Plant Nutrition
An understanding of some basic concepts about plant nutrition is necessary in order to
understand fertilizers.
Essential Plant Nutrients
Fertilizers contain one or more essential plant nutrients. A plant nutrient is considered essential
if plants are unable to survive or reproduce (flower and set viable fruit) without it. There are 16
nutrients known to be essential for plants. Of these 16 essential plant nutrients, 13 are referred to
as mineral, because plants obtain them predominantly from minerals in the soil. The remaining
3 essential plant nutrients are referred to as non-mineral, because plants obtain them from air and
water rather than from the soil. Fertilizers are added to soil to increase the amount of one or
more of the 13 essential mineral plant nutrients.
Mineral plant nutrients are divided into two major groups: macronutrients are those nutrients
required in relatively large amounts by plants, whereas micronutrients are those nutrients
required in relatively small amounts by plants.
A complete list of the 16 essential plant nutrients is shown in Table 1, below. Letters in
parentheses are symbols commonly used as abbreviations for the nutrients. Most symbols are
simply the first letter or a combination of two letters of the nutrient’s name, but three of the
symbols are derived from Latin (kalium = potassium; ferrum = iron; cuprum = copper).
Macronutrients are further classified as either primary or secondary. Primary macronutrients
include N, P, and K, whereas secondary macronutrients include Ca, Mg, and S. Fertilizers are
said to be complete if they contain each of the three primary macronutrients, whereas incomplete
fertilizers are lacking one or more of the primary macronutrients.
Page 45 of 112
Symptoms of Nutrient Deficiency & Excess
Two terms commonly used to describe symptoms of nutrient deficiency and excess are chlorosis
and necrosis. Chlorosis is a yellowing of plant tissue due to a lack of chlorophyll, the pigment
that gives plants their green color. Necrosis is the death of plant tissue, which generally has a
brown color and sometimes is referred to as “leaf burn” or “leaf scorch.”
Symptoms of chlorosis and necrosis may occur over the whole surface of a leaf, or they may be
marginal or interveinal. Marginal symptoms occur on the edges (margins) of leaves and often
include spots or patches near the edges. For example, marginal necrosis is the death of the edges
of leaves (Figure 1). Interveinal symptoms occur between leaf veins. For example, interveinal
chlorosis describes leaves with green veins and light green or yellow tissue between the veins
(Figure 2).
Brown
necrotic tissue
Light
green
interveinal
areas
Green living
tissue
Figure 1. Marginal necrosis.
Dark
green
veins
Figure 2. Interveinal chlorosis.
Below is a brief description of how individual nutrients affect plants, including common
symptoms of deficiency and excess. The nutrients listed here are limited to those with symptoms
of deficiency and excess that can be recognized easily by visual inspection of plants.
Note: the two most common soil fertility issues in lawn, garden, and landscape soils are
nitrogen deficiency and improper soil pH (both are described in more detail below).
Nitrogen (N): N promotes plant growth and, because it is a component of chlorophyll, also
promotes green color. Almost all plants contain chlorophyll, although the green color is
masked by other pigments in some plants, such as those with red or purple leaves.
Deficiency: stunted growth and yellowing of normally green leaves, beginning with older
leaves and quickly progressing to the whole plant if not corrected. Nitrogen
deficiency is the most common nutrient deficiency.
Excess: excess N promotes excessive plant growth that may be weak and susceptible to
disease or insect infestation. Excess N also delays and reduces flowering and fruit
production. Some N fertilizers are salts, and excess salts may result in wilting,
marginal necrosis, and, in severe cases, plant death.
Page 46 of 112
Phosphorus (P): Because P is part of the fundamental processes of energy storage and
transport in plants, it is necessary for every aspect of plant growth. Although P frequently is
said to promote root growth, this simply is not true. A lack of P will reduce plant growth—
including roots—and the addition of P in the form of fertilizer can alleviate this reduced
growth; however, the addition of P to a plant that already has adequate P will not promote
additional root growth beyond that which is normal for the plant.
