Organic Matter

Organic matter in soil consists of the remains of plants and animals. When temperature and moisture conditions are favorable in the soil, earthworms, insects, bacteria, fungi, and other types of plants and animals use the organic matter as food, breaking it down into humus (the portion of organic matter that remains after most decomposition has taken place) and soil nutrients. Through this process, materials are made available for use by growing plants.

The digested and decomposing organic material also helps develop good air-water relationships. In very sandy soil, organic material occupies some of the space between the sand grains, binding them together, and increasing water-holding capacity. In a finely textured soil (e.g., loam, clay loam), organic material creates aggregates of the fine soil particles, allowing water to move more rapidly around these larger particles. This grouping of the soil particles into aggregates, or peds, makes it easier to work.

Organic matter content depends primarily on the kinds of plants that have been growing in a soil, the long-term management practices, temperature, and drainage. Soils that have native grass cover for long periods usually have a relatively high organic matter content in the surface area. Those that have native forest cover usually have relatively low organic matter content. In either case, if the plants are grown on a soil that is poorly drained, the organic matter content is usually higher than where the same plants are grown on a well-drained soil. This is due to differences in available oxygen and other substances needed by the organisms that attack and decompose the organic material. Soils in a cool climate often have more organic matter than those in a warm climate.

Water and Air

All water in the soil ultimately comes from precipitation (rain, snow, hail, or sleet), entering the soil through cracks, holes, and openings between the soil particles. As the water enters, it pushes the air out. Oxygen is taken up by plant roots for respiration. If air is unavailable for too long, the roots will die.

Some water is used by plants, some is lost by evaporation, and some moves so deep into the soil that plant roots cannot reach it. If it rains very hard or for a long time, some of the water is lost through run-off.

When organic matter decomposes in the soil, it gives off carbon dioxide. This carbon dioxide replaces some of the oxygen in the soil pores. As a result, soil air contains less oxygen and more carbon dioxide than the air above the soil surface.

Carbon dioxide is dissolved by water in the soil to form a weak acid. This solution reacts with the minerals in the soil to form compounds that can be taken up and used as nutrients by the plants.

Table 2-4: Examples of soil structure

Depth

The effective depth of a soil for plant growth is the vertical distance into the soil from the surface to a layer that essentially stops the downward growth of plant roots. The barrier layer may be rock, sand, gravel, heavy clay, or a partially cemented layer.

• Very shallow: Soil surface is less than 10 inches from a layer that retards root development
• Shallow: Soil surface is 10 to 20 inches from a layer that retards root development
• Moderately deep: Soil surface is 20 to 36 inches from a layer that retards root development
• Deep: Soil surface is 36 to 60 inches from a layer that retards root development
• Very deep: Soil surface is more than 60 inches from a layer that retards root development

Soils that are deep, well drained, and have desirable texture and structure are suitable for the production of most crops. Deep soils can hold more plant nutrients and water than can shallow soils with similar textures. The depth of a soil and its capacity for nutrients and water frequently determine the yield from a crop, particularly annual crops grown through the summer months. Gardeners with very shallow soil often benefit from establishing raised beds.

Plants growing on shallow soils also have less mechanical support than those growing in deep soils. Trees growing in shallow soils are more frequently blown over by wind than are those growing in deep soils.

Northern Neck Shoreline Evaluation Program

By Ian Cheyne, Extension Master Gardener, Northern Neck

In the early 2000s, Northern Neck Master Gardeners (NNMG) began to engage in living shoreline installations. They recognized that, in the right situations, this was a way for homeowners to protect their property and also contribute to the restoration of the Chesapeake Bay. Where homeowners and plants are involved, Extension Master Gardeners (EMGs) should be too.

In 2004, the opportunity arose to participate in a project to install a demonstration living shoreline in Reedville in Northumberland County. The project involved Bethany United Methodist Church’s waterfront property adjacent to the Reedville Fishermen’s Museum. The eroding shoreline and stormwater runoff from the church and its parking lot were contaminating the adjacent creek. To address these issues, a living shoreline was proposed. The implementation was a collaboration among between the church, the Fishermen’s Museum, the Virginia Institute of Marine Science (VIMS), NNMG, and several other local non-profits. The project was successfully completed in 2005 and has been maintained by NNMG volunteers since then. The installation demonstrates the benefits and beauty of a living shoreline.

Enthused by their initial involvement, the EMGs wondered how they could do more. Then opportunity struck. In 2009, the Great Recession resulted in state budget cuts and one victim was the VIMS homeowner shoreline advisory service. Seeing an opportunity and already having a relationship with VIMS, the EMGs proposed to offer a similar service on the Northern Neck. VIMS welcomed the idea and agreed to provide training and consulting support. Thus, the Shoreline Evaluation Program (SEP) was conceived.

Before the program launched in 2012, the SEP team received training from VIMS and also qualified as EMG Water Stewards, an advanced training available to current Extension Master Gardeners. They documented necessary procedures, performed practice evaluations around the Northern Neck, and developed a strategic plan.

The SEP mission is to encourage homeowners to adopt shoreline protection methods that also protect habitats and their adjacent waters. To that end, SEP offers shoreline management advice through individual on-site evaluations and educational outreach to the public.

