FAQ − Frequently Asked Questions about Biobeds


What do I have to consider for preparing a good biomixture?

1. The proportions, by volume, must be 2 parts straw, 1 part soil and 1 part peat (or compost).

2. Good mixing of the biomixture components. This gives a homogeneous substrate to support the growth and activity of the pesticide-degrading microorganisms and good sorption capacity. In addition, with proper mixing the water holding properties of the biomixture can be utilized fully. This is important as it is in the water film around the particles where most of the breakdown occurs.

3. The length of the straw. The use of long straw, for example, uncut straw, gives a heterogeneous biomixture which may give lower activity and inadequate sorption capacity. In addition, longer straw results in lower levels of straw in the biomixture as the volumetric weight is lower than for shorter straw. As a rule of thumb the size of the straw should be around 5 cm.


Can a biobed with freshly mixed biomixture be used directly?

Answer: It is recommended that freshly mixed biomixture not be used for at least one month so that it can undergo pre-composting.This will allow a proper establishment of the microorganisms and the grass layer, particularly ifthe strawis toorough.


How often do I need to replace the biomixture?

Answer: Studies have shown that in Sweden the biomixture should be replaced every 5 to 8 years. The frequency might be shorter in warmer countries.


What do I do with the spent biomixture?

Answer: The spent biomixture removed from the biobed should be placed on an impermeable material. Similar material should also be used to cover the pile in case of a heavy rain storm. Any run-off from the spent biomixture should be collected and dispersed on vegetative land. The spent biomixture pile should be located in a corner of the farmyard or other suitable out-of-the-way place.

The spent biomixture should be kept at environmental conditions for one year to promote the degradation of any possible pesticide residues left in the material.


How can the microbial processes in the fresh biomixture be speeded up?

Answer: Save some of the spent biomixture to mix it with the new one. This way the active microorganisms can be transferred to the new mixture.


Why is green grass on the top so important?

Answer: The grass layer helps to regulate the moisture of the biobed by creating upward transport of water and maintaining good levels of oxygen for the microorganisms. In addition, root exudates can support co-metabolic processes, enhancing the degradation capacity of the biobed. Absence of the grass layer gives poor evapotranspiration and can generate a hydrophobic crust on the top of the biobed, which decreases microbial activity.

The grass layer is also an excellent demonstration tool since it reveals herbicide spillages. Grass damage is often observed at sites where the concentrated pesticides are handled, below the middle of the sprayer tank from surface runoff, below pesticide-contaminated wheels, and below faulty tubes and leaking nozzles.


What should I do if the grass layer is damaged?

Answer: Damaged and withered grass must be replaced to maintain a good water balance and reduce the risk of leakage.