Deficiency: Phosphorus deficiency initially may go unnoticed due to the lack of dramatic
symptoms such as chlorosis or necrosis. Stunted growth, leaf drop of older (lower)
leaves, and delayed flowering of plants may be indications of P deficiency. In some
plant species, P deficiency may result in a purple coloration of foliage.
Excess: There are no visual symptoms directly associated with excess P.
Potassium (K): K is used by plants primarily to regulate water movement. Potassium may
also play a role in disease resistance and general stress tolerance, such as moisture and
temperature extremes.
Deficiency: K deficiency begins with marginal chlorosis, progressing to necrosis, and
appears first on older (lower) leaves.
Excess: Most K fertilizers are salts, and excess salts may result in wilting, marginal
necrosis, and, in severe cases, plant death.
Calcium (Ca): Ca is a component of cell walls and, as such, is required for all plant growth.
Deficiency: Ca deficiency results in reduced plant growth. Calcium deficiency usually is
associated with low soil pH (see “Soil pH” on page 48). Blossom end rot in tomatoes
and bitter pit in apples are the result of Ca deficiency in the fruit, although these
disorders do not always indicate a deficiency of Ca in the soil.
Excess: There are no visual symptoms directly associated with excess Ca. Excess Ca
often is associated with high soil pH.
Magnesium (Mg): Mg is a component of chlorophyll.
Deficiency: Interveinal chlorosis, developing first on older leaves.
Excess: There are no visual symptoms directly associated with excess Mg. Some Mg
fertilizers are salts, and excess salts may result in wilting, marginal necrosis, and, in
severe cases, plant death.
Boron (B): B is necessary for pollination and several other plant processes and components.
Boron is the only micronutrient that leaches from soil appreciably in moderate soil pH
conditions.
Deficiency: deformation or death of new shoots; abnormally shaped fruit, or failure to set
fruit.
Excess: necrosis of interveinal leaf tissue.
Iron (Fe): Fe is necessary for the formation of chlorophyll but is not a component of
chlorophyll.
Deficiency: interveinal chlorosis of new leaves. Iron deficiency usually is the result of
high soil pH which causes iron in the soil to be unavailable to plants.
Excess: Most plants show no visual symptoms as a result of excess Fe, although excess
iron applied to lawns may turn foliage black.
Page 47 of 112
Zinc (Zn), Manganese (Mn), and Copper (Cu): Similar to iron, the availability of Zn, Mn,
and Cu all are affected by soil pH.
Deficiency: interveinal chlorosis. Zn, Mn, and Cu deficiency usually are the result of
high soil pH which causes Zn, Mn, and Cu in the soil to be unavailable to plants.
Excess: Generally only a problem in soils with a pH below about 5.5.
Soil pH
pH is a measure of acidity. A pH of 7 is neutral; pH levels below 7 are acidic; pH levels above 7
are alkaline (Figure 3). The pH of western Oregon soils generally is between 5 and 7.
Figure 3. Soil pH.
Soil pH affects plants indirectly through its control on the availability of nutrients and also some
other elements in soil. Figure 4 shows the availability of essential mineral plant nutrients over a
pH range from 4 to 10. In Figure 4, the wide portion of the bands corresponds to maximum
nutrient availability; the narrower a band, the less available the corresponding nutrient(s). Most
plants grow best in soil with a pH between about 6 and 7, which corresponds to reasonably high
availability of all 13 essential mineral nutrients. The availability to plants of Mn and also
aluminum (which is not a plant nutrient) increase with decreasing pH and easily can reach levels
toxic to many plants in low pH soils.
Figure 4. Soil pH effects on nutrient availability.
Page 48 of 112
The following is a list of optimal pH ranges for several common groups of plants.
