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QUALITY TESTS AND THEIR INTERPRETATION

Three chemical tests (the determination of the pH, the salinity and three forms of mineral nitrogen) and two biological tests (the open and the closed cress tests) (fig. 1) are presented in the video. You will find the values needed for the interpretation of these tests in the table below (according to the Swiss directive 2010 for the compost and digestate quality).

For the determination of the dry matter (DM) of the compost, dry a sample at 105°C for one day.

Other important quality parameters are the content of other mineral nutrients such as P2O5, K2O, Mg and Ca, and the carbon content of the compost. Analysis of these parameters are more complicated and therefore a sample has to be analyzed by a specialized laboratory. In general, laboratories which analyze soil can also analyze compost. For the interpretation of these results, national guidelines have to be consulted. Often, but not always, the interpretation is integrated in the analysis report of the laboratory.

COMPOST MOISTURE

A compost has to be humid to allow the microorganisms to be active. If the compost is too dry, no microbial activity is possible and the transformation (composting) process of the compost is stopped. If the compost is too wet, undesired microbial processes under anaerobic (= absence of oxygen) conditions will occur and the compost will possibly have a bad smell and contain phytotoxic acids.



A simple test to control the moisture content of a compost is the ‘fist test’. You take a handful of compost, squeeze it firmly and then open the fist.



If the compost is too dry, the compost will then fall apart.



If the moisture content is normal, then compost stays together.



In case the compost is too wet, then water will run out of your fist when you squeeze the compost. Depending on the situation, you can take the needed measures, such as adding water to the compost or cover the compost.