Soil Analysis

Soil Physical Analysis

The physical properties of soil influence physical processes (such as infiltration, erosion, and nutrient cycling) as well as biological activity.

Texture

When analysing soil texture, pedologists use a chart called a soil triangle.

Different textures modify physical processes in the soil. For example:

• Sandy soils are fast-draining, but have a limited water holding capacity and low nutrient content.

• Silt soils are more fertile than sandy soils, and better at holding water.

• Clay soils are great at retaining water but drain poorly.

Farmers prefer loam soils (also called agricultural soils). They have the beneficial properties of the other three types. Loam soil retains moisture, nutrients, and humus.

Analysing texture helps farmers maximise their crop yield. If the soil traits are not desirable, farmers can add sand or compost to change its texture.

Water Content

Desirable loam soils have a fast infiltration rate and a good water-holding capacity. Measuring the soil infiltration rate and moisture content is another indicator of soil texture. If necessary, the soil texture can be rectified using sand or compost.

Bulk Density

High levels of compaction is undesirable. Compaction reduces pore size, inhibiting root growth, water storage, and oxygen availability.

The best treatment for soil compaction is prevention. However, compacted soil can amended by adding sand, or breaking up the topsoil with a shovel.

Soil Nutrient Analysis

The chemical and biological properties of soil influence plant growth and community composition.

pH

You might remember from GCSE Chemistry that pH is a measure of acidity or basicity of a solution.

Soil pH is closely linked to nutrient availability. Every plant has an optimal pH range (usually between 6.5 to 7.5) that enables nutrient uptake. Outside this range, plant growth may be stunted.

Measuring soil pH is important to maximise crop yield. If the soil is too acidic or alkaline for the target plant to take up essential nutrients, farmers can add lime or aluminium sulfate.

Humus

We learned earlier that humus is the dark organic matter in soil, formed by decomposition of plant and animal matter. Humus improves soil health and fertility. It's approximately 60% carbon, and contains other useful nutrients like nitrogen, phosphorous, and sulfur.

Farmers measure the humus content of their soil. If it's low, they add compost and organic matter to improve fertility.

Macronutrients

What are the main macronutrients in the soil, and what are they used for?

Analysing the level of soil macronutrients lets farmers know if the plants will suffer from nutrient deficiencies, which can be rectified by applying fertiliser.

Soil Sample Microbial Analysis

Many organisms live in soils. Plants, fungi, invertebrates, and even mammals!

But by far the most abundant soil organisms are microbes.

Soil microbes are essential to the soil community and the ecosystem as a whole. They carry out important processes, such as:

• Nutrient cycling

• Purifying water by removing pollutants

• Recycling carbon into soils

• Decomposing organic matter

• Forming symbiotic relationships with plants

Soil microbial activity can be measured using direct microscopy, colorimetric analysis, or fumigation techniques.

Soil Analysis: Methods

How are soil properties analysed?

Texture

Soil texture is analysed using one of two methods, depending on the particle size.

• Sieving: If soil particles are greater than 0.075 mm in diameter, they are sieved to determine the grain size distribution. The sieves are made of woven wires with square openings of various sizes, ranging from 4.75 mm to 0.038 mm.

• Sedimentation: if the soil particles are smaller than 0.075 mm in diameter, they are measured by their rate of sedimentation (i.e. their settling rate in a liquid medium). Heavier particles settle first – their size is determined by density and particle size.

Once the texture has been analysed, it is compared to the soil triangle to classify the soil type.

Water Content

Two techniques can be used to analyse soil water: infiltration rate and moisture content.

• Infiltration Rate: unlike most other soil analysis techniques, this takes place on site. An infiltration tube is inserted into the ground until it forms a seal. Water is added to the tube, and its infiltration time is measured. Infiltration time is dependent on soil composition and bulk density.

• Moisture Content: a fresh soil sample is weighed. It's allowed to dry (overnight, or using an oven at 105ºC). Once dry, it's weighed again. The difference between the two masses is the soil moisture content (expressed as a percentage).

Bulk Density

To measure soil bulk density:

• Weigh a dry sample of soil.

• Measure the volume of the soil.

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

• Bulk density is typically expressed in g/cm³.

pH

Soil pH can be measured in two ways. The first takes place on site, using a handheld digital probe.

Alternatively, place a sample of soil into a test tube. Add barium sulfate, distilled water, and indicator solution.

Compare the colour of the solution to an indicator chart to determine the soil pH.

Humus

To measure humus:

• Weigh a dry sample soil sample.

• Burn off the humus in the soil using a furnace or Bunsen burner.

• Weigh the soil again.

• The difference between the two masses is the humus content (expressed as a percentage).

Macronutrients

Add a soil sample to an extractant solution and mix by shaking. Then, filter the liquid content, and analyse it for the presence and concentration of elements. The number obtained is known as the soil-test index.

Microbial Analysis

Soil microbes are analysed using one of three methods.

• Direct Microscopy: soil samples are suspended, and microbial matter is counted under a microscope. Each type of microorganism is measured to calculate the microbial biomass per gram of dry soil.

• Colorimetric Analysis: a machine called a colorimeter uses the colour changes of a solution to determine its concentration.

• Colorimetry is based on Beer-Lambert's law. According to the law, the amount of light a colour solution absorbs is directly proportional to its concentration as well as the length of the light path through the solution.

• Fumigation Techniques:

  • Incubation: measuring the amount of CO₂ released by a soil sample during a predetermined incubation time.

  • Extraction: the soil is fumigated using chloroform. Microbial constituents (C, N, S, and P) are immediately extracted and quantified.

I hope that this article has explained soil analysis to you. The physical, chemical, and biological properties of soil can influence plant growth and community composition. As a result, it's important to know how to analyse soils.

^{1. Encyclopaedia Britannica, Humus, 2020}

^{2. MD Sahadat, Properties and Behavior of Soil - Online Lab Manual, PressBooks, 2022}

^{3. Nutrient Stewardship, Soil pH and the Availability of Plant Nutrients, 20174. UK Centre for Ecology & Hydrology, Why do soil microbes matter?, 2022}