The educational outreach includes an annual shoreline management seminar, presentations to individual community groups, and a presence at farmers markets and other local events. To date, SEP has conducted over 300 individual shoreline evaluations and interacts directly with approximately 600 people each year through its evaluations, seminars, and outreach. The program is also continually adapting to changing legislation and the growing effects of climate change.

Collaboration has been a key ingredient to the program’s success. The living shoreline project was a collaboration. The relationship with VIMS continues as a collaboration. Recently, the James City County Williamsburg Master Gardeners in partnership with the Colonial Soil and Water Conservation District decided to launch SEP in their district. NNMG has played a role in training and transferring know-how to the new program, and the groups will maintain a collaborative relationship.

It has been a tremendously rewarding experience to see attitudes towards waterfront property management gradually change as homeowners realize they can protect their property in ways that can also contribute to the restoration of the Chesapeake Bay.

Soil Testing

The purpose of a soil test is to supply the homeowner with enough information to make wise decisions about the purchase and application of lime and fertilizer. A soil test from Virginia Tech will provide information on the soil pH and the plant available levels of phosphorus, potassium and seven other essential elements or nutrients. A Virginia Tech Soil Test report will also provide an estimated Cation Exchange Capacity (CEC), which gives an indication of a soil’s ability to retain nutrients (Ca⁺⁺, Mg⁺⁺ & K⁺) against leaching. Soil tests should be performed if such tests have never before been conducted or if past soil test results are unavailable. A soil test is unnecessary more often than every 3-4 years.

The accuracy of the soil test is a reflection of the sample taken. Be sure the sample is representative of the area to be treated; sampling should be specific and occur for one landscape type or use (e.g. vegetable garden, turf, landscape bed). Sample the soil from 10 or more random areas of the garden to a depth of 6 inches. Avoid sampling unusual areas such as those near gravel roads, manure or compost spots, brush piles, or under eaves. Place the samples in a clean plastic pail or container, and mix the soil thoroughly. Contact your local Extension office for soil sampling boxes and proper sampling procedures. For more information, visit the Soil Testing Lab website.

A soil test is the only way to determine if nutrients must be added or if a pH adjustment is needed. The soil test results suggest how much of each nutrient to add. Without a soil test, any application of fertilizer could be detrimental to the landscape and surrounding environment or ecosystem. Over-application or application of unneeded materials could result in salt injury to plants, cause nutrient imbalances unsuitable for plant growth, and has the potential to become an environmental pollutant.

Most nutrients move very slowly through the soil profile, so to be most effective, they should be incorporated into the top 4-6 inches. However, these elements can be surface applied, but the nutrients will be less readily available to the plants.

Magnesium may be deficient, especially in low pH soils. If magnesium levels and soil pH are low, dolomitic limestone can be used to raise the pH and supply the needed magnesium. To add magnesium without affecting the pH, Epsom salts (MgSO₄) can be applied, either by incorporating into soil or as a soil drench (dissolved in water and applied near roots). Potassium can be surface applied, if needed. It is possible to over-apply potassium which can lead to deficiencies of other nutrients, particularly magnesium, so follow soil test recommendations closely.

Nitrogen is the nutrient that most frequently limits plant growth, and is often the only nutritional element that accelerates the growth of ornamental plants.

Unfortunately, nitrogen is also the most challenging nutrient to manage. Unlike other nutrients, it is not possible to accurately determine from a soil test how much nitrogen will be available during a plant’s growth. Nitrogen readily moves or leaches in the soil. The challenge is to provide adequate nitrogen levels when needed to meet the plant growth requirements. A soil test will recommend the amount and time to apply nitrogen based upon established annual requirements for the specified plant to be grown.

Nitrogen can be supplied with two different approaches and both work very well. Nitrogen can be applied as (1) a water soluble form which includes liquid feed and granular fertilizers or (2) slow release forms. See chapters on specific plant groups (i.e., lawns, woody ornamentals, etc.) and the chapter on water quality for more information on fertilizer application.

Cation Exchange Capacity (CEC)

The cation exchange capacity (CEC) is a measure of the net negative charge per unit of clay. Nutrient cations (such as CA⁺⁺, Mg⁺⁺, K⁺), which have a positive charge, are readily attracted to clay particles which have a net negative charge. A water molecule’s positive polar ends are also attracted to clay. Humus also has a high CEC and may also be adsorbed to clay. If the CEC is too low, cations and water are not adsorbed strongly and are easily drained or leached away. Soils with little clay, such as sand textures, and highly weathered clays and oxides have very low CEC. The CEC determines a soil’s ability to adsorb nutrients, and is related to potential fertility. Although many factors affect cation exchange between the clays or humus and the plants, we are most interested when nutrient cations and anions (which have a negative charge) are exchanged across root cell walls and taken into the root. The plant root releases hydrogen ions (H⁺), which then replaces the cations in the soil for uptake into the plant. Uptake of phosphate anions requires some energy expenditure by the plant.

Soil pH

A pH is a reading taken from a scale that measures the hydrogen (acid-forming) ion activity of soil or growth media. The reading expresses the degree of acidity or alkalinity in terms of pH values. The scale of measuring acidity or alkalinity contains 14 divisions known as pH units. It is centered around pH 7 which is neutral. Values below 7 constitute the acid range of the scale and values above 7 make up the alkaline range.