Annual landscape plants: 6.0 to 7.0
Azaleas & Rhododendrons: 4.5 to 5.5
Blueberries: 4.5 to 5.5
Fruit trees: 6.0 to 7.0
Grapes: 5.5 to 6.5
Herbaceous ornamentals: 6.0 to 7.0
Lawns: 5.5 to 6.5
Shade trees: 5.0 to 6.5
Vegetables: 6.0 to 7.0 (Brassicas, including broccoli, cabbage, cauliflower: 6.5-7.0)
Woody shrubs and vines (for example, roses, clematis): 5.5 to 7.0
Adapted from Oregon State University Extension Circular 1560-E, “Acidifying soil for
blueberries and ornamental plants in the yard and garden west of the Cascade mountain range
in Oregon and Washington.”
In cultivated soils, including lawns, gardens, and landscapes, leaching and addition of N
fertilizers gradually decrease soil pH. See “Liming Materials” on page 59 for information about
amendments that raise soil pH.
Fertilizers
Now that we have established a basic understanding of plant nutrition, let’s apply this
information to fertilizers.
Fertilizer Grade
The grade of a fertilizer refers to the percentage, by weight, of primary macronutrients in the
fertilizer. For example, the “20-20-20” shown on the sample label in Figure 5 indicates the
fertilizer contains 20% N, 20% P, and 20% K (see “Phosphate (P2O5) and Potash (K2O)” on page
50 for more information).
Figure 5. Sample label.
Page 49 of 112
Guaranteed Analysis
While the grade can be useful for quickly locating a product of interest, the Guaranteed Analysis
lists complete details about the contents of a fertilizer product, including both the amount of each
essential plant nutrient in the fertilizer and the materials from which those nutrients are derived
(Figure 6).
Figure 6. Guaranteed Analysis.
By law, any product marketed as a fertilizer must be labeled with a Guaranteed Analysis, and
fertilizers must contain each nutrient in at least the amount listed in the Guaranteed Analysis.
Manufacturers know that most customers are not interested in the details of a Guaranteed
Analysis, so usually it will be printed in small font, often on the back of product packaging.
Phosphate (P2O5) and Potash (K2O)
Primary macronutrients are listed in fertilizer grades and Guaranteed Analyses as Total
Nitrogen (N), Available Phosphate (P2O5), and Soluble Potash (K2O). While the nitrogen
content is listed simply as N, the phosphorus and potassium contents are listed in the form of
their oxides, P2O5 and K2O, respectively. Ironically, the P and K in fertilizer products never
take the form of these oxides. Nonetheless, this is the standard by which P and K contents of
fertilizers are listed on product packaging. The difference between P and P2O5 or between K
and K2O is not significant under normal circumstances, because the instructions on product
packaging already account for these differences.
Page 50 of 112
Ratio
Figures 8 and 9 represent an “all purpose” fertilizer with a grade of 20-20-20. A general
characteristic of all purpose fertilizers is that the proportion of each primary macronutrient (N, P,
and K) is approximately equal. A simple way of comparing the proportion of N, P, and K in
various fertilizer products is to compare the ratio of primary macronutrients—that is, how much
N a product contains relative to P and K. For example, a product with a grade of 20-20-20 has a
1-1-1 ratio of N-P-K, which means that for every 1 part N there is also 1 part P and 1 part K.
The ratio of a fertilizer is determined by dividing all three numbers in the grade by their greatest
common factor. In the case of 20-20-20, the greatest common factor is 20, so dividing the three
numbers in the grade by 20 yields a ratio of 1-1-1.
As another example, let’s find the ratio of a “bloom booster” fertilizer with a grade of
10-50-10. The greatest common factor of the three numbers in the grade is 10. Dividing the
three numbers in the grade by 10 yields a ratio of 1-5-1. A different bloom booster fertilizer may
have a grade of 0-50-30, which has a ratio of 0-5-3. Some general characteristics of bloom
booster fertilizers are that they have little or no N, a moderate to high amount of P, and a low to
moderate amount of K. These characteristics hold true for the fertilizer products with ratios of
1-5-1 and 0-5-3.
