Don’t Bother with Nails
Acid-loving plants need iron but rusting metal won’t help
A Bay Weekly reader told me he throws a handful of nails in the bottom of each planting hole whenever he plants trees or shrubs. The tradition has been handed down from grandpa to grandson. The purpose, he says, is “to provide an adequate supply of iron to the roots, of course.”
He could not tell me if nail size, such as ten-penny, finish nails or shoe tacks, made any difference. He had no preference for rusty nails or new nails.
It took me a while to convince him that adding nails to the planting hole had no beneficial effect other than disposal of rusty old nails. For the growth of plants, nails do nothing. Iron is available to plants only in its ferrous state. When nails rust they form iron oxide, which cannot be absorbed by the roots of plants.
Iron is a very important nutrient in the growth of plants. It serves as a catalyst in the formation of chlorophyll. It is most readily available to the roots of plants when the pH is 5.5 and below. Acid-loving plants have a great affinity for iron in their diet.
Most soils in Maryland have an adequate supply of iron. But there are areas — in and around Hagerstown and Frederick, for example — where iron in soils is limited. In such instances iron can be supplied as iron sulfate, chelated iron or iron in a polyflavanoid.
Symptoms of iron deficiency most often occur on the new growth at the tips of the branches. The tissues between the veins will appear bleached while the mature tissues will appear normal.
Similar symptoms can be caused by repeated applications of hardwood bark mulches because they contain large amounts of manganese, which is released in the soil as the mulch decomposes. When the manganese level in the soil exceeds the iron level in the soil, the manganese inhibits iron uptake by the roots. Unless you have the soil tested for both iron and manganese, you won’t know you have a problem. This is another reason why I discourage hardwood bark mulch in landscapes.