The measurement scale is not a linear scale but a logarithmic scale. That is, a soil with a pH of 8.5 is ten times more alkaline than a soil with a pH of 7.5, and a soil with a pH of 4.5 is ten times more acid than a soil with a pH of 5.5.

The pH condition of soil is one of a number of environmental conditions that affects the quality of plant growth. A near-neutral or slightly acidic soil is generally considered ideal for most plants. Some types of plant growth can occur anywhere in a 3.5 to 10.0 range. With some notable exceptions, a soil pH of 6.0 to 7.0 requires no modification of soil pH to improve plant growth.

The major impact that pH extremes have on plant growth is the availability of plant nutrients and concentration of the plant-toxic minerals. In highly acidic soils, calcium, phosphorous, and magnesium become tied up and unavailable, and manganese can be available in toxic levels. At pH values of 7 and above, phosphorus, iron, copper, zinc, boron, and manganese become less available.

Ground limestone and rarely sulfur are used to adjust pH that is not in the ideal range for plant growth. Limestone raises pH and sulfur lowers it. To avoid under or over applying these materials, applications should be based on a soil test. Preferably, any applied lime should be thoroughly tilled into the soil prior to planting.

Table 2-5 presents some approximate amounts of ground limestone needed to increase the pH of five soil types. Table 2-6 presents some approximate amounts of ground sulfur needed too decrease the pH of two soil types. These values are only representative and should not be taken as recommendations. To avoid under or over applying liming material, applications should be based on a soil test. In order to ascertain a more precise lime requirement recommendation, the Virginia Tech Soil Testing Laboratory not only performs a soil pH using distilled water, but it also determines a pH using a buffered solution to measure the total (active plus reserve) acidity and buffering capacity for a particular soil sample. Preferably, any applied lime should be thoroughly tilled into the soil prior to planting. As with soil testing for fertilizer needs, amendments to improve aeration and/or drainage should be applied prior to testing soil pH.

Table 2-5: Approximate amount of limestone needed to increase the pH of the upper 7 inches of soil to 6.5

Table shows the amount of lime to apply in lbs./1000 square feet. Lime recommendations are based on using a ground limestone with a neutralizing value of 90%

Table 2-6: Approximate amount of ground sulfur needed to decrease the pH of the soil to 6.5

Sulfur recommendations are based on using a ground sulfur material containing 95% S.
This information was provided by D.A. Bailey, Department of Horticulture Science, NCSU

If the soil pH is too high and must be lowered, elemental sulfur or aluminum sulfate can be incorporated into the soil to reduce alkalinity. If only a small decrease in pH is required, acid-forming fertilizer such as ammonium nitrate can be used as a nitrogen source.

Most ornamental plants require slightly to strongly acidic soil. When grown in soils in the alkaline range, these species may develop trace metal deficiencies (such as iron or manganese, trace metal deficiency symptoms will first appear on new leaves).

While there is likely plenty of iron in the soil, when the pH is not in a favorable range for the plant being grown, the iron is unavailable to the plant. This problem can be corrected by applying chelated iron. The term chelate comes from the Greek word for claw. Chelates are chemical claws that help hold metal ions, such as iron, in solution, so that the plant can absorb them. Different chemicals can act as chelates, from relatively simple natural chelates like citrate to more complex, manufactured chemicals. When a chelated metal is added to the soil, the nutrient held by the chelate will remain available to the plant for a longer period of time than if added as a salt form such as iron sulfate.

Most nutrients do not require the addition of a chelate to help absorption. Only a few of the metals, such as iron, benefit from the addition of chelates.

Building Healthy Soil

Well-formed soil structure can have a positive impact on nutrient retention. Organic matter is a great soil improver for both clay and sandy soils. Good sources of organic matter include manures, leaf mold, sawdust, and straw. These materials are decomposed by soil organisms. Various factors such as moisture, temperature, and nitrogen availability determine the rate of decomposition through their effects on these organisms. Adequate water must be present, and warm temperatures will increase the rate at which the microbes work. The proper balance of carbon and nitrogen is needed for rapid decomposition. Fresh green wastes, such as grass clippings, are higher in nitrogen than dry material. The addition of nitrogen may be necessary if large amounts of undecomposed high-carbon substances such as dried leaves, straw, or sawdust are used. In the process of breaking down the organic matter. Nitrogen is used by the microbes and, therefore, may become deficient in the plants.

Tilling

Tilling is a good way to loosen soil and break up soil clods in new beds as well as in the yearly preparation of annual flower and vegetable gardens. It is also a good opportunity to incorporate organic matter and nutrients into the soil, though excessive tillage can be detrimental to soil organisms and soil structure. The best time to till is when soil is very slightly moist. Tilling in the fall for spring planting helps kill insects and weeds and allows winter freezing and thawing to help build soil structure.

However, tillage of the soil, especially deep tilling practices, can negatively affect the physical and biological properties of the soil. For more about alternatives to tilling, see “No-till or low-till methods” in Chapter 9 “The Vegetable Garden.”

Compost

Organic matter should make up 5% or more (by weight) of a healthy soil. Many Virginia soils, especially urban soils, are low in organic matter. The regular application of compost to lawns and landscape beds will add organic matter and improve the overall health of the soil.