In Search of the Perfect Fertilizer
Although some generalities loosely characterize the ratios of certain types of fertilizer products,
such as the all purpose and bloom booster products described above, no perfect ratio or grade
exists for any particular use or plant type. A good way to think about fertilizers is this:
fertilizers don’t make plants do anything, such as grow, flower, or fruit; fertilizers simply allow
plants to do what they do naturally by supplying the basic building blocks needed for growth
and reproduction (flowering and fruiting). Genetics (DNA) provide the blueprint that directs
plant growth, and fertilizer simply ensures there are adequate raw materials in the soil for
plants to fulfill the directions in their genetic blueprint. No matter what the blueprints call for,
a carpenter can’t build a sprawling mansion if given only enough materials for a modest
cottage; similarly, a plant will not achieve optimal productivity (growth, color, flowers/fruit,
etc.) if it cannot obtain an adequate amount of nutrients.
A safe and simple way to select an appropriate fertilizer is to read and follow the directions on
product packaging. Package size and price can help to narrow the choices available.
Application Rate
The application rate of N-containing fertilizers generally is based on their N content. This is
because N promotes plant growth, and excess N (beyond what is required for optimum growth
and productivity) promotes excessive plant growth that may be weak and susceptible to disease
or insect infestation. Excess N also delays and reduces flowering and fruit production. While N
doesn’t make plants grow, plants generally will make use of N when it is available, regardless of
whether or not it is in excess of that needed for optimum growth and productivity; this is referred
to as luxury consumption.
Page 51 of 112
N Forms
The N content of a fertilizer is expressed on the product label as total N, and that amount is
further divided among up to five different categories in a Guaranteed Analysis:
Ammoniacal N
Nitrate N
Urea N
Other water soluble N
Water insoluble N
For example, Figure 7 (excerpted from Figure 6) shows that the 20% N contained in the labeled
fertilizer consists of 1.97% nitrate N and 18.03% urea N.
Figure 7. Fertilizer N content.
A significance of these various categories of N is that they are not all equally available to plants.
Plants can directly make use of only 2 forms of N: ammoniacal (often referred to as ammonium)
and nitrate. Thus, ammoniacal and nitrate N both are forms of N that can be used directly by
plants at the time of fertilizer application. All other forms of N must be converted either to
ammonium or nitrate before plants can use them. This conversion is a natural process resulting
from microbial activity in the soil.
Urea generally is considered to be readily available to plants at the time of application, because it
is quickly converted to ammonium by soil microbes under normal growing conditions (this
conversion is somewhat slower in cold temperatures). Urea may be present in a sulfur- or
polymer-coated form that is slowly released into plant-available form as the coating degrades,
thereby allowing longer intervals between fertilizer applications, such as for lawn fertilizers.
When sulfur- or polymer-coated urea is present in a fertilizer product, the amount of urea in
sulfur- or polymer-coated form will be identified in the Guaranteed Analysis (usually at the
bottom).
Other water soluble N, sometimes referred to as slowly available water soluble N, is considered
readily available to plants at the time of fertilizer application and includes all water soluble forms
of N other than ammoniacal, nitrate, and urea N. The methylene urea used in Scotts lawn
fertilizers is one of the most commonly available forms of other water soluble N.
Water insoluble N includes all insoluble forms of N, including N contained in organic matter.
Water insoluble N is not considered to be readily available to plants at the time of fertilizer
application; instead it is slowly released into plant-available form as a result of microbial activity
in the soil, thereby allowing longer intervals between fertilizer applications.
Organic N sources, such as compost, manure, etc., may contain both water insoluble N and other
forms of N, such as ammoniacal N. As described above, the other forms all are considered readily
available to plants at the time of application, but water insoluble N is not. Microbial activity
releases organic N into a plant-available form, so environmental conditions must be suitable for
microbial activity in order for organic N to become available to plants following fertilizer
Page 52 of 112
application. Cold and/or dry conditions slow the rate of microbial activity and may slow the
release of organic N to plants until such time as warmer and/or wetter conditions prevail.
Regardless of the N form(s) in a fertilizer, whenever possible, incorporate N-containing
fertilizers into the top 1 to 2 inches of soil or irrigate with ¼ to ½ inch of water following N
fertilizer application in order to minimize the loss of N to the atmosphere.
