Soil analysis provides answers of public safety to engineers and architects, of history to archaeologists, of science to geologists, and of plant health, crop productivity to farmers and gardeners.
Soil analysis is a scientific process to assess soil nutrient level and composition. This is a management tool that allows 1) deciding which crops to plant, 2) identifying nutrients deficiency 3) detecting the presence of helpful or harmful bio-organisms 4) identifying excess fertilizer, and 5) avoiding drought or root suffocation.
By look, touch, or feel, experienced plant lovers and crop growers can tell if soil is good for planting. However, for soil contents that we can’t sense by – not even with years of experience, soil analysis is necessary.
First, let’s look at the first step in soil analysis: how to properly take a soil sample.
Knowing the soil in which you grow your plants is extremely important, especially if you grow them in large amounts or for financial reasons. Indeed, plants (if not for hydroponic growth) develop in the soil through which they “eat” (absorb minerals), drink, and breathe.
A bad soil for nutritional or drainage purposes can cause your crop to fail, losing months of effort, and lots of money.
All of this can be avoided with a soil analysis.
Before even dropping one single seed in a field, it is important to know if that seed is in the right environment to become a fully developed plant. A soil with a low pH (acidic) might be ideal for blueberries, but quite harmful for the majority of herbs.
A nutrient-poor soil can be good for a lavender plantation, but inadequate for pumpkins or squash that are known to be heavy feeders.
Many plants require an adequate level of nutrients in the soil to optimally grow. A soil rich in nitrogen might prevent plants from developing fruits, while a potassium deficiency soil will cause a large variety of problems.
If the crops are not growing or present some strange issues, one of the culprits might be a low level of nitrogen or potassium in the soil. The only final way to discover, and then solve the problem, is a soil analysis.
Microorganisms, despite a few gardeners being skeptical regarding their role in the soil, it is widely accepted in science that they can contribute to better and faster plant growth. Indeed, they improve the decomposition of organic material (turning into plant nutrients) they enhance the uptake of plant nutrients, and much more.
Hence, having an idea of which microorganism flora is present in your soil is the first step to understanding if there is something you need or you want to do to change or promote a different balance.
Once the plants have ended their life cycle, what you end up with is empty soil. Is it really empty? Probably not as the plants and the nutrients have interacted with each other for a season or more.
A soil that has been by accident overfertilized might be a problem for future plants that are going to grow there. So, before the beginning of new planting reasons, a health check through a soil analysis is key for the plant.
Some plants are heavy feeders, while others do not need almost any nutrients. This is also important if you do crop rotation. Perhaps you can go for a heavy feeder crop first, and alternate with a low demanding one by using the same soil with little to no addition of fertilizer. However, to be sure, you still need to do some soil sampling.
Plants need water to thrive. If you plan to grow lavender or rosemary, these are pretty hardy herbs that can survive with little water for a while. Hence, a lossy and gritty soil with lots of drainages is fine for them.
However, the same does not apply to basil for instance or even worse to pumpkin or large fruit-producer plants. These need a large amount of water and, more importantly, a soil able to retain them. You can drop as much water as you want but if you have very drained soil (high in sand content) your basil will not grow, and your pumpkin might not produce any fruit. The soil needs to hold the water to give the plants time to drink it.
The opposite is also true. A very clay soil will prevent the plant roots to exchange gases with the rest of the environment making them suffocate and drought. This is because clay acts like cement, creating a block of very solid material.
Without a soil analysis (in this case, you might not need any special equipment) it is difficult to understand if your soil is adequate.
The accuracy of your soil test depends on how you take soil samples. Here are some basic guidelines:
- Tools and materials: Use a clean soil probe, auger, or spade to take each soil sample. Prepare resealable plastic bags or containers for storing the samples.
- Area: Choose an area with similar soil and planting history.
- Soil depth: First, clear the surface residue where you will collect soil samples. Then, collect at least 10 soil samples randomly from no more than 8 inches of topsoil. For crop farms, collect from the depth of the tillage.
- Sample preparation: Mix, crush, and air-dry your soil samples. Then, place the soil samples in a clean plastic container.
There are 2 major technique for soil sampling: 1) Cup Sampling and 2) V-Cutting.