The carbon in compost feeds a variety of beneficial bacteria, fungi and other organisms. As these organisms feed they convert nutrients into plant available forms. For example, nitrogen in ammonium (NH₄⁺) and ammonia (NH₃^–) is converted to the more plant available form of nitrate (NO₃^–).

Some specialized fungi form symbiotic relationships with plants that provide plants with nutrients and water, essentially acting as an extension of the root system.

As compost-fed soil organisms thrive they break up compacted soil and exude natural glues that allow soil particles to form aggregates. These aggregates help preserve pore space that allows for the passage of water, gasses and roots.

The organic matter in compost can also act as a reserve for food and water. This is especially important in sandy soils where high infiltration rates and low cation exchange capacities would otherwise require frequent applications of water and fertilizer.

The quality of compost is not uniform. Both home-made and commercial compost can vary in chemistry from batch to batch. While there is an effort to standardize commercial compost, it is not mandatory and does not cover home-made compost.

Compost should be dark, brown, fluffy, and crumbly with an earthy odor when ready to use. It should not be moldy, have a rotten smell, or be decomposed to a point of being powdery. Except for some woody pieces, the source materials that went into the compost should not be recognizable and finished compost should have a stable pH near neutral and be low in soluble salts.

Cover Crops

Another source of inexpensive soil improvement that should not be underestimated is the cover crop. Cover crops, sometimes called green manures, can be used to increase organic matter, break up hard soils, suppress weeds, prevent erosion, and hold and increase soil nutrients. Most cover crops have multiple uses. Cereal rye, for example, can absorb soil nutrients and hold them over the winter while also suppressing weeds.

The best time to seed a cover crop depends on the species being planted and when the previous crop is harvested. Often, cover crops are planted in the garden in the fall for incorporation in the spring. In a fall garden, plant cover crops between the rows and in any cleared areas. Cover crops can also be used in late spring and summer to suppress weeds or provide a nitrogen boost before the next crop is planted.

Cover cropping provides additional organic matter, holds nutrients that might have been lost over the winter, and helps reduce erosion and loss of topsoil. Legume cover crops can increase the amount of nitrogen in the soil and reduce fertilizer needs over time. A deep-rooted cover crop allowed to grow for a season in problem soil can help break up a hardpan and greatly improve soil tilth. Planting multiple species of cover crops together has shown to increase the diversity of soil organism communities.

Incorporate green manures at least 2 weeks before planting vegetables. In most cases, cover crops should not be allowed to go to seed.

The regular addition of manures, compost, cover crops, and other organic materials can raise the soil nutrient level to a point where the need for synthetic fertilizers is greatly reduced. This highly desirable soil quality does not come about after a single or even several additions of organic material and requires a serious soil-building program. For more information, see Chapter 9: “The Vegetable Garden.”

Table 2-7: Table of common cover crops

Soil Food Web

The soil food web is built of a wide range of organisms. This complex system includes organisms such as bacteria, algae, fungi, protozoa, nematodes and micro-arthropods, as well as earthworms, insects, small vertebrates, and plants. From the microscopic to the visible, each organism plays an important role in this ecosystem. Decomposition of organic matter, sequestration of nitrogen, fixing of nitrogen from the atmosphere to make it available for plants, and aerating the soil are just a few of the specific tasks these organisms can play in the soil food web.

Understanding Fertilizers

There are 16 elements essential to plant growth, as described in Table 2-2 and 2-3. Carbon, hydrogen, and oxygen come from air and/or water. Nitrogen, phosphorous and potassium are considered fertilizer macronutrients because plants require them in a relatively large quantity for maximum growth. Calcium, magnesium, and sulfur are secondary macronutrients but usually are either present in sufficient quantities or are added coincidentally with other materials (e.g., lime). The other seven nutrients, called micronutrients, are just as important but are required in smaller amounts. To review the roles of the elements in plant growth, please see the section, “Physiological and Biochemical Functions of Essential Elements in Plants” in Chapter 5 “Abiotic Stress.”

If plants are low in any of these elements, they exhibit signs of nutrient deficiency. High levels may contribute to nutrient toxicity. Some of these symptoms are given in the discussion of nutrients in Chapter 5 “Abiotic Stress.”

Fertilizer Analysis

Some fertilizer packaging includes an analysis of nutrient content, represented by three numbers which refer to the percentage by weight of nitrogen (N), phosphate (P₂O₅), and potash (K₂O) respectively.

Often, to simplify matters, these numbers are said to represent nitrogen, phosphorus, and potassium, or N-P-K. We should remember that it is not N-P-K, but N-P₂O₅-K₂O. For example, a bag of 10-10-10 fertilizer contains 10% N, 10% P₂O₅, and 10% K₂O by weight. For example, if we have a 100-pound bag of this fertilizer labeled 10-10-10, there are 10 pounds of N, 10 pounds of P₂O₅, and 10 pounds of K₂O in that bag.

In some fertilizers, a filler is sometimes added to bulk up the fertilizer. This is done to lower the analysis, or dilute the concentration, allowing a more even spreading pattern as compared to trying to spread a small amount of a high analysis fertilizer over an area. A filler is generally an inert material such as sand or ground corn cobs. Some fillers are used to enhance the fertilizer’s handling qualities.