Complete Fertilizer Blends
Complete fertilizers—those that contain each of the primary macronutrients—often are blends of
multiple fertilizer materials, which is necessary to achieve a particular grade. For example,
Figure 8 (excerpted from Figure 6) lists the individual fertilizer materials in the 20-20-20
fertilizer blend from the examples above.
Figure 8. Individual fertilizer materials.
Many different blends of complete fertilizers are available. In general, these blends loosely can
be grouped into one the following categories of fertilizer products.
All Purpose: These products generally have a 1-1-1 ratio (for example, a product with a
grade of 16-16-16). All Purpose fertilizers safely can be used on most plants, when used
according to label directions.
Bloom Booster: These products are used to ensure adequate nutrients are available to plants
when needed for flowering. Most Bloom Booster products contain little or no N, a moderate
to high amount of P, and a low to moderate amount of K.
Rhododendron/Azalea/Camellia: Sometimes referred to by terms such as, “acid-loving
evergreen fertilizer,” these products are formulated specifically for plants that grow best in
acidic soils. The ratios of these products vary, but generally they contain…
Elemental S, which lowers soil pH.
Micronutrients.
Some N in slow-release form.
Rose: These products tend to have a 1-2-1 ratio and contain both micronutrients and some
slow-release N. Although the 1-2-1 ratio is common among rose fertilizers, there is nothing
about this ratio that makes it particularly appropriate for roses. Some fertilizer products
labeled for roses also contain an insecticide that helps manage common insect pests, such as
aphids.
Starter: These products generally contain high amounts of P. The reason for this often is
said to be that P makes roots grow, but this is misleading, at best. More accurately, since P is
very immobile in the soil, starter fertilizers can be used to help ensure adequate P is available
to young plants when root growth is limited by cool soils, such as in the spring.
Page 53 of 112
Some starter fertilizers contain vitamin B1, which sometimes is said to reduce
transplant shock and stimulate new root growth. However, research is lacking to support the
notion that vitamin B1 provides any such benefits to plants.
Some starter fertilizers contain plant hormones, such as Indole Butyric Acid (IBA) or
Naphthalene Acetamide (NAD), which may increase rooting and plant growth.
Vegetable: These products often contain low amounts of N and higher amounts of P and K,
sometimes in a 1-2-2 ratio. Since application rates of N-containing fertilizers generally are
based upon their N content, vegetable fertilizers supply relatively high amounts of P and K
without supplying an excessive amount of N. P is very immobile and K is only somewhat
mobile in the soil, so tilling vegetable fertilizer into the soil prior to planting increases the
amount of P and K throughout the top 4 to 6 inches of the root zone rather than only at the
soil surface.
Lawn: These products generally contain high N, low P, and low to moderate K. The N
content generally is around 30%, and some of this N generally is in slow release forms that
help the fertilizer remain effective for about 6 to 8 weeks following application. Lawn
fertilizers with significantly less than 30% N generally either are organic (for example,
10-2-6) or do not contain slow release N (for example, 12-5-5, 15-0-10).
There are several specialty lawn fertilizers available.
Weed & Feed: These products contain one or more herbicides to help control
existing broadleaf weeds. These products will not control grassy weeds, and they will
not prevent new weeds from sprouting. Weed & Feed products should not be applied
when temperatures are expected to be above about 85 F within a day or two
following application (refer to individual product labels for specific directions).
Crabgrass Preventer: This product contains a pre-emergent herbicide that helps
prevent crabgrass from becoming established. Crabgrass is a summer annual,
meaning that it dies off in winter and sprouts from seed each spring. Crabgrass can
occur in lawns in the southern Willamette Valley, but it doesn’t thrive in the mowed,
watered, and fertilized environments typical of most lawns. Crabgrass is most
common in recently disturbed areas, which can include new lawns.
SummerGuard: This product contains an insecticide that helps control insect pests.
Brown
Spots
in Lawns
Brown
Spots
in Lawns
We are fortunate to have relatively few insect pests that damage lawns in the
southern Willamette Valley. Although insects often are blamed for brown spots in
lawns in summer, brown spots more often are the result of watering issues (too
much, too little, too often, not often enough, etc.).
