How Does A Garden Grow?

For chemists, gardening is a real busman's holiday, as Nina Morgan discovers.

From seed germination to weed and pest control, chemistry lies at the root of all aspects of gardening - whether you're organic or not. Plants, like all living things, are essentially chemical factories. Thanks to photosynthesis, plants are pretty self-sufficient when it comes to organic chemicals. But for inorganic nutrients they have to rely on regular deliveries of the right feedstocks in order to function properly.

Plants require a wide range of mineral nutrients for healthy growth and reproduction. The main ones, often referred to as macronutrients, include N, P, K, Ca, Mg and S. Micronutrients - also essential, but needed in much smaller amounts - include Fe, Mn, Cu, Zn, Bo, Cl, Mo and Ni. Some plants also require traces of Si and Co. Each element may perform several functions within the plant: eg forming part of the organic structure; catalysing enzyme reactions; acting as a charged carrier to maintain electrochemical balance; and regulating osmotic pressure.

Getting dug in
As far as plants are concerned it doesn't matter whether these inorganic nutrients come from a fertiliser manufactured in a chemical factory or one 'manufactured' in an animal's gut or in a compost heap. The important thing is that they are available in a form that the plant can absorb.

The chemical properties of soil play a key role. Plants generally obtain all of their essential nutrients in the form of inorganic ions dissolved in the soil solution - the water in the pore spaces between the soil grains - rather than from the solid soil. For a soil to be fertile - that is, capable of supporting healthy plant growth - it is not enough for the soil particles to contain the necessary elements; these also need to be present in a form that the plants can take up.

Soil is a complex substance made up of particles of mineral and organic matter, with spaces in between filled with water and air. The organic fraction is often loosely referred to as humus and includes both some living and all of the dead material.

The exact composition of the mineral matter, largely derived from the breakdown products of rocks, depends on the local geology. Coarser particles tend to be composed predominantly of quartz (SiO₂), which is more or less chemically inert. So, as far as plants are concerned, the finer-sized fraction is of greater interest. This comprises clay minerals (hydrous aluminium or magnesium silicates); amorphous aluminosilicate materials; sesquioxides (hydrated oxides); and humus - all of which carry an electrical charge. The total charge - or cation exchange capacity (CEC) - and the balance of ions together determine the ability of the soil to replenish some nutrients to the soil solution from where they are taken up by plants.

The acid test
The CEC also determines the pH of the soil. Managing soil pH is important because it affects the availability of plant nutrients (Fig 1). The best range of nutrients is available at pH 6-7, and the optimum soil pH for many garden plants is around 6.5. But thanks to soil acidification - a natural process that occurs when rainwater reacts with atmospheric CO₂ to produce a solution of dilute carbonic acid - soil pH tends to decrease over time.

Fig 1. Relationships between the availability of plant nutrients and soil pH. The wider the band the more available the nutrients

plant nutrients

As soils become more acidic the availability of many plant nutrients is altered. For example, the amounts of exchangeable calcium and magnesium decreases, while the amount of exchangeable aluminium increases. In addition, the negative charge on humus decreases and the positive charge on sesquioxides increases. The effect on some plants can be dramatic. Low pH also affects the activity of many soil organisms, resulting in a decrease in the availability of N, P and S, and the accumulation of undecomposed organic matter.

The commonest way of increasing soil pH is by adding lime (CaCO₃), but adding too much lime can bring problems of its own. Plants vary in their pH tolerance and the foliage of so-called acid lovers, such as rhododendrons and azaleas which prefer a pH of 4-5, can turn yellow if the soil pH is too high. This happens because the alkaline conditions reduce the solubility of iron and manganese, so the plants cannot absorb enough of these elements to remain healthy. Many gardeners solve this problem by applying chelated fertilisers that contain iron, copper, manganese and zinc in a sequestered form that remains soluble at higher pHs.

Various kits are available at garden centres to help monitor pH and other chemical characteristics of soil. But since soil characteristics can vary considerably over just a small area, a more effective way to monitor garden chemistry is to keep a close eye on the plants. Some plants show clear symptoms when an essential nutrient is missing from the soil.

In the UK any deficiencies are more likely to be in the micro- rather than macronutrients. If N, P and K are provided then there are usually sufficient amounts of other nutrients in the soil to allow for good growth. This explains the continuing popularity of inorganic chemical fertilisers such as Growmore.

Dig for victory
Successful gardening depends on more than ensuring the right chemical balance in the soil. Soil texture is just as important. The ideal soil should retain water, yet drain well, be easy to dig, but neither too sandy nor 'clayey'. In practice most gardeners have to make do with what geology and weathering have given us, so we're on the lookout for materials that we can add to improve soil texture. Materials added to soil to change its structure fall into two main groups: adhesives that bind together soil particles to increase the number and size of soil crumbs; and natural materials like composts and manures which, in addition to conditioning the soil, also add nutrients.

