With foliar feeding, instead of watering a liquid fertilizer into the soil, it is sprayed (in solution) onto the leaves of the vegetable or fruit crop.
Research has found that a leaf can be a very efficient organ of absorption. The amounts absorbed may at first seem relatively small, but to offset this, the efficiency is high.
So plants are able to absorb essential elements through their leaves as well as their roots. The absorption from the leaves takes place through their stomata and also through their epidermis. Transport is usually faster through the stomata, but total absorption may be as great through the epidermis.
Do not attempt to totally replace a properly constructed soil fertility program with foliar feed as a plant cannot absorb sufficient quantities of most nutrients via the foliage to satisfy its full requirements for those nutrients. On the other hand, it is a fast way to supply micro-nutrients, such as Boron and Manganese, which the crops don't need in such quantities, but which may not be so readily available in the soil.
If the crops are under stress from drought, pest-attack or disease, it's definitely worth giving them a squirt of encouragement by foliar feeding. An ailing plant can perk up quite visibly after spraying with, for example, liquid seaweed, OPF or Vinasse.
Purposes of Foliar Feeding:
- To improve plant health and development when nutrient uptake from the root system is suppressed or delayed
- To alleviate physiological stress after or before exposure of the plant to unfavourable weather events
- To supplement overall nutritional status of plants grown under intensive conditions or low soil fertility conditions
- To supplement nutrient supplies during critical peak demand periods
- Do not apply foliar feeds during the warmest period of the day, during hot weather conditions, or to wilted plants. So best foliar feed very late in the evening or even very early in the morning. Dew will help absorbtion.
- Foliar sprays dry quickly when applied at high temperatures, reducing absorption of the nutrients in solution.
- High light intensities can improve foliar uptake.
- High humidity favours nutrient uptake through the leaves in two ways; by decreasing the rate of drying of the applied nutrient solutions, and by causing the cuticle to absorb water from the atmosphere and swell, which results in the formation of more polar pores.
- Aging leaves develop thick cuticles that hinder foliar uptake. Young developing leaves have thin cuticles and are therefore more efficient at foliar uptake.
- Higher spray volumes result in more uniform coverage and more effective foliar feeding.
- Foliar applications should wet the entire canopy, especially the new leaves.
It is important to understand specifically what the crop needs!
Soil tests are very useful in ascertaining the nutrient status of a field, but even if a nutrient is present, it may not be available to the crop. Tissue testing is a technique that is used more often these days as it can focus on which nutrients are actually limiting crop growth.
The most popular general organic foliar feed is Seaweed Extract, as it is high in trace elements and it also contains natural growth stimulants. Research has shown that applications of Seaweed Extract make plants less susceptible to pests and diseases, including the dreaded potato and tomato blight. The reasons for this aren't completely understood, but it seems likely that it, not only makes the plant stronger, but the micro-organisms in the solution compete with the spores and bacteria that cause disease.
Foliar Feed Application
Don't make your foliar feed too strong as there's a risk of scorching the leaves when salts in the solution are left on the leaf surface. The same dilution that you use for your liquid fertiliser will be safe, always read the instructions first, as dilution rates vary between products: Liquid Seaweed 0.3%, Vinasse 1% and OPF can be used up to 2%.
Research has indicated that water droplet size is not important as far as absorption of the nutrients is concerned. On the whole, though, heavier drops slide off leaves more easily, so, if you have a choice, a finer spray or mist (Sprayer) is better.
Any vegetable with leaves can benefit from a foliar spray. Those vegetables with particularly robust leaves (indicating a thick and waxy cuticle or outer layer of leaf) like cabbage, are unlikely to absorb as much of the feed as other vegetables with softer leaves, but there will still be some benefit and any run-off will go to the roots.
The leaves of vegetables in the greenhouse are softer, therefore they can absorb the foliar feeds better.
- Spray the whole plant, and make sure you cover both the upper and lower surface of leaves.
- Don't spray in direct sunshine, as the drying of the salts in the solution may result in leaf scorch.
- Avoid overhead irrigation until the spray has dried, and do not to spray just before rain, as extra water will dilute the solution and wash some of it away
Liquid feeding is especially effective if a drip irrigation system is set-up, but liquid feeds at the right solution can also be administered with a watering can or our Birchmeier Fertilizer Mixing Sprayer.