A soil test commonly refers to soil analyses performed for a variety of reasons. Some soil tests are conducted for geotechnical purposes while others are performed for use in agricultural settings. The soil tests performed for gardens are similar to the tests conducted for farmers.
Testing your garden soil can provide you with the information you need to ensure the success of your garden. This is particularly true if you are setting up a new garden. A basic soil test will tell you the soil pH, cation exchange capacity, the amount of organic matter, and the levels of major nutrients such as phosphorus, potassium, and magnesium.
However, soil testing can be technical in nature. In order for a soil test to be an effective way to improve your garden, it is essential that you understand how to properly collect samples and how to interpret the results of the soil test. The rest of this blog post will help you understand how soil testing works.
Is a Soil Test Worth It?
You may be wondering why you should bother going through the trouble of getting your soil tested. In my opinion, it depends on your situation. Some people live in an area where their native soils are very fertile. For example, my parents, who live in Central Indiana, successfully grow different types of plants year after year without any fertilizer. Gardeners who are fortunate enough to be in that type of situation probably do not need to worry about soil testing.
For people who live in an area where the soil is less than ideal, a soil test can be a great tool because it is a helpful way to understand what is going on in your soil. A soil test can also be really helpful when you are gardening for the first time or planting a new crop.
Performing a soil test every couple of seasons will provide you the information you need to identify nutrient deficiencies, determine your soil pH, and determine the amount of organic matter. Knowing this information will help you understand what soil amendments should be added to your garden.
How Often Should I Test My Soil?
The University of Maine Cooperative Extension office recommends testing your soil once every three years. However, there is nothing stopping you from testing your soil every year if you find the information helpful or if you enjoy poring over the results. It is a good idea to keep all soil tests so you can identify any changes in soil fertility over time.
How Much Does A Soil Test Cost?
The cost of a soil test depends on a number of factors including whether you are using a home kit or a laboratory, the type of soil test you are ordering/performing, and your geographic location.
In general, a soil test will cost between $20 and $50 at a laboratory.
A home test kit will cost between $10 and $100 depending on what you are trying to test. If you just want to test soil pH, you could purchase a small kit from Lowes for less than $10. On the other hand, a larger kit such as the Harris Seeds Professional Soil Test Kit (Large) costs about $100. The Professional Soil Test Kit allows you to test for pH, nitrogen, phosphorus, and potassium.
There are also soil moisture meters and soil pH meters available on Amazon for between $15 and $30. These types of meters can be helpful, but they do not provide information about the nutrients in the soil.
What Does A Soil Test Tell You?
A soil test will help you develop an understanding of your soil’s current fertility and health. It takes the guesswork out of fixing any potential soil fertility problems in your garden. The information below will help you interpret the information provided in a typical soil test.
Cation Exchange Capacity (CEC)
Cation Exchange Capacity (CEC) is one of the first numbers you should look at on a soil test because it gives you a general idea of the soil’s ability to supply nutrients to plants.
CEC is the sum of exchangeable cations that a soil can absorb at a specific pH. It is typically expressed in terms of milliequivalents per 100 grams or centimoles per kilogram. Many cations function as essential nutrients for plants. Because soil particles are negatively charged, these positively charged nutrients but allow these to exchange with other positively charged particles in the surrounding soil.
In general, soil with a higher CEC will hold more nutrients and more water than soil with a lower CEC. A CEC is highly related to the texture of the soil. A high CEC is typically associated with soils containing a higher clay content. The table below shows typical CEC values for different types of soil.
| Soil Group | CEC (meq/100g) |
|---|---|
| Light-colored sands | 3-5 |
| Dark-colored sands | 10-20 |
| Light-colored loams and silt loams | 10-20 |
| Dark-colored loams and silt loams | 15-25 |
| Silty clay loams and silty clays | 30-40 |
| Organic soils | 50-100 |
Although there may be more nutrients in a soil with a high CEC, that does not necessarily mean that these nutrients are available to the plants.
CEC is an inherent soil property and is difficult to change. To improve nutrient uptake, make sure the soil pH is within the optimum range for the particular plant you are growing.
