University of Pittsburgh at Bradford
Science In Motion
Chem/Bio Lab 028
Soil Macronutrient Testing
Introduction:
Soil is an often overlooked and abused natural resource. Just as life is dependent
on air and water, life is also dependent on soil. Soil provides the nutrients needed for
plant growth, houses microorganisms responsible for decomposition of organic matter, and
retains and purifies water.
There are five factors that determine the quality and composition of natural soil:
1. living matter (plants, animals, and microorganisms)
2. climate (cold, heat, snow, rainfall, and wind)
3. parent materials (fineness of particle size, chemical composition)
4. relief (slope and land form), and
5. time.
Because of different climates, parent material, etc., there are many different types of
soils. However, three main components are found in most soil types – sand, silt, and clay.
These components of soil are referred to as fractions. When testing soil composition, we
are concerned with how much of each of the three fractions is present in the soil sample.
A fourth fraction, organic matter, is important in determining the biological, chemical, and
physical characteristics of the soil but is not used to determine the soil texture.
The components of soil are arranged in a series of zones called soil horizons. Each horizon
has a distinct texture and composition. Mature soils have at least three horizons while
new or underdeveloped soils may have no horizons.
Adapted from Lamotte Soil Testing Kit
1
Good soil management practices include both chemical and physical conservation of the soil
to optimize its growth capacity. Plants require both mineral and non-mineral nutrients for
growth. Hydrogen, oxygen, and carbon are non-mineral nutrients plants obtain form the
air. Mineral nutrients, found in soil, are dissolved by water and absorbed through a plant’s
roots. The three primary macronutrients (macro – needed in large quantities) required for
growth are nitrogen (N), phosphorous (P), and potassium (K). Another factor that affects
plant growth is soil pH. The pH of the soil will determine the availability of the nutrients
needed for growth. Macronutrients tend to be less available in soils with low pH.
During this lab activity students will learn techniques for soil collection and preparation
for soil testing, and will perform tests to determine the composition of the soil, as well as
soil pH, potassium, phosphorous, and nitrogen.
Lab Activity 1: Soil Texture Unit
The texture of the soil is dependent on its composition – percent sand, silt, and clay. Since
these can occur in varying amounts, there are many different types of soil textures. A
soil textural triangle (below) can be used to determine the soil type
(http://nowlin.css.msu.edu/software/triangle_form.html)
The amount of time required for particles of different size to settle in the soil separation
tubes is the basis of this test. Sand particles are the largest in size and clay particles are
the finest. Clay soils are considered finely textured while sandy soils are considered
Adapted from Lamotte Soil Testing Kit
2
course textured. Sandy soils hold more air and less water and nutrients while clay helps
increase water and nutrient retention. Too much clay however will make the soil compact
and water logged. The ideal soil will have a mixture of the three fractions.
To determine the soil texture, 50ml soil texture separation tubes are used. The three
different fractions will be separated by placing 15ml of the soil sample into one tube along
with Texture Dispersing Reagent (TDS) and water. The TDS will help separate the
fractions and time will allow the different particle sizes to precipitate to the base of the
tube(s).
Lab Activity 2: pH Module
pH refers to whether a substance is acidic or basic. The scale ranges from zero to 14. pH
of 7.0 is neutral with values < 7.0 being acidic and values > 7.0 being basic. pH affects the
availability of nutrients to plants so therefore can be used to determine the production
potential of the soil. Low pH levels prohibit plant growth. Soil with low pH levels can be
treated with lime to increase pH and hence increase plant growth. pH preferences of
plants can vary, though for most plants the desirable pH range is 6.0 – 6.5. At this
level, microbial growth is also at its maximum.
To determine the pH of soil, the soil is mixed in a flocculating reagent. The resulting
solution is then treated with a standardized pH indicator that reacts with the solution to
create a different color for different pH values. The colored solution is then placed in
color comparator to assign the correct pH value of the solution.
Lab Activity 3: Nitrogen Module
Nitrogen is one of the primary macronutrients required by all living cells. It is a
component of proteins, enzymes, and DNA and is involved in metabolic processes and
energy transfer. It is also found in chlorophyll, the green pigment in plants responsible for
photosynthesis. Nitrogen enhances plant growth, improves the quality of crops, and
increases seed and fruit production. Nitrogen in the soil is usually supplied by
decomposition of organic material, commercial fertilizers, and nitrogen-fixing bacteria.
The amount of nitrogen present in soil is reported in pounds per acre. The desirable
amount varies between crops however; too much nitrogen can have adverse effects.
To test nitrogen level in soil, a small amount of the soil sample is mixed with universal
extracting solution. The extracting solution removes minerals from the soil. The soil is
Adapted from Lamotte Soil Testing Kit
3
filtered from the suspension. The solution is then tested with nitrate test reagents. This
results in a color change in the solution that is compared to standards printed on a color
chart.
Lab Activity 4: Phosphorus Module
Phosphorus is also essential in photosynthesis and is involved in energy transfer. It
improves root development, rapid growth, and encourages blooming. Phosphorus also
increases resistance to disease. An excess amount of phosphorus does not harm plants.
Phosphorus also improves microbial activity in the soil. Phosphorus can be added to soil
through fertilizers and bone meal. Soils should contain at least 75 pounds of
phosphorus per acre.
To test phosphorus level in soil, a small amount of the soil sample is mixed with universal
extracting solution. The extracting solution removes minerals from the soil. The soil is
filtered from the suspension. The solution is then tested with phosphorus test reagents.
This results in a color change in the solution that is compared to standards printed on a
color chart.
Lab Activity 5: Potassium Module
Potassium plays a vital role in the physiological and biochemical functions of plants. It is
absorbed in large amounts by plants. It is used to build proteins; increases resistance to
disease by strengthening stalks and stems; increases cuticle (waxy layer) to prevent water
loss; helps prevent wilting; and enhances fruit size, flavor, texture, and development. The
presence of clay in the soil increases the amount of potassium present. Potassium can be
added to soil via fertilizers and organic material. The amount of potassium required in soil
depends on the crop. The amount of potassium present is reported in pounds per acre.
To test potassium level in soil, a small amount of the soil sample is mixed with universal
extracting solution. The extracting solution removes minerals from the soil. The soil is
filtered from the suspension. The solution is then tested with potassium test reagents.
The treated test sample will gradually be added to a test tube until the black line on a
“potash” plate is no longer visible. The amount of potassium present will be represented by
the amount of solution required to block out the line.
Adapted from Lamotte Soil Testing Kit
4
Lab Activity 6: Life in the Soil
Various organisms, including bacteria and fungi, make their homes in the soil. Many of
these are beneficial to the soil and to the growth of plants. In this activity you will
observe and make note of any organisms present is a small sample of the soil being tested
using a stereoscope.
Soil Analysis Report
NAME: _________________________________________________
CLASS: _______________________ DATE: __________________
SAMPLE Number _______________
Source _______________
CHARACTERISTICS
Color
______________
Odor
______________
ANALYSIS
pH
Nitrogen
Phosphorus
Potassium
_______________
_______________
_______________
_______________
CLASSIFICATION
% Sand
_______________
% Silt
_______________
% Clay _______________
Soil Type _______________
OBSERVATIONS
_______________________________________________________
_______________________________________________________
_______________________________________________________
Adapted from Lamotte Soil Testing Kit
5