The information contained on fertilizer labels has been well standardized, and the consumer is protected by state laws requiring manufacturers to guarantee the claimed nutrients. In some cases, a fertilizer will contain secondary nutrients or micronutrients not listed on the label because the manufacturer does not want to guarantee their exact amounts.

On fertilizer labels, the initials WIN and WSN stand for Water Insoluble Nitrogen and Water Soluble Nitrogen. Water soluble nitrogen (WSN) dissolves readily and is usually in a simple form, such as ammoniacal nitrogen (ammonium, NH₄⁺) or nitrate (NO₃^–) nitrogen. Nitrogen, which will not dissolve readily, may exist in other forms in the fertilizer. Water insoluble nitrogen (WIN) is referred to as a slow-release nitrogen (SRN) source and delivers nitrogen at different rates according to the amount and kind of material in its composition. WIN is also sometimes referred to as slowly-available nitrogen (SAN).

The best fertilizer to use depends on many factors, such as the nutrients needed, soil structure, soil chemistry, and method of applying the fertilizer.

Complete versus Incomplete

A complete fertilizer contains nitrogen, phosphorus, and potassium. Common examples of complete fertilizers are 10-10-10 and 20-10-5. An incomplete fertilizer is missing one or two of the major components. Examples of incomplete fertilizers are indicated on the following chart.

Manufacturers can blend different incomplete fertilizers to create many different fertilizer ratios found on store shelves.

The fertilizer ratio indicates the proportion of nitrogen, phosphate, and potash contained in the fertilizer. The specific fertilizer ratio you will need depends on the soil nutrient level, and the plants being grown.

Table 2-8: Common incomplete (agricultural) fertilizers

Fertilizer Formulation

Fertilizer formulation determines how quickly nutrients will be available to plants. Some of the formulations available to the homeowner are: water-soluble powders, slow-release pellets, slow-release collars or spikes, liquids, tablets, and granular solids.

Liquid fertilizers come in a variety of different formulations, including complete formulas, special incomplete formulations, and types that offer just one or two micronutrients. All liquid fertilizers are made to be diluted with water, some are concentrated liquids, and others are powder or pellets. Growers of container plants often use liquid fertilizers (which typically contain more WSN) at half concentration twice as frequently, so that the plants receive a more continuous supply of nutrients.

Special-Purpose Fertilizers

When shopping for fertilizer, you will find fertilizers packaged for certain uses or types of plants such as Camellia Food, Rhododendron and Azalea Food, or Rose Food. Some of the compounds used in these fertilizers are chosen because they have an acid reaction, or provide the plant with its preferred nutrient source, such as ammoniacal N, so they are especially beneficial to acid-loving plants where soil is naturally neutral or alkaline.

A soil test should be performed before the purchase of any expensive, special-purpose fertilizers. It is not possible to make a blanket statement that one fertilizer is best for every area. It is true that different plants use different nutrients at different rates. What is unknown is the reserve of nutrients already in the soil. This will change with soil type and location.

Slow-release fertilizers

Plants can absorb nutrients continuously, so it is beneficial to provide them with a balance of nutrients throughout their growth. Perhaps the most efficient way to achieve this is to apply a slow-release fertilizer, which releases nutrients at a rate that makes them available to the plants over a long period. Slow-release fertilizers contain one or more essential elements and can be categorized according to their release mechanism. The three major types of nutrient release mechanisms are: (1) materials that dissolve slowly, (2) materials requiring microorganisms to release nitrogen, and (3) granular materials with membranes made of resin or sulfur to control the rate of nutrient release into the soil.

Sulfur-coated urea is a slow-release fertilizer with a covering of sulfur around each urea particle. The rate of nitrogen release is determined by the thickness of the coating. Sulfur-coated urea applied to the soil’s surface releases nitrogen more slowly than if incorporated into the soil. This material generally costs less than other slow-release fertilizers, and it supplies the essential element sulfur.

When fertilizer products coated with multiple layers of resin come into contact with water, the layers swell and increase the pore size in the resin so that the dissolved fertilizer can move into the soil. Release rate depends on the coating thickness, temperature, and water content of the soil. There is often a large release of fertilizer during the first 2 or 3 days after application. Release timing can be from 0 to 6 months, depending on the coating.

Slow-release fertilizers need not be applied as frequently as other fertilizers, and higher amounts can be applied without danger of burning. Plants may use the nitrogen in slow-release fertilizers more efficiently than nitrogen in other forms, since it is released over a longer period of time and in smaller quantity. Slow-release fertilizers are generally more expensive than other types, but they require much less frequent application than conventional fertilizers.

Urea formaldehyde and sulfur-coated urea have been used as turf fertilizer, while resin-coated fertilizers are predominantly used in container growing.

Caution should be used when applying slow-release fertilizers around trees or shrubs as this can encourage late summer growth that may not harden off completely, exposing the plant to winter damage.

