6
Chemistry In The Garden
6.1 Insecticides The general definition of a pesticide is a substance which is capable of selectively killing one or a group of insects without harming the rest of the biological community. Desirable properties for insecticides An ideal insecticide should have the following properties:  very high toxicity to target species – meaning that very little of the substance need be added to achieve the desired effect  low toxicity to mammals and non target species – selectivity is critical  controlled persistence – the substance should remain active for a specific time, then degrade  easy degradation to harmless products Problems associated with the use of insecticides These are obviously closely linked with the desirable properties in that basically this is a list of how current insecticides fall short of being ideal. These include:  environmental persistence and contamination – the long‐term (20 year plus) stability of some insecticides has led to their accumulation in the environment in various food chains; chronic exposure to the compounds leads to toxic effects in animals; this was highlighted in the book Silent Spring, written by an American biologist in the late 1950s, pointing out the problems being caused by overuse of the organochlorine insecticides  killing of non‐target species – insects that “good” in the environment can also be wiped out by the insecticide, unless it is very selective; this can rebound up the food chain as species which rely on the insects for their food, starve  development of resistant species – you will be familiar with the term LD50 from your OH&S module; in general terms, what it means is that a particular dose of a poison will not kill all the population; it will in fact leave the most resistant; when these organism breed, the next generations are all resistant, thus requiring higher doses of the poison, until a point is reached where it becomes almost ineffective Major groups of insecticides In general insecticides may be classified into groups according to their chemical structure and physiological action. These groups are:  organochlorine compounds  organophosphorous compounds  carbamates  plant extracts  pheromones Organochlorine Insecticides These were very popular during the middle of the twentieth century, but their low biodegradability and their suspected role in causing cancer has led to most of them being either banned or greatly restricted in use in many western countries. Some of the more important organochlorine insecticides include DDT, Lindane (also called BHC and Gammaxane), methoxychlor, heptachlor, dieldrin, aldrin and chlordane. Some such as DDT are still extensively used in parts of Africa as they are inexpensive and highly effective against locust plagues. 6. Chemistry In The Garden (a) (b) Cl
Cl
CH3O
H
Cl
Cl
Cl
Cl
(c) OCH3
H
Cl
Cl
(d) Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
FIGURE 6.1 Organochlorine insecticides (a) DDT (b) methoxychlor (c) lindane (d) chlordane Organophosphorous insecticides The term organophosphorous insecticide refers to a very broad range of substances with greatly differing selectivity, activity, environmental persistence and modes of action. The central phosphorous atom forms a double bond to either oxygen or sulfur and three single bonds to chains beginning with an oxygen atom. They are also known as phosphate esters, because they are based on phosphoric acid. Some of the more important commercially used organophosphorous insecticides include parathion, malathion, dichlorvus, rogor (dimethoate) and TEPP (tetraethylpyrophosphate). All are inexpensive to produce, and most are still commercially available. Dichlorvus, which used to be a major component of surface sprays such as Baygon and in the old Shelltox insect strips, is no longer used in this country as it is a suspected carcinogen. (a) (b) O
O
S
CH3CH2O
P
O
OCH2CH3
CH3CH2O
NO 2
P
CH3CH2O
O
P
OCH2CH3
OCH2CH3
FIGURE 6.2 Organophosphorous insecticides (a) parathion (b) TEPP IPC 6.2 6. Chemistry In The Garden Insecticides Derived from Natural Products There is a growing trend in the insecticide industry towards the use of substances derived from natural products. The most important of these are the group of synthetic compounds, based on the pyrethrins, which were originally extracted from an African chrysanthemum plant). These substances have the advantages of: high toxicity to insects 
very low toxicity to mammals 
short lifetimes 
