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FAQs: Soils & Soil Testing

Topics pertain to physical and chemical properties of soils, how they are tested and modified to improve plant growth

How will soil testing help solve a plant problem?
For routine analysis, soil testing is used to determine whether soil conditions and nutrients are in a good range for growing the plants or crops you want. If nutrients levels are low or excessive and/or if the soil texture, structure or pH is less than optimal, then some action would be called for to correct the problem. This is all best done before plants are installed, but that infrequently happens. More commonly, plants are purchased without a hint of what shape the soil is in. Problems that appear are usually after 2 to 3 growing seasons. Plants become indicators of these soil problems by expressing symptoms including discoloration, lack of growth, branch dieback, leaf burn or curl etc. These symptoms are also similar for problems caused by bacterial, fungal or viral pathogens as well as certain pests.

It is difficult to distinguish the symptoms of a nutrient deficiency from that of mite injury. One of the best ways to begin the process of diagnosis is to get soil test. This the most fundamental information you can start with and often times helps to distinguish whether the soil or other cultural conditions are involved in a pattern of symptom development. From experience, we know that a very high percentage of the problems found with plants in residential settings are due to cultural conditions like poor soil, improper light exposure, excessive or deficient rainfall, temperature extremes, pest/disease control or fertilization practices. A soil test will begin to define how important the soil is in contributing to the possible complex of conditions leading to poor plant performance.

How do you compare results from different soil labs?
If the same soil sample is sent to several soil laboratories, no doubt you may receive different results when comparing individual tests. The first thing to realize is that there is going to be inherent variability with any test, within and between any two laboratories. This is due to differences in technique, equipment and procedures. One may report 35 pounds of phosphorus per acre and the other 55 pounds. Which is correct? The idea about soil testing is to establish the amount/availability of a nutrient in the soil and whether that level is relatively low or not with respect to the crop/plant you wish to grow. Each laboratory will have it's own standards and those should be based upon good experience of soils and plant performance. When it comes to agricultural crops, that experience is very important because of the cost to add fertilizers and amendments. For ornamentals, fruit, vegetables and lawns, the standards may range.

Other sources of variability come with the sample itself. Analytical techniques are designed to detect very small amounts of each nutrient. If one sample is not exactly identical to the other, then that introduces a source of variation and the numbers will differ. Another source of variation is with techniques. Each laboratory will develop a set of procedures for any given analysis. In the case of pH, you might find one laboratory doing a pH using a buffer. Another will use water and yet another will use a salt solution. The water pH might vary 0.5 units between techniques. Who is correct? The end result is that any recommendation should be standardized and relative to the technique and the crop/plant. The bottom line is that variation exists and only through repeated testing will significant and reliable trends emerge. The most important part is to have some consistency between tests and receive an understandable report with recommendations that make practical sense.

What is soil pH?
The measure of pH is relative to determining the soil's acidity or alkalinity. The pH scale is logarithmic and spans from 1.0 to 14.0. A pH of 7.0 is considered neutral, being neither acidic nor alkaline. The range for good plant growth is typically in the area of 6.3 to 7.0. However, there are many plants that do well in a broad range of pH values. Likewise, there are some plants which prefer acid soils and some that prefer alkaline soils. Knowing what pH your plant will do best helps to approach the challenge of amending the soil of affect a pH change. A professional soil test will evaluate the pH and provide a recommendation for the plant you wish to grow.

Overall, changing the soil pH is not easy. Soils tend to be good buffers and resist change. When the pH is too high (alkaline), sulfur is commonly used to lower the pH. Sulfur forms sulfuric acid in the soil when combined with water. Elemental sulfur comes in a dust or a pellet form. The latter is the safest way to spread this material. During the growing season, never apply more than 5 pounds per 1,000 sq. ft. at one time. Otherwise, the plant may exhibit leaf burning. If more is needed to low the pH, then wait six weeks to apply the rest. Incorporation is required to speed up the reaction. If your soil is acidic and needs to be adjusted towards the alkaline side, then we commonly apply lime. This material causes the hydrogen in the soil to combine with carbonate in the lime and form water. This neutralization causes the pH to elevate and therefore, become more alkaline. Liming soils was once done as a routine practice, but without a soil test indiscriminate use of lime can damage plants and soils. The best thing to do is get a soil test first, then apply the amendment that adjusts the soil to favor the growing conditions for the plant you wish to grow.