What it’s about: This is an easy, low-tech way to take one soil sample if you have a small or a medium-sized garden.
Why it’s important: This is for indoor gardeners or for those with small gardens where soil testing is needed using simple materials.
4 Steps for cup sampling soil:
- Collection: Fill up a cup from the top 6 inches of your garden soil. Repeat up to 8 times from different parts of the garden.
- Mixing: Mix all your soil samples thoroughly.
- Storage: In a resealable plastic bag, place about 2 cups of the combined soil samples.
- Labelling: Label the plastic bag with your name, address, soil location, and date. Your soil sample is now ready.
This method can be used whether you intend to use a do-it-yourself soil testing kit to analyze it, or to send it to a laboratory for soil analysis.
PRO TIP: Don’t use a galvanized bucket that can add zinc and change your soil test results. Instead, use a plastic bucket or bag.
What it’s about: This is a soil sampling method that is commonly used in larger farms or in commercial crop fields.
Why it’s important: This soil sampling method is best used when collecting soil samples to be sent for laboratory testing.
4 Steps for v-cut sampling soil:
- Make a V-pit: Insert the spade at an angle into the soil, about 6 to 8 inches deep. Repeat on the opposite side, and remove the soil to form a V-shaped pit about 6 to 12 inches deep (or the depth of tillage).
- Take a furrow slice: Take out a slice of soil (like a ½ inch thick bread slice) from top to bottom of each of the two surfaces of the V-pit. Scoop that into a container.
- Collect soil samples: In a random, criss-cross or zig-zag pattern, repeat and collect furrow slices from at least 8 spots throughout the entire garden or up to 30 spots throughout the entire crop field. Note where each soil sample was taken.
- Bag and label: Whether you’re doing a DIY test or sending the soil samples to a laboratory, label each soil sample accurately:
- On a piece of paper, provide a name and mailing address, date of sampling, and the locations of each soil sample.
- Fold and place the paper inside the resealable plastic bag.
- Outside the plastic bag, attach a copy on another piece of paper.
PRO TIP: Send your soil sample to a soil testing lab in the fall. That’s when they aren’t too busy.
Analyze your soil to know if it is suitable for your plants. For best results, take a soil sample of your garden at least once a year.
After you identify your soil type, many healthy gardening or planting decisions will follow. Don’t worry. Soil analysis is easy.
What it’s about: This is about using common senses to analyze soil: your eyes and fingers.
Why it’s important: You want to know if the soil allows your plant’s roots to grow and develop or if they will struggle, rot or die.
How to do it:
- Rub some soil with your fingers. Look closely. Identify if the soil is crumbly, gritty, or sticky.
- Smell the soil. Fertile soil with a lot of helpful microorganisms smells earthy or pleasant.
Your learning point: if there is an offensive odor that is similar to decay, the soil may need aerating to increase oxygen content.
PRO TIP: Even in a small garden, test results can vary; test soil from different spots in your field or garden.
What it’s about: This is for you to know how fast plant matter in your soil turns into nutrients.
Why it’s important: One critical element of good soil is that soil organisms work fast to quickly decompose organic matter in the soil.
How to do it:
- Dig in: About six months after plant matter is mixed into soil, dig down about 6 inches.
- Look for plant matter. If you see some partly decomposed plant parts, fibers, or dark, decomposing humus, then soil organisms at work.
- Smell the soil. A pleasant, earthy smell means that organic matter is decomposing speedily.
Your learning points: If you smell something that seems a bit sour, then plant matter may be breaking down too slowly because a) the soil is poorly aerated or b) because you don’t have enough active bio-organisms at work in the soil.
What it’s about: This is called the Water Settling Method. This is used to know the basic composition of soil without using chemical or laboratory testing.
Why it’s important: One basic element of good gardening is knowing the exact composition of the soil.
How to do it:
- Add soil to jar: Place about four inches of your soil sample into a quart-sized glass jar.
- Add water: Fill up the jar with water up to the neck.
- Cover it: Screw on the lid tightly.
- Shake well: Shake vigorously and set aside (in a dark place) for about 12 to 24 hours. If the water is still opaque, let it settle for a few more days
- Look closely: The settled layers should show the layers that make up the soil.
