Basic Agricultural Chemistry

Agricultural chemistry is the study of chemistry, particularly organic chemistry and biochemistry as they apply to agriculture—agricultural production, raw product processing into foods and drinks, and environmental monitoring and clean-up. These studies focus on the interactions between plants, animals, and microbes and their surroundings. Agricultural chemistry is a field of agricultural science that examines the chemical compositions and reactions involved in the production, protection, and usage of crops and livestock.

Chemistry is the study of the properties, composition and structure of matter, the changes that occur in matter, and the energy that is released during these changes. In the simplest ways to explain, chemistry can be said to be the study of matter and how different forms of matter combine.

 

Agricultural chemistry

In agriculture, we need to study all aspects of life and nature, and we study chemistry because it helps us to understand the world around us, mostly the things we cannot see with the naked eye.

Everything around you, including a table, a piece of chalk, some food, and water, differs chemically. Our bodies are made up of an extremely intricate mixture of chemicals, all of which react in specific ways to support our ability to move, breathe, eat, and digest food. There are many distinct types of solids, liquids, and gases visible in the outside world. Since chemistry is a natural science, it deals with actual, natural objects that we can touch, see, hear, smell, and feel. Chemistry examines the materials that makeup things as well as how they interact.

Fundamental Terminology and Concepts in Chemistry

Chemistry has its unique terminology and concepts:

Matter is anything that takes up space. It has volume and mass. As agricultural scientists, it is also important to know that the environment is made up of things which are alive (biotic) and things which are not (abiotic). Important abiotic matter includes water, rocks, sand and air. Biotic matter includes all plants and animals. Some things do not fit neatly into the biotic and abiotic definitions. Soil is a good example as it is made up of both a living and a non-living component.

A property is any characteristic that allows us to recognise a particular type of matter and distinguish it from other types.

Atoms are the almost-infinitely small building blocks of matter. Atoms can also be defined as the smallest particles that can be used to describe an element. Each element is composed of a unique kind of atom. Atoms are the smallest particles of matter that cannot be broken down further by chemical means. An atom can be split in a nuclear reaction (for example, in a nuclear power station, or an atomic bomb), but not in an ordinary chemistry laboratory.

 

Parts of an atom

Ions are charged particles that are made when electrons are taken away from or added to an atom.

We see a huge range of matter in our world, but many experiments have shown that all matter is made up of combinations of about 100 substances called elements. An element is a pure substance that cannot be broken down into simpler substances and is made up of one type of atom. An element is basically an atom of a particular substance, which cannot be broken down by chemical means. For example, iron is an element consisting of iron (Fe) atoms.

Currently, 118 elements are known. Elements are denoted by their chemical symbols. The symbol for each element consists of one or two letters, with the first letter capitalised. These symbols are derived mostly from the English names of the elements, but sometimes they are derived from a foreign name instead.

Some common elements and their symbols:

Element	Symbol	Element	Symbol
Carbon	C	Copper	Cu	(from cuprum)
Fluorine	F	Iron	Fe	(from ferrum)
Hydrogen	H	Lead	Pb	(from plumbum)
Iodine	I	Mercury	Hg	(from hydrargyrum)
Nitrogen	N	Potassium	K	(from kalium)
Oxygen	O	Silver	Ag	(from argentum)
Phosphorus	P	Sodium	Na	(from natrium)
Sulfur	S	Tin	Sn	(from stannum)

Different elements (atoms) are defined by the number of protons they possess.

Isotopes are each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in the nuclei of their atoms.

A molecule is a group of two or more atoms which are bound together to form a compound which behaves differently from the original atoms. A water molecule is made up of two hydrogen atoms and one oxygen atom. Hydrogen and oxygen are both gases, yet once combined in a certain way, they form a liquid.

Molecules make up the smallest component of a chemical compound or a new substance. Compounds are substances composed of two or more kinds of atoms (or elements). Most elements can interact with other elements to form compounds. The elemental composition of a compound is always the same (called the law of constant composition).

 

Water molecule

A pure substance (usually referred to simply as a substance) is matter that has distinct properties and a composition that does not vary from sample to sample. A mixture is made up of two or more substances that still have their own chemical identities. Most of the matter we encounter consists of different substances. The substances making up a mixture are called the components of the mixture.

 

Molecular comparison of elements, compounds, and mixtures

 

Some mixtures do not have the same composition, properties, and appearance throughout:

Heterogenous mixture – mixtures that vary in texture and appearance in any typical sample. In other words, a heterogenous mixture is not uniform throughout, and it has two or more states (or phases). This type of mixture can be seen as layers when two or more solutions or substances are mixed. An example of a heterogenous mixture can be oil and water, vegetable soup, and so forth.