Adhesive materials used in commercial soil conditioners include colloids such as alginates, derived from seaweeds. But for those gardeners who prefer a do-it-yourself approach, discussions about improving soil texture and fertility tend to centre on activities such as shovelling...manure.

Garden writers have been advocating the use of muck to increase soil fertility for hundreds of years. For example, Thomas Hill, in his classic book The arte of gardening, published in 1608, included chapters extolling the virtues of dung and outlining the types of manure best for various applications. But Hill's recommendations were based on trial and error and experience rather than chemical analysis.

In the past manure was used to describe any material applied to the soil to improve fertility. These days it is normally used to describe animal dung and urine. All manures are valuable sources of N, P, K, Mg, S and various micronutrients - but the amounts in different types of manure vary considerably. The relative value of a particular manure as a fertiliser depends on the animal species from which it was obtained, the animal's diet, the amount and type of bedding material mixed with the dung, and the degree of decomposition that has taken place before use.

Stable manures - the type most commonly available to home gardeners these days - contain N, P and K in nearly equal proportions and are a good addition to most gardens. But for a real boost, poultry manures are the ones to go for. Chicken droppings, for example, contain higher amounts of N, P and K. And the levels of these nutrients are even greater in pigeon droppings.

But whatever manure you choose, it should be well rotted to reduce the nitrogen levels. The ammonia (NH₄) released from fresh manure is toxic to plants and can cause leaf scorch and root death. In the short term, adding fresh manure directly to the soil will reduce the availability of nitrogen to plants because initially the nitrate (NO₃-) will be immobilised by soil microorganisms. However, fresh manures are excellent additions to the compost heap.

All animals - including humans - are like walking compost accelerators, pre-digesting raw organic material to make an excellent compost precursor, rich in nitrogen and microbes keen to break down organic matter.

Compost chemistry
Many amateur gardeners look on composting as a black art. Some people seem to have that magic touch that allows them to build compost heaps that rot down to sweet smelling friable material in the space of just a few weeks, while others complain about a sodden, smelly heap at the end of the garden that never seems to get any smaller.

But, says Joe Short, a consultant specialising in composting with the Enviros Group, a consultancy that advises local authorities on (among other things) composting projects, there is nothing mysterious about the process. 'Making compost is a bit like baking a cake', he explains. 'Basically you just mix the right combination of raw ingredients together and leave them to cook. It's very much "bucket chemistry".' You don't have to add precise amounts of ingredients or follow complicated steps. Nevertheless, a basic understanding of the process will certainly help you to get good results.'

Composting is essentially a temperature- and oxygen-dependent biologically controlled process that involves the rapid self-heating and aerobic decomposition of organic matter by microorganisms. Successful composting is largely a matter of keeping the microbes happy. Their needs are relatively simple; the basic requirements are warmth, moisture, oxygen, carbon and nitrogen.

Carbon provides both an energy source and serves as the basic building block that makes up around half the mass of the microbial cells. Nitrogen is an important component of the proteins, nucleic and amino acids, enzymes and coenzymes necessary for cell growth and function. Oxygen drives energy production to fuel the aerobic decomposition, and the moisture supports the microbes' metabolic processes.

To create the ideal rapidly decomposing compost heap, aim for a combination of ingredients that will give you a C:N ratio of around 25-30:1. In practice this involves combining carbon-rich ingredients, such as straw, woody garden prunings or shredded and crumpled paper, with nitrogen-rich materials, such as vegetable peelings and grass cuttings, adding water as necessary to bring the moisture content up to 50-60 per cent - then mixing well.

As well as bringing the ingredients into intimate contact, mixing also raises the oxygen levels. For good aerobic decomposition you need to maintain an oxygen concentration of at least 5 per cent in the air spaces of the heap. Building a heap with a porous structure will help maintain oxygen levels by allowing a flow of fresh air. And ensuring that the heap is not too wet will prevent the air spaces from becoming waterlogged.

As the microbes get to work metabolising the 'food' you've provided, they grow rapidly and generate heat. The higher temperatures serve to speed up the biochemical reactions and kill off pathogens and weed seeds.

Sounds simple. But there are a few potential pitfalls. It's important to get the initial C:N ratio right. Too much nitrogen will lead to a build up of ammonia and undesirable odours. Too little nitrogen will affect the growth of the microbes and slow the decomposition process. As the heap decomposes carbon is oxidised to form CO₂, so the oxygen levels decrease. Regular turning will help to keep the oxygen levels up and prevent the heap from becoming anaerobic. If anaerobic conditions develop this will lead to putrefaction and the production of various nasty and smelly compounds, including volatile fatty acids such as acetic acid, propanoic acid and butyric acid; sulphides such as hydrogen sulphide and mercaptans; and amines such as ptomaines derived from partly degraded proteins.