How do you improve clay soil?
All soils are made up of three components; sand, silt and clay. These components are distinguished from each other by their relative size. Sand has the largest particle size and is easily seen by the naked eye. Clay is composed of the smallest soil particles and silt is intermediate. Each has its own set of properties and when mixed in different percentages, contributes to the characteristics of a specific soil type. Soil texture is the term used to describe the relative percentage of sand, silt and clay. There are twelve different texture types including sandy loam, silty loam, clay loam, sandy clay, silty clay, loamy sand, etc. The best soil is one that is loam and consist of nearly equal parts of sand, silt and clay. This balance can be achieved by adding amendments to a certain degree.

In clay soils, the percentage of clay is between 30 and 50%. You can determine this by having your soil tested for a particle size analysis. The properties of clay can be both positive and negative. On the positive side, clay tends to store a large reserve of nutrients because of the number of potential exchange sites where nutrients are attracted. On the negative side, clay tends to hold onto water and dry slowly causing the soil to be wet for prolonged periods. It also tends to repel water when dry and exhibits large cracks when moisture loss is significant. Because of these swell and shrink properties, clay tends to compact down and lose pore space where air and water exchange. If you walk on wet clay, it will stick to your feet and compact so that plant roots will have a difficult time penetrating.

Clay soils can be amended with organic matter as the first step in renovation. The best time to deal with clay soils is in the fall when soil moisture is balanced and it can be tilled with the addition of organic matter. For every 1,000 sq ft, add about 2 cubic yards of compost or peat moss and till to a depth of 8 inches. The addition of organic matter is the best way to break up the clay particles and get them to form aggregates which will allow for more air space and water exchange. Improving clay soil make take several years, but the repeated use of organic matter at the rate of 2 inches over the top and tilled to 8 inches deep is the best approach. Other amendments may also be useful including expanded clay and sand. Expanded clay is formed by heating clay to high temperatures so that it "puffs up". These lightweight, small-sized particles are useful in breaking up clay soils and adding pore space for air and water exchange. Sand should only be used where the percentage applied is greater than 25% by volume. That is about a 2-inch layer over the top of the area to be worked to 6 inches deep. Using smaller amounts may result in making "mortar" instead of more porous soil.

Why add organic matter and how much is enough?
Organic matter is generally referred to as plant or animal remains in different states of decomposition. When leaves are added to soils, they begin to break down with the assistance of bacteria, fungi and other soil organisms. Eventually, organic matter reaches a state of final bread down that we call humus. Adding organic matter is beneficial to the soil in several ways. One of the most important improvements is to cause soils to become structured into small clods or particles. As these particles aggregate, they allow the development of pore spaces that become filled with water and air. When the soil has achieved a state where half of the volume is air and water, then it is much improved for better root growth. Air and water exchange are critical for growing many of our favorite garden plants. Compacted soils without good pore space for water and air exchange are less productive and may support only a certain plant type that are tolerant to extreme conditions.

Organic matter has some nutrients and contributes nitrogen to plant growth as it breaks down. But, perhaps it larger contribution to healthy soil is as a buffer to soil moisture loss and temperature fluxuations. This means that organic matter lessens how rapid moisture is lost from the soil during summer and how fast the soil and plant roots becomes frozen during the winter. Good garden soils should have organic matter contents of around 5 to 10% in the first six inches and this is what you should strive for as you mulch or amend with organic matter. This is considered a good balance for most ornamental plants. It is possible to overdue the application of organic matter. When the content of the soil is greater than 20%, then you may experience problems associated with depletion of nitrogen, too much water held around the root system leading to root rots or a situation where anaerobic decomposition occurs without a proper mix of water and air. The latter generates chemical byproducts that inhibit root growth. Also consider that some plants are quite satisfied with less than 5% organic matter. For these plants, a "rich" soil created by adding lots of organic matter may cause them to grow too well and fall over in a wind or rain storm. This is more commonly seen with native plants that are use to lean soils, low in organic matter and nitrogen.

When should soil be tested?
The timing of a soil test depends upon whether you are working with an established lawn or garden, beginning a new planting or diagnosing a problem. There are different starting times or critical periods for each situation. Also, you need to figure in about two weeks to submit a sample and receive the soil test report.

Established Planting: For routine purposes working with an established planting, the best time to get a soil test is about 6 weeks prior to the most rapid growth period. That means for cool season lawns like bluegrass and fescue, soil tests should be done in early spring or in mid-summer. This is because cool season grasses have two growth periods and the best response to fertilizers will occur at these times. For warm season grasses like zoysia and bermudagrass, samples can be taken in late spring since the growth period for these lawns is in the middle of the summer. For annual, perennial, vegetable and herb gardens, sample should be submitted in the spring, 4 to 6 weeks before planting or signs of new growth. Trees and shrubs are typically fertilized in the spring although, fall fertilization is also acceptable. Soil samples can be taken in early spring or late fall.

