About the scores

SOLISCA was developed by Royal Eijkelkamp to provide an indication of the presence of microorganisms in soil and compost. The four main groups of soil microorganisms are bacteria, fungi, protozoa, and nematodes. These organisms are the basis of the soil ecosystem. A distinction is made between quantity (amount of organisms per gram of soil) and quality (favourable and unfavourable organisms and diversity). Quantity of organisms and the proportions of favourable versus unfavourable organisms both provide valuable information on the overall condition of the soil in the sample.

The vast majority of soil organisms are beneficial to plants. In fact, without these organisms, plants would not survive. Only a small proportion of soil organisms is unfavourable to plants. These are the pathogens and parasites. In healthy soil, the organisms are abundant and diverse and form a complex ecosystem. The many favourable organisms together keep the unfavourable ones in check by competition and predation. Therefore, it is not necessarily harmful when some unfavourable organisms are found in your sample. Yet, larger quantities of unfavourable organisms combined with low total organism quantities, can indicate a problem.

Bacteria

Bacteria are the most numerous organisms in soil. They play a significant role in the building of soil structure. With the adhesives they produce, they form microaggregates that allow water, oxygen, and plant roots to infiltrate deep into the soil. In addition, bacteria retain nutrients in their cell structures. This prevents nutrient leaching from the soil.Bacteria also contribute to nutrient cycling. Plants produce exudates (sugars) which they excrete through their roots, attracting bacteria and other organisms. Larger soil organisms such as protozoa and nematodes eat these bacteria, after which they excrete waste materials. These waste materials are nutrients for plants. Because plants attract bacteria around their root systems, the nutrients are released exactly in the right place for them. Bacteria also help plants to become more resilient against external factors. They do this by forming a protective layer around the roots and aboveground parts of plants. This obviously benefits plant health and growth.

Explanation of the score

The bacterial biomass score is obtained by direct counts. This means that individual bacteria in several microscope fields of view are counted, after which the concentration of bacteria of the entire sample is calculated. The number of counted bacteria is converted into biomass (in μg per gram of soil) by computer calculations. A minimum bacterial biomass of 300 μg /gram of soil is desirable, because only then there are enough bacteria to provide significant nutrient cycling, plant protection and good soil structure. If bacteria are less than 300 ug/g soil, the soil is not sufficiently active to provide these ecosystem service.

The score for bacterial diversity is based on the number of morphological types of beneficial bacteria that were found in your sample. There are twelve morphological types of beneficial bacteria that can be found in soil samples. The desired value is at least four morphological types. The more morphological types found, the higher the diversity score. The bacteria that we count as unfavourable are the ones that thrive in slightly to very anaerobic circumstances, the anaerobic or facultative anaerobic bacteria. Some of these are disease causers, like E-coli. Even though these anaerobic bacteria add up to the number of types found, so theoretically would increase the diversity, we do not count these as such, because aerobic bacterial diversity is what is needed in a healthy and properly functioning soil. In case unfavourable anaerobic bacteria are found in your sample, this will be displayed in your scorecard.

The bacterial biomass score and the bacterial diversity score combined give an indication of the potential of nutrient cycling, nutrient and water retention, and plant protection provided by bacteria.

Fungi

Fungi play an important role in soils. The functions of soil fungi largely correspond to those of soil bacteria. Fungi, like bacteria, are structure builders. With hyphae, and a glue-like substance called glomalin, they bind microaggregates together, creating macroaggregates. Those macroaggregates contribute to good soil structure, which allows water, oxygen, and plant roots to infiltrate deep into the soil. Fungi also store nutrients that eventually become available as plant food. The fungi are attracted to sugars in the rootzones of plants. They are eaten by soil organisms such as nematodes, microarthropods and earthworms, who release waste materials which are plant essential nutrients. Favourable fungi also play a role in increasing the resilience of plants, by occupying plant tissue. This makes plants less vulnerable to external factors. In addition, a special type of fungi called mycorrhiza, creates a symbiotic relationship with the majority of plants. By entering the roots of a plant, the mycorrhizal fungus creates an exchange site. Sugars that are produced by the plant are traded here for nutrients taken up from the soil by the mycorrhizal fungus. This system can be seen as an increase of the plant’s rootzone. Because fungal hyphae can grow over relatively long distances, they can transport nutrients and water from places in the soil that the plant by itself could never have reached. Mycorrhizal fungi are particularly important for the uptake of phosphorus by plants. The presence of mycorrhizal fungi in soils is directly related to the amount of phosphorus plants can uptake, particularly in systems where phosphorus is limited. In addition to building structure and feeding plants, fungi play a role in increasing plants health and resilience. Together with bacteria and other micro-organisms, fungi form a protective layer around the roots and above-ground plant parts. This makes the plants less sensitive to external factors.