Because most microbes work best at temperatures of around 40?C, it's a good idea to aim for a heap that contains at least 1m3 of material. Any smaller and the resulting surface to volume ratio will be too great and the heap will lose heat more quickly than it can hold on to it. If your heap does warm up, you can congratulate yourself on your technique, because it indicates high levels of aerobic decomposition. But remember also that too much heat can kill the microbes - most species cannot survive at temperatures above 65-70?C - so if your heap gets this hot, it's a good idea to turn it to help bring the temperature down.

The actual decomposition process takes place in a number of stages. Early on, the more readily putrescible materials - such as vegetable and fruit scraps - and the simple carbohydrates (starches and sugars) and proteins they contain, are converted by microbes into compounds like fatty and amino acids. These, in turn, are consumed by other microorganisms. As composting proceeds the microbes oxidise carbon to get energy and the C:N ratio gradually declines as carbon is lost as CO₂.

Around two-thirds of the carbon that the microbes consume is given off as CO₂. The remaining third is incorporated along with nitrogen into their cells and released later when they die. In finished compost, the C:N ratio will be about 10-15:1. Aside from CO₂, other byproducts produced by the microbes include water and ammonium. Towards the end of the composting process, nitrifying bacteria convert some of the ammonium into nitrates (NO₃-) that plants can use.

Compost microorganisms operate best at pHs between 5.5 and 8, but maintaining this is usually not a problem. As the microorganisms digest organic matter they release organic acids. This can result in an initial lowering of the pH. But provided enough oxygen is present, these acids are neutralised as composting proceeds, and the finished material generally has a pH in the range of 7 to 8.5. If anaerobic conditions develop, the acids may accumulate. If this happens, mixing the heap to add oxygen is an effective way to reduce the acidity.

Earth to earth
Although the amounts of nutrients that compost contains are often over-estimated, garden compost is a valuable source of humus. Even when only partly decomposed, it still works as an excellent soil conditioner that can help to improve soil structure, support soil life and recycle nutrients.

But the benefits of composting don't stop at the garden gate. Composting is also being increasingly studied as a useful means to dispose of organic waste that might otherwise go into landfill. Of the over 100m t of waste produced by commerce, industry and households in the UK every year, an estimated third of the household waste could potentially be composted. Statutory UK government targets call for local authorities to recycle or compost at least 25 per cent of household waste by 2005. This adds up to 8m t and provides a strong incentive to investigate ways of composting on a larger scale. Now that should really make your garden grow.

Source: Chemistry in Britain

Acknowledgements

Nina Morgan is a freelance science writer based in Oxfordshire.

1. Minerals, vegetables ...

Clay minerals consist of a series of plate-like layers comprising sheets of silicon, aluminium, magnesium and other cations surrounded and held together by hydroxyl groups. Impurities are often found in the layers of clay minerals, resulting in a permanent negative charge. This negative charge is always balanced by exchangeable cations, such as calcium, magnesium, potassium and hydrogen, adsorbed onto the clay mineral's surface.

In contrast, the charge in the aluminosilicate materials - which develops in association with hydroxyl groups that can form at the broken edge of crystals - varies depending on the soil pH. In alkaline soils the charge is negative, but in acid soils the charge is positive and the soil has an anion exchange capacity that can be satisfied by taking up negatively charged ions such as chloride, sulphate and nitrate.

Humus is a mixture of non-crystalline colloids synthesised by microorganisms in the soil from the breakdown of products or alteration of plant and animal materials. Humus colloids consist largely of carbon, hydrogen and oxygen. They have a pH-dependent negative charge. As a result, humus, like clay minerals, adsorbs cations.

Plants take up nutrients such as potassium, calcium, magnesium and copper as the positively charged ions K⁺, Ca²⁺, Mg²⁺ and Cu²⁺.
Phosphorus and sulphur are taken up as the negatively charged ions H₂PO₄ ^- and SO₄ ^2-. In contrast, the availability of nitrogen is controlled more by the action of soil microbes than chemistry. Most plants take up nitrogen from the soil solution as inorganic nitrate (NO₃ ^-) and ammonium (NH₄ ⁺), both of which are replenished by the microbial breakdown of organic matter. In some plants, notably legumes like peas, symbiotic nitrogen-fixing bacteria allow them to take advantage of atmospheric nitrogen gas.

2. Singing the blues

In most areas of the UK, pink is the common colour for hydrangea flowers, but with a bit of clever chemistry, hydrangeas can be induced to produce blue flowers. Although Victorian gardeners often sang the praises of blue hydrangeas, they found that producing them was a bit of an uncertain operation.

