In the natural order of things, plants take nutrients from the soil and the atmosphere as they grow, and as they die and decompose nutrients are returned to the soil. When we remove plants, as food or timber for example, we deprive the soil of these nutrients. If we continue removing without replenishing, the end-result is infertile soil and inferior, or no, crops. Nourishing the soil is the foundation of organic gardening.

Healthy soil is important because it:

  • is the source of most food production
  • influences water quality through run-off
  • helps maintain biodiversity and minimises agrichemical pollution and nutrient leaching
  • is a major carbon store and so reduces atmospheric methane which contibutes to climate change
  • retains water and therefore lessens flooding risk
  • retains water and therefore reduces need for water for irrigation
  • optimises the nutrient content of food and reduces agrichemcial residues

Soil makeup

The ideal soil contains roughly: 45% minerals (particles), 25% water, 25% air, 5% organic matter. Humus in the organic component allows the sol to retain this large percentage of water.

When organic gardeners care for the soil, they are principally nurturing its microbial life. Soil microbes help enliven the soil and allow it to function properly by:

  1. converting organic matter into humus.
  2. giving soil its structure – including water retention, good drainage, aeration, compaction resistance and allowing plant roots to penetrate. As soil microbes break down organic matter to produce humus, they secrete a ‘glue’ that hold soil particles together forming the soil crumb and thus help prevent erosion.
  3. activating the organic nutrient cycle – releasing minerals from the sub-soil, fixing nutrients from the air, making nutrients accessible and transporting nutrients dirently into plant roots. Plants have no digestive systems and so do not produce enzymes to break down substance. They rely on the biological life in the soil to do this for them.
  4. adding to the soil’s ability to combat climate change by oxidising methane, a more potent greenhouse gas than carbon dioxide.
    controlling soil-borne pathogens.
  5. Artificial fertilisers and pesticides inhibit or destroy soil microbial life. Clean, organic composts, such as worm castings and other sources of organic matter with microbial activity, enhance soil microbial life.

Soil organisms

Bacteria are the dominant microbial group in terms of numbers, but other important groups are fungi and mycorrhiza, which form close associations with plant roots.

Larger organisms like worms and snails break up organic matter into small pieces and take them below the soil surface. The various species of bacteria and fungi then secrete very specific enzymes which ‘chop up’ long chain molecules at specifc places to form simpler molecules. A vast diversity of micro-organisms are needed to completely break down organic matter and make excess nutrients available for the plant.

The role of microbes in making nutrients avialable is so important to plants that they actively nurture the development of rich microbial communities around their roots. The energy rich organic acids released by the root feed the microbial populations, and plants spend from 10 – 90 percent of their energy supporting microbes in this way.

Earthworms and other soil dwellers

According to Charles Darwin, “Wormshave played a more important part in the history of the world than most persons would at first suppose.” Worms create tunnels which help air and moisture flows, digest plant debris, soil etc and excrete it as fertile, microbial-rich humus. They are high on the organic gardener’s best friends list and are a good indicator of the soil’s health. Compost worms are different from earthworms adn can only live in the confines o rich decaying organic matter. Worm castings can increase nutrient availability to plants by as much as ten times.

Many other beneficial creatures live in or on the soil including:

  • Slaters – they recycle organic debris and occasionally eat small seedlings or damge roots.
  • Carabid beetles – shiny purple-black backed beetles up to 2.5 cm long are mostly nocturnal and prey on slugs, caterpillars and smaller insect pests.
  • Rove beetles – brown and black, like earwigs without pincers, found under mulches and in compost pile, they eat slugs and grubs.

