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Understanding a Soil Analysis

 

Good nutrient management starts with a sound understanding of soil properties. The following is a reference guide to understanding the components of your AgSource Laboratories soil analysis. It provides a brief description of the essential nutrients along with the various ranges to allow you to effectively interpret your results.

Soil pH

Soil pH is a measurement that determines if the soil is acidic (pH values below 7.0) or basic (pH values above 7.0).  Most soils have pH values in the range of 4.5 to 8.5, however, slightly acidic conditions usually are the most productive.

Acidic

Neutral

Basic

Strong

Moderate

Slight

 

Slight

Strong

Very Strong

pH 4

pH 5

pH 6

pH 7

pH 8

pH 9

pH 10

Buffer pH

Buffer pH, also referred to as Buffer Index, measures the response of the soil to a known amount of lime. Lime is added to soil to neutralize soil acidity and raise the pH.  The Buffer pH values range downward from 7.5 indicating increasing amounts of lime required to raise the soil pH. The lower the value is below 7.5, the greater the amount of lime required.

Phosphorus

Phosphorus (P) is an essential plant nutrient that is often low in soil and must be added as fertilizer. Phosphorus is a key nutrient for seedling growth because it contributes significantly to healthy seed formation and to initial root development after germination. Phosphorus is also directly related to energy transfers in cells that are vital to all growth processes.

The availability of phosphorus in soil is influenced by pH and mineralogy. Different analysis methods are used to give an index of availability in these various soil conditions. Bray is suited for neutral and low pH soil and Olsen is used on high pH (>7.2) samples. Mehlich 3 is used on most types of soil and across the pH range typically found in cultivated soils. Soil test ranges are similar for Bray and Mehlich 3 and lower for Olsen extractions.

Rating

Bray P, ppm

Mehlich 3 P, ppm

Olsen P, ppm

Very Low

0 - 8

0 - 8

0 - 5

Low

9 - 15

9 - 15

6 - 9

Adequate

16 - 20

16 - 20

10 - 13

High

21 - 30

21 - 30

14 - 18

Very High

> 30

> 30

> 18

 

Potassium

Potassium (K) is another essential plant nutrient that is used in large amounts by plants and is typically added as fertilizer for maximized crop growth. Potassium is involved in controlling the flow of water through plants in transpiration and is also generally associated with winter hardiness and disease resistance. The soil test evaluates the readily available, or exchangeable, potassium content of a soil sample.

The common methods for extracting potassium for a soil test are ammonium acetate or Mehlich 3.  These tests provide identical results.  Other methods include extraction with Bray solution or a Mehlich3 extraction of a moist sample (not dried and ground as is typically done for soil testing methods). The following table provides the interpretations for these potassium extractions.

Rating

Ammonium Acetate and Mehlich 3, ppm

Bray K, ppm

Moist Soil K, ppm

Very Low

0 - 120

0 - 70

0 - 50

Low

121 - 160

71 - 90

51 - 85

Adequate

161 - 200

91 - 110

86 - 120

High

201 - 240

111 - 150

121 - 155

Very High

>240

>150

>155

Organic Matter

Organic Matter (OM) refers to the stable by-products of decomposition that occur in the soil. The organic matter content, expressed as a percent, reflects the ability of the soil to supply nutrients, moisture and other physical benefits to growing plants. Productive soils can range from 0.5% up to 10% organic matter, depending on soil texture, local geographic and prevailing climatic conditions.  Changes in organic matter content happen slowly but increases in OM are related to improvements in the health of the soil.

Nitrate-Nitrogen

Nitrate-Nitrogen (NO3-N) is the amount of available nitrogen, tested as nitrate, present in the soil at the time it was analyzed in the laboratory. Because of it's solubility nitrate can leach rapidly from the soil under wet soil conditions. This mobility makes it difficult to predict how much nitrogen will be present throughout the growing season. However, a sample collected in the spring can provide some guidance on how much additional N might be needed as a side-dressing to supply all the crop requirements.