'Blue Hydrangeas are very much admired; partly, perhaps from the difficulty of obtaining them, for no plants can be more capricious,' writes Mrs Loudon, in The ladies' companion to the flower garden, published in 1858. 'Sometimes they come without any trouble at all; sometimes applying any one of the numerous recipes recommended will change the colour, either directly or gradually; and sometimes no care and no recipe has the slightest effect, and the flowers remain pink, in spite of all that can be done to turn them blue.'

The Victorian recipes involved watering hydrangeas with water impregnated with substances such as alum (Al₂(SO₄)₃.K₂SO₄.24H₂O), steel filings, sheep dung, wood ash, peat ash, nitre (KNO₃), carbonate of soda (Na₂CO₃) or common salt (NaCl). But the results were, Mrs Loudon admitted, a bit hit and miss. 'All succeed,' she wrote, 'sometimes'.

Hydrangea fanciers now recognise that the colour change in hydrangea flowers occurs when the plants take up aluminium from the soil - and this is largely controlled by the soil pH. To produce blue blooms in soils where aluminium is present, the soil pH needs to be lowered to 5.2-5.5. Adding organic matter is one way to achieve this, but adding aluminium sulphate is sometimes favoured because, as well as reducing the pH, it also ensures that aluminium is present. If the plants are in the ground, the recommended dose is roughly 3g aluminium sulphate per litre of water. In pots the concentration should be reduced to 1.5g l-1.

3. Liquid human activator

Nightsoil - human faeces and urine - was once highly prized as a high quality fertiliser. In Victorian Britain popular garden reference books were singing its praises. 'Nightsoil is the richest of these manures', reads the entry in Johnson's Gardeners' dictionary, published in 1852, '... if the land be of such quality as to produce without manure five times the sown quantity, then the horse dung manure will yield 14, and human manure 19 and two-thirds the sown quantity.' And if that wasn't enough to convince you of the nutritional value, data from chemical analyses of nightsoils were also published.

These days the use of human manure, whether composted or not, is not officially recommended. But many home gardeners consider human urine - known in some circles as 'liquid human activator' - to be a very useful addition to speed up decomposition on their compost heaps. In the not-so-distant past urine was also prized as a foliar fertiliser.

The fertilising properties of human urine are also the subject of modern scientific studies. For example, a recent project carried out by the Danish Environmental Protection Agency, part of the Danish Ministry of the Environment, examined the potential for using human urine as a fertiliser on organic farms. The study involved collecting urine from the 100 residents of the Svanholm Gods organic farming collective - the largest producers of organic vegetables in Denmark.

Residents' urine was collected using urine separation toilets and analysed to reveal the levels of nutrients (see below). In addition, researchers also measured the levels of pharmaceutical residues, and artificial and natural hormones and heavy metals - all of which might constitute environmental hazards. Their conclusion: human urine shows great potential for use as a fast-acting environmentally benign fertiliser for organic farming which complies with the provisions of the Danish Order on Sludge. (It certainly sheds new meaning on the concept of organic peas and leeks.)

Chemical composition of the urine collected at Svanholm Gods, Denmark:

Total nitrogen 2500 mg l^-1	Ammonium 2200 mg l^-1
Total phosphorus 170 mg l^-1	Potassium 1.2 g l^-1
Sulphur 0.13 g l^-1	Copper 0.30 mg l^-1
Manganese 5.2 ?gl^-1	Magnesium 0.82 mg l^-1

4. Clever composting

Enviros Consulting, a multidisciplinary company that helps organisations with all aspects of composting issues, is just one of a number of organisations in the UK studying the use of composting as a way of reducing the amount of waste going to landfill. Its projects encompass a wide variety of composting issues, ranging from strategy and collection systems to processing technologies and the marketing of end products.

Enviros is currently involved in a number of projects to investigate composting methods for waste disposal. These include:

helping to establish the London Remade organics eco-site at Rainham in the London Borough of Havering, a landfill site operated by Cleanaway.
working with the London Borough of Bexley to monitor the composting of green and kitchen wastes through an in-vessel system. Trials are currently under way using the composts produced as components of growing media to replace peat.
working with researchers in Surrey on a project funded by Waste Recycling Environmental, to investigate the use of concrete tunnels for composting a range of wastes including biosolids, green waste and trade wastes.

Enviros is also involved in several projects to develop markets for compost. These include:

investigating the relative stability of composts produced in different systems to see whether a certain technology can reach a given end point more quickly.
a project funded by Waste & Resources Action Programme to study the use of green compost as an alternative to peat.
investigating the use of compost as a soil improver and mulch in landscaping projects and for planting trees and shrubs.