Homogenous mixture (also called solutions) – mixtures that are uniform throughout when two or more substances are combined. Many common chemicals are homogeneous mixtures, for example, vodka, vinegar, dishwashing liquid, air, and so forth.

Types of mixtures

 

 

Classification of matter. (Source: Chemistry The Central Science; 14th Edition; Brown, LeMay, Bursten, Murphy, Woodward, and Stoltzfus.)
 States of Matter

The properties of matter depend on both the kinds of atoms it is made of (its composition) and how these atoms are arranged (structure). Matter is typically characterised by:

Its physical state (gas, liquid, or solid).

Its composition (whether it is an element, a compound, or a mixture).

The states of matter refer to the different states or forms matter can be encountered in. the states of matter differ in some of their observable properties:

A gas (or vapour) doesn’t have a fixed volume or shape, it fills its container evenly, and it can be compressed to take up less space or expanded to take up more space.

In a gas, the molecules are far apart and moving at high speeds, colliding repeatedly with one another and with the walls of the container. Compressing a gas decreases the amount of space between molecules and increases the number of collisions between molecules but does not change the size or shape of the molecules.

A liquid has a volume that is different from the size of its container, it also takes on the shape of the part of the container it fills, and it cannot be compressed to any appreciable extent.

In a liquid, the molecules are packed closely together but still move rapidly. The rapid movement allows the molecules to slide over one another; thus, liquids pour easily.

A solid has both a shape and volume that cannot be changed and it also cannot be compressed to any appreciable extent.

In a solid, the molecules are held tightly together, usually in definite arrangements in which the molecules can wiggle (or vibrate) only slightly in their otherwise fixed positions. Thus, the distances between molecules are similar in the liquid and solid states; however, in a solid, molecules are mostly locked in place, while in a liquid, they can move around a lot.

 

Molecule arrangement in different states of matter

The change from one state to another can be caused by changes in temperature and/or pressure.

The Periodic Table

The periodic table of the elements is a table of all the known chemical elements arranged in order of increasing atomic number, with elements having similar properties placed in vertical columns. The table shows the atomic number and atomic symbol for each element, and the atomic weight is often given as well:

 

The atomic number of an element refers to the number of protons in the nucleus of an atom of an element. The atomic weight of an element refers to the average mass of the atoms of an element in atomic mass units (amu). Atoms of a given element can differ in the number of neutrons they contain and, consequently, in mass. The atomic symbol is the same as the chemical symbol of an element.

The horizontal rows of the periodic table are called periods and the vertical columns are called groups. The physical and chemical properties of elements in the same group are often similar. For instance, copper (Cu), silver (Ag), and gold (Au) are all in group 11. These elements are less reactive than most metals, which is why they have always been used to make coins all over the world.

The colour code of the periodic table shows that, except for hydrogen (H), all of the elements on the left and in the middle of the table are metals (or metallic elements). Metals have similar properties, such as shine and high electrical and heat conductivity, and all of them, except mercury (Hg), are solid at room temperature (20 – 22 °C). However, all metals become liquids if heated sufficiently.

A stepped line from boron (B) to astatine (At) separates the metals from the non-metals (or non-metallic elements). Note that hydrogen (H) is not a metal, even though it is on the left side of the table. Non-metals generally differ from metals in appearance and other physical properties.

Many of the elements that lie along the line that separates metals from non-metals have properties that are in between those of metals and non-metals. These elements are often referred to as metalloids.

 

The periodic table of elements. (Source: Chemistry The Central Science; 14th Edition; Brown, LeMay, Bursten, Murphy, Woodward, and Stoltzfus.)

Units of Measurement

There are a lot of things about matter that can be measured, or put into numbers. When a number is used to represent a measured amount, the units of that amount must be given. For example, it makes no sense to say that the length of a pencil is 17.5, but saying the length of the pencil in terms of its units, 17,5 centimetres (cm), makes more sense. The metric system is what is used to measure things in science.

 

SI Units

SI base units

Physical Quantity	Name of Unit	Abbreviation
Length	Meter	m
Mass	Kilogram	kg
Temperature	Kelvin	K
Time	Second	s or sec
Amount of a substance	Mole	mol
Electric current	Ampere	A or amp
Luminous intensity	Candela	cd

In 1960, scientists all over the world agreed on a certain set of metric units to use when measuring things. These preferred units are called SI units, and this system has 7 base units from which all other units arise or originate.

 Length and Mass

The metre is the SI base unit of length.

