Nickel is a plant micronutrient. It contributes to nitrogen fixation and the metabolism of urea (a nitrogen containing compound) and is important for seed germination. Nickel is also important for bacteria and fungi, which are both important for good plant growth.
Ideally, for healthy and productive soil the concentration of 1‑20 mg/kg. It would be a good idea to conduct further investigations if the concentration of nickel in your soil exceeds 400 mg/kg.
It was only fairly recently that nickel was recognised as a micronutrient in plants so not a lot is known about symptoms of deficiency.
It does appear though that seeds of nickel deficient plants are often unviable (won't germinate and grow into healthy plants) and that the yield of many crops is reduced. Plants may also show symptoms of
If a soil test reveals that your soil is nickel deficient you should ensure your soil's pH is around 7 (or an appropriate value for plants that require a different pH such as acid loving plants like blueberries and azaleas) and that the soil contains plenty of organic matter. Once you have done this, you can apply a nickel containing fertiliser. In most areas, you're more likely to find nickel in a general trace element fertiliser than a fertiliser designed specifically to increase soil nickel levels but a custom fertiliser provider should be able to provide a suitable product if your soil is severely deficient in nickel. Whatever fertiliser you choose, ensure you follow the directions carefully so that you don't accidentally add too much.
Excess nickel can impede the uptake of other essential nutrients especially iron. It can also inhibit seed germination as well as shoot and root growth. Photosynthesis is impeded by excessive concentrations of nickel and flowers are often deformed under these conditions. Affected leaves may show signs of chlorosis and/or necrosis but this is not always the case.
If your plants are showing symptoms of nickel toxicity, first check that the pH of your soil is appropriate for your plants and if it's not, apply sulfur or lime to lower or raise the pH as appropriate. If toxicity appears to be primarily due to a deficiency in another element (a tissue test combined with a soil test will show this if visual symptoms are not sufficient for diagnosis), the soil should be supplemented with that element. If soil tests reveal excess nickel add lots of organic matter as this can help reduce the amount of nickel being taken up by plants until soil levels lower. Whenever excess nickel is detected in plants or soil, only low nickel fertilisers should be used. It's also a good idea to check that irrigation water doesn't contain high levels of nickel (if in doubt, send some water samples away for heavy metal testing).