Table 2-9: Comparison of slow-release fertilizers, conventional fertilizers, and manure or sludge

Organic fertilizers

Although compost is fertilizer, home gardeners might seek additional sources of organic fertilizer. The nutrients in organic fertilizers are derived solely from the remains or by-products of a once-living organism (and not necessarily “organic” in the sense of the term used in organic gardening). Cottonseed meal, blood meal, bone meal, hoof and horn meal, and all manures are examples of organic fertilizers. When packaged as fertilizers, these products will have the fertilizer ratios included on the label. Some organic materials, particularly composted manures and sludge, are sold as soil conditioners and do not have a nutrient guarantee, although small amounts of nutrients are present. Some are fortified with nitrogen, phosphorus, or potash for a higher analysis. In general, organic fertilizers release nutrients over a fairly long period, first releasing quickly available nutrients and then additional mineral nutrients become available later via decomposition. The potential drawback is that they may not release enough of their principal nutrient at a time to give the plant what it needs for best growth. Organic fertilizers depend on soil organisms to break them down to release nutrients, so most are only effective when soil is moist and warm enough for the soil organisms to be active.

Cottonseed meal is a by-product of cotton manufacturing and is frequently used for fertilizing acid-loving plants such as azaleas, camellias, and rhododendrons. Formulas vary slightly, but generally contain 6% nitrogen, 3% phosphate, and 2% potash. Cottonseed meal is readily available to plants in warm soils, and there is little danger of burn. For general garden use, apply 2 to 5 pounds per 1000 square feet.

Blood meal is dried, powdered blood collected from beef processors. It is a rich source of nitrogen — so rich, in fact, that it may do harm if used in excess. The gardener must be careful not to use more than the amount recommended on the label. In addition to supplying nitrogen, blood meal supplies many of the essential trace elements, including iron.

Urea is a synthetic organic fertilizer, an organic substance manufactured from inorganic materials.

Fish emulsion, a complete fertilizer, is a partially decomposed blend of finely pulverized fish. No matter how little is used, the odor is intense — but it dissipates within a day or two. Fish emulsion is high in nitrogen and is a source of several trace elements. In the late spring, when garden plants have sprouted, an application of fish emulsion followed by a deep watering will boost the plant’s early growth spurt. Contrary to popular belief, too strong a solution of fish emulsion can burn plants, particularly those in containers.

Manure is also a complete fertilizer and varies in nutrient content depending on the animal and its diet, but a fertilizer ratio of 1-1-1 is typical. Manures are best used as soil conditioners instead of nutrient sources. Commonly available manures include horse, cow, pig, chicken, and sheep. The actual nutrient content varies widely, but is highest when manures are fresh. Fresh manure should not be used where it will contact tender plant roots. As it is aged, leached, or composted, nutrient content is reduced. However, the subsequent reduction in salts will reduce the chances of burning plants. Typical rates of manure applications vary from a moderate 70 pounds per 1,000 square feet to as much as half a ton per 1,000 square feet.

Compared to synthetic fertilizer formulations, organic fertilizers contain relatively low concentrations of actual nutrients, but they perform other important functions which the synthetic formulations do not. Some of these functions are: increasing organic content of the soil, improving physical structure of the soil, adding micronutrients, and increasing bacterial and fungal activity.

Fertilizers Combined With Pesticides

Fertilizers can be combined with pesticides by the manufacturer or the applicator but must be applied at the appropriate time and rate. Always read the label carefully.

Applying Fertilizer

Fertilizer Application Rate

The amount of fertilizer applied is determined by the amount of nitrogen because it is the nutrient most easily lost from the soil. In the following chart, you can see how the amount to be applied decreases as the percentage of nitrogen increases.

Nitrogen fertilizers do not burn or damage plants if they are applied correctly. Fertilizers are salts, much like our familiar table salt, except that they contain various plant nutrients. Salts in the fertilizer begin to diffuse or move away from the place where they had been applied. This dilutes the fertilizer and distributes it through a much larger area. If tender plant roots are close to the area where the fertilizer is placed, water will be drawn from these roots and from the surrounding soil. The more salt or fertilizer applied, the more water will be drawn from nearby roots. As water is drawn from the roots, plant cells begin to dehydrate and collapse, and the plant roots burn or dehydrate to a point from which they cannot recover. If soil moisture is limited, most of the water drawn towards the salt will come from plant roots and the damage will be more severe.

Two rules should be kept in mind when applying a fertilizer during hot weather when soil moisture is limited: 1) do not over-apply nitrogen fertilizers, and 2) make sure adequate moisture is present after applying fertilizers high in salts.

Soluble salts accumulate when fertilizer is applied repeatedly without sufficient water to leach or wash the old fertilizer’s salts through the soil. It also occurs when water evaporates from the soil and leaves previously dissolved salts behind. Soluble salts will accumulate on top of the soil in a container and form a yellow-to-white crust. A ring of salt deposits may form around the pot at the soil line or around the drainage hole. Salts may also build up on the outside of clay pots.

As the salts in the soil become more concentrated, plants find it harder to take up water. If salts build up to an extremely high level, water can be taken out of the root tips, causing them to die.

Table 2-10: Fertilizer application rates as determined by goal N content

Timing and Method of Application

Soil type dictates the frequency of fertilizer application. Sandy soils require more frequent applications of nitrogen and other nutrients than do clay-type soils. Factors affecting the type and rate of fertilizer application include: the type of crop, the level of crop productivity required, frequency and amount of water applied, and the type of fertilizer applied and its release rate.