New Garden: If you are beginning a garden either from scratch or renovating an area in efforts to change the planting, then soil samples should be collected in the summer with the anticipation of turning the soil under to prepare the new bed. Tilling or spading the soil will allow for adequate incorporation of amendments. But it is desirable to let the soil settle for a least a couple months prior to planting. The winter will assist with the settling, if you begin in the fall. Spring soil preparation can be difficult due to water-soaked soils -- never till a wet soil or you may destroy the soil structure, commonly referred to as the clods.

Plant Problems: Diagnosing a plant problem typically occurs during the height of the growing season. It is sometimes very hard to determine the cause of the problem which may be due to a pest/disease, attributed to the climate and weather or soil nutrient deficiency/nutrient excess. Diagnosing a plant problem is commonly done at the time the symptoms occur. This is the best time to get information. One helpful tip is to sample from two separate areas; one where the plant/crop is looking poor and the other where the same plant is seemingly doing very well. The comparison between samples will help contrast good growing conditions against poor conditions. The soil sample is one piece of this puzzle and at the very least provides a basis to suggest that the soil either is or is not involved.

Which test is best for my situation?
All soil laboratories will recommend some basic soil test and generally, that should include a measure of pH, available phosphorus as diphosphorus pentoxide, and potassium as dipotassium oxide. Phosphorus and potassium are two of the three macronutrients required by plants for active, healthy growth; nitrogen is the third and these would be the essential elements, otherwise called primary nutrients for plant growth. Nitrogen is usually not measured for routine lawn and garden soil tests. Unless you live in a climate that gets less than 20 inches of annual rainfall, you can assume that nitrogen needs to be applied each year since it is leaches and tends to wash out of the root zone.

Two other important soil measurements to include in a basic test would be pH and percentage organic matter. Soil pH measures the acidity/alkalinity of the soil which controls, in part, nutrient availability to plants. Knowing the pH will also assist in determining plant selection since some plants grow best in acid soils and others in alkaline soils. Additional nutrients important to plant growth include calcium and magnesium. Measuring these along with the soil pH will provide information about the soils' ability to store and hold onto nutrients. This information has some bearing on how much and how often to fertilize plants.

In contrast to the primary nutrients, the secondary nutrients are required in reduced amounts and these include sulfur, iron and zinc. The last group of nutrients considered important for plant growth are the micronutrients including manganese, boron and copper. Several others are commonly listed, but these are either not required by all plants or in needed in only very small amounts.

The basic soil test is good for routine analysis and recommended every 3 to 5 years for lawns or gardens. It will guide your efforts to apply only what is needed for good plant growth without overloading the soil. The EarthCo Basic Soil Test measures pH, percentage organic matter, phosphorus, potassium, calcium and magnesium. Where no soil test records are available for an area or, when significant renovation in a garden area is taking place or, if you are experiencing a persistent plant problem, then a micronutrient analysis would be helpful, providing a complete understanding of all nutrient levels in the soil. The EarthCo Micronutrient Soil Test measures all the essential elements in the Basic Test plus the secondary and micronutrients including iron, sulfur, zinc, manganese, copper, and boron. If you are buying or blending your own topsoil, renovating a very large area that would be considered a significant investment or solving a persistent plant problem, then you should include an evaluation of soil texture (particle size analysis) to know the percentages of sand, silt and clay. This will make amending the soil more meaningful. The EarthCo Basic Plus and the EarthCo Complete Soil Test will give an analysis for soil texture in addition to either the basic or total nutrient analysis.

Why test your soil?
Soil testing is the only scientific way to tell what soil nutrients are available for absorption by plant roots as well as their relative supply in the soil. Soil tests become especially important when new gardens are installed or garden areas are changed from one planting scheme to another. Testing soils becomes even more significant in light the investment made in plants and for the number of years you would expect that investment to perform.

If the soils are poor, then the best time to correct them is before planting - not after. Soil tests also tell you to apply only what nutrients are needed. Guessing about this leads to "blanket" applications of fertilizers and often times excessive nutrient levels. Working towards a good balance of soil nutrients might avoid "induced" nutrient deficiencies and overloading of soils.
While measuring the levels of nutrients is the prime focus in testing your soil, a professional soil test should also provide information about the acidity or alkalinity of the soil. The expression of acidity/alkalinity is call pH. It is measured using a variety of methods depending upon the individual laboratory and its analytical practices. These laboratory differences will affect the acceptable pH range for growing certain types of plants, but generally, the pH should range between 6.3 and 7.0.

Soils can be tested at any time during the year. Fall is ideal, allowing time to get the testing done, apply the amendments and work the soil before the planting season in spring. On a routine basis, soil testing should be done every 3 (lawns, fruit and vegetables) to 5 (flowers, perennials, trees/shrubs) years. More frequent testing is typical when diagnosing plant problems and when determining whether the soil corrections are effective and for how long.

