Soil Texture & Fertility
Plants normally grow in soil. Texture, the characteristic of soil that describes the soil
particle size, has a substantial effect on plant growth. Soil particle size is important in
determining the water holding capacity of the soil and is also important in soil fertility.
Water-holding capacity is
controlled primarily by soil
texture and organic matter. Soils
with smaller particles (silt and
clay) have a larger surface area
than those with larger sand
particles, and a large surface area
allows a soil to hold more water.
The table illustrates waterholding-capacity differences as
influenced by texture. The sandy
soil can quickly be recharged
with soil moisture but is unable
to hold as much water as the soils
with heavier textures. A silt loam will have 2.00 - 2.50 inches of available water per foot,
while coarse sand will have 0.25 - 0.75 of an inch of available water per foot of soil. As
texture becomes heavier, the wilting point increases because fine soils with narrow pore
spacing hold water more tightly than soils with wide pore spacing
The greater surface area provided by finer textures results in the soil holding more water,
also plays a role in soil fertility by having a higher chemical reactivity (the ability to store
more nutrients and supply them to plants, called Cation Exchange Capacity), and usually
having greater resistance to erosion. Coarser textures will have larger packing voids and,
therefore, larger pore spaces; water can move and drain very rapidly through these soils.
Property/Behavior
Sand
Silt
Clay
Surface area to volume ratio
Low
Medium
High
Water-holding capacity
Low
Medium to high
High
Nutrient supplying capacity
Low
Medium to high
High
Aeration
Good
Medium
Poor
Internal drainage
High
Slow to medium
Very slow
Susceptibility to water erosion
Low
High
Low
(After Brady and Weil, 2008)
Organic matter percentage also influences water-holding capacity. Organic matter
increases a soil’s ability to hold water, both directly and indirectly. When a soil is at field
capacity, organic matter has a higher water holding capacity than a similar volume of
mineral soil. While the water held by organic matter at the permanent wilting point is also
higher, overall, an increase in organic matter increases a soil’s ability to store water
available for plant use. Indirectly, organic matter improves soil structure and aggregate
stability, resulting in increased pore size and volume. These soil quality improvements
result in increased infiltration, movement of water through the soil, and available water
capacity. A major component of organic matter is the populations of microorganisms that
inhabit the soil. These microorganisms can have both a beneficial and a deleterious effect
on the soil. The presence of certain bacteria performs chemical reactions that make
nutrients available to plants. An example of these would be the nitrifying bacteria that
convert an unusable form of nitrogen, N2, into a useable form, NH3. Populations of soil
microorganisms that are deleterious to the plant include plant pathogens. Pesticides are
applied to agricultural fields to kill the pathogens. Home gardeners can purchase potting
soil that has been sterilized, which has the same effect as pesticides. Technically potting
soil when made up of 100% organic matter is not a soil because it does not contain sand,
silt, or clay.
In this activity you will observe the effect of four different growing mediums, sterilized
potting soil, local top soil (textural class to be determined), clay, and sand on the growth
of rapid radish. Tomorrow, you will explain and possibly defend your answer to the
following questions (all growing media refer to this activity),
1. Which growing medium will have the highest Field Capacity?
2. Which growing medium will have the least Available Water?
3. In which growing medium will plants wilt first?
4. Which growing medium do you believe will be the most fertile?
5. In which growing medium will rapid radish plants show the healthiest growth?
Materials Per 4 person group
Four, four chambered trays
watering tray
Four different growing media
rectangular tray
Six rapid radish seeds
pencil
Planting
1. Moisten each media in a pan with enough water until it feels slightly damp.
2. The class will be divided into three groups that have four teams of two people.
3. Each group will have four, four chambered trays; one tray for each growing
medium.
a. Team One: Control
b. Team Two: Local top soil
c. Team Three: Clay
d. Team Four: Sand
4. Each team will put their growing medium into the each cell of the four cells of the
four chambered tray ensuring that each one is ¾ full. Place the four chambered
tray in the rectangular plastic tray to minimize the amount of growing medium on
the lab tables. Tap the chambered tray to help the medium descend in the cell. DO
NOT PUSH DOWN; doing so will alter the pore space of each growing medium.
5. Place one seed on top of the growing medium in two of the cells. Place two seeds
apart from each other as far as possible in the other two cells. These additional
seeds will be used for transplants or seeds that didn’t germinate.
6. Place the chambered trays on an empty watering tray and carefully water them
with ___ ml of water. Watering will cause the medium to go down.
7. Place a pinch of medium to cover the seeds. No more than 1/8” of medium should
cover the seed(s).
8. On one side of plant label, with a pencil, write the
a. Team’s name, growing medium, amount of nutrients
9. Growing chambers should be covered and placed on the platform of the watering
tray underneath growing lights.
Watering
The amount of water has to be constant for every plant; therefore we will water plants
only when the control groups decide that their plants need water. At that point they will
decide how much to water each plant.
Transplanting (Day 2 or 3)
Any extra seedlings need to be transplanted to cells where the seed did not germinate.
This should be done carefully, by making a small hole with forceps or a pencil where the
seedling is to be planted. Carefully extricate the seedling from the medium using forceps,
place it gently in the hole, and then push medium gently over the roots so that the
seedling is secure.
Fertilizing
Each group will receive fertilizer that will be added when plants are watered. Each plant
will receive 3 ml of fertilizer.
Fertilizer Extension Lab
Group A will receive no fertilizer
Group B will receive the proper amount of fertilizer
Group C will receive 4 times the proper amount of fertilizer.
The 3ml of fertilizer added to the plants will be subtracted from the amount of water to be
given to the plants.
Each group will observe possible eutrophication based on the different levels of
nutrients.
Staking (Day 10)
The stake is inserted in the growing medium and the plant is propped up against it,
secured by a twist-tie. The twist-tie should be looped tightly around the stake, but
LOOSELY around the tender stem of the plant at a node so the leaves can help keep the
plant in place.
Observations: In addition to the specific details about the plant growth, observe and record any attributes different
from usual or unexpected. These will be beneficial in the lab report to explain unexpected results.
# of plants< 5 cm
Compared to the control the plants are
.
(color, shape, turgidity)
# of plants >5 cm but < 10cm
# of plants > 10 cm
Day 1
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Title
Each person needs to turn in a typed report
Hypothesis (use headings)
A few sentences will suffice to explain the observations and knowledge that have led to the
stated hypothesis.
Data & Analysis (use headings)
Use the data collected during the experiment to analyze and summarize the condition
and overall health of the plants in the various growing media. The primary task of analyzing the
data is to wind up with a few numbers or observations from the many that were collected
during the experiment. The report should not display the data in the form it was collected
during the experiment, (i.e. notebook or prepared chart for the experiment). Instead, the data
should be presented graphically. And the drawing isn’t enough. The graph needs to be explained
in terms of what it represents. The data and summary go hand in hand; it tells the reader how a
decision was made to accept or reject the hypothesis.
Conclusion (use headings)
 Restate your hypothesis. “We hypothesized that.”
 State whether the hypothesis was supported or rejected. Summarize the major finding
of the experiment to back up your statement. “The hypothesis was rejected because it
was found that.”
 Explain how the variables interacted to produce results you expected and didn’t expect.
 Offer a plausible hypothesis for unexplained or unexpected results.