What is the Soil Life Monitor used for?

The biological parameters of Soil Life Monitor are analyzed using the PLFA method. PLFA stands for phospholipid fatty acids. These fatty acids occur in the cell membranes of living organisms. Different groups of organisms have unique compositions of PLFAs. By measuring and quantifying PLFAs, it is possible to obtain a fingerprint of the soil food web. For example, the cell membranes of fungi consist of different PLFAs than those found in bacteria. The PLFAs present are measured and quantified using gas chromatography-mass spectrometry (GC-MS).

Eurofins Agro measures PLFAs using near-infrared spectroscopy (NIRS): a quick, innovative, and reliable method. NIRS analysis uses PLFA GC-MS as the wet chemistry reference method. By analyzing many samples – both with wet chemistry and with NIRS – the NIRS analysis is calibrated to optimally measure PLFAs in soil samples.

The best time to take a sample for Soil Life Monitor depends on the purpose of the analysis. For plots being monitored over time, the best time to take samples is at roughly the same time every year and under similar conditions each time. Soil life is generally more active in the growing season under warm, moist conditions. In winter, soil life activity slows down. In addition, very dry conditions can kill off or largely inactivate soil life.

The best way to take a Soil Life Monitor sample depends on the substrate.
 

PLFAs are known to degrade rapidly and are therefore indicative of living microorganisms. However, the rate at which the fatty acids degrade depends on environmental factors, particularly temperature. 

Soil Life Monitor

No, the PLFA method can only distinguish between groups of microorganisms such as actinomycetes and arbuscular mycorrhizae. PLFA analysis provides a fingerprint of the soil food web. Plating methods and DNA techniques are more suitable for identifying specific species.

Soil Life Monitor

The target values indicate how the sample scores compare with similar soils or matrices and are based on percentiles of samples taken in practice. The target values of outdoor and greenhouse soil samples are furthermore corrected based on their organic matter content. The target values of soils that are poor in organic matter are lower and lie closer together than soils that are rich in organic matter.

Soil Life Monitor

Actinomycetes are a special type of bacteria. They form thread-like filaments similar to hyphae, but are not fungi, as they have no cell nucleus and their cell walls contain no cellulose or chitin.

Arbuscular mycorrhizae (AM) are fungi which can establish a symbiotic association with around 80% of all plant species. The fungi grow in and around the roots and deliver nutrients and water to the crop in exchange for sugars. The larger the mycelium (the network of fungal threads, or hyphae) around the roots of a crop, the more nutrients, such as nitrate and phosphate, the plant can absorb.

Crops that establish very little or no symbiotic association with AM are plants from the Cruciferae (e.g. brassicas) and Chenopodiaceae (e.g. spinach and beetroot) families.

The PLFA analysis provides insights into the biomass of the active mycelium (network of fungal threads or hyphae) of arbuscular mycorrhizae in the soil. The fatty acid 16:1ω5 is used for this purpose. 

Protozoa are single-celled microorganisms which contain a nucleus (eukaryotes). The most important function of protozoa is to make nutrients available to the crop by 'grazing' on micro-organisms (particularly bacteria). Protozoan activity is highly dependent on the presence of moisture in the soil. The action radius of protozoa is limited to water films and water-filled pores. Small pores can protect bacteria against protozoan grazing.

Bacteria can be divided into two different groups: gram-negative (gram-) and gram-positive (gram+). Gram-positive bacteria are generally larger than gram-negative bacteria and can form spores. This makes them more resistant to drought and water stress. Gram-positive dominant populations (>1) are more common at the beginning of the growing season and balance out again when the soil conditions become more favorable. Gram-negative dominant populations (<1) are often associated with other forms of stress, such as ploughing and the use of pesticides. Gram-negative bacteria can tolerate these types of disturbance better because they have an outer membrane.

The fungal-to-bacterial ratio indicates the ratio between the total fungal biomass and the total bacterial biomass (expressed in g C/kg soil). The ratio can also be used as an indicator for the extent of disturbance. In general, undisturbed ecosystems have a higher fungal-to-bacterial ratio than disturbed systems. Biological and low-input systems have a higher fungal-to-bacterial ratio than conventional, enriched systems. Forms of disturbance such as tillage, removal of crop residues and grazing cause this ratio to drop.

The Shannon-Wiener index (SWI) is a measurement of the ecological diversity of the groups of organisms that occur in the soil. The index uses the number of species and their abundance as input. The lowest value of the SWI is 0 (only 1 species present), and the maximum is dependent on the number of species when they are all present in the same abundance. The index is based on the six groups that are measured with Soil Life Monitor analysis: gram +, gram -, actinomycetes, saprophytes, mycorrhizae and protozoa.

Microbial biomass is a quantified total of a large number of fatty acids. Fungi and bacteria make up the largest proportion of this but do not contain the entire microbial biomass. The unit of the parameters measured is mg PLFA/kg soil. The biomass of fungi, bacteria and microbial biomass in mg C/kg soil is calculated using a conversion factor known from literature.

The maximum duration of an SoilMonitor analysis is three weeks after arrival of the sample at the Eurofins Agro lab in Wageningen.