- Measure: Measure each layer.
Your 5 learning points: This test gives you additional information to the touch-smell test as well as the organic decomposition test.
- Loamy soil: You will see clear with layered particles at the bottom of the container, with the topmost layers of the finest particles. For instance, perfect loam should show:
- about 45% sand at the lowest layer
- about 25% silt as the next layer above sand
- about 25% clay as the next layer above silt
- about 5% organic matter, including light plant matter floating on top.
- Sandy soil: You will see mostly clear water and a layer of particles at the bottom of the container.
- Sand layer is more than 60%: Your plants may not get enough adequate water and nourishment. You may need to add organic matter (e.g., manure, compost).
- Sand layer is less than 40%: Your plant’s roots may be struggling or suffocating in the dense soil. You may need to mix a lot of organic matter evenly into soil (don’t just add them into furrows or drop them into holes in the soil).
- Clay & silty soil: You will see cloudy water and a layer of particles at the bottom of the container.
- Peaty soil: You will see slightly cloudy with particles floating on the surface and a thin layer of particles at the bottom of the container.
- Chalky soil: You will see pale grey water with a layer of gritty, whitish fragments at the bottom of the container.
For more information, ask your local agricultural office representative or experienced farmers or gardeners in your area.
What it’s about: This is a method of examining the plants to know if the soil type and content are appropriate to the plant’s needs.
Why it’s important: Above the ground, what is one way to know that you have soil with good tilth and structure, water supply, and bio-organisms? The answer: vigorous plants.
How to do it:
- Choose some healthy plants that show a uniform size and color.
- Compare them against the normal plant development and health in your area.
- Note: This test is not reliable if you planted late, recently had a drought, or a pest infestation.
Your learning points: This test tells you if your soil is not appropriate for your plant, or if the soil needs enhancements or amendments.
What it’s about: This is a method of examining the roots of plants to know if the soil type and content are appropriate to the plant’s needs.
Why It’s Important: How roots look like tells you a lot about soil quality. Healthy roots indicate that soil has enough water and good drainage, crumbling and good aeration, as well as biological organisms. Here’s a simple procedure:
How to do it: Whether for a farm or garden, springtime is when you can best apply this method.
- Choose a plant in the soil you want to analyze. Any plant, such as a weed, will do.
- Dig in and loosen the soil at maximum root depth.
- Pull it up: pull up the plant you selected for analysis.
- Examine: Look closely at how the root is developing.
Your 3 learning points: Close examination of plant roots can provide you with some key information about your soil. For instance:
- Healthy soil: If you see roots with fine, white strands, the plant is healthy.
- Waterlogged soil: If you see mushy and brown roots, the soil may need better water drainage.
- Infested soil: If you see roots that are shorter or stunted compared to healthy roots, the soil may be infested with pests that gnaw at roots, or other disease-causing pathogens.
What it’s about: This is a way to know the soil’s rate of water drainage or retention in order to determine if this rate is appropriate to your plant’s needs.
Why It’s Important: This test tells you if your soil holds standing or excess water in the soil that can cause plant roots to rot. The soil should drain or retain water at a rate that is appropriate for your plants, and avoid water runoff that can cause loss of soil and nutrients.
How to do it:
- Dig in: Dig a hole in your soil, about 12 inches wide, long, and deep.
- Fill up: Completely fill the hole with water.
- Wait until the water is completely drained.
- Repeat: Again, fill the hole with water.
- Time it: How much time does it take for the water to drain completely?
- Repeat: Repeat the process once every 30 minutes or each the hole is drained.
Your 2 learning points: The second rate of drainage indicates the soil’s drain rate.
- High drain rate: If, at the second round, it took less than two hours for water to empty, then your soil drains quickly.
- You can add organic material to the soil to retain water for longer periods.
- You can switch to plants or crops that require less water.
- Low drain rate: If any water remains in your hole after six hours, then your soil drains slowly.
- You can switch to plants or crops that require more water.
- You can use raised garden beds to improve soil drainage at root areas.
What it’s about: Water availability refers to the amount of water that your soil can hold at any given time.
