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Understanding pH management and plant nutrition

Understanding pH management and plant nutrition

Part 1: Introduction

Bill Argo, Ph.D.
Blackmore Company, Tel: 800-874-8660, Int’l 734-483-8661, E-mail: bargo@blackmoreco.com
Originally printed in 2003 in the Journal of the International Phalaenopsis Alliance, Vol. 12 (4).

Plants are basically water surrounded by a pretty
package. If we place 100 lbs. of healthy living plant
material into a special oven to remove all the water, we
will have only about 10 lbs. of dry plant material left.
In general, plants are about 90% water and 10% dry
matter.
The 10 lbs. of dry plant material that we have left is
made up of carbon (C), hydrogen (H), oxygen (O), and
a number of inorganic salts. If we take the 10 lbs. of
dry plant material and remove all the carbon, hydrogen,
and oxygen, there will be about 1 lb. of ash left. Thus,
plant nutrition is the direct management of about 1% of
the plant by weight.
The ash that is left is composed of the essential plant
nutrient. However, these nutrients are not all taken up at
the same rate. The essential plant nutrients can be
separated into two groups, macronutrients and
micronutrients. Macronutrients are found at relatively
high concentrations in the plant tissue and include
nitrogen (N), phosphorus (P), potassium (K), calcium
(Ca), magnesium (Mg), and sulfur (S). Micronutrients
are found at much lower concentrations in the tissue
than macronutrients and include iron (Fe), manganese
(Mn), zinc (Zn), copper (Cu), boron (B), and
molybdenum (Mo).
These twelve essential plant nutrients are commonly
provided by various fertilizer sources, which includes
not only the water-soluble fertilizer, but also can
include the irrigation water and container substrate.
There are several other nutrients that are considered as
essential for normal growth including sodium (Na),
chloride (Cl), Nickle (Ni), and possibly chromium (Cr).
However, these later four essential plant nutrients are
not required by plants in large amounts. Because they
are often found as contaminants in a number of different
fertilizer sources, it has not been demonstrated that that
they have to be specifically apply to plants.

Substrate pH and plant nutrition
The term pH is a direct measurement of the balance
between acidic hydrogen ions (H+
) and basic hydroxide
ions (OH-), and can be measured with a pH meter. The
pH of a solution can range between 0 (very acidic) and
14 (very basic). At a pH of 7.0, the concentrations of
H+
and OH- are equal, and the solution is said to be
neutral.
When growing plants in containers, the pH range
commonly found in the solution extracted from the
substrate is much narrower, from about 4.5 to 8.5. The
recommended substrate pH range from growing plants
in containers is even more specific, around 5.8 to 6.2,
depending on the crop.
The reason that the pH of the solution in the substrate
is so important is that it affects nutrient solubility. Using
Figure 1 as an example, the solubility of micronutrients
(iron, manganese, zinc, boron) and phosphorus decrease
with increasing substrate pH.
Substrate pH can also be an indication of problems.
For example, low pH can be an indication that sufficient
lime was not added to the substrate, or that a fertilizer is
being used that is too acidic for the water quality. High
pH can be an indication that too much lime was added to
the substrate or that there is too much alkalinity left in the
irrigation water.
Substrate pH can also affect the uptake of nutrients
by the plant. Iron (Fe) uptake generally decreases with
increasing pH because it precipitates out of the soil

solution at higher pH levels. Phosphorus (P) also will
precipitate out of solution at higher pH levels.
Phosphorus uptake will be further reduced above a pH
of 7.2 because any phosphorus left in solution is
converted into a less available form. Nitrogen (N)
uptake can be indirectly affected by medium pH
because low pH decreases nitrification (conversion of
ammoniacal nitrogen to nitrate nitrogen) or the
conversion of urea to ammoniacal nitrogen.

Plants and nutrient uptake
Plant species differ in their ability to take up
nutrients at a given pH level. While there are not good
examples with orchids, there are good examples with
other plants produced in containers.
For example geraniums and African marigolds are
very efficient accumulators of iron (Fe) and manganese
(Mn), and are often grown at a relatively high substrate
pH (6.0 to 6.8) compared to most container grown
crops. The high pH reduces iron and manganese
solubility, which limits the uptake, and prevents toxicity
problems.
At the other end of the spectrum are plants like
rhododendrons, blue berries, and petunias, which are
very inefficient at taking iron from the soil solution, and
are often grown at a relatively low substrate pH (5.2 to
6.2). The low pH increases iron solubility, which
increases the uptake, and prevents deficiency problems.
There is a third group of plants, like poinsettias,
chrysanthemums, and impatiens that can be grown over
a relatively wide range of pH’s (5.5 to 6.5) without
showing any deficiency or toxicity problems.

While I don’t know it for sure, I would guess that
orchids are like all other plants. Some species will
perform better when grown at a low pH, some will
perform better when grown at a high pH, and for some,
it will not matter. However, for each of these groups,
the acceptable range where they will grow and perform
the best will be relatively narrow and will be similar
that of other plant species. If you had to choose a pH
range to grow all orchids, then the recommended range
would 5.8 to 6.2, again, just like all other crops.

pH management and plant nutrition.
` Many growers make the assumption that
growing in containers is like growing hydroponically.
Unless water is constantly dripping out of the bottom of
the container, then it is not like hydroponics. Others
consider growing in containers like growing outside in
soil. It is not like that either.
Research has shown that the pH and nutritional
management of container grown crops, including
orchids, is affected by the interaction of a number of
different factors, including the water quality, water-soluble fertilizer, and the substrate. In the next issue, I
will discuss water quality.

 
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