Agrochemicals - Herbicides, Soil Chem and Pesticides¶

Herbicides¶

Are substances which kill plants. They tend to be non-selective in their approach.

Plants have a waxy ‘cuticle’ on their outside which protects them from becoming dehydrated, however a surfactant is typically require to penetrate it.

Beware of wetting agents/spreaders such as “wettasoil” which will make the herbicide spread through the soil and kill lots of unintended plants

Glyphosate¶

Is a non-selective weed killer that is absorbed mostly through the leaves of the plants, however partially through the roots as well.

It inhibits the enzyme EPSP synthase, which is necessary for the production of certain amino acids. It’s cost can vary anywhere from $0.03/g for bulk supplies to $2.15/g for dilute ‘ready to use’ products.

Soil Chemistry¶

Soils are mostly made up of oxides of Si, Al, and Fe, with their classification primarily being carried out based on particle size.

The definitions of these types of soil particle are:

Clay	Silt	Sand
$< 20\:\mu m$	$20\:\mu m - 0.5\:mm$	$>0.05\:mm$

And the percent of particles within each of these ranges determines the type of soil

Loamy sand is generally the best for plants, but all soil needs organic material as well to be useful. This means that bacteria, fungi, and actinomycetes in good soil should number ~1,000,000,000/g of soil and will mineralise the organic matter, acting like a slow release fertiliser

Clay¶

Is acidic, has a high surface area, adosrbs nutrients and holds water. This may seem like a beneficial trait, but if the nutrients are adsorbed to the clay then they’re not being solubilised into the water for the plants to utilise them.

Sand¶

Is neutral, has a much lower surface area, does not adsorb nutrients as much and releases water. This is closer to what we need for plants, as the nutrients are going to be solubilised, but if there’s too much drainage, than the nutrients will all get washed away.

pH¶

It’s really important to control the pH of the soil, as this is going to control the chemistry that happens within the soil, as well as leading to specific physical effects.

In acidic soil, the nutrients will be very soluble and will wash away much faster than they can be taken up by the plant.
In basic soil, the nutrients will form their insoluble hydroxide form and will also be unavailable to the plants.

We therefore need to add different components to the soil to control the pH

Decreasing the pH¶

Alum ($\ce{KAl(SO4)2.12H2O}$) - hydrolyses the water, using up $\ce{OH-}$, and decreasing the pH
Aluminium sulphate ($\ce{Al2(SO4)3}$) - acts in much the same way as the alum
Sulphur anions ($\ce{S^{2-}}$) - can be used, but can have an unpleasant odour
Leaves can be added, which will liberate acid when decomposing, particularly oak leaves, pine needles, citrus fruit peels and nutshells

Increasing the pH¶

Lime ($\ce{CaO}$), aka, quicklime
Slaked lime ($\ce{Ca(OH)2}$) - can burn plants, so needs to be applied only to the soil
Agricultural lime ($\ce{CaCO3}$) is limestone, and is the preferred agent for increasing the pH. the $\ce{Ca^{2+}}$ ions are also highly beneficial to clay soils
Dolomite contains $\ce{CaCO3}$ and $\ce{MgCO3}$ which are good for a slow release
Wood ashes contain potash as $\ce{K2CO3}$, which will both fertilise and basify the soil

Insecticides¶

Insects make up ~76% (w/w) of the total animal mass in the world, many of which are considered pests to agriculture. Chemical agents are therefore implemented to control their growth.

DDT¶

The best and worst of these is probably DDT, which is a colourless, odourless, hydrophobic compound that won Paul Hermann Mueller the Nobel prize. Its usage has since been famously protested by people such as the marine biologist Rachel Carsen.

It was first used on a large scale in the late 1930s and was used to control mosquito populations to reduce the spread of malaria and typhus (mortalitry rates of malaria dropped from 1 in 10 to 1 in 1000).

The big issue however is that while DDT has a short half-life of only 4-6 weeks in water, in fatty environments (where it prefers to be), its half life is 2-15 years. Given this long half-life, DDT is also known to biomagnify as it climbs up the food chain, causing greater issues as it goes.

While DDT in itself may not be harmful to animals, its degradation products can be. In humans specifically, DDT can have oestrogenic effects that can cause hormone disruption. It is however quite toxic to other animals, such as filter feeders.

In insects, DDT works by forcing the sodium channels open, preventing the nerves from being switched off. It is however, possible for insects to develop a resistance to DDT, which is also a big concern.

It’s not widely used as an insecticide, however since its use is most important in developing countries where mosquito borne viruses are a particular concern, it’s the most affordable and effective option.

Combating Biomagnification¶

Modern chemical pesticides are specifically designed with functionalities that will degrade into soluble products, allowing them to be metabolised by the body and excreted safely, one example of these is methocychlor, which replaces the phenolic chlorine substituents of DDT with methyl ethers to promote metabolism.

There’s also the option of adding additives, such as piperonym butoxide which makes the pesticide more effective by deactivating the enzyme that’s essential for detoxifying the organism.

Other Chemicals Used in Agriculture¶

Most fruits will stop ripening once picked, with the exception of avocados and bananas, which will only ripen once picked. When this ripening occurs, the plant undergoes a ‘climacteric’ rise in respiration that make it more susceptible to attack by disease. One solution to this is to make sure that the fruit don’t ripen on the tree. Instead we can control the ripening process by:

Inhibiting climacteric rise with reduced $\ce{O2}$ and temperature
Increasing climacteric rise with Ethylene gas