Watering in the early morning helps prevent excessive
evaporation and also avoids windy conditions that distort sprinkler
head spray patterns. Most lawns do well when watered every other
day in summer. In the hottest part of summer, lawns in the
southern Willamette Valley require approximately 1 ½ inches of
water per week, which amounts to about ½ inch of water every
Green Grass
other day. EWEB supplies Green Grass Gauges to help determine
Gauge
how much water sprinklers deliver to a particular area. Place a
Green Grass Gauge in the area(s) with brown spots to determine if
those areas receive too much or too little water.
53 of 112
111
Page 54
Winterizer: These products generally contain less N and more K than other lawn
fertilizers. The lower N avoids excessive growth that may be more susceptible to
diseases in cool, wet weather. Higher K helps ensure there is an adequate source of
this nutrient which has been said to help with stress tolerance during cold, wet
weather, although research supporting this is lacking.
Moss Control: These products contain iron sulfate, which helps control moss. Since
these products only contact the top of moss plants, often they are less effective at
controlling moss than liquid moss control products that more thoroughly cover moss
plants. However, the fertilizer in these products ensures the lawn has the nutrients
needed to compete with moss. The best results are obtained when liquid moss killers
are applied after de-thatching. De-thatching physically removes up to about 70% of
moss, leaving the remaining moss exposed to be covered more easily with a liquid
moss control product. Follow with a regular lawn fertilizer for fast recovery.
Starter: Starter fertilizers generally contain less N and more P than other lawn
fertilizers. See “Starter” and “Vegetable” fertilizers above for more information.
Heterogeneous Versus Homogeneous
Most fertilizers are heterogeneous, meaning not every granule in the product is the same. One
granule may contain N, another P, another K, and so on. Scotts® lawn fertilizers are
homogeneous, meaning every granule is the same and contains some of each nutrient in the
product.
In theory, homogeneous fertilizers are superior, because they guarantee each nutrient is
distributed evenly over the area to which they are applied. In practice, however, heterogeneous
fertilizers are equally effective and generally cheaper.
Homogeneous products may provide better delivery of Weed & Feed products. There are two
reasons for this: (1) every granule contains the herbicide; and (2) the granules in homogeneous
fertilizers tend to be smaller than the granules in heterogeneous fertilizers, and smaller granules
are more likely to stick to the leaves of weeds and are less likely to bounce off.
Low P and No P Fertilizers
P generally is the nutrient that limits biological productivity in lakes and streams. When runoff
from rain or irrigation carries P to a lake or stream, an algae bloom may result. As that algae
dies, decomposition of the algae consumes the oxygen dissolved in the water. This process is
known as eutrophication and can result in mass fish mortality due to a lack of dissolved
oxygen.
Eutrophication has been a major issue in the Chesapeake Bay and Great Lakes regions,
primarily due to large swine and poultry industries that dispose of manure via land-application
as fertilizers. Although the Pacific Northwest (PNW) does not have extensive confined animal
feeding operations, some instances of eutrophication have been documented in the PNW.
Many fertilizer manufacturers are beginning to reduce or eliminate P in fertilizers, including
products marketed in the PNW. In particular, several lawn fertilizers contain no P. While P is
not needed by lawns in large quantities, P is an essential nutrient, and some PNW soils have P
levels below that considered optimum for lawns. Mulch-mowing helps reduce the need for
fertilizer P by returning grass clippings to the soil.
Page 55 of 112
Component Materials Used in Fertilizer Blends
Some of the fertilizer materials used in complete fertilizer blends are available separately as well,
which can be particularly useful in correcting specific nutrient deficiencies. For example, Epsom
salts consist of magnesium sulfate and can be used as a source of plant-available Mg. The
following tables list fertilizer materials that commonly are available. Table 2 lists organic
materials; Table 3 lists synthetic materials; Table 4 lists inorganic materials.