Why it’s important: Well-aerated, porous soil provides more adequate water supply to plants between waterings by resisting water evaporation.
How to do it: Do this test after a soaking rain.
- Keep track of how much time passes between the rain and your plants exhibiting signs of thirst.
Your 3 learning points:
- If your plants need less watering than what is usual in your area, your soil is properly aerated.
- If your plants require more watering than typical for your region, your soil is probably compacted.
- Different results may show in different areas of a field or in various spots in a garden.
PRO TIP: Go a bit high-tech and insert into the soil (at plant root depth) a device called a tensiometer. This is a sealed, water-filled tube topped with a vacuum gauge and has a porous ceramic bottom to measure soil moisture tension.
What it’s about: This test for soil structure and tilth is also called the Squeeze Test. The term “tilth” refers to a summary of your soil’s composition, drainage, aeration and moisture.
Why It’s Important: Soil rich in organic matter tends to be open and porous where air and water can move freely, and where plants develop healthy and strong roots.
How to do it:
- Choose a part of your field or garden where the soil is not too wet or too dry.
- Dig a hole in damp soil, about 6 to 10 inches deep.
- Pick up a handful of earth.
- Squeeze it: Cup both hands together and squeeze to compress the soil.
- Release it: Stop squeezing and look at the soil in your hand. Does the soil hold its shape?
- Poke it: Gently poke the soil with your finger. Does the soil easily fall apart?
- Observe: Is the soil more cloddy, more powdery, or more granular?
Your learning points:
- The best soil for growing most plants is organic soil with clumps that allow air and water to move around plant roots.
- Thus, if after some slight pressure, different sized crumbs or chunks of soil (aggregates) stay clumped together, the soil is generally healthy and rich in organic aggregates.
- Dry or infertile soil: If it doesn’t form a smooth lump: the soil is too dry, too sandy, or lacking in soil bacteria.
- Sticky soil: If the soil is sticky, slick, and doesn’t break apart when poked: the soil is too hard, too wet, too dense, or too high in clay content.
- Hard soil: If the soil’s aggregates are not easy to break apart: the soil is too hard.
- Spongy soil: If your soil feels like a sponge (springy) then it has high peat content.
- Sandy soil: If your soil feels gritty and easily falls apart, then it has high sand content.
- Silty soil: Soil that holds its shape for a short while and feels smooth to the touch has high loam or silt.
What it’s all about: This is a test to measure degree of soil workability or compaction. This test can help determine when soil particles are too close together (high density) with too little space between them.
Why it’s important: Heavily compacted soil less water infiltration (soil may be too dry) and less water drainage (soil may be too waterlogged), either of which may inhibit the growth of plant roots, water availability, and restricts the circulation of earthworms and other soil microorganisms.
Method 1: The Wire Plunge
How to do it:
- Get a piece of straight metal wire about 2 feet long.
- Plunge the wire firmly but slowly into untilled garden soil.
- Mark the wire at ground level when it stops and bends.
- Pull out the wire.
- Measure the length that sank easily into the soil.
Your learning points:
- Ideal soil: A foot or more of easily penetrable soil is ideal.
- Highly-compacted soil: The sooner the wire bends, the higher the soil compacted level.
- More than 12 inches: If the soil in your garden is easily penetrated for about a foot or more, then the roots of your plants can easily grow and expand outwards.
- Less than 11 inches: You may have high-clay, compacted soil where roots cannot easily grow outwards.
- Some causes of compaction may be walking, weight of machines or vehicles on the ground.
- To soften hard ground, tilling helps. However, aerating by adding coconut coir, compost, or sphagnum peat can provide a more lasting solution.
- Hard clods: If tilling or crumbling your soil results in plate-like clumps or hard clods, then the soil is not very workable. It will prevent the circulation of water and vital nutrients.
What it’s all about: This is a test to measure how many earthworms are working to enrich your soil.
Why It’s Important: More earthworms means more air and water circulation, more organic material, more castings that provide more enzymes, organic matter, bacteria, plant nutrients, enzymes, and more chemicals that bind particles and improve soil tilth.
How to do it: Do this when the soil is damp – not too dry or wet.
