Please have a look at this researchgate link.

https://www.researchgate.net/post/What_is_the_physiological_effect_on_plants_if_the_soil_is_contaminated_by_a_small_or_large_amount_of_cyanide

 Dear Olatunde,

It is expected that concentrations in the soil are low, because the cyanide ion is not adsorbed or retained securely in the soil and that many microorganisms can degrade free cyanide into carbon dioxide and ammonia.

According to the Environmental Protection Agency-EPA (2012), the acceptable limit of cyanide level in the agricultural soil is about 40 mg CN- equivalent/Kg soil. 

With my best regards

Prof. Bachir ACHOUR

Doc ayodele, Cyanide ion is highly mobile in soil [just like NO3-N] and are also degradable by some soil microbe spp, however its concentration in the soil must be low as its accumulation in the soil tend to destroy beneficial microbes who normally carry out the degradation process. As for critical threshold levels, there are various limits for plants, soil, animal, even air. The problem is, in our country we don't have set limits yet to my understanding. Most times researchers adopt FAO/WHO, US-EPA and sometimes China-SEPAC heavy metal limits which are recommendations based on their own region/environmental conditions. If you are going to work with a set limit, just know it isn't universal.