Organic, Inorganic, and Synthetic
The term “organic” often is used in vague, misleading, or even incorrect ways. The fertilizers
listed in Table 2 are organic because they are carbon-based, whereas the fertilizers listed in
Table 4 are inorganic because they are minerals. Both the organic and inorganic fertilizers
listed in Tables 2 and 4 are naturally occurring, which is in contrast to the synthetic fertilizers
listed in Table 3.
The chemical definition of “organic” (carbon-based) is not the same as the legal definition of
“certified organic.” Certified organic fertilizers are certified based upon the guidelines of the
National Organic Program (NOP) and include both carbon-based and non-carbon-based
fertilizers, including many of the inorganic fertilizers listed in Table 4.
See http://www.ams.usda.gov/AMSv1.0/nop for more information about the NOP.
A significant difference between organic and synthetic products can be seen in their
Guaranteed Analyses. The concentration of nutrients in organic products generally is lower
than the concentration in synthetic products. In practical terms, this means generally it takes
more of an organic product than it would a synthetic product to supply a given amount of
nutrients. For example:
Fertilizer
type
Organic
Synthetic
N content
of fertilizer
5%
30%
Pounds of fertilizer
to yield 1 pound N
20
3⅓
Page 56 of 112
Page 57 of 112
Page 58 of 112
Micronutrients
When micronutrients are added to soil in the form of water-soluble salts, such as iron sulfate,
most micronutrients quickly react to form insoluble compounds that are unavailable for uptake
by plants. When included in fertilizers, micronutrients usually are in a chelated (pronounced
“KEE-leyt-ed”) form, as shown in Figure 9, below. Chelated nutrients are bound to molecules,
known as chelating agents, that remain available in the soil for uptake by plants. Once inside a
plant, chelated nutrients are released from these molecules in a form the plant is able to use.
EDTA (EthyleneDiamineTetraacetic Acid) is a common chelating agent.
Figure 9. Chelated micronutrients.
Liming Materials
Liming materials are used to raise the pH of soil. Three liming materials commonly are
available: (1) lime; (2) dolomite; and (3) hydrated lime.
Lime: Sometimes referred to as “agricultural lime” or “ag lime,” this liming material consists
of calcium carbonate and is the most common form of liming material available.
Dolomite: Sometimes referred to as “dolomite lime” or “dolomitic lime,” this liming
material consists of calcium magnesium carbonate.
Hydrated Lime: This liming material consists of calcium hydroxide, and it is slightly more
effective than lime or dolomite at raising soil pH. In other words, a given amount of
hydrated lime will raise the soil pH slightly more than the same amount of either lime or
dolomite. Additionally, hydrated lime completes its reaction with soil faster than lime or
dolomite. Hydrated lime sometimes is used to neutralize odors, such as in outhouses.
Hydrated lime is caustic and easily can harm plants (and animals, including humans) if it
comes in direct contact with them. Follow all label directions and precautions when using
hydrated lime.
Lime and dolomite both are available in powdered or prilled form. Prilled refers to the process
of forming pellets from powdered material. Lime and dolomite are slow to complete their
reaction with soil, so prilled liming materials are used instead of simply grinding raw limestone
to a larger particle size, because the finer particle size has more surface area to react with the
soil. Additionally, prilled liming materials are easier to spread than powdered liming materials,
Page 59 of 112
especially on lawns. The binders used to hold prilled pellets together are water soluble, so the
granules break apart quickly when they encounter soil moisture. In general, fall is a good time of
year to apply liming materials, because ample time remains for the liming materials to react with
the soil prior to the next growing season.
Lilly Miller SuperSweet is a prilled lime product available in 25-pound bags. At this
time, Jerry’s does not carry a powdered lime product.
Lilly Miller SoilSweet is a powdered dolomite product available in 40-pound bags.
Espoma Garden Lime is a prilled dolomite product available in 5-pound bags.
SuperSweet and SoilSweet are equally effective in raising soil pH. Espoma Garden Lime
is slightly more effective than SuperSweet and SoilSweet at raising soil pH. In other
words, a given amount of Espoma Garden Lime will raise the soil pH slightly more than
the same amount of either SuperSweet or SoilSweet if applied to the same size area. In
practical terms, however, the difference in these products is so slight as to be negligible.
