8 Important Factors for Understanding Soil Test Results

November 14, 2021 11 min read

soil moisture meter - understanding soil testing results

You cleared an area for your garden, picked out what you wanted to grow, collected soil samples, and sent them off to a soil testing laboratory. You're ready to plant your garden, but now you have a paper full of letters and numbers, but no idea what to do with the information.

You can rest assured that you're not alone. Many gardeners end up confused by their soil test results, and they don't know what to do with the results in their hands. It is important to organize your soil test report so you have a better understanding of what your soil needs are.

Soil tests are a great way to check soil conditions and make sure your soil is healthy. The soil test report will tell you if there are nutrient or pH deficiencies in your garden so you can amend them as needed.

This article will explain what the test results mean, and how to use them to improve soil health for better plant growth.

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Understanding Soil Testing Results

Your soil report will be comprised of various sections that break down the components of your soil to a micronutrient level.

The soil report will tell you what the soil is lacking if it has too much of something, and how well the soil is holding nutrients for plant growth.

When you receive your soil report, look for soil pH, soil reaction, and Cation Exchange Capacity (CEC).

If your soil has a high CEC, it will prevent the leaching of minerals from the soil. You will also want to look for an optimal soil pH -- somewhere between 6.0 and 7.0 (slightly acidic).

It is important to have a soil pH that is neither too acidic nor too alkaline because soil that is either of these extremes can be toxic to plants.

In addition, you will want to make sure your soil contains adequate levels of macronutrients and micronutrients before deciding what soil amendments you'll need for your soil.

Let's break down what each section of the soil report means.

understanding pH and soil reaction results on soil test report

pH and Soil Reaction

Soil pH is the measure of soil acidity or alkalinity. It measures on a scale from 1 to 14 with 7 being neutral, meaning there is no soil acidity or soil alkalinity present. More acidic soils have less than 7 pH and soils that are more alkaline have over 7 pH.

When soil is too acidic or too alkaline, it can affect soil life and limit plant growth.  Soil life refers to all the organisms in the soil that help fertility and soil structure by breaking down organic matter and soil particles.  

If soil pH is too low, soil life can be suppressed or even killed off because the soil is too acidic. If soil pH is too high, soil organisms will have a difficult time functioning on your soil because it's so alkaline.

Soil reaction refers to how well the soil holds nutrients for plant growth.

A soil's reaction can be acidic, neutral, or alkaline. A soil with a neutral soil reaction has an even balance of nutrients and soil particles so it holds water evenly and provides a good environment for soil organisms.

Acidic soil reactions are generally considered to be those below 6 on the pH scale; however, if your soil has a soil reaction of 4.5 or below, it is considered extremely acidic.

This soil is generally unable to grow most vegetables because soil life cannot survive in this soil environment.

Alkaline soil reaction refers to a soil that has a pH above 7-7.5. It may be difficult for many soil organisms to live in alkaline soil because soil particles are bound together tightly by these soils.

Other sections of the soil report will be referenced to soil reaction, so it's important to know if your soil reaction is acidic, neutral, or alkaline.

Most vegetables prefer a neutral soil pH; however, beets require acidic soil (4.5-6.0) and tomatoes need slightly more alkaline soil (6.5-8.0).

If your soil's pH falls outside this optimum range, you may have issues getting your soil to the ideal sweet spot for most vegetables.

Proper Soil pH Level for Vegetable Plants

Acidic (pH 4.5-6) Beets, Broccoli, Cabbage, Carrots, Cauliflower, Celery Root, Collards, Kale & Spinach

Alkaline (pH 6.5-7.0) Beans, Squash, tomatoes, and Peas

Neutral (pH 7.0) Corn, Okra, Onions & Garlic, Peppers & Eggplant, Potatoes, Radishes, Turnips, and Watermelon.

macro nutrients - understanding soil test results

Macronutrients

Macronutrients are the nutrients plants require in the largest quantities. These three elements are key for proper plant development and growth (these also include moisture content).

Nitrogen(N) is needed to build new leaves, stems, and roots. Nitrogen deficiency may cause yellowing or browning of leaf tissue with purplish stems. Plants may be stunted or slowly growing.

Phosphorus(P) is required by plants for root formation and good fruit development. Phosphorus levels may decrease quicker than nitrogen because it shows up in higher values when testing your soil.

It is important to ensure fertility and balance as soil phosphorus deficiencies can cause poor growth and fruit drop.

Potassium (K)is vital for the development of roots, fruit, and tubers; good disease resistance; proper structure; and regulation of water uptake.

Potassium deficiency leads to smaller plants with a general yellowing or browning of the leaves. Plants may also be stunted and less resistant to disease.

secondary macronutrients

Secondary Macronutrients

Calcium, magnesium, and sulfur are essential nutrients for plants. They're also called "secondary" nutrients because they're required in smaller quantities than nitrogen, phosphorus, and potassium - the primary nutrients that are used by plants.

Calcium(Ca) is key in regulating cell division that affects the overall growth of the plant as well as leaf production.