Caring for the soil – crop rotation

Crop rotation is something of a lost art. It is a traditional method used to cleanse, protect and replenish the soil and involves growing different annual or perennial crops in the same area on a planned systematic basis. It is the opposite of continuous cropping, i.e. growing the same crop in the same area year after year. Crop rotation is useful in:

  • improving and maintaining good physical, chemical and biological soil conditions
  • reducing erosion
  • eliminating the need for nitrogen fertiliser by including legumes in the rotation
  • disrupting the build-up of insect populations and disease life cycles
  • discouraging weeds
  • encouraging healthy root systems which retrieve nutrients from the soil and minimize leaching
  • maximising use of nutrients e.g. following a nitrogen-fixing legume with a crop that needs high nitrogen soil content.

A Simple Crop Rotation*

Plot APlot BPlot CPlot D
Year 1RootcropsPotatoesLegumesLeafy Vegetables
Year 2PotatoesLegumesLeafy VegetablesRootcrops
Year 3LegumesLeafy VegetablesRootcropsPotatoes
Year 4Leafy VegetablesRootcropsPotatoesLegumes

Quick growing plants such as lettuce salad crops can be fitted in any gaps. Tomatoes, corn, and curcubits (pumpkins, cucumbers etc) fall into the leafy vegetable part of the rotation.

* Henry Doubleday Research Assocation, UK.

Green crops

A green crop is a quick growing plant sown thickly on vacant ground or within a vegetable rotation and then dug into the top-soil. Within the soil it breaks down to humus and release nutrients.

Green crops are usually planted in late autumn, grow through the winter and are dug-in in spring. It is most important that the green crop is still soft and watery when dug-in so that it breaks down easily. Allow three weeks after digging-in before planting the next crop. If space or time is at a premium the green crop may be bettter harvested and composted, or left as mulch to allow immediate planting of the crop to follow. Green crops have several functions:

  • as a protective cover for the soil
  • to gather nutrients for use by crops to follow
  • to maintain humus levels
  • to preserve soil structure
  • as shelter and a food source for beneficial insects

Useful green crops

  • Legumes – important for maintaing fertility; add nitrogen to the soil. Lucerne and clover are particularly good being deep rooting. Also useful in compost.
  • Lupins – excellent nitrogen fixers; deep roots mine nutrients from the subsoil.
  • Comfrey – use as a permanent stand and harvest for green manure or compost. Comfrey roots penetrate three metres or more into the subsoil, mining nutrients and replenishing topsoil. It regenerates rapidly from even the smallest piece of root, so DO NOT dig it in.
  • Winter Cereals – produce large bulk through the winter but if left quickly become woody; strong root systems improve soil structure.
  • Annual grasses – similar to winter cerreals but generally produce less growth in winter.
  • Weeds often have strong, deep root sytems which improve soil structure and gather nutrients from the subsoil. But seeds left in the soil after digging-in are likely to cause problems for the following crop.



Composting is more than a recycling scheme. It is a creative, transformative process that takes spent or waste matter and tranforms it into a living substance.

In traditional Western agriculture, animal manures and green crops were the main fertilisers, and compost as such only seems to have been promoted and investigated when people like Sir Albert Howard (who devised the Indore method) developed composting techniques.

Compost heaps are often referred to as “factories” or “colonies” because they encompass legions of microbial and insect workers. As well as enriching soils with nutrients, compost also contibutes energy and life through this microbial activity. This is what distinguishes it from the fertiliser in a bag. Wrapping compost in sealed plastic deprives the micro-organisms of life-supporting elements and so they die. The best option is to make your own or get fresh delivered from a guaranteed source.

There are many variations on the compost recipe and as long as you observe the principles, there’s plenty of room for individual variations.

The essential composting process transforms organic materials through microbial and chemical activity into humus. Micro-organisms involved include bacteria, fungi, yeasts, algae, protozoa and enzymes. Each require specific conditions in which to live and work. What goes into the compost heap, and in what proportion, must provide those conditions or the result is putrefication rather than fermentation.

Slow or hot?

