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Soil Quality

Unless you're a soil scientist, all soil might look the same to you. How can scientists tell if one soil sample is better than another? Well, they can test the soils' quality. This isn't a difficult process – some of these tests you could easily do yourself in class!

Get ready to dig deep into this article!


Soil Quality: Definition

Let's begin with a definition.

Soil quality is the ability of a soil to perform functions essential to biotic species, including humans.

Basically, soil quality is a measure of how well the soil does what we want it to do. Quality soil is not only fertile, but possesses suitable physical and biological properties.

Characteristics of High-quality Soil

What characteristics are associated with high-quality soils?

Characteristic
Description
Fertility
The ability of the soil to sustain agricultural plant growth.
Depth
Adequate volume and root space of the soil.
Rich in Nutrients
Plant growth requires nutrient availability.
Effective Drainage
The ability of the soil to allow water to pass through it.
Biodiversity
The diversity of soil organisms.
Resilience
The ability of the soil to recover rapidly from disturbance.

Human Impact on Soil Quality

Unfortunately, human activities have negatively impacted soil quality.

The primary cause is population increase, as this increases the demand for agriculture and land development.

We'll look at agriculture and soil quality a little later, so keep reading.

The impacts on soil quality can be categorised into erosion, compaction, and salinisation.

Soil Erosion

Soil erosion is the wearing away of the upper layer of soil.

There are some natural causes of soil erosion, such as rain splash, wind blow, and surface runoff. However, human activities, especially agricultural, have enhanced the rate of soil erosion.

Consequences of soil erosion include:

  • Reduced productivity

  • Flooding

  • Sedimentation

  • Desertification

  • Landslides

  • Atmospheric pollution

Soil Compaction

Soil compaction occurs when the upper layer of soil becomes squashed into an impermeable layer.

A common source of soil compaction is heavy agricultural machinery.

Consequences of soil impaction include:

  • Poor root growth, leading to limited uptake of water and nutrients

  • Decreased water uptake

  • Decline in structural stability

Soil Salinisation

Soil salinisation occurs when soluble salts are retained in the soil.

Anthropogenic causes include overuse of fertilisers, sea level rise, and irrigation with salty water.

Consequences of soil salinisation include:

  • Waterway contamination
  • Plant stress
  • Soil erosion
  • Flooding
  • Biodiversity loss

Effect of Agricultural Practices on Soil Quality

One of the major causes of soil quality decline is industrial agricultural practices. Let's look at some of these in detail.

Ploughing

Ploughing is the breaking up and loosening, and turning over of soil.

Traditional ploughing techniques impact soil quality. Soil is exposed to wind and water, leaving it susceptible to erosion.

Furthermore, ploughing disturbs soil biota (e.g. bacteria, fungi, and invertebrates) that make soil naturally fertile. It also releases the carbon stored in soil into the atmosphere in the form of carbon dioxide, a greenhouse gas.

Fertiliser Use

Long-term fertiliser use can actually cause a gradual decline in soil quality. Organic matter becomes degraded, and soil acidification is common. Plus, excess fertiliser runs into water bodies, causing eutrophication.

Eutrophication is excessive richness of a water body, leading to algal overgrowth and oxygen depletion.

Pesticides

Pesticides are fairly self-explanatory – they're chemicals used for killing pests. These include insecticides, fungicides, and herbicides (weedkillers).

Pesticides severely impact natural soil communities. Overuse can kill beneficial soil microorganisms, such as mycorrhizal fungi.

Mycorrhizal fungi form symbiotic relationships (positive interactions between different species) with plants. The fungi act as root extensions, helping plants to maximise their nutrient uptake. In return, plants provide the mycorrhizae with carbohydrates and moisture.

Heavy Machinery

Heavy agricultural machinery, such as tractors and combine harvesters, result in soil compaction. Topsoil is squashed into a dense, impermeable layer.

Effects of soil compaction include restriction of root growth, poor aeration and drainage, and increased risk of erosion. Plants struggle to absorb adequate nutrients and water, leading to stunted growth.

Testing Soil Quality

How can we test soil quality?

