Bulk Density May 2016

Summary

This educational video script explains the process of collecting soil samples to measure bulk density, a key indicator of soil compaction affecting plant growth and water infiltration. It details the importance of careful sample collection to avoid altering density readings and discusses the impact of soil moisture on density measurements. The script also includes a laboratory demonstration to illustrate how water content affects bulk density, emphasizing the need for standardized, moisture-free samples to ensure accurate soil analysis.

Takeaways

• 🌱 **Soil Sampling for Bulk Density**: College students learn to collect soil samples to measure bulk density, which indicates how compacted the soil particles are.
• 🔍 **Importance of Bulk Density**: Bulk density is crucial for understanding soil chemistry, water infiltration, and plant growth as it reflects soil compaction.
• ⚖️ **Correct Sample Collection**: To ensure accurate bulk density measurements, students must carefully cut off excess soil without pressing it into the cylinder to avoid falsely high readings.
• 📏 **Volume Measurement**: The volume of the soil sample, including both particles and pore space, is a key factor in calculating bulk density, with the cylinder volume given as 347.5 cubic centimeters.
• 🌿 **Soil Composition**: Soil is composed of solid particles with varying shapes and sizes, and the space between them, called pores, can be filled with air or water.
• 💧 **Impact of Water Content**: The water content in soil can significantly affect bulk density measurements, as it adds weight and can vary due to weather and precipitation.
• 🌧️ **Soil Saturation**: During rainfall, soil can become saturated, filling all pore spaces with water, which is a critical condition for understanding how water affects soil density.
• 🔥 **Moisture Elimination**: To standardize bulk density measurements, soil samples are dried in an oven at 105 degrees Celsius to remove moisture content and measure dry weight.
• 🧪 **Laboratory Demonstration**: A demonstration using pebbles and a beaker illustrates how adding water to a soil sample increases its mass and affects bulk density calculations.
• 📐 **Bulk Density Calculation**: Bulk density is calculated as the mass of the dry soil sample divided by its volume, providing a standardized measure of soil compaction.

Q & A

What is the purpose of collecting a soil sample for bulk density testing?

Collecting a soil sample for bulk density testing helps to understand how compacted the soil particles are, which significantly affects soil chemistry, water infiltration, and plant growth.

How is a soil sample collected for bulk density measurement?

The soil sample is collected by driving a hollow cylinder into the ground, ensuring that the soil is cut off level with the cylinder without pressing it in to avoid falsely high bulk density readings.

What is the significance of bulk density in soil science?

Bulk density is significant in soil science as it measures soil compaction and is an important concept in both soil physics and soil chemistry.

What is included in the volume of a soil sample when calculating bulk density?

The volume of a soil sample for bulk density calculation includes both the soil particles and the pore space between the soil particles.

Why is the term 'bulk' used in the context of soil density?

In soil science, 'bulk' refers to the whole sample, which includes soil particles, plant roots, and the air space between them, representing the density of the soil matrix.

How does the presence of water in soil pores affect the calculation of bulk density?

Water in soil pores can significantly affect the calculation of bulk density because the mass of water is about one gram per cubic centimeter, which is not negligible and can change the mass of the soil sample.

How can changes in soil moisture content impact the measurement of bulk density?

Changes in soil moisture content can cause fluctuations in the mass of bulk soil samples, making the bulk density measurements inconsistent and less representative of the soil matrix.

What is the laboratory demonstration used to understand the effect of water content on bulk density?

The laboratory demonstration uses pebbles in a beaker to represent a soil sample, where water is added to the pore space to simulate rainfall and saturation, demonstrating how water content can increase the mass and affect bulk density.

How is the bulk density of a soil sample calculated?

Bulk density is calculated using the formula: bulk density equals the mass of the sample divided by the volume of the sample. The mass is determined after drying the sample to remove moisture content.

Why is it necessary to dry the soil samples before calculating bulk density?

Drying the soil samples is necessary to remove moisture content and ensure that the mass measured represents the dry weight of the soil particles, allowing for standardized and accurate bulk density measurements.

What is the expected bulk density range for soil samples taken from the same field?

