Page 12 - 2015 Spring Gazette

MAGNESIUM DEFICIENCY:
Clues & Cures
Image: Magnesium deficiency on geranium
By Rick Yates, GGSPro Technical Services Manager
Magnesium is the central element in chloro-
phyll, making it a crucial nutrient for maximiz- up this group. Iron deficiency symptoms, like magnesium, are often
ing photosynthesis, as well as maintaining
vibrant green foliage. Like calcium, magne-
sium is required by plants in large enough
quantities that it’s not considered a micronu-
trient, but rather a secondary nutrient. Only
nitrogen, phosphorus and potassium (N,P,K)
are needed in larger quantities by plants.
Given magnesium’s key role in chlorophyll
production and function, it’s probably no surprise that one of the key
symptoms of magnesium deficiency is an interveinal yellowing, often
referred to as chlorosis. Other nutrient deficiencies can cause similar
symptoms, so what can be done to help determine if a magnesium
deficiency is the source of trouble? The location on the plant where
these symptoms are occurring is a strong clue.
Some nutrients are very mobile in plants with the ability for older,
lower leaves to give up their bounty in order to supply the newly
forming leaves at the growing tips of the plants. This group includes
nitrogen, phosphorus, potassium and magnesium. With each of
these mobile nutrients, it’s most common for deficiency symptoms
to appear first on the lowest leaves as nutrients are given up for the
youngest growth.
Other nutrients are considered immobile, meaning that the
lower leaves cannot give them up to supply the youngest leaves.
In this case, the deficiency symptoms appear first in the youngest
Left Image: Iron deficiency on petunia
Right Image: Location clues chart
12
foliage. Boron, calcium, copper, iron, manganese and zinc make
described as an interveinal yellowing. Observing the location on the
plant where symptoms began can help you narrow down the pos-
sibilities between magnesium (lower foliage) and iron (upper foliage).
Magnesium is leached easily from soilless media, so it’s preferable
to supply it on a continuous basis. The dolomitic limestone used
in most mixes to adjust the pH does contribute some magnesium,
and most, but not all, water-soluble fertilizers contain magnesium.
The question is whether or not enough is being provided to optimize
foliage color and plant performance. To assess magnesium levels,
consider the levels in both the media and in the fertilizer water to
determine if enough is being supplied to the crop on an ongoing
basis. Water sources typically have at least some magnesium in
them already. Be sure to take that into account in addition to the
amount provided by the fertilizer to determine if additional magne-
sium would be beneficial. For most crops, 30-50 ppm of magne-
sium in the irrigation water is sufficient (includes clear water and
fertilizer). Magnesium is safe even if somewhat more than is required
is provided, although large overdoses could lead to competition
with potassium. Magnesium sulfate (i.e., Epsom salts) is an inex-
pensive source of water-soluble magnesium and is compatible with
water-soluble fertilizers that do not contain calcium. To aid in calcu-
lations, 1 oz per 100 gal of Epsom salts provides about 7.5 ppm of
magnesium. For tank mixing with fertilizers that do contain calcium,
magnesium nitrate can be used instead. The rates are similar to
Epsom salts but the cost is somewhat higher.
Media analysis can be used to verify that your adjustments have
been effective. 40-100 ppm of magnesium is an acceptable range.
Calcium levels should be 2-3 times higher than the magnesium as
a rule of thumb. Pulse applications of 8 ozs per 100 gals of Epsom
salts can be made to quickly correct a magnesium shortfall detect-
ed in the media analysis. The addition of Epsom salts will have a
slight impact on the EC of your fertilizer solution.
The 3rd Edition GGSPro Tech Reference Guide includes a new
chapter, “Water Quality and Crop Nutrition”, which addresses
the most common nutrition problems, including magnesium
deficiency. This chapter also includes many pictures of nutri-
tional disorders, charts and tables to make fertilizer calculations
easy, and instructions on conducting in-house soil tests. It also
includes specific information to assist you to calculate the impact
of additional Epsom salts on the EC of your irrigation water. To
purchase a GGSPro Tech Reference Guide, contact your Griffin
sales representative or our customer service team.