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:
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?
| Technique | Description |
| Improve humus content | Add compost or organic matter to soil enables humus to accumulate, thus improving the soil fertility. |
| Prevent compaction | Avoid using heavy machinery (if possible). Alternatively, loosen compacted soil mechanically (via milling, raking, or digging). |
| Alter soil composition | Add silt, sand, or clay to alter the soil composition. This affects the infiltration rate and nutrient content. |
| Add fertiliser | Add fertiliser to increase soil nutrient content, enhancing plant growth. |
| Plant legumes | Legumes form symbiotic relationships with nitrogen-fixing soil bacteria. Growing legumes helps to increase soil nitrogen content. |
| Crop rotation | Crop 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.
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