The expected bulk density range for soil samples taken from the same field is likely to be close to each other since they are from a similar soil matrix and have been oven-dried to remove soil moisture for standardized results.

Outlines

00:00

🌱 Understanding Soil Bulk Density

This paragraph introduces the concept of soil bulk density, explaining its significance in soil chemistry, water infiltration, and plant growth. Students are shown collecting soil samples using a hollow cylinder to measure compaction. The process emphasizes careful cutting to avoid altering the sample's density. The paragraph reviews the formula for density, noting that bulk density accounts for both soil particles and the pore space between them. It also discusses the impact of water content on bulk density, highlighting the need to standardize measurements by eliminating moisture.

05:04

💧 Impact of Water Content on Soil Bulk Density

The second paragraph delves into how water content in soil affects bulk density. It uses a laboratory demonstration with pebbles in a beaker to illustrate the concept. The dry weight of the pebbles is measured, then water is added to simulate rainfall, filling the pore spaces. The saturated sample's weight is then compared to the dry weight, showing a significant increase due to the added water. The paragraph explains how this affects bulk density calculations and the importance of using dry weight for standardized measurements.

10:08

🔍 Calculating and Standardizing Soil Bulk Density

The final paragraph focuses on the process of calculating bulk density by using the dry weight of soil samples. It describes how samples are oven-dried to remove moisture, ensuring standardized results. The paragraph provides an example calculation using a sample's dry weight and the known volume of the sample container, resulting in a bulk density of 1.3 grams per cubic centimeter. It concludes by emphasizing the importance of standardized bulk density measurements for understanding soil characteristics.

Keywords

💡Soil Sample

A soil sample is a portion of soil that is collected for analysis to determine various properties like texture, structure, and chemical composition. In the video, students collect soil samples by driving a hollow cylinder into the ground, which is crucial for testing bulk density. The process must be done carefully to ensure the sample's integrity and representativeness of the soil's actual conditions.

💡Bulk Density

Bulk density is a measure of how compacted the particles are in the soil, which significantly affects soil chemistry, water infiltration, and plant growth. The video explains that bulk density is calculated by dividing the mass of the soil sample by its volume, including both soil particles and the pore space between them. It's a key concept in soil physics and chemistry, as it provides insights into soil compaction.

💡Pore Space

Pore space refers to the empty spaces between soil particles. The video emphasizes that the volume of a soil sample includes both the soil particles and the pore space. Pore spaces can be filled with air or water, and their presence affects the bulk density of the soil. Understanding pore space is essential for assessing soil health and its ability to retain water and nutrients.

💡Soil Compaction

Soil compaction is the process by which soil particles are pressed closer together, reducing the pore space and increasing the soil's bulk density. The video mentions that compaction has a significant effect on soil properties, and it's important to measure it accurately to understand soil conditions. Over-compacted soil can impede water and air movement, affecting plant growth.

💡Soil Chemistry

Soil chemistry involves the study of chemical processes and components in the soil, which are influenced by factors like compaction and bulk density. The video script indicates that bulk density affects soil chemistry, as a denser soil can alter the availability of nutrients and the overall chemical balance necessary for plant growth.

💡Water Infiltration

Water infiltration is the process by which water penetrates the soil surface and moves through the soil profile. The video explains that soil compaction, as measured by bulk density, affects water infiltration rates. Proper infiltration is vital for plant health and the overall hydrological cycle.

💡Plant Growth

Plant growth is influenced by various soil properties, including bulk density. The video script mentions that how compacted the soil is can significantly impact plant growth, as it affects the availability of water, nutrients, and air to the plant roots. Understanding and managing soil compaction is, therefore, a key aspect of agricultural and environmental management.

💡Soil Matrix

The soil matrix refers to the entire soil system, including soil particles, plant roots, and the pore spaces between them. The video uses the term to describe the context in which bulk density is measured, emphasizing that bulk density represents the density of the whole soil sample, not just the solid particles.

💡Soil Saturation

Soil saturation occurs when all the pore spaces in the soil are filled with water, typically during or after a rainfall event. The video script describes a laboratory demonstration where pebbles in a beaker represent a soil sample, and water is added to simulate soil saturation. This concept is important for understanding how water content can affect soil properties and bulk density.