To convert the results from ppm into pounds/acre (lbs/acre) use the following formula:

Sample depth (in inches) X 0.333 = conversion factor (CF)
CF X NO3-N ppm = NO3-N, pounds/acre

Example:

Soil test = 7 ppm NO3-N
Sample was collected to a depth of 6 inches

6 X 0.333 = 1.99  This is the conversion factor (CF)
1.99 X 7 ppm = 14 pounds/acre of NO3-N

Ammonium-Nitrogen

Ammonium-Nitrogen (NH4-N) is the amount of available nitrogen, tested as ammonium, present in the soil at the time it was analyzed in the laboratory.  When used along with a nitrate-nitrogen test this measure of ammonium-nitrogen provides a complete evaluation of the readily available nitrogen in the soil.  Ammonium is held in the soil by adsorption to the clay surfaces so it does not leach but it is subject to nitrification, the biological process of conversion to nitrate.  Plants can utilize both ammonium and nitrate for their growth.

The conversion from ppm to pounds/acre is the same as for nitrate, as detailed above. 

Available Nitrogen is determined by adding the amount of nitrate-nitrogen (NO3-N) and ammonium-nitrogen (NH4-N) together.  When expressed as lbs/acre it provides a measure of the pounds of nitrogen from fertilizer and mineralized organic sources available to the crop in the layer of soil represented by the depth of sampling.  Results from a deeper sample(s) can be added to the result from a surface sample to provide a measure of the nitrogen available in a soil profile.

Calcium

Calcium (Ca) is an important constituent of plant cell walls, thereby giving overall strength to the plant. Calcium is also essential for good root development and may serve to neutralize some toxic compounds present in the plant. Calcium is abundant in mineral soils with pH above 6.0. Because calcium is the major component of limestone added to raise soil pH, it is adequately supplied to plants in the management of acidic soils as well.  Calcium can be added to soil without raising the soil pH with applications of gypsum.

Magnesium

Magnesium (Mg) is an essential plant nutrient that is typically abundant in soils, especially when soil pH is 6.5 and above. Magnesium is an integral part of the chlorophyll molecule, which means it is essential for photosynthesis. Magnesium is also associated with phosphorus uptake and utilization within the plant. As was noted above, if the soil requires lime to raise the pH, then ground limestone is added as the liming material. If magnesium is low it can be increased in the soil with a dolomitic liming material. 

Rating

Calcium, ppm

Magnesium, ppm

Low

0 - 500

0 - 50

Medium

501 - 2500

51 - 150

High

> 2500

> 150

Sodium

Sodium (Na) analysis provides information for use in reclaiming saline-alkali soils. Whenever Na exceeds 5% of the base saturation of a soil, water infiltration rates can be reduced and plant growth impaired. Gypsum, epsom salts and elemental sulfur are common amendments for aiding water infiltration rates in alkaline and sodium affected soils.

Cation Exchange Capacity

Cation Exchange Capacity (CEC) is a measure of the ability of the soil to store and release nutrients.  Elements such as Ca, Mg, K, Na and hydrogen (H) are held to the soil particles by their positive charges (as cations), attracted to the negative charges of the soil particles.    Because clay and organic matter are the major source of negative charges in the soil the CEC values also reflect the soil’s texture and composition.

Estimated Texture

CEC value, meq/100g

Sand

0 - 7

Loamy Sand

8 - 12

Sandy Loam/Silt Loam

13 - 20

Loam

21 - 28

Silty Clay/Clay Loam

29 - 40

Clay

   > 40

Organic soil (peat)

50 - 100

Base Saturation

Base Saturation (BS%) compares the proportion of the CEC that is filled with basic cations (K, Ca, Mg, Na).  The acidic cation is hydrogen (H) which increases in concentration as the soil becomes more acidic.  The proportion of the base saturation met by each of these cations is an indication of the nutrient supply and condition of the soil.  Typical ranges for each cation are shown below.  If the soil pH is above 7.2 there may be more than 100% base saturation because of the solubility of calcium and magnesium in the soil.

Cation

% Saturation

Potassium (K+)

2 - 7

Calcium (Ca2+)

60 - 75

Magnesium (Mg2+)

10 - 20

Sodium (Na+)

0 - 10

Hydrogen (H+)

0 - 12

Sulfur

Sulfur (S) is an essential plant nutrient because it is an integral part of certain amino acids and, therefore it is necessary in the formation of proteins. Measurements of sulfur in the soil are based on an extraction of sulfate, the soluble, most readily available form. This form is subject to leaching and therefore a soil analysis indicates the available S at sampling time.  Organic matter in the soil provides some of the crop’s requirement as it undergoes normal mineralization during the growing season.