The amount of lime needed for a particular purpose depends on many factors, including
the current soil pH, needed soil pH, soil texture, etc. The best guide for lime addition is a
laboratory soil test (see “Soil Test Kits” on page 61 for more information).
For vegetables, if soil pH is below 6.0, apply 5 to 10 pounds of liming materials per 100 ft2 (50
to 100 pounds per 1,000 ft2) and till in prior to planting.
As a general rule for lawns, do not exceed 50 pounds of liming materials per 1,000 ft2 per
application and 100 pounds per 1,000 ft2 per year unless the need is indicated by a laboratory soil
test. For rates over 50 pounds per 1,000 ft2, split the total amount to be applied between two
applications, one in the fall and the other in the spring, to avoid accumulation of liming materials
at the soil surface. Rates of 50 or more pounds of liming materials per 1,000 ft2 may be needed
to raise the pH of a soil initially, but these rates will not be necessary on an annual basis. Annual
additions of about 10 to 15 pounds of liming materials per 1,000 ft2 generally will be sufficient to
maintain a proper soil pH. Addition of excessive amounts of liming materials eventually will
raise the soil pH too high, which may result in a deficiency of micronutrients, such as iron.
Liming materials never should be applied at the same time as N-containing fertilizers unless they
are tilled in immediately following application, because the materials can react to form ammonia
gas that is lost to the atmosphere, thus decreasing the effectiveness of both the liming material
and the fertilizer. If both liming materials and N-containing fertilizer need to be applied without
tilling, such as on a lawn, apply the fertilizer first, then apply the lime a week or two later.
Nitrogen is water soluble and will be carried into the soil via rain or irrigation, whereas liming
materials will remain on the soil surface much longer following application.
Liming materials often are said to control moss and mushrooms in lawns. This simply is not
true. Lawns may not thrive in soils with a pH lower than about 5.5, but moss will thrive in soils
with a pH ranging from below 5.5 to over 7.0 if it has shade, moisture, and compacted soils.
Until these issues are corrected, moss will continue to thrive. Fungi are ubiquitous, meaning
they are everywhere, but only their reproductive structures—mushrooms—are noticeable in
lawns. There are no pesticides or soil amendments that will control mushrooms, but mushrooms
generally don’t persist for long periods of time, so simply knocking them down with a rake
generally provides adequate control.
Page 60 of 112
Gypsum
Gypsum is calcium sulfate and a good source of both calcium and sulfur in a form that neither
raises nor lowers soil pH. Gypsum often is said to break up clay soils, but this simply is not true
in the Pacific Northwest (PNW). It is true that sodic soils (high in sodium) can be improved by
the addition of gypsum. However, the high rainfall of the PNW limits sodic soils to areas east of
the Cascade Mountains, so it is safe to say gypsum will not break up clay soils in the PNW.
Soil Test Kits
The soil test kits commonly available on retail shelves provide a qualitative (descriptive, such as
“high,” “medium,” or “low”) assessment of soil nutrient concentrations. These kits commonly
include tests for N, P, K, and pH. The results of these tests are of questionable accuracy, at best.
Laboratory soil tests are preferred. Additionally, while a test kit may reveal the pH of a soil, that
result does not provide any information about how much lime is necessary to raise the soil pH to
a desired level.
Laboratory soil tests provide quantitative results, meaning they provide nutrient concentrations,
in units such as parts per million (ppm). Without proper interpretation, laboratory analyses are
meaningless to most customers. Most soil testing laboratories will provide interpretations, for a
cost. Standard laboratory soil tests measure P, K, Ca, Mg, pH, and SMP buffer pH. (SMP
simply stands for the three people who created the test: Shoemaker, McLean, and Pratt.) The
SMP buffer pH provides the information necessary to determine how much lime is needed to
raise the soil pH to a desired level. B also can be added to the standard suite of tests, which is
useful when growing fruit trees, berries, and Brassicas (broccoli, cabbage, cauliflower, etc.).
Page 61 of 112