Nature’s own composting process is usually cool and slow – like the one seen on the forest floor where most of the ingredients and the environment tend to be on the dry side so debris needs time to break down. You can simply keep adding to a compost heap or bin – lawn clippings, garden waste etc – and it will compost eventually. But it takes a long time, any smell in the process and the end product may be inferior. This “trickle” composting is probably best done through a worm farm, expecially kitchen scraps, unless your compost heap or bin is vermin proof.

If you do opt for the slow process, follow the basic layering principles in the Hot Heap and boost occasionally with a generous helping of lawn clippings, animal manure or stinging nettle to keep it all cooking. When the container is full, or the heap high enough, remove any ready compost from the bottom, mix the rest well with a booster, add water to moisten or carbon material to dry. Replace in bin, or turn into a second bin, and leave to mature.

Bins & heaps

We talk about compost “heaps” but many small-scale composters use bins. Buy the plastic variety or make your own from untreated timber, or simply place stakes in the ground as a frame for wirenetting (cover the sides with cardboard or similar to keep moisture and heat in).

Ideal size for a bin or heap is 1m x 1m x 1m. Below this the volume is too small to heat up.

Hot heap

The basic “Indore” (the location in India where Sir Albert Howard carried out a lot of his research) method is the classic hot method. How long it takes to mature depends on the season, ingredients and degree of microbial activity. Quicker results are guaranteed with a method from the University of California. Both use animal manure which quickly heats up and improves the compost. But if you can’t get chemical-free manure, use other nitrogen-rich materials.

When compost is properly made, heat rapidly builds up as the thermophilic bacteria and fungi get to work, breaking down organic matter. Protein is broken down into amino acids and then ammonia; carbohydrates convert to simple sugars, organic acids and carbon dioxide. The fungi mop up the free ammonia and reconstitute the amino acids.

The heat build-up is a result of the oxidation of organic substances and the manufacture of carbon dioxide and water. As molecular bonds are broken and reformed, energy is released, temperatures reach 55 degrees or more allowing organic matter to be broken down, and undesirables like pathogens and weed seed to be destroyed.

Oxygen is essential at this stage, so aeration is important both in builidng the heap and turning it to keep air circulating.

Once the heat drops, different bacteria come in together with worms and insects to refine and enrich the compost. Compost worms are particualry important as they digest organic matter mixing it with their intestinal juice which is rich in hormones and enzymes. The resulting castings of fertile humus are said to be five times richer than the most fertile soil.

What to compost

Compost needs quite specific materials.

Green material, woody material, manure; but no fats or meat. Ultimately, you should be able to compost most things, except anything containing heavy metals and longer-lasting pollutants. Sewage sludge is highly undesirable and is not permitted under New Zealand’s organic standards. Really good compost, especially where biodynamic starters are used, can break down chemical residues e.g. from pesticides, but seek expert advice before composting suspect material.

Carbon-nitrogen ratios are important. Carbon is the plant’s stored energy in the form of starches and sugars fixed from carbon dioxide and stored solar energy. The nitrogen component is linked primarily with protein and is what the micro-organisms need to build up their workforce. In the composting process, carbon is converted to carbon dioxide and released. Nitrogen is taken up into the bodies of the organisms themselves.

Too much carbon slows the release of energy, so the heap is slow to break down and may never break down completely. Too much nitrogen allows the heap to heat up but the nitrogen is lost as nitrogen gas or ammonia.


Like beer, bread or yoghurt, compost seems to benefit from a “starter” to get the microbial activity going. Some good ones are:

  1. A shovelful of compost from a good existing heap.
  2. A half a kilo of fertile garden soil added to one litre of water and left overnight before adding to the compost.
  3. Cow dung – one part dung to ten parts water.
  4. Fermentedstinging nettle or comfrey – use rainwater.
  5. The biodynamic compost preparations 502-507, from the Biodynamic Association.