- Find: Look for earthworm burrows (see photo here) or casts (see photo here) on the topsoil.
- Dig: Dig 6 inches deep into the soil.
- Count: Count the worms you find in the chunk.
Your learning points:
- Worm count: A big population of earthworms means that your soil is very rich in nutrients. If you find at least five earthworms, that great. Three earthworms mean you’re still okay.
- Zero worm count: If you don’t find any earthworms, the soil may not have enough organic matter for them to feed on. You should treat your soil as a living organism that needs nourishment.
PRO TIP: Earthworms don’t like hot soil, so you won’t find them in the desert or even in fertile gardens in the Southwest.
What it’s all about: This is a test to measure the population of thriving bacteria, fungi, invertebrates and insects that indicate soil quality. They don’t leave casts or burrows like earthworms.
Why it’s important: If your soil contains an abundance of living organisms, they break down more plant residues, create more soil nutrients, and lessen pests or pathogens.
PRO TIP: You can test your soil for organic matter to know the percentage of organic content. Ideal is 5 percent, but a range from 1 to 3 percent works well enough.
How to do it: Note that most soil organisms hide from sunlight. Be gentle when you dig into the soil to find them.
- Dig at least 6 inches deep into the soil In various areas of your garden.
- Turn the soil over gently and sift the materials.
- Watch the soil for about 4 minutes.
- Count the number of insects, spiders, ground beetles, worms, centipedes, beetles and other living organisms you can see.
Your learning points: Healthy soil is full of animal life.
- There should be at least half a dozen bugs, ants, millipedes, and so on.
- If you count less than 10 organisms, your soil is low on animal life.
- If you can, introduce and encourage living organisms to your soil. For instance, the fungus Mycorrhizae will aid your plants in the absorption of water and nutrients and worms will help speed up the composting process and help spread fertilizer through the soil.
But that’s not all. Here’s why you need laboratory technicians to help you with soil analysis.
- One tablespoon of soil contains about 50 billion organisms you can only see with a microscope.
- One gram of soil contains from 10 to 5,000 nematodes, from 1,000 to 500,000 algae, from 1,000 to 500,000 to protozoa, from 5,000 to 1,000,000 fungi, from 1,000,000 to 20,000,000 actinomycetes, and from 3,000,000 to 500,000,000 bacteria.
- Aside from all that, it’s also quite difficult (even impossible) to see microbes, mites, nematodes, springtails, viruses, and yeast in soil.
What it’s about: You want to know the nutrients removed by previous plants, how much to add, and how to avoid over-fertilizing. Also, as we know, problems happen when there’s too little or too much of one nutrient or another.
Why it’s important: By testing your soil, you determine its exact condition so you can fertilize more effectively and economically. Along with hydrogen, oxygen and carbon in air and water, plants need at least 17 soil nutrients. Here’s a quick list:
- 6 Primary Nutrients: Carbon (C) 45%, Oxygen (O) 45%, Hydrogen (H) 6%, Nitrogen (N) 1.5%, Potassium (K) 1%, Phosphorus (P) 0.2%
- 3 Secondary Nutrients: Calcium (Ca) 0.5%, Magnesium (Mg) 0.2%, Sulfur (S) 0.1%
- 9 Micronutrients: Iron (Fe) 0.01%, Chlorine (Cl) 0.01%, Manganese (Mn) 0.005%, Boron (B) 0.002%, Zinc (Zn) 0.002%, Copper (Cu) 0.0006%, Molybdenum (Mo) 0.00001%, Nickel (Ni) 0.00001%, Cobalt (Co) 0.00001%,
How to do it:
- Test your soil every now and then throughout the year.
- Use a soil test kit to assess primary nutrients (N-P-K) as well as pH levels.
- You can buy soil test kits online. Here are some options.
