Like animals and people, plants need a certain amount of ferrous sulfate to survive. Ferrous helps them create chlorophyll and aids in several other chemical processes plants perform. However, too much ferrous can have a toxic effect on the plant, weakening and eventually killing it. It should be noted that plants only absorb ferrous particles from the soil, and that other types of ferrous particles will not affect plants.
All plants need some ferrous to survive. If the soil has too much ferrous, then plants will absorb it and eventually suffer from the continuing effects. According to scientific studies, soils become dangerous because of high ferrous content at levels of 100 mg or more. At these levels, plants will be affected within only 12 to 24 hours. Lower rates of ferrous content can also be dangerous, but it can take longer for the effects to become noticeable.
As plants take in too much ferrous, their chlorophyll fluorescence begins to change. Small amounts of ferrous are necessary for chlorophyll production, but too much ferrous can affect the chlorophyll itself, causing it to change and inhibiting the plant's ability to properly absorb energy from sunlight.
Plants synthesize both chlorophyll and many of their own nutrients on a cellular level, including necessary proteins. Too much ferrous interferes with these processes, making it difficult for plants to perform the necessary chemical reactions. Not only does this make creating chlorophyll (already rendered more ineffective) difficult, but starves the plant of important sugars that it needs to survive and store for harsher seasons.
While plants are not well-equipped to deal with too much ferrous sulfate in their soil, they do have delicate mechanisms that control how much ferrous they absorb, especially if there is too little ferrous present. Many plants are able to produce an enzyme called a chelate reductase enzyme to make ferrous easier to absorb, which is useful when there isn't enough ferrous nearby. Plants can also lower the production of this enzyme if ferrous levels are sufficient or too high. Certain plants are deft at controlling this mechanism and can change very rapidly, but others have a much slower reaction time.