Percent Base Saturation of the CEC
Some soil tests also report the percent base saturation. This value represents the percentage of soil exchange sites occupied by calcium, potassium, magnesium, and sodium ions. The percent base saturation increases with increasing soil pH. The availability of calcium, potassium, and magnesium also increases with percent base saturation.
Soil Organic Matter (SOM)
Another value listed on most soil tests is the amount of organic matter in your soil. This value also provides another clue about how well your soil is able to supply nutrients to plants without the help of fertilizers or other soil amendments.
Soil organic matter (SOM) refers to the part of the soil that is comprised of decaying plant and animal tissue. Most agricultural soils contain between 3 and 6 percent organic matter. Sandier soils tend to have a low percentage of organic matter. The amount of organic matter tends to increase with clay content.
SOM is an important component of soil because it provides nutrients such as nitrogen, sulfur, and some micronutrients. These nutrients become available to plants during a process called mineralization. In addition, SOM improves soil structure and increases water holding capacity.
Although SOM levels are an indication of soil health, soils with high SOM content are not always healthy. Some soils have a lot of organic matter but have poor aeration. This prevents the organic matter from breaking down in a way that those nutrients become available to plants. The soil may also be wet which makes it difficult to grow many crops successfully.
If you find that your soil has low levels of SOM, the easiest way to add organic matter to your garden is to apply compost. Compost refers to a pile or bin of organic matter, such as grass clippings and vegetable scraps, that are in varying degrees of composition. Finished compost smells and looks like fresh soil.
Typically, gardeners dig in compost while tilling (1-2 inches of compost for every 3-5 inches of soil). However, you can also apply compost as a mulch (2-4 inches) if you are practicing no-till gardening.
Soil pH
pH is a measure of acidity or alkalinity. Acidity is a measure of the concentration of hydrogen ions, and alkalinity describes the ability to neutralize hydrogen ions. A pH of 7.0 is considered neutral. pH values lower than 7.0 indicate acidic conditions, and pH values greater than 7.0 indicate basic (alkaline) conditions.
Soil pH has a significant impact on plant health. This is because nutrients that plants need to survive become unavailable outside of certain pH ranges. For example, plants can experience iron deficiencies (even if there is enough iron in the soil) if the pH is too high. In general, most home gardens will do well if the soil pH is between 5.6 and 6.5. However, some plants prefer a growing environment with a pH outside of this range. For example, blueberries prefer to grow in more acidic conditions with a pH between 4.0 and 5.0.
In addition, pH levels that are too low or too high can cause certain plant nutrients to become toxic. For example, molybdenum becomes available in toxic amounts at very high pH levels.
Keep in mind that adding fertilizer typically decreases the pH of the soil.
If the soil pH is too high in your garden, you can lower the pH by applying organic mulch or sphagnum matter. There are also acidifying fertilizers available such as ammonium sulfate or urea. If the soil pH is too low (too acidic), raise the soil pH using lime or wood ash.
Buffer pH/Buffer Index
Another pH value listed on some soil reports is the buffer pH/buffer index. This value does not tell you anything about the soil. Laboratories perform this test for the purpose of providing lime recommendations.
If your soil has a high pH, you may notice that the buffer pH/index column is blank. When the soil pH is above 6.8 or 6.9, the soil laboratory assumes that you will not be applying lime in order to raise the pH. Therefore, there is no reason to add a buffer pH/buffer index value.
Recommended Lime Application
The amount of lime needed to raise soil pH depends on the soil pH as well as the buffering capacity. The buffer capacity describes the soil’s resistance to a change in pH. This means soil with a higher buffer capacity will need more lime to change the pH.
