Healthy Soil-- Healthy Roses: Soil Chemistry
Dr. Lakshmi Sridharan
A rose
gardener, who has a thorough understanding of soil chemistry can certainly grow
healthy roses. Soil chemistry is
related to soil pH, mineral composition (inorganic compounds), and organic
matter in the soil.
The alkaline,
neutral, or acid nature of a soil is related to its pH. What exactly is pH? The symbol pH is derived from the German
word, ‘potenz Hydrogen’ meaning "Power of Hydrogen.” Pure water at
equilibrium shows an equal number of hydrogen (H+) and
hydroxyl (OH-) ions.
The pH of a solution is related to its hydrogen ion concentrations. When
an acid is in an aqueous environment, it dissociates into more H+ ions than OH - ions. An acid, therefore,
carries a positive charge. A base or an alkali dissociates into more hydroxyl
ions than hydrogen ions. Hence, a base
carries a negative charge. It is
convenient to express the hydrogen ion concentration in a solution by a
logarithmic pH scale. The pH values
range from 0 to 14. The lower the pH value, the greater is the hydrogen ion
concentration. A solution, at pH 7 is neutral, while at pH below 7, it is
acidic, and at pH above 7, it is alkaline.
Most of the living organisms, including plants, prefer a pH in the range
of 6.5 to 7. 5. Roses grow well in an
acid soil with a pH of 6.5 to 6.8. As
the pH scale is logarithmic, a change of one unit in the pH scale represents a
10-fold change in acidity or alkalinity.
A soil with a pH of 4.0 is 10 times more acidic than a soil with a pH of
5.0, 100 times more acidic than a soil with a pH of 6.0, and 1000 times more
than a soil pH of 7.0. This is one of the major reasons to use caution when
attempting to increase or lower soil pH.
An addition of sulfur to an alkaline soil will lower its pH and an addition
of lime to an acid soil will increase its pH.
While adding lime or sulfur to alter soil pH, closely monitor soil pH.
Simple, easy
to handle kits are available in nurseries.
Take soil samples from several
locations in the garden. Follow the instructions on the kit to measure the
pH. Once you determine the pH of garden soil, you may add the necessary
amendment to alter the pH for a healthy growth of roses. Soil pH has a decisive role in the
availability of soil nutrients to a plant and the nutrient absorption by a
plant.
The acidity
or alkalinity of a soil, depends upon a number of factors: the source of soil
particles, the structure of the soil, the chemical composition, and the
distribution of positive and negative charges.
A clay or a sandy soil may be alkaline or acidic. If the soil pH is too high (highly
alkaline), or too low (highly acidic), some nutrients become insoluble,
limiting the availability of these nutrients to the root system. An excess of
calcium in an alkaline clay soil locks up the mineral nutrients, magnesium,
manganese, iron, zinc, etc. and drastically reduces the availability of these
nutrients for root absorption. For this reason, inorganic fertilizers are
ineffective in an alkaline clay soil, where nearly 80 percent of applied
inorganic nutrients can be locked out from plants. Furthermore, because of
the high retentivity of clay, inorganic fertilizers may build up to a
level that is toxic to plants. A sandy
soil is usually acidic. In an acid sandy soil, calcium, phosphorus, and nitrogen
are usually deficient and less frequently, magnesium and molybdenum are
deficient. Unlike as in a clay soil, a
careful use of inorganic fertilizers can correct mineral deficiencies in a
sandy soil. However, applied in excess,
inorganic fertilizers may destroy soil microorganisms and earthworms. Nitrogen,
phosphorus, and potassium deficiencies may occur both in acidic and alkaline
soils. In an acid soil, clay or sandy, calcium is commonly deficient, and to a
lesser extent, magnesium and molybdenum may also be unavailable. One can not
simply assume that a clay soil is alkaline or a sandy soil is acidic. It is imperative to do soil pH tests to find
out what exactly the soil pH is. In addition to the soil type (clay, sand, or
silt), the organic matter present in a soil, affects the amount of materials
required to change its pH.
The chemical
composition of a soil consists of organic and inorganic compounds present in
it. The earth’s crust contains 92 chemical elements. However, only sixteen
elements are absolutely essential for plant growth. The essential inorganic nutrients are: carbon (C), hydrogen (H),
oxygen (O), nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium
(Ca), magnesium (Mg), iron, manganese Mn), molybdenum (Mo), boron (B), zinc
(Zn), copper (Cu), and chlorine (Cl).
These elements are deemed essential elements as they satisfy the three
principal criteria listed below: (1) absolutely essential for completion of
life cycle - to germinate, grow, and reproduce (flower, and set seed); (2)
cannot be replaced with another element with similar properties; and (3) direct
involvement in plant metabolism. As
mentioned earlier, carbon, hydrogen, and oxygen are absolutely essential for
the syntheses of all organic compounds that are structural and functional
components of a plant (e. g. carbohydrates, lipids, proteins, nucleic acids,
etc). Nitrogen is a component of
protein, amino acids, nucleic acids, etc.
