How Useful Really is Foliar Feeding?

Research at Michigan State University in the 1950’s concluded that like roots, plant leaves are also highly efficient in absorbing nutrients. Soon after this discovery, agricultural companies began producing products designed to deliver nutrients to plants through foliar application, and heavily promoting the method. Today a wide range of foliar feeding products – both organic and synthetic - are on the market.

But just how effective is foliar feeding?

While the 1950’s research at Michigan State was useful in illustrating how nutrients move within plant tissues, and how efficiently plant leaves can absorb nutrients, many of the subsequent advertising claims from foliar fertilizer makers – especially those that suggest that foliar fertilizers have the ability to deliver more nutrients to a plant than soil fertilizers - are exaggerated. Often wildly.

The reason is that while plant leaves are remarkably effective at absorbing nutrients, they still cannot absorb nearly enough nitrogen, phosphorous, and potassium to supply all of the plant’s needs. While it is true that materials applied directly to a leaf are more likely to enter a leaf in larger quantities than the same material applied to the soil, materials applied to the leaf do not necessarily travel throughout the entire plant as effectively as they do through root uptake. They often remain in the same or adjoining tissues but travel no further.

More recent studies have indicated that micronutrients – primarily iron, manganese,  zinc, copper, magnesium, molybdenum, boron, and calcium – are the only minerals that are effectively absorbed by plants during foliar feeding, and that micronutrients applied in excess of a plant’s needs can actually injure or kill the plant.

So while foliar feeding is an effective way to quickly deliver important micronutrients to plants, it should never be seen as a shortcut around the much more important, long term issue of building and maintain soil fertility.

Thinking about a supplemental foliar feeding regimen in your garden? Remember to :

-choose a low analysis organic seaweed or fish emulsion product, or make your own compost tea, which will have little risk of burning plant leaves as the solution evaporates. 

-remember that in order for the foliar spray to stick to the leaves long enough for the nutrients to be absorbed, a wetting agent must be added to the solution. A couple squirts of dish soap can even be used as a wetting agent.

-it’s best to apply foliar sprays in the morning or evening (never in direct sunlight) to avoid the possibility of leaf burn that can occur when spraying plants at hotter times of the day.

-spray the entire plant including the tops and undersides of leaves as well.