Infiltration Rate

The infiltration rate is the speed that water enters the soil. Testing filtration rate provides an indicator of soil structure, and highlights any compaction. You can test infiltration rate by inserting an infiltration tube into the ground until it creates a seal. Add water to the tube, then measure its infiltration time.

Soil's infiltration time is dependent on its composition and bulk density.

Soil Structure

Soil structure is the arrangement of solids and pore spaces within soil.

Not to be confused with soil texture – the proportion of sand, silt, and clay particles.

Soil structure is analysed using the slake test. It measures the stability of soil, and the ability to maintain its structure to support plants and soil biota. To perform the slake test:

  • Collect two chunks of topsoil – one from a cultivated area, and the other from an uncultivated area.

  • Take two jars and mould wire meshes over the top of them. Place the chunks of soil into the jars.

  • Fill each jar with water, submerging the soil.

  • See if the soil disintegrates. If it does, it has a poorer structure and a lower organic matter content.

Soil Compaction

You can test for the level of soil compaction by calculating bulk density:

  • Weigh a dry sample of soil.

  • Determine the volume of the soil using a measuring cylinder.

  • Calculate the bulk density using the equation: dry weight ÷ volume.

  • Bulk density is typically expressed in g/cm3.

Humus Content

Humus is dark organic matter in soil, formed by the decomposition of plant and animal matter. It's associated with fertility and soil health. To test humus levels:

  • Weigh a dry sample soil sample.

  • Burn off the humus in the soil using a furnace or Bunsen burner. As it burns, it will give off a strong peaty smell.

  • Weigh the humus at regular intervals. Once the mass has stopped decreasing, all the humus has been burned off.

  • The difference between the initial mass and the final mass is the humus content (expressed as a percentage).

It's important to measure humus using a dry soil sample. Otherwise, heating the soil will cause water to evaporate from the sample, skewing the mass measurements.

Improving Soil Quality

How can farmers (or keen gardeners) improve their soil quality?

TechniqueDescription
Improve humus contentAdd compost or organic matter to soil enables humus to accumulate, thus improving the soil fertility.
Prevent compactionAvoid using heavy machinery (if possible). Alternatively, loosen compacted soil mechanically (via milling, raking, or digging).
Alter soil compositionAdd silt, sand, or clay to alter the soil composition. This affects the infiltration rate and nutrient content.
Add fertiliserAdd fertiliser to increase soil nutrient content, enhancing plant growth.
Plant legumesLegumes form symbiotic relationships with nitrogen-fixing soil bacteria. Growing legumes helps to increase soil nitrogen content.
Crop rotationCrop rotation (including a season of fallow) allows soil to recover between growing seasons.

Legumes include peas, lentils, chickpeas, soybeans, and peanuts.

Soil and Water Quality Management in Aquaculture

What is aquaculture?

Aquaculture is the controlled cultivation of aquatic organisms for food.

Soil is important in aquaculture. Most aquaculture ponds are built from, or in, soil. Quality of the pond soil is positively associated with the productivity of the aquaculture system. When soil conditions are poor, production is limited. However, when the soil conditions are good, the pond is rich in nutrients and can support a stable population of phytoplankton (the basis of the marine food chain).

I hope that this article has explained soil quality for you. Remember that soil quality is the ability of a soil to perform functions essential to biotic species, including humans. Soil quality is impacted by human activities, including agriculture.

Soil Quality - Key takeaways

  • Soil quality is the ability of a soil to perform functions essential to biotic species, including humans. High-quality soil is fertile, deep, rich in nutrients, biodiverse, resilient, and has effective drainage.
  • Human activities have negatively impacted soil quality. These detrimental impacts are categorised into erosion, compaction, and salinisation.
  • Agriculture is a major cause of declining soil quality. Ploughing, agrochemicals, and heavy machinery disrupt the soil and its biological communities.
  • Testing soil quality can be tested by measuring infiltration rate, soil structure, soil compaction, and humus content.
  • Improving soil quality aids plant growth and helps the soil recover from disturbance.
  • Soil quality is positively associated with aquaculture productivity.

Frequently Asked Questions about Soil Quality

Soil productivity can be measured by testing infiltration rate, compaction, humus content, and analysing structure.

