Potato Review Group

Contents

Background

Application of magnesium to soil

Foliar application of magnesium

Further information:

Magnesium notes

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Background

  1. Magnesium is the most important micro-nutrient for potatoes.
  2. Magnesium deficiency can result in a lower number of progeny tubers and a lower rate of tuber bulking.
  3. Magnesium deficiency may increase pigment development during bruising.
  4. Magnesium is important for membrane formation and a deficiency may increase susceptibility to bruising.
  5. Magnesium deficiency which reduces yield also increases the propensity to bruise.

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Application of magnesium to soil

  1. Potatoes require 80 – 100 ppm Mg in soil.
  2. It can be expected that availability of magnesium from applied fertiliser will be related to solubility.
  3. Epsom salts (MgSO4.7H2O) has greater solubility than magnesium sulphate (MgSO4), which has greater solubility than dolomite (CaMg[CO3]2).
  4. If soil is dry, nutrient availability will be restricted and foliar application may be necessary. Loss by leaching can occur on sandy soils.
  5. To raise soil magnesium concentration by 1 ppm, 1 kg Mg ha-1 should be applied:
    • Mgfert = 100 – Mgsoil (ppm)
    • (assuming a target of 100 ppm in soil)

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Foliar application of magnesium

Principles

  1. Most crops may benefit from an application at emergence, as a small root system at this time limits uptake.
  2. A minimum of 500 g Mg ha-1 is required for uptake via foliage.
  3. Application of 1000 g Mg ha-1 can be expected to increase the concentration in foliage for a longer duration.
  4. Uptake of foliar applied nutrients can be improved by admixture of a suitable nutrient uptake additive.
  5. At sites with an endemic magnesium deficiency, or if soil is dry, apply at emergence and repeat during the season if necessary.
  6. Do not apply magnesium when stolon tips are swelling.
  7. Magnesium may be applied as:
    • “Bittersaltz” (MgSO4) which provides 10% Mg
    • formulated micro-nutrients but as the concentration of Mg may be low, check that sufficient Mg will be applied.

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Determining requirements

  1. In order to determine whether foliar application of magnesium is likely to be effective, a critical concentration is required.
  2. The Figure below shows a list of target concentrations for different potato plant parts at different growth stages. Note that critical concentration depends on both of these factors.
  3. Having determined that a plant is deficient in magnesium, the next step is to determine how much is required to effect an increase in concentration to the target value. This requires information about biomass growth and efficiency of uptake.
  4. Biomass growth and hence leaf coverage of soil can be estimated from previous year’s growth and a knowledge of the plant; uptake efficiency is likely to depend on several factors including formulation of the specific product used.
  5. See Foliar micronutrition 2017 for a quantitative discussion.

Critical concentrations of magnesium

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Further information

Magnesium notes

Foliar magnesium 2020 (Background; results of experiment in 2019; using tissue tests)

Calcium and magnesium 2018 (Includes: physiology, nutrient interactions, influence on soil structure)

Foliar micro-nutrition 2017 (Includes requirements for foliar application of magnesium)

Magnesium and biostimulation 2012 (Physiology, interaction with other nutrients and requirements)

Magnesium 2004 (A series of experiments showed little response to application of magnesium to soil)

Magnesium nutrition 2001 (Magnesium physiology and application to soil and foliage)

Foliar micro-nutrients 2000 (Includes magnesium applied with other micro-nutrients: single applications may be more effective than mixes)

Micro-nutrients 1999 (Includes magnesium applied with blight fungicides: no effect of foliar application to crop with sufficient magnesium)

Foliar nutrient uptake 1997 (Uptake of magnesium in experiments was improved by a suitable surfactant)

Magnesium nutrition 1996 (Magnesium is the most important micro-nutrient for potatoes)

Foliar application of nutrients 1994 (No effect of foliar application to crop with sufficient magnesium)

Notes for 1993 (Chapter 9: Foliar application of magnesium was beneficial at a site with endemic deficiency)

 

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