Soils 101 and Understanding the Soil Analysis Michael J. Curry, BS, PLA, ASLA GreenSite, Inc. [email protected].

Soils 101 and Understanding the Soil Analysis

Michael J. Curry, BS, PLA, ASLAGreenSite, [email protected]

The Ideal Soil Composition

The Soil Ecosystem-

It is important to understand the complex relationships within the living system known as soil. Soil environments vary by location in terms of content, structure and chemical make-up.

Critical aspects of soil Physical properties

Structure, Texture, and Density Soil Profile Soil layers interface

Chemical properties pH, EC, CEC, C:N, and Nutrients

Organic properties Organic Matter Soil Biology

Soil Texture-

Refers to the relative proportion of mineral particles, of various sizes in a given soil.

Soil Texture

Soil Structure and Aggregation

Structure refers to the arrangement of soil particles, their shape, size and stability.

This plays a major role in how the soil functions.

Large aggregations called Peds!

Sand, silt, clay, and OM particles glued together

Bulk Density or Compaction-

Bulk density is a measure of a soils mass per unit volume of soil. It is used as a measure of soil wetness, volumetric water content, and porosity. Factors that influence the measurement include; organic matter content, soil porosity, and the soil structure. These factors will in turn control hydraulic conductivity.

Bulk density is used to used to calculate the total porosity. The higher the bulk density the more compacted the soil. A soil that has a well developed structure will become less

dense as porosity increases; as a result the bulk density of the soil will decrease.

Permeability and Porosity

Permeability (aka-Saturated Hydraulic Conductivity (ksat) & Infiltration Rate) is the ability of water to move down through soil and at what rate.

Soil porosity is the quantity of voids in a soil sample.

Permeability and Porosity are directly influenced by the soils texture, structure, and density as well as the type and distribution of organic matter.

The number of pores as well as the size of pores affect permeability.

Larger pores allow for more movement and more readily available water for plant growth, but smaller pores do not. However, the largest pores are not necessarily most conducive to growth.

Soils with a fine texture hold more water but medium-textured soils are actually best for growing plants.

Soil Density and Effects of Compaction

Type of Material Bulk Density (grams/cm3)

Normal soils 1.0 to 1.6

Soils with restricted root growth 1.4 to 1.6

Bricks 1.4 to 2.3

Soil commonly found at construction sites 1.7 to 2.2

Total Pore Space % Bulk Density (grams/cm3)

58 1.1

55 1.2

51 1.3

47 1.4

43 1.5

40 1.6

36 1.7

Chemical Properties

The chemical composition of soils are important in soil and plant health. These affect how nutrients circulate through soil.

Chemical composition depends on pH or acidity content, soil structure and the chemical activities that take place between soils, soil organisms, and plants.

The chemical properties of soil are easier to modify than physical properties, especially after the soil is installed.

Therefore, design emphasis should be placed on soil physical properties.

Nutrients-

There are 14 plant nutrients that are essential for plant growth.

There are only 3 that you need to worry about most of the time. Nitrogen Phosphorus Potassium

pH

• pH is one of the most important chemical properties. It affects nutrient availability and microbial activity. Most plants and microorganisms prefer a pH range of 6-7.

• pH is a measure of the active hydrogen in soil. The presence or absence of hydrogen determines whether the soil is acidic or alkaline.

It is important to know the optimum pH range for the plants to be grown, however…

Don’t expect a pH range of 6-7 if you are in the SW US or along the limestone belt from Western Massachusetts to Iowa. You probably won’t get there.

Plants tend to be more tolerant to alkaline, rather than acidic soils.

Buffer pH (or Buffer Index)

• Buffer pH is the resulting sample pH after a liming material is added. This liming material is called the buffer solution and it acts as an extremely fast- acting lime. Each soil sample receives the same amount of buffering solution; therefore the resulting pH is different for each sample. To determine a lime recommendation, we look at the difference between the original soil pH and the ending pH after the buffering solution had reacted with the soil.

 • If the difference between the two pH measurements is large, it

means that the soil pH is easily changed, and a low rate of lime will be sufficient. If the soil pH changes only a small amount after the buffering solution has reacted, it means that the soil pH is difficult to change and a larger lime addition is needed to reach the desired pH for the crop.

Lowering pH

Know that lowering pH of alkaline soils is difficult and often unnecessary.

Sulfur products can be used but this is not sustainable.

Nutrient deficiencies associated with high pH are often not present. This depends on the sensitivity of the plant (e.g. Pin Oak). It may be more economical and effective to treat for nutrient deficiencies with fertilizer rather than continually trying to adjust the pH.

Bio-Stimulants can also be used to aid the plant in nutrient uptake

You have to know what is driving the pH up. If you have even a small amount of free calcium carbonate in the soil, reducing the pH will be

impractical, if not impossible!

Excess Carbonate

• This measures the amount of free limestone in the soil.

