Which is the best microscope for soil analysis

Serious compost tea brewers should have a microscope in their toolkit to see exactly what they are brewing. Zeroing in on samples of your product is the only way to be sure you’re applying tea that is full of the right kind of microbes for healthy soil.

You don’t need a PhD to master the microscope – far from it. With a microscope and a patient eye, you can learn to analyse your compost tea efficiently and effectively.

What do I need to get started?

You don’t need a really high-tech piece of kit to get started in soil biology testing. A simple compound light microscope with magnification of at least 400x (40X objective and 10X eye pieces = 400X total magnification) is sufficient to see the major groups of microorganisms.

To choose the right piece of kit, however, you need to be aware of a major difference between sampling a soil sample and sampling compost tea.

Organisms in soil samples tend to settle to a particular depth of field. In compost tea, they tend to be located throughout the depth and breadth of the sample. This makes them tricky to see because they’re mainly transparent. So you need a combination of a brightness-adjustable LED light and an iris diaphragm to ‘shadow’ them and show up their features – size, shape and movement.

XSZ-107T Soil Biology Microscope Bundle

Minimum specs of a suitable light compound or brightfield microscope

• 4X and 40X objective lens
• 10X eyepiece
• Abbe 1.25 NA condenser
• Iris diaphragm
• Mechanical stage

However, by considering a few options you can do your job more effectively and comfortably.

A compound light microscope is a microscope with more than one lens and its own light source. In this type of microscope, there are ocular lenses in the binocular eyepieces and objective lenses in a rotating nosepiece closer to the specimen.

Objective lens

One of the most important parts of a compound microscope, as it is the lens closest to the specimen. The objective relays a real image of the object to the eyepiece. This part of the microscope is needed to produce the base magnification. In general, it is responsible for:

• creating the image
• determining the quality of the image produced
• controlling the magnification
• overall resolution.

Usually you will find 3 or 4 objective lenses on a microscope. They almost always consist of 4X, 10X, 40X and 100X powers.

Most microscope objectives come in three basic designs: achromatic, semi-plan, and plan. Both achromatic and semi-plan objectives are acceptable for sampling compost tea.

Achromatic objectives produce clear images from the centre of the field of view with some blurring as you move to the outer edges. Semi-plan objectives have an 80% flat field.

A parfocal lens is a lens that stays in focus when magnification and focal length is changed. By focusing correctly using the lowest objective (4X), the 40X objective can be swung into place with minimal refocusing. This means you can switch more quickly between magnifications to identify faster moving organisms. Most modern brightfield microscopes are parfocal.

Eyepiece (ocular)

Microscopes come in different configuration according to the eye pieces – monocular, binocular and trinocular. They work in combination with objectives to further magnify the intermediate image so you can observe specimen details.

• The monocular has only one eyepiece. Objects viewed through a monocular microscope will always look flat and without depth.
• The binocular, as expected, is a microscope with two eyepieces in its head, and is very common in every lab.
• The trinocular is very similar to the binocular as for its properties, but with 2 standard eyepieces and 1 camera eyepiece in which an external camera is placed. This can broadcast vision to a computer and take photos or video.

For longer sessions or more frequent viewing, the best microscope is binocular, because you won’t need to keep one eye closed. Using an ocular microscope for extended periods may strain your eyes and cause headaches.

Condenser

The function of the Abbe condenser is not to magnify light in any way, but to manipulate its direction and angles of reflections. It gathers light from the microscope’s light source and focuses it into a beam – a bit like a multi-function torch. You can use it with a broad beam of light or focus the same amount of light into a brighter but narrower beam.

As the light gets condensed, it hits the sample in a more organised fashion. Thus, when the light leaves the specimen and goes to the objective lens on the other side, the specimen will appear more clear and vivid when you look at it with your eyes. You’ll get a good idea of the kinds of bacteria, fungi or protozoa that are in your compost tea.

Condensers are vital parts of any microscope system, and their relationship to the light and objective lens is the ultimate decider of how well a microscope will function. The condenser sits directly below the stage and contains one or more lenses.

Iris diaphragm

Most high-quality microscopes include an Abbe condenser with an iris diaphragm. Combined, they control both the focus and quantity of light applied to the specimen. The iris diaphragm has an adjustable opening size – like the iris in the human eye, which can dilate and constrict in relation to the size of the pupil. Narrowing the diaphragm creates a darker image, but with more contrast, while widening the diaphragm creates a lighter image, but with less contrast.

Increasing the contrast with surrounding materials improves the view of microbes at various depths within the sample. This is particularly important in measuring the tiny, mainly transparent, organisms in compost tea.

For instance, the distance between the top of your slide and bottom of your coverslip is approximately 15 microns. The size of bacterium is 1 micron. So these organisms have a lot of room to move. Without the iris diaphragm, when you are viewing at one depth, you may not see organisms at a different depth.

By having light come up and travel through the organisms, we are trying to ‘shadow’ them. Imagine someone in a nylon tent. With a weak light, we may be able to make out there is someone there because of blurry movement, but with a strong, focused beam we can make out the features and be able to tell it is a person. In the same way, we can identify the different microbes, using morphology – shape, size and movement. The best microscopes will make these as clear as possible to aid identification.

The diaphragm can be found near the bottom of the microscope, above the light source and the condenser, and below the specimen stage. It can be controlled through a mechanical lever, or with a dial fitted on the diaphragm.

Stage

The flat platform where you place your slides. Stage clips hold the slides in place. If your microscope has a mechanical stage, you will be able to move the slide around by turning two knobs. One moves it left and right, the other moves it forward and back.

Other important concepts

• Resolution – not to be confused with magnification, microscope resolution is the shortest distance between two separate points in a microscope’s field of view that can still be distinguished as distinct entities.

If the two points are closer together than your resolution, then they will appear ill-defined and their positions will be inexact. A microscope may offer high magnification, but if the lenses are of poor quality the resulting poor resolution will degrade the image quality.

Microscope resolution is the most important determinant of how well a microscope will perform and is determined by the numerical aperture and light wavelength.

Resolution comes from a combination of the quality of the lenses and the operator’s ability to manage the light source for optimum viewing of a specific sample.

• Light source – older microscopes used mirrors to reflect light from an external source up through the bottom of the stage. However, most microscopes now use a low-voltage bulb. The best microscope will use an LED light source to provide a brighter, ‘cleaner’ light than halogen.

What to look for when purchasing a microscope for compost tea

If you want a real microscope that provides sharp, crisp images, then stay away from the toy stores and the plastic instruments! There are many high-quality, student-grade microscopes on the market today. They have a metal body and all glass lenses. One of the most important considerations is to purchase your instrument from a reputable source.

One of the best microscopes for viewing compost tea is the XSZ-107T microscope, which not only exceeds the minimum specs needed for sampling with ease and comfort as recommended by Dr Elain Ingram from The Soil Food Web.