These characteristics make them absolutely perfect for indoor sprays that will inevitably come into contact with humans. Some of the more important synthetic pyrethrins include permethrin, tetramethrin and allethrin. However, in the case of surface sprays, it means that they must be applied regularly. 6.2 Fertilisers A fertiliser is any material, organic or inorganic, natural or synthetic, that is added to soil to supply one or more nutrient elements. Because of intensive cropping, soil fertility can only be maintained by the addition of fertiliser. Nutrients are classified as macro or micro on the basis of their content in normal plants: macronutrients have levels of greater than 500 mg/kg. TABLE 6.1 Macro‐ and micronutrients in plants Macro Micro 
nitrogen 
boron 
phosphorous 
iron 
potassium 
manganese 
calcium 
copper 
magnesium 
zinc 
sulfur 
molybdenum 
chlorine Types of fertilisers As mentioned above, fertiliser can be in a number of different forms. The most important division is between not natural versus man‐made, but organic and inorganic. Organic fertilisers have the macronutrients – especially the key three: N, P and K – but also organic matter which enriches the soil. Inorganic fertilisers, which are generally more processed, only provide the macro‐ and micronutrients. Inorganic fertilisers For most inorganic fertilisers, which are a mixture of salts of the various nutrients, the N:P:K grade is an important measure. This is simply the %w/w of the three elements (with P expressed as P2O5 and K expressed as K2O). For example, a fertiliser with a grade of 10:6:8 is composed of 10% N, the equivalent of 6% P2O5 and the equivalent of 8% K2O. The fertiliser does not actually contain these forms of P and K; they are simply the standard way of reporting their levels. It is like the reporting of water hardness as mg CaCO3/L even though there is no calcium carbonate as such in the water. IPC 6.3 6. Chemistry In The Garden To convert between the actual (total) and reported (grade) for levels of P and K, use the following factors. x 0.44
%P
÷ 0.44
Fertiliser
Grade
÷ 0.83
%K
x
0.83
The levels of the nutrients varies greatly, depending on the intended use for the fertiliser. General purpose fertilisers often contain only the macronutrients (and most will not have magnesium). There are many different fertilisers designed for particular applications: 
complete – have the full set of macro‐ and micronutrients, trace element mixture – micronutrients only 
specific plants – formulations for plants requiring nutrients in different proportions to normal, 
eg rose, citrus, azalea & camellia) 
slow release – have a special organic coating on the fertiliser granules. The coating slowly decomposes, allowing a gradual release of the fertiliser over a number of months. This reduces the need for careful attention by the gardener, but importantly provides a steady flow of nutrient to the soil, rather than a major burst, then nothing soluble – all components are soluble, providing a quick tonic for plants, but one that is readily 
lost by leaching; ideal for pot plants and flowering plants; these tend to be high in nitrogen (above 25%) to give the extra boost in growth The distinction between the different forms of the particular nutrients – ammonium vs nitrate for N, water soluble vs citrate soluble/insoluble for P – is important because of the different availability of the various forms. Citrate solubility for phosphorous is a measure of the solubility in a solution of neutral ammonium citrate, which presumably is supposedly to be a simplified representation of soil solution. Nitrogen industrially is fixed by a chemical reaction from N2 to ammonia. From there, it is converted into ammonium by acidification, usually with nitric acid, to give ammonium nitrate, which has the two useful forms of nitrogen for plants. The sulfate and phosphate forms are also used in fertilisers. All ammonium salts are very soluble. One other form of nitrogen in fertilisers is urea (NH2CONH2), which slowly decomposes in the soil to form ammonium. This of course is generally converted to nitrate before uptake. Phosphorous is primary available from large deposits of phosphate rock, which is essentially mineralised bird and bat faeces. Some South Pacific islands, like Nauru, are almost completely composed of this rock. The mineral phosphorous is very insoluble, and essentially useless as a fertiliser. It is processed by treatment with phosphoric acid into triple superphosphate, which is calcium dihydrogenphosphate. This is water‐soluble, with a fertiliser grade of 0‐45‐0. Ammonium phosphate, formed by the reaction of ammonia with phosphoric acid has a grade of 10‐34‐0. Potassium is present in fertilisers as the chloride salt, which is produced either from wood ash treated with hydrochloric acid, or from mineralised salt deposits. IPC 6.4