• This test is reported as: Very Low, Low, Medium, High, Very High. As the rating increases so the amount of free limestone.

• Changing the amount of excess carbonate in the soil is difficult and

economically impossible to do. However, it can be important in herbicide selection as well as selection of fertilizer application techniques.

Soluble Salt or Salinity (EC) -

Electrical Conductivity (EC) is a measure of the level of soluble salts a soil contains.

Problems with irrigated soils can occur when there are elevated evaporation rates and low rainfall. This causes salts to build up.

Buildup can also occur due to irrigation water, compost, manure and fertilizers.

It is possible to leach the salt by gradually applying surplus water.

Saturated Paste Extract dSm or mmho/cm

Low Moderate High Very high

< 1.5 1.6-3.9 4-5 > 5

Exchangeable Sodium (Na)-

High sodium levels in the soil can result from poor drainage and soil compaction issues.

Excess sodium in soil can impede plant roots from taking in important minerals such as calcium, potassium and magnesium.

You can manage high levels of sodium by maintaining optimal levels of calcium in the soil.

Gypsum, or calcium sulfate, is often used in sodic soils to remove sodium and help balance pH levels.

Sodium Index is a measure of the portion of the CEC occupied by sodium; a sodium level of less than 15% is acceptable and levels of greater than 15% are excessive and harmful to plants not adapted to saline conditions.

Cation Exchange Capacity (CEC)-

The total amount of exchangeable cations that a particular material or soil can adsorb at a given pH.

Exchangeable cations are held mainly on the surface of colloids of clay and humus, and are measured in milligram-equivalents per 100 g of material or soil.

In general, the higher the CEC, the higher the soil fertility.

Carbon to Nitrogen Ratio

The C/N ratio (C:N) or carbon-to-nitrogen ratio is a ratio of the mass of carbon to the mass of nitrogen in a substance. Carbon-to-nitrogen ratios are an indicator for nitrogen limitation of plants and other organisms.

The carbon to nitrogen ratio for natural soils is typically between 10:1 and 20:1.

High C:N causes Nitrogen draft or pulling N out of the soil to feed the decomposition process.

Percent Base Saturation

The ratio of quantity of exchangeable bases to cation exchange capacity. It is a measure of the amount of electron charges that are occupied by the cations: primarily calcium, magnesium, and potassium vs. aluminum and hydrogen.

A high base saturation reduces soil acidity (or increases alkalinity) and increases supply of other plant nutrients.

By comparing the actual percent with the suggested percentage an idea of which kind of soil amendment (Lime, Gypsum, Sulfur) may or may not be needed.

Nitrate (NO3)

• Nitrate-Nitrogen is the amount of available nitrogen present in the soil at the time it was analyzed in the laboratory.

• Because of it's solubility it can leach rapidly on various soil conditions. This mobility makes it difficult to predict how much nitrogen will be present through out the growing season.

• However, it can be a useful tool for determining nitrogen utilization efficiencies at the end of the growing season.

Phosphorus (P)

Two types of phosphorus extractions are used for analysis, type is determine by the soil pH.

If pH is < 7.2 a Bray I extraction is used, > than 7.2 an Olsen extraction is used.

The interpretation of the two methods are different, the following table defines the two.

Rating bray p (pH<7.2) ppm olsen p (ph>7.1) ppm

Low 1-15 1-19

Low to Adequate 15-25 10-15

Adequate 26-40 16-24

High > 40 > 24

Potassium (K)

• Potassium is a cation which is held on the soil's exchange sites. • The form of potassium extracted is the readily available K. • The following table provides a generalized interpretation for

potassium. Soil textures have a great influence on it's availability. With sand textures high levels of K may be difficult to obtain because of it's ability to leach.

Rating Potassium (ppm)

Low 1-120

Low to Adequate 121-190

Adequate 191-300

High > 400

Zinc – Manganese – Copper - Iron

Below are the tables which gives a general interpretation for these micro-nutrients.

Rating Zinc (ppm)

Manganese (ppm) Copper (ppm) Iron (ppm)

Low 0.1 - 0.8 0.1 - 2.5 0.1 - 0.4 0.1 - 4.5

Low to Adequate 0.9 - 1.2 2.6 - 4.0 0.5 - 0.9 4.6 - 7.0

Adequate to High 1.3 - 3.0 4.1 - 12.0 1.0 - 2.0 7.1 - 20.0

High > 5.0 > 50.0 > 2.0 > 70.0

Sulfur - Boron

• Below are the interpretation tables for sulfur and boron. • Both of these elements are soluble in water and subject to leaching.

Rating Sulfur (ppm) Boron (ppm)

Low 1 - 4 0.1 - 0.5

Low to Adequate 5 - 9 0.0 - 0.9

Adequate 10 - 25 1.0 – 1.5

High > 25 > 2.0

Organic Properties & Soil Biology

A soils organic matter is derived from living things. Decomposer organisms in the soil break down the wastes and remains of plants, animals and other organisms, a process that enriches the soil with nutrients, such as nitrogen and phosphorus, and creates humus. Not only is humus a key component of soil structure, it also retains water and nutrients that plants and other organisms in the soil need.

