Suitability of nutrients for foliar application
- Foliar application of nutrients may help to overcome transient deficiencies, particularly if soil is dry and root growth is restricted. Response cannot be expected if there is no deficiency.
- Suitability of nutrients for application via foliage depends upon mobility of nutrients in plants and their ability to be re-translocated.
- Nutrients with cyclic mobility are readily transported around the plant and may be moved from leaf to leaf and from leaves to tubers.
- Immobile nutrients are unlikely to be moved out of the leaves to which they are applied and may require repeat applications.
- Nutrients with senescent mobility may be moved out of leaves when they senesce but otherwise have little mobility.
- The most suitable nutrients for foliar application are relatively mobile, the least suitable are relatively immobile.
- Foliar application of nitrogen may be damaging and should be avoided.
- In terms of deficiencies, initial symptoms are likely to be evident on older leaves for nutrients which are phloem mobile (e.g. N, P, K, Mg, Cl, and S).
- Initial symptoms are likely to be evident on younger leaves for nutrients which are phloem immobile (e.g. Ca, B, Fe, and Mn).
- Correcting deficiency of phloem-immobile nutrients by foliar application is difficult, particularly if the nutrient is required in the below-ground portion of the plant: there is no mechanism to move it downwards through the plant.
Illustration of xylem and phloem in plants, and list of relative mobility of different nutrients within the phloem. For reference see Foliar micronutrition 2017.
Factors to consider before applying micronutrients to leaves
- Is there a soil chemistry reason why the nutrient availability from soil is poor? Then foliar may be a good option (ideally, first optimise uptake through roots).
- Is it realistic to apply enough through leaves to correct deficiency?
- Is there a chemical form of the nutrient which will stay in solution effectively enough to enter leaf? What evidence for the commercial product you intend to use?
- Is weather appropriate or will nutrient be washed off / evaporate before uptake?
- In times of drought, stomata close so intake of nutrients by leaf is even lower.
- Is the nutrient mobile in phloem? Will it translocate to the relevant place in plant?
Current and future management of identified deficiencies
Considerations for correcting nutritional deficiencies, from Foliar micronutrition 2017.
Mixtures of micro-nutrients
- Ideally individual micro-nutrients should be applied alone.
- Magnesium may not be taken up if leaves already contain > 0.3 % Mg.
- Manganese and zinc should preferably not be applied in combination with magnesium.
- Uptake of zinc may be reduced if applied in combination with manganese.
Timing of early foliar applications
- A number of foliar applications may be required between emergence and the onset of tuber bulking.
- Nutrient applications at emergence cannot be made in response to tissue tests in the growing crop.
- Historic data should be used to determine the probability of deficiency occurring at emergence.
- Tissue tests in the current crop should be used to build up a database.
- In general, applications should be made in the following chronological order: micro-nutrients and some biostimulants (may be damaging during Os); “Fulcrum” (may be less effective during Os); phosphorus (not damaging during Os).
- Further applications of some nutrients may be required during tuber bulking.
Does mancozeb supply nutrients?
- Mancozeb contains both Mg and Zn, however its primary aim is not to provide nutrition. In order to determine whether a product (whose primary purpose is not as a nutrient) provides sufficient amounts of a nutrient through leaves, the following stages should be considered (and can be considered for any product applied to leaves):
- How much (Zn) is applied – total amount?
- What is the chemical state of the (Zn) when it hits the leaf – and can this be taken up through the surface of the leaf?
- How well does it adhere to the leaf?
- What is the behaviour of the (Zn) when it hits soil? This becomes relevant if foliar uptake is poor but root uptake could potentially become an important pathway.
- In terms of Zn in mancozeb, the absolute amount applied over a potato growing season is probably enough to supply the plant with Zn (six applications assumed). However, it is likely that most of this will either be washed off the leaf or if it does enter the leaf, is unlikely to move to another location in the plant, due to lack of mobility in the phloem.
- Furthermore, low leaf coverage of soil through the period when mancozeb is likely to be applied to the field means that of the amount applied to the field, the majority will not hit leaves but instead will land on the soil. From there, the Zn in mancozeb joins the main pool of soil Zn and its availability is influenced by soil chemistry.
- Therefore mancozeb is NOT considered to be a significant source of Zn nutrition to potato plants. Since the main factor in determining this is leaf coverage of the soil, the same is likely to be true of any other nutrient present in mancozeb.
Notes on foliar nutrient applications
Nutrition 2021 (Includes timing of nutrient applications in relation to crop physiology)
Soils update 2019 (Includes: does mancozeb supply zinc?)
Foliar micro-nutrition 2017 (Principles of application; applying combinations of nutrients)
Surfactants for potatoes 2017 (Includes a surfactant to aid nutrient uptake)
Foliar nutrition 2012 (Experiments including foliar application of potassium)
Foliar applied potassium 2012 (Potential for applying potassium to foliage)
Foliar uptake enhancers 2012 (Uptake can be improved by a suitable surfactant)
Foliar micro-nutrients 2000 (Micro-nutrients applied in combination)
Tuber nutrients 1997 (The potential for foliar applied nutrients to be transported to tubers)
Foliar nutrient uptake 1996 (Application of nutrients with other products)