The table below lists the recommended lime rate in (lbs per 50 sq ft) based on buffer pH and target pH. These recommendations were adapted from a document from the Purdue University Soil Science Department.
| Buffer pH/Target pH | 6.8 | 6.7 | 6.6 | 6.5 | 6.4 | 6.3 | 6.2 | 6.1 | 6.0 |
|---|---|---|---|---|---|---|---|---|---|
| >6.8 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 6.8 | 3.2 | 3.1 | 2.9 | 2.8 | 2.7 | 2.6 | 2.5 | 2.4 | 2.3 |
| 6.7 | 5.5 | 5.3 | 5.1 | 4.8 | 4.7 | 4.6 | 4.4 | 4.2 | 3.9 |
| 6.6 | 7.8 | 7.6 | 7.3 | 6.9 | 6.7 | 6.5 | 6.3 | 5.9 | 5.5 |
| 6.5 | 10.3 | 9.9 | 9.4 | 8.7 | 8.5 | 8.3 | 8.0 | 7.6 | 7.1 |
| 6.4 | 12.6 | 12.1 | 11.5 | 10.8 | 10.6 | 10.3 | 9.9 | 9.5 | 9.0 |
| 6.3 | 14.9 | 14.4 | 13.7 | 12.9 | 12.6 | 12.2 | 11.8 | 11.3 | 10.6 |
| 6.2 | 17.2 | 16.6 | 15.9 | 14.9 | 14.6 | 14.2 | 13.7 | 13.0 | 12.2 |
Nutrient Analysis For Garden Soil
Finally, I will talk about how to interpret the nutrient parts of the soil test report. This is the part of the report that describes the amount of plant-available nutrients in the soil.
Some laboratories provide fertilizer recommendations with their soil test reports. I recommend double-checking these recommendations against recommendations posted by your local cooperative extension office.
Phosphorus
Phosphorus is essential for plant growth. It is responsible for storing and transferring energy produced during photosynthesis for growth and reproduction. It is important to note that pH values below 5.5 and above 7.5 limit the amount of phosphorus available to the plant.
There are a few common procedures used to test the phosphorus in soil. The most common tests are the Bray-P1 procedure, the Olsen procedure, and the Mehlich 3 procedure. The Bray-P1 procedure is the most reliable testing method for neutral and acidic soils. However, the Bray-P1 procedure tends to underestimate the phosphorus content in soils with a high pH. The Olsen test is typically only performed on soils with a high pH (>7.2). Although the Mehlich 3 procedure is the newest soil testing method of the three, it is being used more widely as a “universal” testing method.
The table below will help you interpret the phosphorus values listed on your soil test. This table was obtained from Iowa State University’s Extension and Outreach website.
| Bray-P1 or Mechlich 3 (ppm) | Olsen (ppm) | Mehlich ICP (ppm) | |
|---|---|---|---|
| Very Low | 0-15 | 0-10 | 0-20 |
| Low | 16-20 | 11-14 | 21-30 |
| Optimum | 21-25 | 15-17 | 31-40 |
| High | 26-30 | 18-20 | 41-50 |
| Very High | 31+ | 21+ | 51+ |
Low levels of phosphorus can have adverse impacts on plant growth which results in stunted vegetative growth and yield. Young plants exposed to cool, wet conditions are particularly susceptible to phosphorus deficiency problems. This is because phosphorus becomes immobile in soil solution. If your soil test indicates low phosphorus levels, you can increase the amount of phosphorus by adding bone meal, rock phosphate, manure, and phosphate-fertilizers.
High levels of phosphorus can also adversely impact plant health. Excess phosphorus can interfere with the uptake of other nutrients such as iron, manganese, and zinc. Unfortunately, there is no way to actively remove phosphorus from soil. If you have high levels of phosphorus in your soil, avoid adding manure or fertilizers containing phosphate. Another option is to grow nitrogen-fixing plants such as clover or winter peas. This will provide your soil with nitrogen without having to add fertilizer.
Potassium
Potassium is an essential nutrient. It is also a macronutrient meaning large amounts of potassium are required to support proper plant growth. Potassium regulates the opening and closing of the stomata which is essential for evapotranspiration as well as carbon dioxide uptake for photosynthesis. In addition, potassium plays a crucial role in the production of proteins and starches as well as the production of adenosine triphosphate (ATP).
The potassium content of your soil should be between 130 and 175 ppm depending on what you are growing. If your plants are experiencing a potassium deficiency, make sure the soil is within the optimum pH range for the crop you are growing. Another way to treat potassium deficiencies is to apply a fertilizer that contains potassium. Fertilizer potassium is often referred to as potash.
