Organic Gardening - Part 2
| Published: 30th January 2008 12:11 |
Organic Gardening - Part Two of Two Articles
Soil Fertility
This has been covered briefly before in the topical article on soils and composts but the following may complement those notes:
Use of Nutrients:
Elements required by plants are carbon, hydrogen, nitrogen, phosphorous, potassium, calcium, sulphur, magnesium and a range of trace elements.
The air and water supply plants with carbon, hydrogen and oxygen.
Most nutrients required by plants are derived from organic matter.
Nutrients released into the soil solution are the result of the degradation of organic matter by soil organisms.
Suitable forms of organic matter added to the soil are as follows:
Farmyard manure (FYM) - medium to high fertility. Must always be well-rotted before use; this is to stabilise their nutrients, which might otherwise be washed out by rain or precipitation. Also ‘hot' manure i.e. manure taken fresh from the source, still steaming - may have a risk of damaging plants. Source FYM from non-intensive or organic farms- Stable manure (SM) - medium to high fertility. Again, this must be well-rotted before use and care must be taken that stable manure is not too ‘strawy' and that the source is wholly organic
- Leafmould - low fertility but excellent soil conditioner. Leafmould takes longer to break down into humus but may be used as a mulch or dug in
- Seaweed - high in fertility, excellent as a soil conditioner. May be used as a mulch or dug in. Useful on annual beds, fruit trees and bushes, lawns and turf
- Composted straw - low in nutrients, good as soil conditioner when broken down into humus. Use as a mulch
- Spent hops - medium to good fertility. Not always easily available. May be used as a mulch or dug in
- Mushroom compost - medium fertility. Needs to be well-rotted. Tends to be alkaline. Source from organic growers to avoid contamination by pesticides
As stated, for organic purposes, all materials should come from sources uncontaminated by pesticides, herbicides, heavy metals or BSE.
Medium and high fertility materials should be applied in spring and summer only. Their goodness will be wasted if applied near winter when growth rate is minimal and plants enter dormancy. Too much nitrogen applied encourages soft, leafy growth, rendering plants more susceptible to winter frosts and chills.
Green Manures:
Green manuring uses the growing of a crop specifically for mixing with the soil.
Green manures may be mixed with the soil in which they have been grown or they may be harvested and taken to other sites to be used.
Examples of green manures and their uses are as follows:
Plant | Type | Overwinter? | Fix nitrogen? | Sow | Rotation |
Alfalfa | HP | yes | yes | April-July | legume |
Field beans | HA | yes | yes | Sept. - Nov | legume |
Buckwheat | HHA | no | no | April-August | Fits anywhere |
Clover - crimson | HA | no | yes | April -August | legume |
Clover - red, white | HP | yes | yes | April-August | legume |
Fenugreek | HHA | no | no | March-August | legume |
Lupins | HHA | no | yes | March-June | legume |
Mustard | HHA | no | no | March-Sept. | brassicas |
Phacelia | HA | yes | no | May-Sept. | Fits anywhere |
Radish | HA | no | no | Aug-Sept | brassicas |
Rye, grazing | HA | yes | no | Aug-Oct | Fits anywhere |
Rye grass | HA | yes | no | Spr or aut. | Fits anywhere |
Tares | HA | yes | yes | Mar-Sept | legume |
Trefoil | HB | yes | yes | Mar-Aug | legume |
As the table above suggests, the choice of green manure to use depends upon the length of season available, soil type and compatibility with main crop.
One caution remains with the planting of green manures. Often, when decaying in the soil, they release compounds that can inhibit germination of following crops, so it would probably be advisable to plant a crop which has been pre-germinated and grown on in a container.
Green manures, however, are beneficial in that they absorb and retain nutrients which might otherwise be leached out of the soil if the surface is left bare. Additionally, the roots of green manures assist in the development of a better soil structure, forming a covering to protect the top structure and preventing soil ‘pans'.