What to use

Alfalfa/Clovercontains nitrogen, potassium, phosphorus.
Banana plantsthe leaves, trimmings, banana skins/fruit contain phosphoric acid and potash.
Bindweedcontains boron.
Blackberrycontains iron.
Boragecontains potassium.
Bracken fernhas a high potassium content.
Broomcontains magnesium and sulpher.
Chamomilecontains calcium, potassium, sulpher.
Chickweedcontains copper, boron, zinc, phosphorus, iron.
Citrushigh in potash. Oils and resins break down in compost, but better shredded.
Coffee groundscontain minerals, trace elements, vitamins, carbohydrates, sugars.
Comfreycontains phosphorus, calcium, iron, potassium, sodium. Invaluable in the compost, helps get it ‘started’.
Corn cobsweather in the open before shredding or grinding. Holds moisture well.
Dandelion contains silica, potassium (roots)
Dolomite use in place of lime.
Feathersgood nitrogen content. Need to be moist and may need lime/dolomite or extra plant material to aid breakdown.
Fennelcontains copper, potassium.
Fishhigh in nitrogen and phosphorus. Some may be oily. Need plenty of soil and moisture for quick breakdown.
Grasshigh nitrogen content, helps create heat. Spread carefully to avoid putrefication.
Hairmust be very wet. Kept separately and rotted, makes a good insulating mulch.
Hopscontain nitrogen and some phosphorus. High water content. Helps heat compost.
Horsetailhigh in silica, calcium.
Gorsecontains nitrogen.
Inkweedgood source of potassium
Leavesvery high mineral content. Shred leaves and mix with manure or nitrogen-rich material for leaf compost.
Manuremajor traditional component of compost. High in nitrogen. Any animal dung can be used except human (and ideally not cat or dog either).
Nettle (stinging)contains iron, phosphorus. Good compost starter.
Peaswaste from crops, high in nitrogen, good potash content.
Rockdust mineral rich.
Sawdustbest as mulch, needs supplementing with nitrogen, but help aeration and moisture retention. Must be untreated.
Seaweedhigh nutrient value varying according to type. Most are rich in potash and iodine. Some must be soaked before use. Can be composted dry or fresh.
Strawlittle nutrient value, but adds bulk. Large quantities need nitorgen supplement.
Tagasaste high nitrogen content, breaks down quickly and can be coppiced as a compost crop. Excellent too as mulch.
Thistlescontain nitrogen, copper, silicon
Willowcontains calcium
Wood ashadds potash, some phosphorus. Don’t allow ash to stand in rain as water leaches potash.
Woolwaste decomposes when moist. Contains nitrogen, potash, phosphoric acid.
Yarrowcontains sulphur, potassium.

Note: Weeds are a good bulk compost ingredient. If seeding or carrying pests, place in centre of heap where heat is greatest. Can also be applied as foliar feed and act as growth ‘stimulants’ over and above their nutrient value.

Basic method

  1. Site on newly dug soil, in shade.
  2. Make a base of branches, twigs etc. 1.5 – 3.0 m wide x desired length.
  3. Add layer of green material – weeds, plant waste etc, 10 cm.
  4. Add layer of manure, 5 cm.
  5. Sprinkle with topsoil and limestone.
  6. Repeat layers until height is 1.5 m.
  7. Add water to keep all materials moist.
  8. Cover with sacking, hay etc.
  9. Turn heap after 6 weeks and again after 12 weeks.

Quick compost (University of California)

  1. Assemble materials in equal quantities.
  2. Shred everything, including manure, with shredder or mower.
  3. Pile as you shred – no need for layers.
  4. Water pile as you go.
  5. Add occasional sprinkling of rock dust, plus extra nitrogen rich materials if lacking in basic ingredients.
  6. Check over next 24 hours that heap is getting hot. If it’s not, take apart and add more manure or nitrogen-rich material.

