AA - ASPECTS OF AGRICULTURE
The Soil 
p-Block Elements
Fertilisers, Pesticides and Herbicides
Introduction
The Soil
Clay Minerals
Ion Exchange
Effect of pH on Soil
The Effect of Temperature on Reaction Rate
Introduction
Growing crops for food has long been a major problem. The rapidly increasing world population means that the need to provide enough food without destroying the environment is a major challenge facing us.
The nutrients essential for plant growth are:
| Elements used in relatively large amounts | Elements used in relatively small amounts | ||
|---|---|---|---|
| iron | ||
| manganese | ||
| boron | ||
| molybdenum | ||
| copper | ||
| zinc | ||
| cobalt | |||
| chlorine |
The Soil
Soil is a mixture of weathered rock fragments and organic matter. Together they make a porous fabric which can hold both air and water. The quantities of air and water in a soil are variable and their proportions are important in determining how well the soil supports plant growth. The minerals play a vital part, both in retaining water and in making nutrient ions available to plants.
Soil minerals are produced by weathering of the rocks which make up the Earth's crust. Weathering is the action of wind, rain, frost and sunlight. Weathering breaks up the rocks and makes a soil from the uppermost layers. Temperature has an effect on the rate of weathering.
Clays are made up from silicates. A silicate ion, SiO4-, has a silicon atom covalently bonded to four oxygen atoms at the corners of a tetrahedron. Silicon is at the centre of the tetrahedron. Many minerals are silicates in which the SiO4 tetrahedra are linked by a shared oxygen forming chains. Asbestos consists of a double-stranded chain structure. Micas and clays consist of silicate tetrahedrons built up by joining the strands into sheets. The overall structure of the sheet is (Si2O52-)n. Cations such as Na+, Ca2+, Mg2+ and Al3+ are held to the silicate sheets to balance the charge. During weathering aluminium ions can replace silicon (IV) atoms at the centre of the tetrahedra in the silica sheets to give a variety of different minerals. This alters the charge balance so extra cations are needed.
Clay Minerals
Clay minerals contain two different types of sheets:
- tetrahedral sheets based on silica tetrahedra with varying amounts of Al(III) substituted for Si(IV)
- octahedral sheets based mainly on Al2+ and/or Mg2+ ions surrounded by six oxide or hydroxide ions in an octahedral arrangement. The octahedra link together in the sheet by sharing oxygens.
These sheets form into layers in different ways.
- Kaolinite is a 1:1 type clay - each layer is made up of one tetrahedral and one octahedral sheet. The layers are held closely together
by hydrogen bonds. Water cannot get between the layers.
- Vermiculite is a 2:1 type clay - one octahedral is sandwiched between two tetrahedral sheets. Water can get between the layers, hence this expands when wet; montmorillonite is another example. Cations are attracted to both internal and external surfaces of these clays. Many of these cations can be replaced by different ions from soil solution. This process is called ion exchange. For example NH4+ ions can exchange with Ca2+ ions. The balance of charge must be maintained, so two singly charged ions replace one doubly charged ion.
clay-2NH4+(s) + Ca2+(aq)The ability of a clay mineral to exchange ions is measured by its cation exchange capacity. This is the amount in moles of exchangeable positive charge held by 1 Kg of the mineral. Cation exchange capacities vary widely, depending on the surface area of the mineral and the number of charges on the inner and outer surfaces. Typically they are 0.02 - 0.6 molckg-1.
Plant roots withdraw nutrients from the pool of exchangeable cations. There is an equilibrium between those held by clay and those free in the soil solution.
Ion Exchange
In ion-exchange reactions, ions from solution change places with ions held by a solid. The solid may be a synthetic resin or a natural material such as an alumino-silicate clay.
Ion-exchange resins are used to soften water. Water in many regions is 'hard', i.e. it contains calcium and magnesium ions which can cause problems in pipes and boilers. Calcium and magnesium ions in the water change places with sodium ions on the resin.
(resin)2-Ca2+(s) + 2Na+(aq)When the ion exchanger becomes exhausted, it can be regenerated by passing a concentrated solution of sodium chloride through the column. The strength with which an ion is held to a resin depends on the nature of the ion exchanger and the size and charge of the ion. Charge and size of an ion also affect how an ion behaves in solution. An ion which is highly charged and small has a high charge density.
Ions with a high charge density tend to:
- attract water molecules strongly and become hydrated
- attract other ions strongly, forming ionic lattices with large lattice enthalpies and hence higher melting points.
As you go across a period, the atomic radius gets smaller because electrons are going into the same shell and the nuclear charge is increasing. Going down a group the atomic radius increases because there is an extra shell.
When atoms become ions, electrons are added or taken away. A chloride ion, Cl-, is bigger than a chlorine atom because it has gained an electron. A sodium ion, Na+, is smaller than a sodium atom because it has lost its outer electron. This situation changes when ionic substances dissolve in water. The ions attract water molecules and become hydrated. This makes the ions bigger, because it has layers of water molecules around it. The higher the charge density of the ion, the more water molecules it attracts, and the bigger it becomes. Hence an ion which is small in the absence of water becomes large when it is in the aqueous phase. A Mg2+ ion is smaller than a Na+ ion but when hydrated it is the other way round because 5 water molecules surround the Na+ but 15 surround the Mg2+.
Effect of pH on Soil
H+ ions displace Ca2+ and other ions from soil solids. This makes the soil more acidic and reduces the store of nutrient ions. H+ ions also increase the weathering of clay minerals and releases aluminium oxide into the soil. High aluminium concentrations in the soil solution are toxic to crops.
The pH can be raised by adding a base such as limestone or chalk. The soil acts as a buffer, so the amount of base required depends on the buffering capacity of the soil.
The Effect of Temperature on Reaction Rate
Reactions occur when the particles of reactants collide, provided they collide with a certain minimum energy. Not every collision results in a reaction. The combined energy of the reacting particles must overcome the minimum energy barrier, or activation enthalpy.
If the temperature is raised, the particles move faster, so they collide more frequently. How much more frequently can be worked out because the average speed of molecules is proportional to the square root of the temperature. If the temperature is increased from 300K to 310K the average speed increases by a factor of 1.016, but the rate approximately doubles. There must be more than just making the particles collide more frequently. What matters is not just how frequently they collide, but also with how much energy.
At any temperature, the speeds, and hence energies, of the molecules in a substance are spread over a wide range. In a gas they follow the Maxwell-Boltzmann distribution. The distribution changes with temperature. If we consider a reaction with activation enthalpy +50 kJ mol-1, the number of molecules with energy greater than 50 kJ mol-1 is shown by the shaded area under the curve. When the temperature is increased by 10K, a larger proportion of the molecules have more energy than 50 kJ mol-1.
Reactions go faster at higher temperatures because a larger proportion of the colliding molecules have the minimum activation enthalpy needed to react.
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