It also aids in photosynthesis, controls transpiration ratio through stomata, increases water movement between cells, assists in water absorption and utilization by cells, reduces chlorophyll breakdown during senescence (aging), regulates starch synthesis, and sugar content. 

Calcium deficiency in plants may cause necrosis (dying tissue) of young leaves and blossom petals, leaf curling or cupping, fruit cracking, abscission (premature leaf drop), poor roots development, and crooked stems.

Calcium can be found naturally in both organic sources like bone meal and limestone rock dust.

Sulfur(S) assists in root formation, chlorophyll production, and photosynthesis. It also encourages the growth of seed leaves and the proper utilization of nitrogen.

Sulfur helps to regulate enzymes in the process of producing energy from sugars for plant growth. It also aids in bud development and fruit set as well as leaf formation.

Sulfur deficiencies may cause purplish stems on older plants, reduced growth, delayed maturity or slow maturation, flower drop, yellowing between veins and fruits that have a hollow core with brown edges.

The deficiency may not be severe enough to show up on leaf tissue but can be indicated by what happens to flowers or seeds along with poor nutrient uptake during germination.

Iron sulfate is used as an iron source when sulfur is deficient in your soil.

Magnesium(Mg) is a co-factor (a substance needed to carry out a specific function in the body) for numerous enzymes and plays an important role in the regulation of plant metabolism.

Magnesium deficiency may cause interveinal chlorosis on older leaves, yellowing between veins, necrosis (dying tissue), stunting, or poor root formation.

micro nutrients - understanding soil test results

Micronutrients

Iron(Fe) functions as a catalyst for chlorophyll production and affects photosynthesis by transporting electrons from one enzyme to another during photosynthesis. It increases cell division which improves overall plant growth.

Iron levels vary with soil pH level within your soil; it is generally low at neutral pH but high at acidic or alkaline pH levels. 

Iron deficiency in plants may cause yellowing of leaf veins and brown tips, interveinal chlorosis (yellowing between the veins), and failure to make head leaves.

Iron is most commonly deficient in acidic soils with sandy or loamy textures; however, it can be found naturally in organic sources like colloidal phosphate.

Copper(Cu) is essential for the enzyme superoxide dismutase, which protects cells from damage by free radicals and regulates overall plant metabolism.

Copper aids in photosynthesis, chlorophyll production, and proper fruit coloration while reducing sugar content during senescence (aging).

It also plays a vital role in opening stomata to regulate transpiration and water movement between cells.

Manganese(Mn) is a co-factor that aids in photosynthesis by regulating enzymes.

Manganese deficiency may cause premature leaf drop, weak stems, and roots, smaller leaves and seeds along with silvering of older leaves due to chlorosis (yellowing).

Manganese can be found naturally in humic acid and manganous sulfate.

Zinc(Zn) is a co-factor for many enzymes that regulate overall plant metabolism. It controls chlorophyll production, photosynthesis, fruit and seed development, growth, and longevity of leaves.

Zinc aids in the absorption and transfer of nutrients like phosphorus from soil to your plants' cells while helping to reduce transpiration loss through stomata or plant pores.

Zinc also brightens and stabilizes the color of the fruit, increases the yield and size of crops, improves flavor and aroma while increasing disease resistance.

Zinc deficiency may cause marginal chlorosis (yellowing) between leaf veins along with stunting or dwarfing of older leaves. In many cases, zinc deficiency appears as a combination of multiple nutrient deficiencies.

Zinc can be found naturally in zinc sulfate, zinc oxide, and many other chemical forms as well as organic sources like composted plant material.

Zinc deficiency is most common in acidic sandy soils or highly weathered soils with low soil pH levels but may also occur in alkaline soils.

It develops when there is insufficient soil phosphorus available for uptake and is easily corrected by the addition of zinc.

Boron(B) is a co-factor for numerous enzymes associated with plant growth and metabolism. Boron deficiency shows a wide range of symptoms dependent on plant species, varieties, and soil pH level.

It may cause stunted growth, lack or delayed flowering, reduced root formation along with dark or thickened stems and leaves.

Boron occurs naturally in boric acid, sodium borate, and many other chemical forms.

Boron deficiency is most common in sandy soils with low pH levels but can occur in acidic or alkaline soils depending on the availability of phosphate (PO4) for uptake by plant roots.

Chloride(CI) is a co-factor for the synthesis of chlorophyll. Chloride deficiency results in light yellow veins combined with brown spots on leaves and leaf drop or stunted growth.

Chloride occurs naturally from sodium chloride (table salt). Low reported levels in Florida are often associated with stream weathering of bedrock materials containing feldspars, which may contain anywhere from 4%-6% chloride.

Nickel(Ni) aids in the production of enzymes involved with photosynthesis. Nutrient deficiencies are caused by inadequate supply from soil and absorption by plants resulting in a lack of growth, lowered yields, and reduced quality and quantity.

Molybdenum(Mo) functions as a co-factor in the enzyme nitrate reductase, which controls how fast nitrogen is converted to a usable form. Molybdenum deficiency may cause leaves with interveinal chlorosis and may stunt growth.

Molybdenum can be found naturally; it is added to some chemical fertilizers as well as organic sources like composted plant material.