Soil quality is the ability of a soil to perform functions essential to biotic species, including humans.

One of the most important qualities of soil is fertility – the ability to sustain agricultural plant growth.

Soil quality is affected by human activities, such as agriculture and land development.

Soil quality is important because high-quality soil enables agricultural production.

Final Soil Quality Quiz

Question

Define soil quality.

Show answer

Answer

Soil quality is the ability of a soil to perform functions essential to biotic species, including humans.

Show question

Question

Define soil fertility.

Show answer

Answer

Soil fertility is the ability of the soil to sustain agricultural plant growth.

Show question

Question

What is soil erosion? 

Show answer

Answer

Soil erosion is the wearing away of the upper layer of soil.

Show question

Question

When does soil compaction occur?

Show answer

Answer

Soil compaction occurs when the upper layer of soil becomes squashed into an impermeable layer.

Show question

Question

When does soil salinisation occur?

Show answer

Answer

Soil salinisation occurs when soluble salts are retained in the soil.

Show question

Question

What is ploughing?

Show answer

Answer

Ploughing is the breaking up and loosening, and turning over of soil.

Show question

Question

Define eutrophication.

Show answer

Answer

Eutrophication is excessive richness of a water body, leading to algal overgrowth and oxygen depletion.

Show question

Question

Name a disadvantage of pesticides.

Show answer

Answer

Pesticides severely impact natural soil communities, often killing beneficial microorganisms.

Show question

Question

Name the consequences of soil compaction.

Show answer

Answer

Consequences of soil compaction include restriction of root growth, stunted plant growth, poor aeration and drainage, and increased risk of erosion.

Show question

Question

What is infiltration rate?

Show answer

Answer

The infiltration rate is the speed that water enters the soil.

Show question

Question

Define soil structure.

Show answer

Answer

Soil structure is the arrangement of solids and pore spaces within soil.

Show question

Question

The slake test involves submerging soil in water.

Show answer

Answer

True

Show question

Question

How can planting legumes improve soil quality?

Show answer

Answer

Legumes form symbiotic relationships with nitrogen-fixing soil bacteria. Thus, planting legumes helps to increase soil nitrogen content.

Show question

Question

Bulk density is expressed in g/m3.

Show answer

Answer

F

Show question

Question

What is high humus content associated with?

Show answer

Answer

High humus content is associated with soil health and fertility.

Show question

Question

What is the Universal Soil Loss Equation?

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Answer

The Universal Soil Loss Equation is a mathematical model used to predict the long-term average annual rate of soil erosion.

Show question

Question

What is the formula for the USLE?

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Answer

A = R × K × LS × C × P

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Question

Which factors of the USLE cannot be altered?

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Answer

R

Show question

Question

Define runoff.

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Answer

Runoff is the flow of water on the ground when it cannot rapidly infiltrate the soil.

Show question

Question

What does the R factor represent?

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Answer

The R factor represents the erosion potential, based on the rainfall and runoff by geographic location.

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Question

What is the K factor?

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Answer

The K factor is the soil erodibility factor.

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Question

What is the primary influence on the K factor? 

Show answer

Answer

Soil texture

Show question

Question

What does the LS factor represent?

Show answer

Answer

The LS factor represents soil loss dependent on slope steepness and length.

Show question

Question

The steeper and longer the slope, the greater the risk of soil erosion. 

Show answer

Answer

True

Show question

Question

What does the C factor combine?

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Answer

The C factor combines soil loss from a specific crop and soil loss from a specific land management technique.

Show question

Question

Crop type is divided by tillage method to find the C factor.

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Answer

False

Show question

Question

What does the P factor represent?

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Answer

The P factor represents how an agricultural support practice will affect soil loss.

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Question

Define contour farming.

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Answer

Contour farming is growing crops along lines of consistent elevation.

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Question

What do environmental scientists use the USLE for?

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Answer

Environmental scientists use the USLE to help them assess the effectiveness of soil conservation programmes.

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Question

What is the RUSLE?

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Answer

The Revised Universal Soil Loss Equation (RUSLE) is a computerised version of the USLE. 

Show question

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