Explanation of the score

To obtain a value for fungal biomass, length and diameter of fungi are measured. These numbers are converted into biomass by computer calculations. Fungal biomass is expressed in μg/gram of soil. The desired value for fungal biomass depends on your type of land use. For annual plants, vegetables, and grasslands (including sports, golf, and grazing lands for cattle) the target value is at least 75 μg /gram. For shrubs, vines, and bushes (such as grapevines or berries) the target value is at least 300 μg/gram. For orchards or other systems that involve trees the target value is 700 μg/gram or more. These minimal values are desired, because only then there are enough fungi to provide significant nutrient cycling, plant protection and good soil structure. If fungi are less than the minimal value desired, the fungi in the soil cannot provide these ecosystem services properly.

Fungal quality is obtained by measurements of the diameter of the fungal hyphae present in the sample. Fungal diameter is reported in micrometer (μm). The desired value for the average fungal diameter found in a sample is 2,5 μm or higher. In general, the wider the hyphal diameter, the more favourable the fungus.

The scores for biomass and quality of fungi, provide an indication of the potential of nutrient cycling, nutrient and water retention, and plant protection that fungi can supply.

Fungal to bacterial ratio

The fungal to bacterial ratio (F:B ratio) is the ratio between the total fungal biomass and the total bacterial biomass in a soil. Natural ecosystems have specific soil f:b ratios. These depend on various factors, such as the type of vegetation, the rooting density, soil moisture, soil type, the average annual temperature and the average precipitation in an area (He et al., 2020). Disturbed soils, such as agricultural land, are generally highly bacterial. This is because agricultural practices such as ploughing damage fungi much more than bacteria.

Plants that colonize bare soil quickly (those we commonly refer to as weeds) generally require highly bacterial dominated soils. (Weeds therefore often grow well in agricultural fields.) Most types of grasses and vegetables require more fungi. Perennials generally require even more fungi, and trees require the most fungi, and therefore the highest F:B ratios. A high weed density can be a sign that the F:B ratio is too low for the plants that are grown in a (agricultural) field.

The f:b ratio determines what forms of nutrients will be available to plants. In a soil where bacteria predominate, plant available nitrogen is mainly released in the form of nitrate. In a soil where fungi predominate, plant available nitrogen is mainly released in the form of ammonium. Generally, it can be said that woody plant species (mainly trees, shrubs and vines) require more ammonium, while herbaceous plant species (mainly annual, biennial and perennial herbaceous plants) require more nitrate. Less disturbed soils have higher fungal biomass, and therefore have higher fungal to bacterial ratios. This is because fungi are more sensitive than bacteria to soil disturbance.

The F:B ratio of a soil determines which form of nitrogen becomes available to plants. In a bacterial dominated soil plant-available nitrogen is mainly released in the form of nitrate. In a fungal dominated soil plant-available nitrogen is mainly released in the form of ammonium. In general, it can be said that woody plant species (trees and shrubs) require mainly ammonium, while herbaceous plant species (green plants with little or no woody tissue) require mainly nitrate.

Explanation of the score

The F:B ratio on your scorecard is based on the fungal and bacterial biomasses that were measured in your soil sample. An F:B ratio of 0,5 means there is two times more bacterial than fungal biomass in your sample. An F:B ratio of 1 means the fungal biomass equals the bacterial biomass. An F:B ratio of 10, means the fungal biomass is ten times the bacterial biomass, and so on.

Since there is no consensus in science about target values for F:B ratio in different land use and agricultural systems, the F:B ratio in your soil sample cannot be valued, rather it can be considered as an indication of the successional state of your soil. It can be interesting to monitor the F:B ratio in a field, for example when changes in soil management practices are made.

Protozoa

Protozoa in soil can be subdivided into three main functional groups, namely flagellates, amoebae, and ciliates. Protozoa have a role in nutrient cycling because they eat smaller organisms. The surplus of nutrients that they subsequently excrete can be absorbed by plants as food. In a natural system, around 80% of all nitrogen required by plants is supplied by protozoa, which excrete nitrogen and other nutrients after eating bacteria and fungi.

