Foliar feeding of plant nutrients is a relatively new agricultural technique compared to the long history of agriculture, and consequently is often poorly understood. At NTS we have been doing research on foliar applications of nutrients and have obtained some excellent results. Further, we have a large range of products that can be foliar applied, so we want to encourage the use of these products because they can give tremendous benefits to growers. Here are our most frequently asked questions:
1) Can plants absorb nutrients through their leaves to bypass nutrient uptake through the root system?
The answer to this important question was provided in the 1950s by H.B. Tukey & S.H. Wittwer from Michigan State University, USA. They sprayed plants with radioactive potassium (K) and phosphorus (P) and then with a Geiger counter measured the absorption, movement and utilisation of these nutrients within the plant. They found that the nutrients moved at a rate of about 0.3 m/h to all parts of the plants.
2) Are foliar-applied nutrients directly absorbed through the leaves or are they washed off and later absorbed from the soil?
Urea has been applied in trials to banana, coffee, cacao and apple plant leaves. Up to 65% of the urea was absorbed within 25 minutes, with the majority of this being absorbed by the younger leaves and/or by the underneath side of the leaves. Total absorption of the urea occurred in bananas within 30 hrs and in coffee and cacao within 24 hrs. The underneath side of young apple leaves absorbed the urea far better than the underneath side of older leaves. These trials clearly demonstrated that nutrients are directly absorbed through the leaves. In fact, it is becoming a popular practice to foliar apply urea as a cost-effective alternative to side dressing,
3) How are foliar-applied nutrients absorbed?
Leaves have transcuticular pores (i.e. pores between cell structures) and stomata through which nutrient sprays can enter the plant. The transcuticular pores are on both the upper and lower surfaces of leaves and are open all the time, so foliar-applied nutrients are believed to primarily enter through these pores. Stomata are present in far greater numbers on the underneath side of leaves, and if they are open and the spray is directed to the underneath side, this can be a good entry point for the nutrients. (The uptake efficiency was 10 to 12 times better through the leaves than through the roots). Pasture grasses and sugarcane have stomata distributed on both sides of the leaf so there is not the same requirement for spraying on the underside of these leaves.
4) Is it only the leaves that absorb nutrients?
No. The original experiments of Tukey and Wittwer showed that buds, twigs, the trunk, flowers and fruit all absorbed nutrients. For example, there is an advantage in spraying boron on deciduous trees, like cherries, during dormancy, to ensure that this important mineral is present at good levels for flowering (when it is most required).
5) Do wetting agents improve the efficacy of foliar feeding?
Yes, it is always a good idea to add a wetter/sticker to the spray to help spread the product so that it is absorbed more efficiently. Our spray oil, Cloak™, has proven a great tool to maximise the benefit of foliar fertilisers.
6) Is foliar fertilising just for “touch ups” or can more profound responses be achieved?
Foliar fertilising is perfect for bypassing soil-based lockups to address trace element deficiencies very effectively. However, it can also be used to deliver major elements with poor mobility, like calcium, directly into the fruit where they are required. Foliar feeding of tomatoes during flower set can dramatically increase fruit production and foliar sprays of boron immediately prior to flowering can have a major impact on the flower to fruit ratio (and subsequent yield).
7) Does foliar application of nutrients have drawbacks?
Foliar application involves time and machinery that is not required for fertigation. However, in many cases, there are excesses present in the soil that are reducing the uptake of other minerals through antagonism (antagonism is the decrease in availability to the plant of a nutrient by the action of another nutrient). In this instance, there is little point in ground fertilising or fertigating as you are simply throwing good money after bad. This is where foliars are the tool to choose. Sometimes the value of enhanced nutrient uptake must be offset against the time/cost factor in each individual situation. There is also a problem with trying to apply large amounts of NPK via the foliar route as there is always a potential to burn the foliage with excess salts and acid. Therefore, fertigation may be preferable if a large, rapid NPK boost is required.
8) Can foliar application of nutrients replace soil application?
The jury is still out on this one as there is some citrus research from California which suggests this is possible. However, from a soil life perspective, mineral balance in the soil is critical, particularly the calcium to magnesium ratio, which determines the entry of all-important oxygen into the soil. There are also timing issues involved as some nutrients can be counterproductive when applied at certain stages of the crop cycle. For example, iron should not be applied during flowering of any crop.
Some nutrients are less mobile than others. An interesting example of this is boron (B). In some plants it is very mobile within the plant, yet in others it is immobile. If the plant sap contains sugars such as sorbitol and mannitol, the boron is complexed with these sugars and moves freely through the plant. In citrus plants, sucrose is a major sap sugar and it does not complex with boron, so boron tends to be largely immobile. In this situation it is a good idea to apply boron regularly with each new flush of leaves. It is also a good idea to use kelp regularly in citrus as it is a rich source of mannitol. Fulvic acid should ideally be included with every foliar spray as it has been shown to sensitise the cell membrane to foster a 30% increase in nutrient uptake.
9) Does foliar application have secondary effects?
Foliar applications can have important secondary benefits. When nutrients are provided to foliage it causes the plants to exude more sugars and other compounds into the root zone. This increases microbial activity around the root zone, which in turn enhances the uptake of nutrients by the plant from the soil. This important activity has been barely recognised in any type of agriculture but our research has demonstrated that this is a major benefit of foliar spraying. When you boost chlorophyll density with foliar nutrition the enhanced photosynthesis feeds more beneficial microbes that in turn can deliver more nutrients to the plant.
10) Do hydroponic systems have special needs for foliar feeding?
In systems without soil, such as hydroponics, nutrient interactions can occur within the root zone that makes it difficult for plants to absorb certain minerals due to binding and antagonism between the nutrients. Iron deficiency occurs in many of these crops when they are stressed by low temperatures. Therefore, hydroponics have a special need for foliar feeding. There is also the issue of nitrate oversupply in hydroponics as this is usually the only form of nitrogen used in the two-part hydroponic solutions. The ammonium form of nitrogen can be foliar applied to provide more of a balance (with an associated increase in crop quality).
Washington State University (USA) has done some excellent research on foliar spraying of commercial tree crops, and their website at http://www.tfrec.wsu.edu/Horticulture/nutspray.html has some very useful information.