Phosphorus is an important constituent of nucleic acids, phospholipids,
and the energy-rich compounds (ATP, ADP). Calcium is an integral part of a
plant cell wall. The cations such as K, Cu, Fe, Mg, Mn, and Zn are cofactors
for a number of enzymes. The enzymes are essentially proteins. In addition,
most enzymes have a non-protein component, the cofactor, which is absolutely
essential for its activity. The
biochemical pathways in a plant are under the control of specific enzymes. All
the essential elements are directly involved in plant metabolism.
A plant
needs carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium,
calcium, and magnesium in relatively large amounts. Together, these nine
elements contribute to nearly ninety nine percent of the dry weight of a
plant. Carbon (45%) and oxygen (45%)
together contribute to approximately 90 % of the dry weight of a plant. Hydrogen (6%), nitrogen (1.5%), potassium
(1%), calcium (0.5%), magnesium (0.2%), phosphorus (0.2%), and sulfur (0.1%)
make up nearly 8% of the dry weight of a plant. These nine elements are the macronutrients. The rest of the elements, Cl, Fe, Mn, Mo, B,
Zn, and Cu which contribute to less than 0.1% of the total dry weight of a
plant, are the micronutrients or trace elements. The seven micronutrients are often
expressed as parts per million (ppm) of the dry weight of a plant (Cl - 100
ppm, Fe - 100 ppm, Mn - 50 ppm, B - 20 ppm, Zn - 20 ppm, Cu - 1 ppm, and Mo -1
ppm). Except for carbon, and oxygen,
all the other nutrients are available in the soil solution for root
absorption. A root absorbs the
nutrients mostly as ions. The macronutrients, such as potassium, magnesium, and
calcium are available in soil solutions as cations: potassium as K +, magnesium as Mg++, and calcium as Ca++. Nitrogen is unusual in that it is available for absorption both as
a cation, ammonium (NH4+)and
as an anion, nitrate (NO3-). Plants absorb phosphorus and sulfur as anions--phosphorus as H2
PO4- and PO4= ions, and sulfur as sulfate, SO4=. The
micronutrients that occur predominantly as cations in the soil solution are iron
(Fe+++), copper (Cu++), manganese (Mn++), and zinc (Zn++). Molybdenum (MoO4=) and chlorine (Cl-) occur predominantly as
anions. Boron occurs as the neutral
species, borate--H 3BO 3.
Soil
minerals, decomposed organic matter, and chemical fertilizers serve as the
nutrient source for plant use. The availability of soil nutrients to a plant,
largely depends upon the ability of the soil to hold these nutrients on its
adsorption sites (The binding of nutrient ions to negatively or positively
charged soil particles is known as adsorption), and exchange these nutrient
ions with other ions in the soil solution.
The exchangeable cations neutralize the negative charges on the soil
particles and exchange or equilibrate readily with others in the soil solution.
These cations include ammonium-nitrogen, potassium, calcium, magnesium, iron,
manganese, zinc and copper. The Cation Exchange Capacity (CEC) refers to the
ability of the growing medium, the soil, to hold the exchangeable ions within
its structure. The CEC of a soil represents the total number of positive
charges from the exchangeable cations that neutralize the negative charges on
the soil particles. The CEC depends upon the percent of clay in a soil. A sandy soil has negligible CEC,
whereas a soil with a high organic
matter (e.g. humus) has a CEC 10 t0 20 times higher than a sandy soil. Humus and vermiculite with a high CEC are far superior to highly
weathered tropical clay, which is mainly composed of iron and aluminum (Al)
oxides. The tropical clay soil, in
which iron and aluminum oxides predominate, has a very low CEC, therefore not
good for healthy plant growth.
The
adsorption strength (ability to bind with soil particles) of cations varies: Al
> H > Ca > Mg > K. (i.e. Aluminum
has the highest adsorption strength and
potassium has the lowest adsorption strength.) The adsorbed cations, the
acid-forming H+ and Al3+, acidify the soil, and tend to dominate the other
adsorbed cations in very acid soils, whereas the base-forming Ca, Mg, K and NH
4 neutralize soil acidity and
dominate in the neutral and alkaline soils.
Anion
Exchange Capacity (AEC) is as important as CEC. Hydrous oxides of iron and aluminum can have positive sites on
their surfaces, which hold these anions in exchangeable positions. This occurs
in laterite clay because of its high hydrous oxide content.
The
macronutrients as well as the micronutrients must be available in sufficient
quantities for a healthy growth of roses, yet some are often lacking or equally
undesirable - excessive in a garden soil.
Even when present in sufficient quantities, nutrients may not be not be
available for plants, because of high or low pH, cation or anion exchange capacity of soil or climatic conditions. I will discuss the role of nutrients in healthy growth of roses in the forth coming issue of the Criterion.