Nutrients taken up by green manures are subsequently released into the soil when the green manures are incorporated into the soil.
Additionally, green manures smother weeds by germinating quickly and can assist in controlling pests. For example, frogs, beetles and other natural predators appreciate the cool, damp cover provided by a green manure and, to a certain extent, insects may be confused by a green manure crop placed between food crops. In this way, they act similarly to companion plants.
A full green manure crop is the equivalent of one barrowload of farmyard manure.
Green Manures should always be incorporated with the soil before they begin to flower. Younger plants will always rot down more quickly, so it is always best to dig them in or use them as a mulch/compost activator before growth begins to thicken up. There is also the risk if the plants are left too long that seed may form and ripen.
Some green manures are able to utilise nitrogen from the air and so enrich the soil with this nutrient when mixed in and this can supply 10-50g/m2 per year (see table above).
Organic Fertilisers:
As distinct from manures, organic fertilisers add only a small, insignificant amount of organic bulk to the soil, so they do not significantly improve soil structure.
They do, however, provide a range of nutrients to the soil to be taken up by plants. The key difference between organic and inorganic fertilisers is that organic fertilisers return nutrients to the soil, whereas inorganic fertilisers primarily provide food to the top growth of the plant.
Amongst the most commonly used and readily available organic fertilisers and manures is as follows:
fertiliser | N | P | K | uses |
Dried blood | 10-13% |
|
| Boost top (foliage)growth spring |
Hoof and horn | 13% |
|
| Slow release supply of nitrogen where strong growth needed |
Bonemeal | 4% | 22% |
| Promotes strong root growth |
Steamed bone flour | 0.8% | 27% |
| " |
Shoddy | 2-15% |
|
| Boost top foliage |
Wood ash |
|
| 2.5-12% | Flowers and fruit |
Farmyard manure | 0.6% | 0.25% | 0.8% | Soil conditioner |
Poultry manure | Up to 4% | Up to 4% |
| Soil conditioner |
Seaweed | 0.5% | 0.9% | 1.2% | Mainly soil conditioner |
Crop Rotation:
This is the practice of growing crops on a different site each year in sequence.
Different crops have different rooting depths and so will tap plant nutrient reserves at different levels in the soil.
The main reasons for rotating crops are to avoid nutrient depletion and the resulting build-up of pests and diseases in the soil through monocropping. Alliums, brassicas, potatoes ideally need 48 months between planting dates in the same soil.
Liming of Soils:
The use of slaked (hydrated) lime and quicklime is prohibited in organic systems.
Use ground limestone (calcium carbonate) or dolomite limestone (calcium magnesium carbonate).
It is usually necessary to apply lime to soils which are too acid. Also, lime assists in the flocculation (breaking up) of particles in a clay soil.
Glossary of Terms:
Mulch: a covering over the soil, generally to a depth of at least 3in., to improve soil structure and suppress weeds.
HA: Hardy annual. A plant that completes its growing cycle -
seed >germination>flower>seed in one year. Hardy means that it will withstand frost.
HHA: Half-hardy annual. A plant that completes its growing cycle in one year but is susceptible to damage by frost.
HP: Hardy perennial. A plant that dies back to its base in winter but re-grows in spring and continues each year in this manner.
HB: Hardy biennial. A plant that completes its growing cycle from one year to the next - seed May/June>germination>transplanted autumn. Biennials need a period of vernalisation (cold spell) to encourage flowering. An example of a biennial plant (but not in the context of green manuring) is the wall flower.
Leached: washed away.
NPK: The three major nutrients required by plants for growth. Balanced fertilizers contain these nutrients in almost equal measures.
N= Nitrogen, the major nutrient needed for encouraging top growth.
P= Phosphorus, the major nutrient needed to encourage strong roots, in particular.
K= Potassium (potash), the major nutrient needed to encourage flowering and fruiting
Martin Horne
e-mail me on martinjhorne@hotmail.com with your thoughts and questions.
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