Worm bins

Worm bins are ideal for managing food scraps and smaller items of garden waste. They are great for smaller urban sections, taking up next to no room. You can buy ready-made worm bins, or you can make your own variation of a worm farm:

  1. Find a big container, ideally one with a hole in the bottom e.g. an old bath or kitchen sink, or adapt a large plastic container. Or construct one from untreated timber.
  2. Place a bucket underneath the hole.
  3. Place a 10 cm layer of bedding material along the bottom – shredded newspaper or cardboard, old carpet etc. and sprinkle with a layer of compost
  4. Moisten slightly. Worms like a dark moist environment, temperatures between 18°C and 26°C.
  5. Put in food scraps and other suitable organic waste. Avoid too much citrus, onions/garlic, meat, dairy foods.
  6. Introduce the worms to their new home.
  7. Cover with something fairly strong – carpet, thick cardboard, untreated timber.
  8. Add food scraps etc regularly and make sure the conditions are OK – see No. 4.


  1. Stop feeding worms until all feed has been consumed.
  2. Move bedding to one side and put in new bedding and food on cleared side.
  3. Leave for a few days until worms migrate to new bedding and food.
  4. Remove old bedding which is now ready to use as fertiliser.
  5. Remove the liquid fertiliser from the bucket as necessary.

Remove worms if they get to be too many – put them in the compost heap, or give them to friends for their worm farms.


A well-made compost, particularly if it contains animal manures, meets most, if not all a soil’s nutrient needs. Adding compost to the garden each year is eventually all most organic gardeners need to do. Sometimes, though, the soil may need supplementary fertiliser. For instance:

  • When very depleted, particularly if specific nutrients are low or missing.
  • For crops with high requirements of particular nutrients.

The most important requirements for organic fertilisers are that they are free of toxic chemicals and that they are biologically active – one of the main reasons compost is preferred. Once anything has been packaged in plastic and stored for any length of time, the biological activity has died off. If using commercial fertiliser products, mix with a bit of good compost so that microbial life is stimulated.

Fertiliser made from municipal sludge (sometimes called “biosolids”) may be sold as “organic”, but should not be used in the organic garden. Sludge contains heavy metals that contaminate the soil and the food grown in it.

Buy only certified organic fertilisers. Or make your own. Liquid fertilisers made from fish combined with mineral-rich seaweed (often kelp) are particularly useful.

Animal manures

An important fertiliser in all traditional agricultural systems, it must be free of chemicals such as drenches and antibiotics and it should be composted (BioGro standards). Recommended manures are cow, poultry, sheep, pig, horse. Droppings from cats and dogs are not usually recommended.

Worm casts

Worm casts or “vermicasts” act similarly to compost and a small amount applied to soil is very effective. They also make a great tea for liquid fertilising.

Rock dusts

These are great for mineral replacement and enhancement and, together with plant debris and the bio activators found in dung, make a complete fertiliser – as long as natural time frames and plant symbioses are respected. Don’t apply them and then plant straight away. You need to allow time for the microbes to work. But unless the soil is already biologically active, rock dusts, like dung, plant debris and virtually any other organic matter, is best composted.

Lime raises the pH and lowers acidity but if you’re using rock products, it’s best to wait a year or two before using lime – the acidity in the soil helps break down the rock. And be sure to buy rock phosphate not reactive rock phosphate – the latter has an acid product added to speed up the breakdown.


Leaves take time to break down. Store in containers for a year before use. A bit of compost or grass clipping mixed in helps them decompose.

Fish fertiliser

Buy from an approved, certified organic supplier or make your own if you’re really ambitious! Liquid fertilisers made from fish combined with mineral-rich seaweed (often kelp) are particularly useful. Fish is a complete fertiliser as it contains all the essential minerals needed for plant growth. It is a concentrate and needs to be diluted and then applied as a foliar feed.

Note: Avoid fertilisers containing formalin – an additive to stop fermentation which is toxic.

Liquid fertilisers

Liquid fertilisers are very easy to make. All you need is a suitable container and a supply of fresh, clean water (preferably not town supply with its chlorine, fluoride etc).