- Analog Test Kits
- Luster Leaf 1880 Rapitest 4-Way Analyzer: can be used to test for soil fertility, water, pH, light; No limit on testing
- 3-in-1 Soil Moisture/Light/pH Tester: does not test for NPK; can be used to test for light, water, pH
- Atree Soil 3-in-1 Soil Tester Kit : does not test for NPK; can be used to test for light, water, pH
- Chemical Test Kits
- Rapitest Premium Soil Test Kit 1663: can be used to test for NPK and pH; 80 total tests
- Luster Leaf 1601 Rapitest Soil Test Kit: can be used to test for NPK and pH; 10 tests for each type
- (Digital) Luster Leaf 1847 Rapitest Digital Plus: Instantly tests pH; can be used for unlimited testing
- Laboratory Test Kit
Your learning points: If you really want to see an accurate picture of your soil analysis, you’ll need to contact a laboratory for soil analysis. Here’s why:
- As you can see from the list of why this test is important, soil nutrients are in very minuscule amounts and are quite difficult to test.
- To make it more complicated, each of these are affected by cations and the soil’s cation exchange capacity (CEC).
- These are best measured using accurate laboratory instruments.
What it’s all about: The basic pH test measures your soil’s acidity and alkalinity. In some cases, it can also measure the major nutrients in the soil.
Why it’s important: The pH level of your garden soil determines how much nutrients your plants will be able to absorb from the soil, the number of friendly soil organisms living in your soi, and so on.
How to do it:
- Buy a pH test kit at a local plant shop, nursery, garden center, a horticultural or agricultural supply store. Follow the instructions on the kit.
- You can also buy pens or meters that can test for soil pH (acidity, alkalinity) and EC (electrical conductivity or soluble salts). You will need to buy a standardizing solution (buffer) as well. Follow instructions for adjustments to environmental conditions.
PRO TIP: Most vegetables and ornamentals grow well in a slightly acidic soil with a pH between 6 and 7.
Your learning points: A DIY test kit can provide you with an estimate of the pH and nutrient levels in your soil. See a photo of the pH scale here. Meanwhile, here’s what you need to know.
- Measurement: Soil pH is measured on a scale from 0 to 14.
- Interpretation: The standard pH level for garden soil is between 4.0 and 8.5.
- Neutral soil: A pH level of 6.7 to 7.3 indicates neutral soil.
- Acidic soil: A pH level of 6.6 or lower indicates acidic soil
- Basic soil: A pH level higher than 7.3 indicates the soil is basic.
- Soil for buffering: A pH level of 6.0 or lower means that you need to do a buffer index to know how much lime will be needed to raise the pH to 6.8.
- Adjustment: Regardless of your soil pH, you can adjust the level to make it more suitable to your plants.
- General measures: Most nutrients and minerals are abundant in soil with a pH between 6.5 and 7. However:
- Type of plants: The correct soil pH depends on the plants you are growing. Some plants flourish in more acidic soil, others in less.
- Type of soil additives: Soil pH testing will tell you if you need to add lime or sulfur to your soil.
- Types of water in the area: Areas with soft water have acid soil while areas with hard water have alkaline soil.
Laboratory Soil Analysis
What’s this about: In addition to easy soil analysis that you can do yourself, you can also send a soil sample to a soil testing laboratory for more accurate results and more complete information about your soil.
Why It’s Important: A laboratory analysis of soil uses chemicals to estimate what and how much nutrients remain available to plants, the potential toxicities and trace minerals in the soil, and identify why plants don’t grow well.
At the same time, the soil analysis can help you save money by recommending specific amounts of water and fertilizer, reduce environmental effects of soil amendments, and protect the environment from contamination by runoff and leaching of excess fertilizers
Who can test my soil? You can consult a State-by-State List of Soil Testing Labs or check out the database of Alternative Soil Testing Laboratories in your state. You can also ask your local cooperative extension office to test your soil sample. In some states, you may have to pay a small fee).
How much does it cost?
A soil nitrate test may cost from $10 to $20. Tests for toxins and environmental cation can cost from $30 to $50.
Importance: Soil analysis is important in effective gardening as well as in large-scale crop farming.
Soil sampling: The proper soil sampling procedure is an important first step in soil analysis. Two methods are described at the start of this article.
Soil analysis: There are different ways to analyze soil, each providing different types of information. At least 14 methods are described in this article.
Laboratory analysis: Laboratory analysis of your soil samples can provide you with more accurate and more complete information about your soil.
That was it from me. I wish you a happy gardening!
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