Rating

Sulfur (S), ppm

Very Low

0 - 2

Low

3 - 6

Adequate

7 - 11

High

12 - 16

Very High

> 16

Zinc

Zinc (Zn) regulates energy use and chlorophyll production in plant cells. Because plants only require small amounts of zinc it is considered a micronutrient element. Soil tests for Zn accurately predict if a crop will respond to additions of zinc fertilizer.

Rating

Zinc (Zn), ppm

Very Low

0.0 - 0.5

Low

0.5 - 0.9

Adequate

1.0 - 2.9

High

3.0 - 5.9

Very High

> 5.9

Copper

Copper (Cu) is an important part of chlorophyll production in plants and is essential to many enzymes as well. Copper occurs in low concentrations in soil and deficiencies of copper are often found in very acidic soils with naturally high levels of organic matter (such as peat or muck soils). 

Rating

Copper (Cu), ppm

Very Low

0.0 - 0.2

Low

0.2 - 0.7

Adequate

0.8 - 1.1

High

1.2 - 2.4

Very High

> 2.4

Iron

Iron (Fe) is a common element in many soils.  As a plant micro-nutrient iron is required as an integral part in chlorophyll production and is a part of many enzymes. Testing the soil for extractable iron helps to indicate the probability that iron deficiency may occur.  But iron availability is controlled by soil factors such as pH and oxygen supply around plant roots.

Rating

Iron (Fe), ppm

Very Low

0 - 4

Low

5 - 9

Adequate

10 - 15

High

16 - 24

Very High

> 24

Manganese

Manganese (Mn) activates the enzymes that are involved in photosynthesis. Manganese availability in the soil is influenced by soil pH and organic matter content. Low pH increases manganese availability in soil as does increasing organic matter at higher pH ranges. However, soils that formed from organic material, such as muck soils, are acidic and may have very low levels of plant available manganese.

Rating

Manganese (Mn), ppm

Very Low

0 - 3

Low

4 - 7

Adequate

8 - 11

High

12 - 29

Very High

> 29

Boron

Boron (B) is required in the metabolism and movement of sugar within the plant. Very little of this element is required for plant growth but plants are also susceptible to boron toxicity at relatively low concentrations in the soil.  The availability of this micronutrient is most limited on sandy soils with low organic matter.  Available boron in excess of 5 ppm can be toxic to plants.

Rating

Boron (B), ppm

Very Low

0.0 - 0.2

Low

0.2 - 0.6

Adequate

0.7 - 1.1

High

1.2 - 1.9

Very High

> 1.9

Soluble Salts or EC (Electrical Conductivity)

Soluble Salts or EC (Electrical Conductivity) is a measure of the salts in a soil that are soluble in water.  It is determined by measuring the conductivity of a soil and water slurry in a 1:1 ratio.  Soils have a wide range of salt levels, but generally, high salt levels are associated with soils where rainfall is limited and soils with poor drainage conditions.  The salts accumulate at the soil surface rather than leaching throughout the profile. If irrigation water contains a medium or high amount of salt the accumulation process increases. High salinity, or salt content, can cause damage on salt sensitive crops.

These terms soluble salt and EC in soil are used interchangeably. Soluble salts is usually expressed in mmhos/cm and EC is typically expressed as dS/m but the units are exactly equivalent.

 

Soil Texture

Degree of Salinity

Sand to Loamy Sand

Sandy Loam to Loam

Silty Loam to Clay Loam

Silty Clay Loam to Clay

 

 ------mmhos/cm or dS/m------

Non-saline

 0 - 1.1

 0 - 1.2

 0 - 1.3

 0 - 1.4

Slightly Saline

1.2 - 2.4

1.3 - 2.4

1.4 - 2.5

1.5 - 2.8

Moderately Saline

2.5 - 4.4

2.5 - 4.7

2.6 - 5.0

2.9 - 5.7

Strongly Saline

4.5 - 8.9

4.8 - 9.4

5.1 - 10.0

5.8 - 11.4

Very Strongly Saline

> 8.9

> 9.4

> 10.0

> 11.4

Excess Carbonate

Excess Carbonate is the measure of the amount of free limestone in the soil.  Knowing the relative carbonate content can be important in herbicide selection as well as selection of fertilizer application techniques.  However, changing the amount of excess carbonate in the soil is difficult and economically impossible to do.  The test measures the reactivity of the carbonate in the presence of acid.

Rating

Excess Carbonate

Very Low

Not detected

Low

Some detected

Adequate

Moderately reactive

High

Highly reactive

Very High

Very highly reactive