Plants absorb the bulk of mineral nutrients and water through their roots, and photosynthesis, the process by which plants create their fuel, occurs in the leaves. So what about foliar fertilisers – can plants take up nutrients through their leaves?
The major pathway for nutrient uptake is by roots. Leaves have a waxy covering which actually restricts the entry of water, nutrients, and other substances into the plant. To a limited extent nutrients applied to leaves can be absorbed and used by the plant, but for the major nutrients (N, P, K) the quantity absorbed is small relative to plant needs. If the plant already has plenty of N, P and K foliar application will not have any beneficial effects. In fact, if concentrations of nutrients in the foliar spray are too high, then leaf damage can occur and in severe cases may kill the plant. That means that foliar application of these three nutrients can only supply a very small fraction of the total needed by the plant.
A distinction needs to be made between foliar fertilization and the application of soluble nutrients in liquid form where the majority of the nutrients taken up by plants are absorbed by the roots from the soil. With heavier liquid fertiliser spraying there will be considerable runoff from the foliage and it will soak into the soil. In this case there would be some nutrient absorption through leaves, but the majority of the nutrients used by the plant would actually be taken up by roots. From the plant’s perspective, this is essentially the same process that occurs when dry fertilizer is added to the soil. It will be a more expensive and time consuming process than a dry fertilizer application.
A classic example of effectively using foliar fertilizers is for micronutrients such as iron. At high soil pH levels, iron is not available to plant roots even though high levels of iron may be present in the soil. Under high pH conditions, iron chlorosis or interveinal yellowing occurs on young leaves. A way to alleviate the chlorosis temporarily is to apply inorganic salts such as iron sulfate or chelated forms of iron directly to the leaves. Applications of iron are most effective shortly after leaf emergence and only on those leaves that have been sprayed. Iron will not move to leaves emerging after a spray, so new leaves will be chlorotic. Foliar iron sprays will need to be reapplied yearly, unless steps are taken to change the underlying soil pH problem.
Important points about foliar fertilization:
- Routine use of foliar fertilizers without a documented need is not recommended.
- Foliar fertilization is unable to meet the total plant requirements for the major nutrients nitrogen, phosphorus, and potassium.
- Foliar fertilizers are only effective when soil problems occur that restrict nutrient availability - such as iron availability in high pH soils.
- Foliar fertilization should not be used as a substitute for good soil fertility management. Have your soil tested and fertilise according to soil test recommendations.
Modified from: Carl Rosen and Peter Bierman. Department of Soil, Water, and Climate, University of Minnesota.