If you want to avoid applying chemical fertilizers to your garden, consider applying an organic source of potassium such as kelp, wood ash, or banana peel.
Keep in mind that the plants’ response to potassium fertilizer is unlikely if the potassium of your soil exceeds 90 ppm fruit crops or 175 ppm for vegetables.
Calcium
Like the other nutrients discussed so far, calcium plays an important role in healthy plant growth. Specifically, it is involved in important processes such as cell wall formation, cell division, and pollination. It also plays an important role in the way plants respond to drought or heat stress. Finally, calcium protects plants from diseases by enabling the plant to stand attacks from fungi and bacteria.
The calcium content of soil greatly depends on soil texture. Sandy soils typically have a calcium content between 400 and 500 ppm while clay soils tend to have a much higher calcium content – 2500 ppm. If you recently applied lime to your soil, you may see inflated calcium levels on your soil report.
Another thing to keep in mind when looking at your soil report is that calcium is more available at a higher pH. If you maintain a soil pH within the recommended range for the type of plant you are trying to grow, a calcium deficiency is very unlikely.
In the event that your soil does have low levels of calcium (<400 ppm), apply lime to your garden.
Magnesium
Magnesium is another essential plant nutrient. It has several important roles in plant growth. For example, magnesium is involved in the production of chlorophyll which is essential for photosynthesis.
Most soils with low levels of magnesium (<31 ppm) are acidic and also have low levels of calcium. In general, your soil should have a magnesium content between 25 and 45 ppm.
Keep in mind that low levels of magnesium can sometimes be attributed to soil conditions such as low soil temperatures or dry soil.
If your soil has low levels of magnesium, apply compost to the soil. You can also apply Epsom salts or calcium-magnesium carbonate to your garden in the autumn or winter. For a “quick fix” during the summer, you can also try using a magnesium leaf spray.
Micronutrients
Sometimes soil reports for lawns and gardens will report some micronutrients such as zinc (Zn), copper (Cu), iron (Fe), or manganese (Mn). These elements are called micronutrients because plants require very little amounts of these nutrients.
The table below will help you interpret your soil test results. This table was obtained from a University of Arkansas Research and Extension document entitled Understanding the Numbers on Your Soil Test Report.
| Mn (ppm) | Cu (ppm) | Zn (ppm) | |
|---|---|---|---|
| Very Low | <1.6 | ||
| Low | <40 | <1.0 | 1.6 – 3.0 |
| Medium | 3.1 – 4.0 | ||
| Optimum | 4.0 – 8.0 | ||
| Above Optimum | >8.0 |
The level of extractable micronutrients will not necessarily influence your fertilizer application. In many cases, the soil pH should be adjusted so that these micronutrients are available for plant uptake.
It is also important to note that a very high level of a particular micronutrient does not necessarily indicate toxicity. For example, high levels of manganese are only toxic if the soil has a pH below 5.2.
How To Collect Soil Samples
Before testing your soil, you must collect soil samples from your garden.
It is important to properly collect soil samples properly so that the soil test report accurately reflects the condition of your garden. The steps listed below are generic. I recommend contacting your soil testing lab for specific instructions.
I recommend testing your soil prior to planting. Specifically, it is a good idea to test your soil in late summer or early fall. This will give you plenty of time to plan ahead and treat your soil with any necessary fertilizers or soil amendments.
Materials
- A soil probe or auger (you can use a sharp spade or trowel if you do not have a soil probe or auger)
- A clean labeled container for each “zone”
Step 1: Develop a Sampling Plan
First, develop a sampling plan. I recommend drawing a diagram of your property and marking where you are going to sample. This diagram could be hand-drawn or a printed aerial map from Google Earth.
Because a soil sample represents only a very small portion of your garden, a strategic soil sampling plan is a crucial part of ensuring that your soil test report will accurately represent what is going on in your garden.
On your diagram, indicate the different sections of your garden. Each zone should be tested separately. For example, collect a set of soil samples from your flower garden and a separate set of soil samples from your vegetable garden. Areas that receive significantly more shade than other areas might need to be tested separately as well.
In addition, areas that have been managed differently should be sampled separately.
How Many Sub-Samples Should You Collect From Each Area?
