- Blue spruces to be bluer;
- Hydrangea to bloom better and/or have bluer flowers;
- Fruit trees to produce more;
- Oaks suffering from chlorosis (yellow leaves) to green up;
- Plants to profit from an iron supplement;
- Alkaline soils to become more acid;
- And probably dozens of other effects.
And none of the above are true.
It’s easy enough to understand the basic concept behind this myth. If a nail rusts, it’s because it contains iron and iron is one of the minerals that plants need to grow. And once people understand that iron is essential to plant growth, imaginations run wild.
The problem is that the iron produced by rusty nails is iron oxide, an essentially insoluble compound. And since it is insoluble, its iron won’t be absorbed plants. So even if you fill the ground with rusty nails, it changes nothing for the plants nearby… but it does put you at risk of tetanus should ever you scratch yourself on a rusty nail when you garden in the sector.
At any rate, iron is an abundant element in nature and is rarely lacking in garden soil. It is also usually available in a sufficiently soluble form for plants to absorb it, at least when the soil’s pH is somewhat to highly acidic (below 7). If the soil is alkaline, however, (a pH of 7 or above), iron may be present, but fixed to soil particles in such a way that it is not available to plants. This will cause iron chlorosis (interveinal yellowing) in many plants.
When this occurs, however, rather than applying iron in the form of nails, you should first consider trying to reduce the soil’s pH, perhaps by sulfur applications or by mixing peat moss into the soil. Alternatively, you can spray a solution of iron sulphate directly onto the foliage (most plants readily absorb minerals through their leaves) or water the soil with iron chelate, which is a form of iron that is readily absorbed by plant roots. But in no way can rusty nails be useful to plants.
And by the way, adding iron to the soil, even in a chelated form, will do nothing to make a blue spruce bluer. Its color is determined genetically and you can’t change it. But that’s another garden myth worth debunking. Maybe one day soon!