Selenium is an essential nutrient in animals and humans but is not generally considered to be essential to plants. It has been found to be beneficial to some plants, however anything more than a trace amount can cause symptoms of toxicity. Selenium is also toxic to animals and humans if excess amounts are ingested. Selenium is typically low on acidic soils and in high rainfall areas.
If your soil contains selenium at a concentration greater than 200 mg/kg, it would be a good idea to investigate the reason why. It would also be advisable to have some plant tissue from edible plants grown in your garden analysed and to then seek expert advice about any health effects of consuming the produce if the level of selenium is elevated.
Some soils are typically deficient in selenium ‑ Australian and New Zealand soils are good examples. Plants are unlikely to show any symptoms if they aren't absorbing selenium so if you have any doubts about the amount of selenium in your plants, you'll need to have your soil or plants tested. Some laboratories cannot detect the quantities typically present in soil (they can only detect levels if they're in excess of normal amounts) so ensure that the procedures used by the laboratory you choose are sensitive enough (ask them if the test will be able to tell you whether your soil is deficient in selenium). If your soil is deficient in selenium, you can apply a selenium containing fertiliser but be sure to follow the instructions very carefully so that you don't add too much.
Even if your soil contains selenium, you may also wish to test some plants to ensure that the selenium is being taken up by your plants. If your soil contains selenium but your plants don't, ensure that the soil pH is correct and that all the other plant nutrients are correctly balanced.
Note that low selenium in plants is only really an issue if a large portion of your diet consists of food harvested from your garden (meaning that you may not consume sufficient quantities). A blood test can useful in ensuring that you are consuming sufficient selenium. If you mostly grow ornamentals, you shouldn't have to treat any detected soil deficiencies.
Plants suffering from selenium toxicity may be stunted and may die earlier than usual. Leaves of affected plants may exhibit chlorosis or may dry and wither. Some plants can safely accumulate very large amounts of selenium while others cannot so do not be surprised if adjacent plants are affected in different ways.
A soil test can confirm toxic levels of selenium in the soil and a tissue test can confirm toxic levels of selenium in plants. Guidelines may differ in your area but generally, if the concentration of selenium in your soil is greater than 200 mg/kg it is likely that your plants are suffering from an excess amount of selenium and it would be worth ordering a plant tissue test.
If your soil contains excess selenium, investigate the cause. Your soil may for example, be being contaminated by water runoff from nearby industrial processes. Once the source of the problem is identified, you may take steps to prevent further contamination. If your soil naturally contains excess levels of selenium, ensure the pH of your soil is roughly neutral and that it contains plenty of organic matter and appropriate amounts of all the essential nutrients. If some plants still exhibit symptoms of toxicity, select species that are more selenium tolerant.
If you eat large amounts of home grown produce and a soil or tissue test revels excess levels of selenium, you may wish to have a blood sample tested for selenium to ensure you are not ingesting excess levels.