.5 None 1.0 2.5 4.0
Not more than 1 pound (0.46 kg) of sulfur per 100 sq. ft. (9 sq. m.) should be applied in one treatment. If the soil is clay loam, heavier applications of sulfur will be necessary. Repeated applications of sulfur should not be made more often than once every 8 weeks. Sulfur oxidizes in the soil and mixes with water to form a strong acid that can burn the roots of plants and should be used with caution. It is easier to raise the pH of soil than it is to lower it.
Organic Matter Effect
As organic matter decomposes, minerals are slowly converted to salts that dissolve in water and become available for plant roots to absorb. Using overly acidic compost won't usually do any long-term damage to the soil, but using one that's too alkaline might. Regular applications of good-quality compost help maintain neutral soil pH. High-pH composts often contain carbonates, usually in the form of limestone (calcium carbonate). In naturally alkaline soil (most common in drier regions), avoid using high-pH compost because other nutrients, such as phosphorus and zinc, will become unavailable.
Tree Growth
Trees grow within a limited range of pH values. It is difficult to change the pH in a landscape situation as compared to an agricultural situation where the soil is turned frequently and soil amendments are easily added. In the landscape situation, limestone or sulfur can be conveniently added only during the planting process. Therefore, it is better to plant trees that tolerate the existing pH rather than trying to change it after planting, unless the entire root zone is replaced with soil having the desired pH. To do otherwise may result in nutrient deficiencies that would affect plant growth and survival.
Trees Planted in Alkaline Soil
Trees that are tolerant of alkalinity can get their needed nutrients through a process that acidifies the soil around their roots. At planting time include well-decomposed organic matter and soil sulfur in the planting mix of an over-sized planting hole. The organic matter naturally tends to moderate the alkaline conditions, especially over time.
Sulfur is only available in pellet form. When intact, the pellet has a limited effect and is overly concentrated in that spot. To counter this condition, add the sulfur to the backfill piles and turn the pile once. The pile must be at least somewhat moist. Allow the pile to sit for a few hours so the soil moistens the sulfur pellets and the sulfur becomes softer and prone to disintegration. When the soil is turned a second time and again in the planting hole, the pellets break apart and disintegrate. This process of allowing the sulfur to soften greatly enhances its effectiveness.
The final step in the process of creating and maintaining better soil pH levels in our environment is in the use of organic mulch. The moderating effects of this soil treatment are slow and gradual, but are profound. Even if we properly prepare soils, with alkaline irrigation water any moderating effects are eventually lost and pH levels begin to climb again. The use of the organic (usually wood-chip) mulches counters this natural and inevitable climb in pH levels and helps to build and maintain soils that are nutrient rich, dark, pliable and of course, of lower pH.
Instead of trying to correct the soil, it is much more cost effective and sustainable to grow trees that will tolerate alkaline soil such as those on the second list below. One recent development for growing trees in alkaline soils is the introduction of Redpointe Maple Acer rubrum ‘Frank Jr.’ PP 16769. This tree was selected for its beauty and size as well as its extreme resistance to chlorosis in high pH soils. Expect to see similar alkaline tolerant trees to be introduced in the future.
Trees that Tolerate Soil more Acidic than pH 4
Scientific name Common name
Abies spp. Fir
Betula nigra River birch
Carpinus betulus European hornbeam
Chamaecyparis obtusa Hinoki false cypress
Chionanthus virginicus Fringe tree
Cornus florida Dogwood
Crataegus mexicana Mexican hawthorn
Cupressus lindleyi White cedar
Fagus grandifolia American beech
Franklinia altamaha Franklinia
Gordonia lasianthus Gordonia
Grevillea robusta Silk oak
Halesia tetraptera Carolina Silverbell
Ilex verticillata Winterberry
Jacaranda mimosifolia Jacaranda
Liquidambar styraciflua Sweetgum
Maackia amurensis Maackia
Magnolia virginiana Sweet bay
Oxydendrum arboreum Sourwood
Pinus rigida Pitch pine
Quercus spp. Oak (especially pin oak)
Sassafras albidum Sassafras
Stewartia pseudocamellia Japanese Stewartia
Styrax japonicus Japanese snowbell
Symplocos paniculata Asiatic sweetleaf
Taxodium mucronatum Montezuma baldcypress
Taxus canadensis Canada yew
Tsuga spp. Hemlock
Trees that Tolerate Soil more Alkaline than pH 8
Scientific name Common name
Abies spp. Fir
Acacia longifolia Acacia
Acer spp. Maple
Aesculus spp. Horsechestnut
Amelanchier spp. Serviceberry
Asimina triloba Pawpaw
Betula spp. Birch
Carpinus spp. Hornbeam
Casuarina equisetifolia Australian pine
Celtis australis Mediterranean hackberry
Celtis occidentalis Hackberry
Cercidiphyllum japonicum Katsura
Cercis canadensis American redbud
Chamaecyparis spp. Cypress
Cladrastis lutea Yellowwood
Cornus kousa Kousa dogwood
Davidia involucrate Handkerchief tree
Eucommia ulmoides Hardy rubber tree
Ginkgo biloba Ginkgo
Gleditsia triacanthos Honeylocust
Gymnocladus dioicus Kentucky coffeetree
Hamamelis virginiana Witch hazel
Holodiscus discolor Holodiscus
Kalopanax pictus Castor-aralia
Koelreuteria paniculata Golden rain tree
Laburnum x watereri Goldenchain
Liquidambar styraciflua Sweetgum
Lonicera tatarica Tartarian honeysuckle
Maackia amurensis Maackia
Magnolia spp. Magnolia
Malus spp. Crabapple
Nyssa sylvatica Tupelo
Ostrya virginiana American hop hornbeam
Phellodendron amurense Amur corktree
Pinus spp. Pine
Prunus spp. Cherry, plum
Pseudotsuga menziesii Douglas fir
Pyrus calleryana Callery pear
Quercus macrocarpa Bur oak
Quercus muhlengergii Chinquapin oak
Phoenix canariensis Canary Island date palm
Platanus x acerifolia London plane tree
Platanus occidentalis American sycamore
Robinia pseudoacacia Black locust
Sciadophitys verticillata Umbrella pine
Styphnolobium japonica Scholar tree
Syringa spp. Lilac
Tamarix gallica Tamarix
Taxodium distichum Bald cypress
Tilia spp. Linden
Ulmus spp. Elm
Washingtonia robusta Mexican fan palm
Zelkova serrata Japanese Zelkova
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