One of the harmful effects of high concentration of cyanide in growing substrate would be negative impact on photosynthesis through disturbing electron transport in the thilakoids membrane.

please take a look to a paper i've attached

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Solomos and Laties report in PPhys 76 depicts plant tolerance to cyanide  if a respiration alternate path is present, but bacteria and plants are quite resistant targets for cyanide and its heavy metal complexes compared to oxygen obligate consumers like all animals are.

Cyanides in water and soil form hydrogen cyanide that eventually is volatilized into the air and dispenses quickly upwards due to its low density. Micro-organisms in water and soil convert some of the cyanide to less harmful chemicals; some form metal-cyanides.

However, soil concentrations are expected to be low as the cyanide ion is strongly adsorbed and retained in soils, and numerous micro-organisms are able to degrade free cyanide to carbon dioxide and ammonia. As well, some cyanide residues have been detected in food treated with cyanide fumigants. Although cyanides can be found in fish from contaminated waters, the cyanides readily decompose upon heating, and cooked foods contain little or no cyanide. Certain bacteria, fungi, and algae also produce cyanides. Cyanide is an important component of vitamin B-12. Which is an essential vitamin in our diet that prevents vitamin B-12 deficient anemia (iron-poor blood). The cyanide in vitamin B-12 is tightly bound and not harmful. 

Cyanide is a salt or ester of hydrocyanic acid, containing the anion CN − or the group —CN, which is generally extremely toxic to any superior vertebrate organisms. Most of the time, cyanide is found in soil as a result of extraction of gold mining (surface or from sinkhole operations) through milling process of tons of soils and, at the same time it is a powerful poison to humans as well as to wildlife, especially fish (concentrations >20microgram per liter in water) - causes fish death. However, the immediate effects on plant life of cyanide exposure vary tremendously depending on the species of plants. As a result of high cyanide concentrations, plant respiration is inhibited as well as nutrition absorption from soil leading, subsequently,  plant death. In smaller concentrations, cyanide can diminish new growth and can affect germination of seeds; nevertheless, there are some plants, such as cassava, sorghum, cabbage, flax, cherries, almonds and beans already naturally contain small amounts of cyanide that may cause gasses (flatulence) in human beings. Cyanide is not only persistent, but also highly mobile in soil, meaning that it has high potential to affect plants and other organisms in soil rather than being bound up by soil particles. At low concentrations, soil micro-organisms convert cyanide into hydrogen cyanide and other compounds that evaporate out of soil. At high concentrations, however, cyanide is toxic to the very micro-organisms responsible for its conversion into evaporative forms, meaning that cyanide not only remains in soil where it can damage plants but also can easily find its way to groundwater. Actually, soil remediation studies reported that Si reduce the toxic effects in plants caused by heavy metals due to Si bound cyanide and transform it into non-toxic compounds that allow to plants to regrowth soil already contaminated with larger amounts of cyanide. Other reports has indicated that applications of Quick-Sol - 36% Si - have been used to reduce the toxic effect of cyanide in soil. Sodium silicate [Na2SiO3 = Quick-Sol(TM)] at 1 to 3 liter ha-1 reduce cyanide concentration to non-toxic levels in soil.

Can soil microorganisms converts the cyanogenic glycosides in soil to hydrogen cyanide which can evaporate to the air?

It is very important to state that Cyanide is highly dangerous and toxic compound for most living organisms including human being because it forms very stable complexes with some metals that are essential for protein function, such as: Cu2+, Fe2+, and Mn2+ (e.g. Lethal single dose for vertebrates ranges 35-150 μmol•kg-1).

The waste of some industrial factories are the most common source of cyanide, although plants, fungi and bacteria are also natural producers of cyanide too. However, cyanogenic glycosides are compounds that due to enzymatic processes become the precursor of cyanide compounds. Since cyanogenic glycosides are a group of nitrile-containing (α-hydroxynitril) that is used by microorganisms, fungi, or plants in order to growth. Scientific reports indicate that there are more than 60 different cyanogenic glycosides that can be found in plants of beans, broccoli, cabbage, cassava, eggplants, onion, peas, pear, potatoes, and almond kernels. Nevertheless, cyanide in soils may be inhibited by soil microorganisms that metabolize this compound through a biodegradable pathway in order to convert cyanide into ammonium (NH4+) which it is an important source of nitrogen for many plant species. However, ammonium is also toxic to most crop species and its application as the only one nitrogen source is uncommon. Ammonium is a toxic compound that can be metabolized by microorganism leading to hydrogen cyanide (HCN). It is known that HCN is a constituent of tobacco smoke, for instance. Human being, through leukocytes produce HCN during phagocytosis, so therefore it exacerbate bacteria or fungi death. Generally, hydrogen cyanide is a weakly acidic that ionizes in water leading to cyanide anion (CN-) that used to interfere with iron-containing respiratory enzymes. Cyanide mobility in soil occurs by volatilization and distribution due to the microorganism actions. Hydrogen cyanide interferes plant respiratory processes, nevertheless plants defense mechanism occurs right away by bounding HCN to a non-toxic form such as cyanogenic glycosides that are stored in the cell vacuole. The enzyme glycosidase that is present at the cytosol, catalyzes the hydrolysis of the glycoside, so cyanogenic glycoside becomes to be in contact with the residue of glycosidase disrupted when cell is wounded by animal or microorganism attack. As a result, hydrogen is volatilized and the remaining cyanhydrin constituent is unstable and decomposing very rapidly. Although, the presence of cyanide in the environment causes a serious environmental problems due to the formation of stronger and stable metal-cyanide complex that make essential metals unavailable to the organisms, the proliferation of soil microorganism and bacteria lead to produce organic compounds in order to inhibit the toxic effect of HCN within the plant cell. Since cyanide is a natural biodegradable compound, for its degradation and disposable out of plant tissue, a biological treatment might be an excellent alternative.

Article Segie?? I, Bogatek R.. Cyanide action in plants-from toxic t...