Soil Testing and Analysis

June 15, 2011 | by Ethan (email) |

Soil testing measures characteristic and the amount of specific nutrients in your lawn or garden. Common tests identify the levels of phosphorus, potassium, calcium, magnesium, sodium, sulfur, copper and zinc within a given sample. Knowing these results equips you to change soil acidity with lime and add appropriate levels of nitrogen, phosphorus and potassium, thereby encouraging a thriving lawn and plant life. This post will explore soil characteristics, testing and how to interpret those results. Click here to skip down to interpreting soil analysis results.

Soil Classification

Soil texture refers to the presence of silt, clay and sand also known as soil separates. Soil separates are distinct because each falls into a certain range of particle sizes. Clay particles have the smallest diameter, followed by silt. Sand particles are the biggest. It makes sense that soil should be classified by the levels of soil separates as each has different chemistry, porosity (water-holding) and permeability (water-providing).

• Clayey Soil
  Clay soil is composed of the smallest particles with a diameter less than 0.002mm. Clay soil is extremely sticky and plastic (moldable) when moist. It’s hard to break up and is often referred to as “Cohesive soil” meaning the particles are tightly packed and adhere to each other. The structure is that of overlapping plates or plate-like. This type of soil has excellent water-holding capacity but very poor water-providing capabilities making it difficult for vegetation to make use of the water. It will often stay saturated long after rainfall or snow has melted. Clay soil is typically alkaline with a pH ranging from 7 – 9. Some flowers like Day Lilies and Black-Eyed Susan actually prefer clayey soil, and Bermuda Grass is hardy enough to grow in clayey soil.
• Silty Soil
  Silty soil particles fall in the middle, ranging from 0.002–0.05mm in diameter. Silt particles are somewhat plastic, but will break into small bits, feeling smooth and silky. These particles are granular, resembling cookie crumbs. Silt is often found on flood plains as it is water-soluble and will travel downstream. Silty soil features decreased porosity and significantly increased permeability as compared to clay soil. pH ranges from acidic to alkaline so a soil analysis is very important. With all these characteristics, a wide variety of plant species can thrive in silty soil.
• Sandy Soil
  Sandy soils fall into a spectrum of very fine to course. All particles are larger than clay or silt, ranging from 0.5 – 1.0mm in diameter. Sandy soil is loose and granular with very weak plasticity. Sandy soil porosity is comparable to clay because sand has several large pores while clay has many, many small pores. The larger pores and increased particle size mean sandy soil is extremely permeable, releasing water very quickly. Vegetation planted in sandy soil needs to be drought resistant as sand releases moisture so readily. The blanket flower is a tough, drought-resistant plant. Fescue and Bermuda Grass are good choices for grass in sandy conditions.

Loam is a term used to describe soil that has relatively equal concentrations of clay, silt and sand (20,40,40 concentration respectively). Loam is ideal for gardening, growing crops and grasses as it has high levels of nutrients, retains waters well and maintains high permeability.

Did you know that every state, Puerto Rico and the Virgin Islands has a state soil? Much like state flowers and birds, a state soil is a soil with special significance. Here in Maryland, the state soil is called Sassafras. It was established in 1901 making it one of the oldest soil series in the U.S. Click here to find your state soil.

Soil Texture Triangle

A soil texture triangle is divided into 12 sections, and is used to standardize soil classification. Each side is scaled for the percent of soil separate found. Clay percentages are mapped from left to right (yellow line). Silt percentages are read from the upper right to lower left (green line) and sand is read from the lower right to the upper left (blue line). You only need the intersection of two soil separate levels to determine the correct assignment. For instance, 30% clay and 40% silt result in a clay loam classification and indicate 30% sand.

Soil Analysis

Soil testing is a means to determine soil fertility. It takes a “snapshot” of the amount of nutrients, composition and level of contaminants present at the time of the sample. It also provides information to optimize your soil in the future.

Field Test Soil Texture
Soil texture can be determined in the field by feeling for sand particles and using plasticity and flexibility to estimate the amount of silt and clay. An experienced hand will be able to provide an instant, accurate soil texture, but most homeowners will require additional help. To try and approximate your soil texture, use this flow chart provided by The National Resources Conservation Service (NRCS).

Send in a Sample
In some areas, homeowners can actually mail in soil samples to be analyzed by a lab. This is a great option as long as the samples are taken appropriately. Samples should be taken at the right time and depth to obtain the correct mix for an accurate reading. Here are some tips to keep in mind:

• Plan ahead- Allow for a few months after testing to take action. For instance, if you apply lime, leave enough time for it to take effect before planting. Keep in mind what you are planting. For instance, cool season grasses should be fertilized in early spring or late fall. Plot a timetable to match.
• Clean equipment- Make sure all the equipment used to gather samples is thoroughly cleaned. Avoid using brass, bronze or galvanized tools as these will contaminate the samples.
• Number of samples- Collect around seven samples from each area to be tested, and mix the subsamples together.
• Separate areas- Keep unique areas separate. For example, don’t group the vegetable garden and front yard together. Test them individually.
• Depth- Most samples should be taken at a four inch depth.

Professional Soil Testing
A professional service will analyze your soil and make recommendations about lime application (quantity and frequency), fertilizer and aeration. Incorporating this information will ensure better growing conditions, improved efficiency and minimize runoff.

Professionals specializing in soil analysis will have access to various test kits to determine factors like pH, levels of soluble salts, phosphorus, potassium, nitrogen, iron, calcium, magnesium, sulfur, organic matter, soil texture and temperature. These kits and supplies can easily cost hundreds of dollars.

Interpreting Soil Test Results

All of the results describe the amount of a nutrient that is available for plant uptake. Most test results will include fertilizer recommendations (nitrogen – phosphorus – potassium), lime recommendations and additional comments.

pH
pH indicates the soil acidity. A value of 6.6 or lower is considered acidic, and higher than 7.3 is considered basic. Neutral soil falls in between. If your soil is acidic, a buffer index will determine how much lime should be applied to bring the pH up to neutral.

Nitrogen (N)
Nitrogen is a primary nutrient for plant growth and is supplied naturally through precipitation, manure and organic matter. Fertilizer can be applied to increase the amount of N present but care should be taken to avoid over-application resulting in excess NO3.

Phosphorus (P), Potassium (K), Zinc (Za), Iron (Fe), Copper (Ca) and Manganese (Ma)
These levels are reported in parts per million (ppm). Phosphorus and potassium should test higher than 40 and 220 respectively. Small quantities of zinc are sufficient and should measure above 0.3. Soil with high pH may have lots of iron, but it is unavailable for plants. Copper and manganese have not been confirmed as a contributing factor in all areas.

Sodium
High levels of sodium can cause soil to be hard and cloddy when dry and to take water very slowly.

Calcium
Soils with a neutral pH usually have adequate calcium. Low calcium can be increased with lime.

Soluble Salts
Soluble salts levels at 2,600 ppm or more are classified as saline. These can be caused by excessive fertilization, insufficient watering and poor drainage.

Organic Matter
Organic matter is reported as a percent of the total soil. Ideally, it would be around 5% however it’s often closer to 1 to 3%.

Soil Triangle image courtesy of U.S. Department of Agriculture