What is soil texture?
The mineral portion of a typical, unamended soil makes up about half of the volume of good soils. The other half is composed of air space, water and a small amount of organic matter. The mineral part acts like a magnet for many of the plant nutrients. These nutrients are released to the soil water for plant uptake and this process is dependent upon several factors that keep the nutrients in a balanced supply. The mineral portion that regulates nutrient supply is composed of sand, silt and clay particles. These differ from each other on the basis of size - sand having the largest particle size and clay the smallest.

The mixture of sand, silt and clay gives soil different qualities and affects how well plants grow. For example, soils that are high in clay (greater than 20%) tend to be hard or cement-like when dry, sticky when wet and easy to compact so that air and water space is decreased. On the other hand, clay soils have an excellent ability to hold onto nutrients and therefore, serve as a "bank" for fertilizers, storing nutrients on exchange sites until they are released and made available for uptake by plant roots. Sandy soils are very porous and provide easy water penetration and drainage. However, they are poor at holding nutrients and therefore, typically require more frequent fertilizer applications to keep plants supplied with sufficient nutrients. Silt soils are moderately good at storing nutrients, holding water, and are easy to till or turn under in preparation for planting. They tend to compact like clay and need to be amended with organic matter to favor air and water exchange.

The balance of sand, silt and clay also determines the productivity of the soil. Measuring soil texture to determine the percentages of each is a good idea if you are starting a new garden, buying soil from a supplier or attempting to diagnose a plant problem. Your soil test report should give you a percentage of all three components and classify the soil as one of twelve different types ie. silt loam, silty clay loam, sandy clay, clay loam etc. This information tells you what to expect in the way of drainage, ability to hold nutrients and how much to fertilize. The report should also tell you what you may need to add to improve the soil tilth and structure.

What is the composition of soil?
The average garden soil is composed of minerals, organic matter, water and air space. The mineral portion is what remains after thousands of years of parent rock being degraded by geologic changes and weathering. Mineral particles of different sizes make up the three components of soil namely, sand, silt and clay. The nutrients that plants require for growth are associated with these minerals attached to exchange sites that are like magnets. These hold onto the nutrients until they enter the soil water and the nutrients become available for the plant roots to absorb. Soil tests measure the availability of these nutrients to plants in a given soil and whether the plant will have these in sufficient supply to support healthy growth.

Organic matter is derived from once living organisms including plants, microbes like bacteria and fungi and small animals like earthworms and immature insects. Plant leaf material and dead roots make up the major portion of organic matter in soils. Organic matter also has exchange sites where nutrients will attach and be made available for plant uptake once they enter the soil water. Organic matter in good garden soils is usually present in amounts from 5 to 10 percent by volume. The benefits of organic matter are significant for plant growth. It moderates the loss of soil moisture, improves water and air exchange as well as increases drainage by causing the soil particles to form clods and encourages the growth of microorganisms.

In healthy soil, the amount of air space should be about 50% of the total volume; the rest being the mineral and organic portions. Soil water and air space in the soil are both important for good root growth. Soils with less pore space for air and water exchange would be called "compacted" and these soils are more difficult for many plants grow in because they limit root development. Compacted soils need to be loosened by core aeration, tilling or turning over and by adding a soil conditioner like sand, organic matter or expanded clay.

Can I test the soil myself?
There are a number of ways that soils can be tested for their properties and nutrient status. Garden centers sometimes sell "kit" type soil tests that require you to sample the soil, add reagents and read the results by comparing color charts. This takes some patients and practice to do it properly as well as to accurately interpret the results. The cost of these kits varies with the contents and what tests they can do, but generally they are equivalent to a professional soil test.

The Cooperative Extension Service has been testing soils for farmers raising agricultural crops for many years. Their purpose has been to help farmers understand how much fertilizer to apply to a given field in order to attain a certain crop yield or how to modify soil characteristics like pH and organic matter content in order to optimize plant growth. Some states have privatized this service meaning that the Extension service no longer operates as a soil testing laboratory. In these states, you must seek out a private soil testing company. They should have some reputation for quality of results and be affiliated with a standardization or certification program.
Overall, getting your soil tested professionally means that you have hired someone with reliable experience in testing soils and interpreting the results for the plant/crop in question. The real value to a professional soil test for home gardeners is in the recommendations that come back. These need to be understandable and specific enough to leave no doubt about what you should do next. It is not helpful for the reports to offer information that is either useless or adds confusion. Before you buy a professional soil test, get a sample report from the laboratory. A sample EarthCo Basic soil test report can be viewed by clicking on "Report" under "Ordering A Soil Test".