Flagellates and amoebae found in soils are in almost any case beneficial and indicate aerobic conditions. Ciliates in soils, on the contrary, are undesired. The presence of ciliates indicates anaerobic conditions, which is unfavourable to plant growth. In most cases, when substantial significant numbers of ciliates are present in a sample, it means that the soil where the sample was collected was compacted.

Explanation of the score

Protozoan numbers are obtained by direct counts. This means that individual protozoa in several microscope fields of view are classified and counted, after which the concentration of the different types of protozoa per gram of soil is calculated. Favourable and unfavourable protozoa are displayed separately on the scorecard.

Quantities of protozoa are expressed in numbers per gram of soil. A high number of favourable protozoa results in a high score, while a low number of favourable protozoa results in a low score. For unfavourable protozoa the opposite is true. The desired number of protozoa depends on your land use. For annual plants, vegetables, grasslands (including sports, golf, and grazing lands for cattle), orchards, or other types of trees the target range is >10.000 beneficial protozoa per gram of soil. For shrubs, vines, and bushes (such as grapevines or berries) the target range is >50.000 beneficial protozoa per gram of soil. For any type of system ideally no unfavourable protozoa (ciliates) are found.

Nematodes

Nematodes are non-segmented roundworms that are present in all ecosystems on earth. There are millions of nematode species of which only a small fraction is unfavourable to plants or livestock. Soil nematodes are important mainly because of their contribution to nutrient cycling. Like protozoa, they predate on bacteria, fungi, or other smaller microorganisms and release plant available nutrients. Since nematodes need less nitrogen than protozoa, they release even more plant available nitrogen after eating other soil organisms.

Nematodes can be divided into four main groups, namely bacterial feeders, fungal feeders, root feeders and predatory nematodes. Only root feeders are unfavourable to plants. They damage plant roots by eating or penetrating them. The other three types of nematodes are beneficial. When the biology in a soil is healthy and abundant, root feeding nematodes are outcompeted by other organisms. Additionally, predatory nematodes and certain types of fungi keep root feeders in check because they attack and eat them.

Explanation of the score

Nematode numbers are obtained by direct counts. This means that individual nematodes in several microscope fields of view are classified and counted, after which the concentration of the different types of nematodes per gram of soil is calculated. Favourable and unfavourable nematodes are counted separately. A high number of favourable nematodes per gram of soil results in a high score, while a low number results in a low score. For unfavourable protozoa the opposite is true. The desired number of beneficial nematodes is >100/gram for any type of land use. Ideally, no unfavourable nematodes are found.

Our method

Our method includes manual sample preparation, microscopy, and morphological classification of organisms. It assesses the main organism categories of bacteria, fungi, protozoa, and nematodes, as well as subcategories within those main categories. Organisms are identified based on morphological characteristics, such as shape, size, structure, and colour. We categorize these organisms as either favourable or unfavourable. Quantification of the different organism groups is performed by direct counts (bacteria, protozoa and nematodes) or length-diameter-measurement (fungi).

We think it is important to optimize our method. We do this by reconsidering and improving every step in the process, and by keeping ourselves informed with current scientific knowledge of soil biology and analytical methods. Soil microbiology science is developing rapidly these days. It is therefore possible that this report will look different in the future. If you have any suggestions on how we could improve our method, or this report, we appreciate your feedback.

Improve your soil biology

A healthy soil biology provides overall health and resilience of any natural or cultivated system. In case your soil biology is not optimal, this could negatively affect your soil and plants, in terms of soil structure, water retention and drainage capacity, nutrient supply, plant health and yields.

When there are unfavourable soil organisms such as ciliates, anaerobic bacteria or fungi with a diameter below 2.5 µm in your soil, this usually indicates reduced oxygen conditions. In many cases, an oxygen-poor soil is a result of soil compaction. Compaction occurs, for example, when heavy machines are used, or when a deep compaction layer has been formed by frequent ploughing. In that case it can be worthwhile to change some of your soil management practices, to protect and enhance the soil biology.

There are several measures that can positively influence soil biology, such as reduced tillage, the use of cover crops and green manures, the use of mulch materials, adding high-quality compost or compost liquids, controlled traffic farming, crop rotation, strip cultivation, and the reduction of chemical inputs. The use of woody mulching materials such as straw or wood chips will especially encourage fungal growth, since woody materials are fungal foods.