Molybdenum deficiency is most common in acidic sandy soils with low pH levels but may occur in alkaline and clay soils if phosphate (PO4) availability for uptake by plants roots is restricted.

organic matter - compost - understanding soil test results

Organic Matter

Organic matter is not a nutrient, but it helps keep soil healthy. It provides the physical structure for the soil and holds nutrients in the solution. Organic matter also keeps moisture in the soil.

You can add organic matter to your soil by adding compost, rotted animal manure, or any other materials that contain high amounts of carbon.

These materials store nutrients and organic matter in a form your plants can use without the addition of fertilizers.

Soluble Salts

Excess soluble salts can interfere with the availability of other nutrients in the soil. This happens when people use too much water, or when the soil particles leak into the root zone.

Deficiencies in soluble salts are very uncommon because plants can take up nutrients through their roots, even if they have a lot of salt.

Soluble salt in soil can be corrected by adding gypsum to the soil. This will help leach the salt through the plant root zone.

You can also apply organic material to raise a saline soil profile.

Cation Exchange Capacity (CEC)

The Cation Exchange Capacity (CEC) is a measure of how well the soil can hold on to plant nutrients. It's based on the amount of positively charged particles in the soil and their effect on other nutrient ions.

These "bases" are mainly clay-like materials such as those found in kaolinite, bentonite, and vermiculite.

Soil texture is the most important factor in determining a soil's CEC. As the percentage of clay-like material increases, so does the CEC.

Generally, most soils with more than 20% clay have good to excellent levels. Soils that contain high percentages of organic matter (which are negatively charged) can reduce the CEC.

Alkaline soils have higher CEC, while acidic soils are low CEC soils.

CEC should be tested once every few years. If the soil hasn't been tested for a long time, it's beneficial to have the soil tested again.

This will ensure that you're using the right soil amendments (such as lime or sulfur) and fertilization practices for your soil type.

High Cation Exchange Capacity is good for the soil. It strengthens the soil and makes it easy to grow plants. High Cation Exchange Capacity can also lower the pH of the soil.

Low cation exchange capacity is bad for the soil, which means that it doesn't hold water well. It also means that it leaches out important minerals.

This is bad for gardeners because it is very difficult to grow anything in this kind of soil.

Cation Exchange Capacity soil can be corrected by adding soil amendments such as lime, sulfur, and iron. This will help decrease the soil pH.

You should also add soil amendments like peat moss, compost, and leaf mold to increase CEC.

improve your soil - understanding soil test results

Improving Your Soil

The soil analysis will give you a lot of data about the nutrient levels in your soil, but now what do you do with this information?

Each state has a cooperative extension office that is managed by a state university.

This is also the office the manages the county master gardener program.

You can contact your county extension agent and they will be able to give you exact recommendations for the amount of fertilizer to apply based upon your area and the test results from your garden.

Conclusion

Soil tests are an important tool for understanding soil composition.

The soil test report can be used to determine what nutrients your soil may need, how much fertilizer should be applied, and whether or not you have high cation exchange capacity soil.

It's also a helpful way of determining if there are any deficiencies in soluble salts that might require correction with gypsum or other amendments.

Soil testing is something worth doing every few years to ensure the health of your garden!

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frequently asked questions - understanding soil test results

Frequently Asked Questions

Where can I have my soil tested?

If you are located in the United States, your state agricultural research university should have an extension office located in your county. They typically manage the Master Gardener program and provide soil testing as a service.

There are also soil testing kits you can purchase that will provide materials and instructions for collecting your soil. The kit includes a pre-paid envelope so you can mail your soil sample to the lap.

Soil test results will be available online along with fertilizer recommendations based on your soil.

What do my soil test results mean?

Your soil test results will tell you what nutrients your soil is lacking. It also gauges the pH level and soil reaction.

How often should soil be tested?

Your soil should be tested one or two times every year, especially if it hasn't been done in a while. Testing your soil once a month will ensure that you're using the right soil amendments and fertilization practices for your soil type.

How do I amend the soil?

There are two options for soil amendment: soil sterilization and soil enrichment. To enrichen soil, add materials that contain high amounts of carbon (such as compost). This will increase the soil's organic matter and supply nutrients for your plants to use over time. For soil sterilization, add soil amendments such as lime (to increase pH) and sulfur (to decrease pH).

How do I know if there are soil deficiencies?

If plants die or fail to produce fruit, your soil might be deficient in nutrients. You should send a soil sample to the lab for testing. If soil is deficient in one or more soil nutrients, a soil test report will tell you what soil amendments need to be added and how much to add.

How do I collect soil?

Collect soil samples from the top 12 inches of soil using a soil probe or shovel. Be sure that it's recently watered -- the soil is easier to sample when it's wet. Collect soil samples from at least four locations around your garden. Keep the samples separated.

What are normal soil levels?

Your soil should have more than 2% organic matter, a soil pH between 6.0 and 7.0, and soil reaction between 4.5 (very slightly acidic) to 8.0 (slightly alkaline). Your soil also needs to contain two or more of the following: calcium, magnesium, potassium, phosphorus, and sulfur.


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