Each zone indicated on your sampling plant represents a single soil test/composite sample. Each composite sample will be comprised of several sub-samples. The number of sub-samples depends on the size of the zone. For example, you should collect 10-15 cores/sub-samples from a yard and 4-6 cores/sub-samples from a flower bed.
Step 2: Collect the Samples
After developing your sampling plan, it is time to actually collect the samples. Before collecting soil samples, remove any mulch, leaves, or thatch from the soil surface. Then take the appropriate number of sub-samples for each zone.
Sample garden and landscape beds to a depth of 6-8 inches below the soil surface. You only need to sample turf areas to a depth of three inches below the soil surface. Make sure you spread out the sub-samples in such a way that your sample represents “average” conditions. For example, sample light and dark areas.
In addition, collect samples in between row crops in your garden. This will ensure that you avoid collecting samples in areas where you have applied fertilizer.
Put all of the sub-samples into the container labeled for the appropriate zone.
Step 3: Dry and Mix Samples
After collecting the samples, make sure the soil in each container is mixed together. You want the soil in each bucket to be fairly homogenous so it represents the “average” conditions of each zone.
Shortly after collecting the samples, break up any lumps in the sub-samples and remove any rocks or debris. Then allow the samples to dry at room temperature for a couple of days until the soil is dry. Do not send wet soil to the testing facility! You should not be able to squeeze water from the soil sample.
After the soil samples are dry, crush and mix the soil samples together in each container.
Step 4: Submit The Samples To Laboratory
After you have mixed the samples, remove about one pint of soil from each bucket. Then place the soil sample in the container or bag provided by the soil testing laboratory. Fill out any associated forms or provide any information requested by the laboratory.
If the testing facility does not provide a soil testing kit, put the soil in a zip lock bag.
Where To Get Soil Tested
I recommend contacting your local Natural Resources Conservation Service (NRCS) Office for a list of reputable soil testing labs. Many universities also provide affordable soil testing through their Cooperative Extension Services. These soil tests will often come with fertilizer/soil amendment recommendations.
Alternatively, you can check with your local garden supply store or nursery to see if they provide soil testing.
Finally, soil testing services are available through a number of commercial laboratories such as:
- Midwest Laboratories
- Woods End
- Control Laboratories
- AGVISE Laboratories
Home Test Kits vs Laboratory Tests
Home test kits are available at Home Depot or Lowe’s. These test kits are widely available, are often cheaper than laboratory testing, and will provide immediate results.
While these test kits are inexpensive and widely available, they are not as accurate as laboratory testing. Another limitation of these tests is that the results of home test kits are categorical (e.g, low, medium, high) rather than numerical.
A study conducted by researchers at North Dakota State University investigated the accuracy of three different home soil test kits: the LaMotte Complete Soil Test, the Luster Leaf Rapitest Soil Test Kit, and the Luster Leaf Rapitest 4-Way Analyzer. The results of the study indicated that home soil test kits tend to underestimate nutrient content. This means that using the results of garden test kits can lead to over-applying fertilizer unless you are using recommendations calibrated to that particular test kit.
Another study conducted by researches at the Ventura County University of California Cooperative Extension determined the accuracy of four soil test kits by comparing the kit results to the results of laboratory testing (Faber et al. 2007). A summary of the findings is tabulated below. Accuracy is defined as the frequency of match with analytical laboratory results. I also added a column that lists the cost of each kit.
| Soil Test Kit | Accuracy | Cost |
|---|---|---|
| La Motte Soil Test Kit (La Motte Co., Chesteron, MD) | 94% | $65 |
| Rapitest (Luster Leaf Products, Woodstock, IL) | 92% | $10 |
| Quick Soiltest (Hanna, Woonsocket, RI) | 64% | $26 |
| NittyGritty (La Motte Co., Chesteron, MD) | 36% | $36 |
Citation:
Faber, Ben & Downer, James & Holstege, Dirk & Mochizuki, Maren. (2007). Accuracy Varies for Commercially Available Soil Test Kits Analyzing Nitrate–Nitrogen, Phosphorus, Potassium, and pH. HortTechnology. 17. 10.21273/HORTTECH.17.3.358.