Mass is a way to measure how much matter something has. The kilogram (kg) is the SI base unit of mass. Mass is not the same as weight. Mass is a measure of how much matter there is. The weight of an object is measured by the force with which that object (of a certain mass) is attracted towards the centre of the earth, i.e., weight is the force that gravity puts on this mass. For example, an astronaut weighs less on the Moon than on Earth because the Moon has less gravitational force than Earth. On the Moon, however, the astronaut’s mass is the same as it is on Earth. Although mass and weight are often used interchangeably, their meaning is different: mass is a quantity of matter, and weight is a force.

Temperature

An object’s temperature, which is a measure of how hot or cold it is, is a physical property that shows which way heat flows. Heat always flows naturally from a substance that is hotter to one that is colder. So, when we touch something hot, we feel a rush of heat, which tells us that the object is hotter than our hand.

In science, the Celsius and Kelvin scales are used to measure temperatures. The Celsius scale was originally based on the assignment of 0 °C to the freezing point of water and 100 °C to its boiling point at sea level.

The SI temperature scale is the Kelvin scale, and the kelvin (K) is the SI unit of temperature. On the Kelvin scale, 0 is the temperature at which all thermal motion stops. This temperature, called “absolute zero,” is the coldest possible. Absolute zero is – 273,15 °C on the Celsius scale. The units on the Celsius and Kelvin scales are the same size. A degree Celsius is the same size as a kelvin. Therefore, the Kelvin and Celsius scales are related according to:

K=°C+273,15

The freezing point of water, 0 °C is 273,15 K.
The common temperature scale in the United States is the Fahrenheit scale, which is not generally used in science. Water freezes at 32 °F and boils at 212 °F.

 

Comparison of Kelvin, Celsius and Fahrenheit temperature scales

Chemical Bonding

When two or more atoms, ions, or molecules come together or are drawn to each other, this is called chemical bonding. This leads to the formation of a chemical compound. This attraction could be caused by ions with opposite charges or by an electrostatic force between two or more atoms. There are many ways for a chemical bond to form, and these interactions are what keep the chemical compound together. The chemical structures made by these bonds are then named and used in chemistry and science.

Ionic, covalent, and metallic bonds are the three main types of chemical bonds:

 

Three main types of chemical bonds

Inorganic and Organic Compounds

Inorganic (or non-organic) compounds do not contain carbon (C), whereas organic compounds always contain carbon usually bonded to hydrogen (H).

 

Organic vs Inorganic compounds. (Source: sciencenotes.org)

In chemistry, the words “organic” and “inorganic” mean something different than when they are used to describe food and plants. Organic chemistry and inorganic chemistry are based on compounds that are made of organic and inorganic matter, respectively. Organic chemists study the way organic molecules are made and how they react with each other. Inorganic chemists study things like salts, metals, and minerals, which are not living things.

The main difference between these two types of substances is that organic compounds always have carbon (C) in them, while most inorganic compounds do not. Because there are inorganic compounds that contain carbon, the presence of carbon alone is not enough to call a compound organic. Most organic compounds have C-H bonds, which are made up of carbon atoms that are joined to hydrogen atoms. There are also oxygen atoms in a lot of organic compounds.

Example of Organic Compounds

Compounds made within living organisms are organic molecules. The main classes of organic compounds are carbohydrates, fats, proteins, and nucleic acids (i.e., DNA and RNA).

 

Examples of Inorganic Compounds

Inorganic substances include all pure elements, salts, many acids and bases, metals and alloys, and minerals. Compounds in which a non-carbon atom forms a chemical bond with hydrogen are inorganic.

Neither Organic nor Inorganic

 

Acids and Bases

Sometimes, atoms or molecules have a positive or negative charge. Nature will attempt to correct this electrical imbalance. As mentioned before, atoms with a charge are called ions. An H+ atom is a hydrogen atom with a positive charge. The presence or absence of H+ ions determines whether a liquid is acidic or basic. A notable exception is distilled water as the positive and negative charges in distilled water cancel each other out. The tap water you drink has ions in it. These ions in solution make something acidic or basic. In your body, there are small compounds called amino acids. Those are acids. Citric acid is found in fruits. Vinegar is an acid. Baking soda, however, creates a basic solution.

The pH scale describes how acidic or basic a liquid is. Although there may be many types of ions in a solution, pH focuses on concentrations of hydrogen ions (H+) and hydroxide ions (OH-). The scale goes from values very close to 0 through to 14. Distilled water is 7 (right in the middle). Acids are found between a number very close to 0 and 7. Bases are from 7 to 14. Most of the liquids you find every day have a pH near 7. The pH of many chemicals is more extreme. There are also very strong acids with pH values below one (battery acid). Bases with pH values near 14 include drain cleaner.

Acids and bases and acidity – alkalinity on the pH scale. (Source: Cambridge.org; Agricultural Sciences; Basic agricultural chemistry.)

Very acidic or very basic chemicals are equally dangerous and will burn you very badly if they come in contact with your skin.