A soil ecosystem's health depends on having adequate organic matter

So an ongoing supply

of fresh organic matter

is needed to replace

what the living things

in soil consume.

Organic Properties & Soil Biology-

Soil microorganisms play a critical role in soil health. Fungi and bacteria decompose organic material, and some work symbiotically with plants, providing them with nitrogen or enabling them to absorb more water and nutrients (mycorrhizae).

The activity of soil organisms is influenced by soil temperature, available oxygen, soil pH, and the amount and type of organic matter.

Microbial activity increases with temperature, available oxygen, and with the introduction of organic matter as the energy source to support microbial growth.

Any activity or practice that encourages soil microbial activity will enhance soil health!!!

Soil Sampling and Testing Laboratories-

Sampling methods and the lab you use will vary depending on the material, and what you are testing for.

There are many things to consider, among them are: Chemical Properties Physical Properties Biological properties Toxic compounds Particle size and distribution Structural capacity In stockpiles In place at grade Etc…

Soil Test Analysis

Determine what tests you will need to be performed on the samples.

Determine what lab you will need to send the samples to.

Be sure the samples are collected according to the quantity and method required by the lab, and the testing to be performed.

Submit with plenty of lead time for the specific project.

Soil Sampling

Promotes environmental quality and stewardship, encourages plant growth and health by diagnosing and providing appropriate rates and recommendations for fertilizer and amendments, and SAVES MONEY!

A soil sample must be taken at the right time and in the right way. The tools used, the area sampled, the depth and the correct mix of the sample, the information provided, and packaging all influence the quality of the sample. 

Sample and test soil before starting any new planting.

Sample and test existing and established areas every 3-4 years.

Know that there are different methods

for soil testing that will produce different

results. And different labs often come up

with varying results even when using the

same test methodologies.

You should use a lab that you and/or your soils consultant are familiar and confident with. You will get used to reading a test report you see the same format and methodologies over & over.

What is important is the interpretation. Soil test labs and consultants should make interpretations on the results relative to the test methods they are using.

Soil Testing

Sampling Tools-

Soil Sampling

Understanding the Soil Analysis Report

Reading and understanding a soil analysis test report can be a challenge because the information is detailed and complex, but it can help you optimize treatment and how you amend and maintain the soil.

Correctly using a soil analysis report aids in using the right amounts

of fertilizers, chemicals, and amendments.

Working with a good lab and soils consultant that can provide you with explanations and recommendations is particularly helpful.

The D50 size, called the median grain size, is the grain diameter for which half the sample (by weight) is smaller and half is larger.

Uniformity Coefficient Cu = D60/D10 is 60. Should be 2-4 in a sand based mix. The higher the value, the less uniform, and the greater potential for packing.

Organic Matter Amendment

Supports soil organisms - Restores soil life

Buffers pH, acid or alkaline toward optimal 6.3-6.8

Reduces bulk density (compaction)

Improves water holding capacity

Improves soil structure Increases CEC, nutrient

storage and availability This will happen…over time!

Restore soil life to restore soil functions

Soil biology and the benefits: Improve soils structure Natural nutrient cycling Increased nutrient uptake Plant disease protection Reduction of pests Elimination of synthetic inputs Bio-filtration Erosion control Stormwater detention and moisture retention Reduction of water use Increase in living diversity in the landscape Increase in carbon sequestration into soil Quicker plant acclimation

Organic matter (Carbon) kickstarts the soil ecosystem by providing food and home for organisms!

What is % Organic Matter????

OM is reported in a lab test by loss-on-ignition method.

Most composts and plant materials are 30-60% OM by this method (or 40-70% mineral).

Know the characteristics of the materials you are working with and match the C:N and nutrients of the amendment to your plant needs

Amending Soils with OM-

Adding 10% compost (by moist volume) into the soil does not increase the Soil Organic Matter (SOM) by 10%. It will raise the tested SOM by only 1-3% (depending on the organic content of the compost, and it’s dry density relative to the heavier soils density).

Use coarse compost or Pine Bark fines to amend clay and other fine textured soils.

Add most compost to the upper 8-12” of soil to mimic a natural profile. Rip a little compost deeper when subsoiling to restart the soil life.

Too much compost too deep in the soil profile promotes anaerobic conditions

One “Size” does not fit all

You may need multiple organic soil amendments

Fine OM Coarse OM

References and Resources

Urban, James. Up by Roots, International Society of Arboriculture, 2008

Soil Science Society of America www.iheartsoil.org

Turf and Soil Diagnostics www.turfdiag.com

AgSource Harris Laboratories www.agsource.com

Soil and Plant Laboratoryhttp://waypointanalytical.com/

Michael J Curry, BS, PLA, ASLA , GRP [email protected]