💡Oven Drying

Oven drying is a method used to remove moisture from soil samples by heating them in an oven at a specific temperature, usually around 105 degrees Celsius. The video explains that samples are oven-dried to determine their dry weight, which is necessary for calculating bulk density without the influence of variable soil moisture content. This process standardizes the bulk density measurements across different samples.

Highlights

Students learn to collect soil samples for bulk density testing.

Bulk density indicates soil compaction levels, affecting soil chemistry, water infiltration, and plant growth.

Soil sample collection involves driving a hollow cylinder into the ground without pressing soil into it.

Bulk density is calculated using the formula mass over volume, with the volume of the cylinder being 347.5 cubic centimeters.

Soil is composed of solid particles with varying shapes and sizes, and the space between them, called pores.

Bulk density accounts for both soil particles and the pore space, which can be filled with air or water.

The term 'bulk' in soil science refers to the entire sample, including soil particles, plant roots, and air spaces.

More pore space results in a less dense soil sample.

Soil moisture content can change frequently, affecting the mass and bulk density of soil samples.

During rainfall, soil pores can become saturated with water, displacing air and affecting bulk density.

A laboratory demonstration uses pebbles in a beaker to simulate the effect of water content on bulk density.

Saturation of soil pores with water increases the mass of the sample, thus affecting bulk density calculations.

To standardize bulk density measurements, soil samples are dried in an oven at 105 degrees Celsius to remove moisture.

Drying samples allows for the use of dry weight in bulk density calculations, providing standardized results.

Bulk density is a key measurement for identifying different soil types and understanding soil matrix characteristics.

Sample 14, after oven drying, has a bulk density of 1.3 grams per cubic centimeter, indicating soil compaction.

Similar bulk density results from multiple samples suggest a consistent soil matrix in the sampled field area.