Timing

The type of crop influences timing and frequency of application since some crops are heavier feeders of particular nutrients than others. Root crops (such as carrots) require less nitrogen fertilization than do leafy crops (such as greens). Corn is a heavy feeder of nitrogen, while most trees and shrubs are generally light nitrogen-feeders. A general rule of thumb is that nitrogen is for leafy top growth, phosphorus is for root and fruit production, and potassium is for cold hardiness, disease resistance, and general durability.

Proper use of nutrients can control plant growth rate and character. Nitrogen is the most critical nutrient in this regard. If tomatoes are fertilized too heavily with a nitrogen fertilizer or side-dressed before fruit set, the plants may be all vegetative (shoot, stem, leaf) and no fruit. This is also the case with potatoes, which will show excess vining and poor tuber formation. If slow-release fertilizers or heavy amounts of manure are used on crops that form fruit or vegetables, vegetative growth will continue into late summer, and fruit and vegetable development will occur very late in the season. Late nitrogen fertilization can also prevent perennial plants from becoming cold-hardy, inducing cold weather injury.

Remember that a nitrogen application will have its greatest effect for 3-4 weeks after application. If tomatoes are fertilized heavily on June 1, there may be no flower production until July 1, which will, in turn, delay fruit ripening until late August. For this reason, it is important to plant crops with similar fertilizer needs close together to avoid improper rates of application.

Late fertilization (after July 1) of trees and shrubs can cause new flushes of growth to occur on woody plants that are normally adjusting themselves for the coming winter. This may delay dormancy of woody plants and cause severe winter dieback in new growth. Gardeners should be aware that individual species within these groups vary considerably. Refer to Chapter 17: “Water Quality and Conservation,” for information on fertilizer use and water quality.

Application methods

There are different methods of applying fertilizer depending on its formulation and the crop needs.

Broadcasting

A recommended rate of fertilizer is spread over the entire growing area and left to filter into the soil or incorporated into the soil with a roto tiller or spade. Broadcasting is used over large garden areas or when time or labor is limited.

Banding

Narrow bands of fertilizer are applied in furrows 2 to 3 inches from the garden seeds and 1 to 2 inches deeper than the seeds or plants. Careless placement of the fertilizer band too close to the seeds will burn the roots of the seedlings. The best technique is to stretch a string where the seed row is to be planted. With a corner of a hoe, dig a furrow 3 inches deep, 3 inches to one side, and parallel with the string. Spread half the suggested rate of the fertilizer in the furrow and cover it with soil. Repeat the banding operation on the other side of the string, then sow seeds underneath the string.

For widely spaced plants, such as tomatoes, fertilizers can be placed in bands 6 inches long for each plant or in a circle around the plant. Place the bands 4 inches from the plant base. If used in the planting hole, place the fertilizer at the bottom of the hole, work it into the soil, and add a layer of soil about 2 inches deep over the fertilized soil before putting the plant in the hole.

Banding is one way to satisfy the needs of many plants (especially tomatoes) for phosphorus as the first roots develop. When fertilizers are broadcast and worked into soil, much of the phosphorus is locked up by the soil and is not immediately available to the plant. By concentrating the phosphorus in the band, the plant is given what it needs even though much of the phosphorus stays locked up.

Starter solutions

Another way to satisfy the need for phosphorus when setting out transplants of vegetables and flowers is through the use of a liquid fertilizer high in phosphorus as a starter solution. Follow directions on the label.

Side-dressing

Dry fertilizer is applied as a side dressing after annuals are up and growing. Scatter fertilizer on both sides of the row 6 to 8 inches from the plants. Rake it into the soil and water thoroughly.

Foliar feeding

Foliar feeding (spray of fertilizer applied to foliage) is used when insufficient fertilizer was used before planting, when a quick growth response is wanted, when micronutrients (such as iron or zinc) are locked into the soil, or when the soil is too cold for the plants to use the fertilizer applied to the soil. Foliar-applied nutrients are absorbed and used by the plant quite rapidly. Absorption begins within minutes after application and, with most nutrients, it is completed within 1 to 2 days.

Foliar nutrition can be a supplement to soil nutrition at a critical time for the plant, but not a substitute. At transplanting time, an application of phosphorus spray will help in the establishment of the young plant in cold soils. For perennial plants, early spring growth is usually limited by cold soil, even when the air is warm. Under such conditions, soil microorganisms are not active enough to convert nutrients into forms available for roots to absorb; yet, if the nutrients were available, the plants could grow. A nutrient spray to the foliage will provide the needed nutrients immediately, allowing the plants to begin growth.

Soil and Nutrient Problems

Nutrients as Pollutants

Nitrogen and phosphorus are currently major sources of pollution in our waterways. Excess nutrients come from sewage treatment plants, automobile exhaust, animal wastes, excess nutrients applied to agricultural fields, excess nutrients applied to home lawns and gardens or failing septic fields. These pollutants can come from organic or synthetic sources. Once these nutrients are in the water, they become available for algae growth. A large, quick growth of algae is referred to as an algae bloom. Once the algae has used up the available flush of nitrogen and phosphorus, it quickly dies and bacteria begins to break it down. This bacteria uses oxygen to help break down the algae, causing oxygen levels in the water to crash. Water low in dissolved oxygen causes fish and other aquatic animals to perish.