Adapt this recipe for other plants according to plant needs such as nutrient deficiencies. Compost can also be used and is excellent for disease prevention and general plant health.

Seaweed & comfrey liquid fertiliser

  • Use a food-grade plastic drum with a lid.
  • Fill loosely with seaweed and a few bundles of comfrey leaves.
  • Cover with water, filling to the top.
  • Keep covered and leave until liquid is dark in colour.
  • Dilute to form a pale liquid and use as a foliar feed or water into the soil.
  • When liquid is used up, tip sediment onto the compost or garden beds.

Nutrient deficiencies in plants

NitrogenOld leaves go pale then yellow with red veins. Stunted growth.
PhosphorusStunted growth, pale green leaves. Leaves scorch and wither.
PotassiumLeaves small, bluish green, old leaves yellow patches, scorched edges, curl upwards.
BoronSplit, thin mis-shapen roots.
CalciumYoung leaves cup backwards, white spots on edges, turn brown.
IronVery pale leaves.
MagnesiumBronzed leaves develop yellow patches between veins.
ManganeseProminent veins, yellow patches. Small dead spots, papery areas.
SulphurOld leaves turn purple and shrivel. Young leaves small, yellow.


Nature’s own mulches include weeds, tree and plant litter, fallen leaves, and spent crops. Organic gardeners do the same and mulch around food-producing and other parts of the garden in order to:

  • Stabilise soil moisture levels
  • Stabilise soil temperature levels
  • Prevent soil erosion e.g. from wind and heavy rain
  • Suppress weeds
  • Add humus to create fertile soils
  • Reduce soil compaction and nutrient leaching from heavy rain.

Note: Mulch should be added to moist soils. Mulches high in carbon take nitrogen from the soil in order to break down, so add some compost or grass clippings to avoid this.

Use organic mulches wherever possible. Many inorganic mulches may seal the soil from the elements keeping out moisture and oxygen, depriving soil micro-organisms of their life support system. After using such mulches, soils need to be inoculated again with soil microbes, earthworms and nutrients – lots of compost.

Many mulches need to be weighted down or secured to prevent them blowing away, e.g. newspaper, plastic, leaves.

Add compost or another fertiliser before using any long-term mulch, especially around fruit trees and other producing crops.

Organic mulches

  • Autumn leaves – particularly oak leaves – are a natural mulch but are easier to manage if chopped with a lawnmower and apply as a 10 – 15 cm layer.
  • Garden compost is the best mulch if you have it, though more often it’s like black gold and in too short supply to spread liberally.
  • Grass clippings are high in nitrogen and make a great and usually readily available mulch. Don’t spread them too thickly as they heat up very quickly, don’t use them when too wet or they form a soggy slimey mess.
  • Spoilt hay or straw is excellent, especially for keeping weeds down and encouraging earthworms. Hay can bring in seed though, so opt for straw if possible.
  • Woollen carpet, rugs, blankets etc. Make sure carpet is hessian backed and not treated with chemicals.
  • Woodchips and bark can be used around established trees and shrubs but only after they have received compost or fertiliser. They are best left for a few months to begin decomposing so they don’t tie up soil nitrogen.
  • Corrugated cardboard with no coating is good – cartons must not have contained chemicals e.g. spray residue on fruit. Pull off any bits that have large amounts of adhesive on them. Top with compost if available.
  • Newspaper is readily available – no coloured ink or glossy coating. Lay sheets about one centimetre thick so that they overlap. Top with compost if available. Weight down with timber, rocks etc.

Inorganic mulches

If mulching to remove persistent perennial weeds, the best option is thick plastic left down for several months. Black plastic is particularly good over summer as it builds up the heat and bakes the weeds underneath. However, plastic discourages soil life. Woven weedmats exclude light but let air and moisture through, much kinder to soil organisms. If you intend leaving mat down permanently and planting through it, compost or fertilise soil first.


Content from Organic Gardening – A Guide by Organic NZ, published by the Soil & Health Association.