Transcripts

these college students are learning how

to collect a soil sample that will be

tested for bulk density the soil sample

is collected by driving a hollow

cylinder into the ground bulk density

tells us how compacted particles are in

the soil and that compaction has a

significant effect on such things as

soil chemistry water infiltration and

plant growth here you see the students

cutting off excess soil and plant root

the soil is cut off level with the

cylinder the students must be careful to

cut off the soil rather than pressing it

into the cylinder remember bulk density

is a measurement of soil compaction so

pressing the excess soil into the

cylinder would cause our bulk density

reading to be slightly higher than it is

in the field bulk density is an

important concept in both soil physics

and soil chemistry so let's review this

concept to make sure that you have a

good understanding of how to determine

the bulk density of a soil you will

remember from high school that the

formula for the simple density of a

material is mass over volume the volume

of the cylinder is three hundred and

forty seven point five cubic centimeters

so we're already halfway there in

calculating our bulk density but when it

comes to determining the mass of the

sample bulk density is a little more

complicated than the simple disa T that

you're used to working with in order to

understand the difference let's review a

few basic facts about soil we know that

soil is made up of solid particles that

have different shapes and sizes and in

between these particles there is

empty space and that empty space we call

pores so the first thing you need to

remember about bulk density is that the

volume of the sample includes both the

soil particles and the pore space

between the soil particles if the word

bulk is a little confusing to you don't

worry about that actually it is a rather

odd word to use in this context just

keep in mind that in soil science the

word means hole and bulk density means

the density of the whole sample which

includes soil particles plant roots and

the air space in between them another

term we could use his soil matrix

density we could say that bulk density

is the density of the soil matrix and is

representative of the field from which

we took the sample obviously the more

empty space or pore space that you have

in your sample the less dense the sample

will be and there's another reason why

pore spaces are important to the

calculation of bulk density these empty

spaces in the soil can be filled either

with air or with water we won't worry

about the weight or mass of the air

because that's negligible but the weight

or mass of water as you will remember is

about one gram per cubic centimeter and

that is significant it definitely will

affect our calculation of bulk density

so the water content of the soil is a

problem it can change from day to day

and even hour to hour depending on

weather conditions and the amount of

precipitation this means that the mass

of bulk soil samples taken from the same

field could fluctuate significantly from

day to day and therefore our calculation

of bulk density would also fluctuate so

it wouldn't tell us very much about the

actual care

heuristics of the soil matrix in the

field where we're sampling so how do we

eliminate the random changes in bulk

density that are caused by changes in

soil moisture content to answer that

question let's take a closer look at

what happens during a rainfall event

during prolonged dry periods soil

moisture is evaporated then the pore

spaces of the soil become filled with

air this is especially true with the top

six inches of soil now let's talk about

what happens when a storm system moves

into the area and there is an extended

rainfall the rainwater moves down

through the soil because of gravity and

begins to fill the pore space pushing

out the air if it rains long enough all

of the pores in the soil will be filled

with water this is referred to as soil

saturation or saturated conditions when

the soil is saturated rainfall can no

longer be absorbed have become surface

water runoff if it continues to rain

streams will rise and flooding will

occur

now let's conduct a laboratory

demonstration that will help us to

further understand how water content in

a soil sample will affect bulk density

the pebbles in this beaker will

represent a bulk soil sample the pebbles

themselves will represent the soil

particles and it's easy to see the pore

space in between them the weight of the

beaker and the pebbles is two hundred

and eight point two grams the weight of

the beaker by itself which is our tare

weight is fifty one point eight grams if

we subtract that from the weight of the

beaker and the pebbles then we get the

weight of the pebbles by themselves

which is 156 point four grams we would

call this the dry weight of our sample

because there's no water or moisture

content that we have to worry about all

the pore space is filled with air now we

will simulate a rainfall by pouring

water into the pore space of the sample

at the 100 milliliter mark all of the

pore space has been filled with water

and our imaginary soil sample is

completely saturated the weight of the

pebbles the water and the beaker is two

hundred forty eight point two grams once

again we will subtract the tare weight

of the beaker and we find that the

pebbles and the water weigh one hundred

ninety six point four grams compare that

to our dry weight of the pebbles which

was 156 point four grams and we see that

the water that we poured into the beaker

simulating a rainfall added 40 grams of

weight to our sample keep in mind we are

assuming that our laboratory scales are

properly calibrated so that weight has

the same number as mass therefore adding

the water to our sample also increase

the mass of the sample by 40 grams now

let's calculate the bulk density of both

our dry sample

and our saturated sample this should

help us to understand how moisture

content and the pores of the soil can

affect our bulk density reading here is

our formula bulk density equals the mass

of the sample over the volume of the

sample the volume we already know

because we filled the beaker up to the

100 milliliter level we also know that

100 milliliters is the same volume as

100 cubic centimeters density is almost

always given in grams per cubic

centimeters so let's use that unit of

volume now we will plug in our

measurements of mass our dry weight

measurement was 150 6.4 grams and the

weight of the saturated sample was 196

point 4 grams when we do the bath we see

that there is a significant difference

in our results and that difference is

significant enough to make our bulk

density readings virtually worthless

bulk density is one of the measurements

used to identify different types of

soils so we want our calculation of bulk

density to be both accurate and

standardized the only way we can achieve

this is by getting rid of the moisture

content in the sample but how are we

going to do that

well what if we simply heat up the

sample and evaporate the moisture then

we would always be using the dry weight

of the sample as the mass and in fact

that is exactly what's done to determine

bulk density the samples are placed in

an oven with a temperature of about 105

degrees Celsius the samples will stay in

the oven for several hours or even

several days if necessary to remove

virtually all of the moisture content so

when the samples come out of the oven

and our way to determine mass

we will be measuring their dry weight if

all of the samples are dried and the

mass then is given to us in dry weight

we can then standardize the results and

the bulk density then will give us

important information about the soil

matrix from which the samples were taken

now that we know what bulk density is

and how it's determined let's calculate

the bulk density of one of the samples

taken by the college students let's say

the sample 14 is taken out of the oven

and weighed and the weight is 450 1.75

grams

this would be the samples dry weight and

we would then plug this number into our

formula as the mass of the sample the

result is a bulk density of 1.3 grams

per cubic centimeter we would expect the

other samples to have a bulk density

close to sample 14 this is because they

were all taken close together in the

same field so it is likely that their

soil matrix is very similar and they

were all oven dried to remove soil

moisture which will give us standardized

results