Nitrogen is mobile in the soil. It can easily leach out of the soil or run off the soil surface with the movement of water and easily end up in waterways where it contributes to algae blooms. Phosphorus on the other hand is not usually mobile in the soil. Phosphorus binds with clay particles and organic matter in the soil. Plants are then able to use the phosphorus associated with organic material and soil particles. Phosphorus usually becomes a pollutant in our waterways when it is washed into streams, and creeks while attached to a soil particle. Thus it is important to limit the amount of soil erosion. Another factor that makes phosphorus toxic to our waterways is the fact that algae are much more responsive to phosphorus than to nitrogen. Thus, a small amount of phosphorus can trigger the same algae bloom as a much larger amount of nitrogen.

Since nitrogen and phosphorus have been identified as pollutants, one might expect Virginia Cooperative Extension to recommend not using fertilizers on lawns and gardens in the future. The answer is much more nuanced than this. When plants have the nutrients they need available to them, they are healthier, grow thicker and denser, and their roots are stronger. They hold soil better and limit erosion. Thus, these plants help to limit the amount of nitrogen and phosphorus that end up in our waterways. Conversely, if plants don’t have enough nutrients to thrive and stay healthy, they are often spindly and small, and they may not be able to hold soils in place as well as we would like and may contribute to soil erosion. Thus, the results of a soil test are invaluable to help determine the need for nutrient amendments, as well as the form and amount needed. Nutrients applied correctly are beneficial. When misapplied, they can become pollutants.

Our communities are making concerted efforts to help limit the excess nutrients that end up in our waterways. Virginia law prohibits the sale of synthetic fertilizers with phosphorus for maintenance purposes. Phosphorus fertilizers are, however, allowed to be used for establishing new plantings or for use on sites that are low in phosphorus according to a soil test.

Areas inappropriate for fertilizer application

Some locations are inappropriate for fertilizer application. They create a high potential for runoff and/or leaching, in which case the fertilizer then becomes an environmental pollutant. Detailed information on each of these types of locations is given below.

• Impervious surfaces: Driveways, sidewalks and other paved areas should not have fertilizers applied to them. During lawn fertilization some fertilizer may spread to these areas and this fertilizer should be swept or blown back into the planting areas. Fertilizers should not be used as deicers in the winter.
• Soils with high water table: Fertilizer is more likely to move into water where there are soils that do not drain well or there is a high water table. Individuals should be cautious about applying fertilizers in these areas.
• Stream and pond banks: In places where a lawn goes directly to the edge of a stream or pond, a buffer should be left in place. An area of turf grass that is 20 feet from the water should not be fertilized and ideally there is an area of vegetation that is thicker and taller than a lawn that is typically mowed weekly during the growing season.
• Well heads and rock outcroppings: Both of these sites have the potential to take surface water directly into local ground water. And as such a buffer or no fertilization zone should be left around these sites.
• Karst bedrock: Many areas of Virginia have limestone based bedrock. This limestone dissolves over time and often there are channels or sinkholes that provide immediate movement of surface water into ground water. Use fertilizers in these areas cautiously.
• Steep slopes: Steep slopes have a high potential to have fertilizers wash away. Slow release fertilizers work best in these situations.

Keeping soils covered with healthy plants is the best way to limit the amount of nutrients that end up in local waterways. But where plants are absent it is helpful to have the soil covered. Hardwood or pine bark mulch and other plant based mulches can be helpful to limit erosion and nutrients reaching water.

Compaction

Compaction is the process in which a stress applied to a soil causes loss of pore space, such as equipment traffic on wet soils. The measure of soil bulk density, which is the ratio of dry weight of a soil sample divided by its volume, determines the bulk density or compaction. As the dry weight increases relative to the volume, porosity in the soil decreases and compaction increases. This situation restricts root, air and water movement and leads to plant stress. Urban soils tend to be moderately to heavily compacted because of the amount of traffic they receive during construction or landscaping.

Urban Soils

Soils in urban environments provide unique challenges. One primary challenge is to provide the functions of the natural environment in a built environment. While some plants are adaptable to a fairly wide range of environmental conditions, others have a narrow range in which they will grow well. For example, urban soils will often have a higher pH than surrounding rural areas due to limestone-containing materials in the urban environment (Bassuk, Curtis, Marranca, and Neal). For plants that require a specific pH range in order to thrive, the higher pH of urban soils can cause stress to the plant. Other challenges of urban soil are compaction and the removal of topsoil during construction. For more about urban soils, see the VCE Master Gardener Tree Steward Manual.

Salinity

Soil salinity falls into two categories:

Primary salinity is the result of natural processes such as erosion. It can also be seen in land near the ocean, where salts are accumulated through salt spray brought by wind and deposited in the soil by rain. Primary salinity can occur over very long periods of time.

Secondary salinity results from human activity and occurs more quickly than primary salinity. Continuous irrigation and fertilization of agricultural areas results in a buildup of salts in the top layers of the soil. Removal of natural vegetation to make space for annual agricultural crops destroys perennials that have adapted to their environment. Removing perennials and planting annuals often requires irrigation that can expose plants to higher salinities. For more information about salinity and the problems it can cause plants, see Chapter 5 “Abiotic Stress Effects on Plant Growth and Development.”