Roehampton Garden Society


WHY DO I NEED THEM? A good compost heap will provide your soil with most of its requirements, however on an allotment which has been heavily cultivated for many years you will need to add some nutrients to encourage healthy plants.


It is very important to know the pH level of your soil to ensure a successful allotment! The pH of soil is a number that describes how acid or alkaline the soil is. A pH of 7.0 is considered neutral, an acid soil has a pH value below 7.0 and above 7.0 the soil is alkaline. Knowing the pH allows you to choose, more accurately the plant suited to that soil or the appropriate additive to add to be able to grow your choice of plants. It is advisable to test your soil frequently with a pH Soil Meter, to maximise healthy plants. These are available at the Store. They are good quality and also come with a very helpful extensive list of plant pH preferences.

WHERE CAN I BUY THEM? Organically and inorganically produced fertilisers are available loose and at highly competitive prices to members of the Society from the Store.We also stock some pre-packed proprietary brands.

HOW MUCH DO I NEED? See individual details – a small 150 g yogurt carton holds about 200g (7oz) fertiliser. A chemical fertiliser should always be used with great care.

Major Plant Requirements (N:P:K)

  • Nitrogen (N): essential for growth and dark green foliage; apply as base or top dressing; particularly to young plants and sparingly to mature plants
  • Phosphate (P2O5): essential for good root development; promotes ripening of fruits and seeds; apply as base dressing or top dressing forked in to reach the roots
  • Potash (K2O): essential for production of flowers and fruits; promotes healthy growth and resistance to frost and fungal diseases; apply as base or top dressing; slow to leach away and therefore apply at any time of year.

ORGANIC FERTILISERS originate from living organisms (plants or animals). They release nutrients slowly and may also improve soil texture. They contain trace elements as well as major nutrients but are unpredictable in what they release. If bought, they may be expensive if they are bulky and weigh heavily, but the cheapest source is always your own compost heap. They maintain long-term soil fertility and are used to prepare soil for planting shrubs and slow-growing plants. Apart from those listed below examples include horse manure which is delivered to our sites and is £1 per barrow load, (don’t add it to young growth until composted down,) and Country Natural Compost (available in the Store and ready to use straight away.)

INORGANIC FERTILISERS are exact in their contents and come either from quarried materials e.g. rock phosphate, limestone and Epsom salts (magnesium sulphate) or are manufactured chemically (e.g. Growmore and superphosphates.) Inorganic fertilisers can quickly remedy a particular deficiency e.g. ammonium sulphate will supply nitrogen. Proprietary fertilisers have different N:P:K ratios for different plants e.g. Cactus fertiliser has a ratio of 8:34:32. The disadvantages of inorganic fertilisers are that they may leach (drain) rapidly from soil after rain, and can damage plants if applied too liberally. They also have no lasting benefit for the soil.

Base dressings are dug into the soil at root depth when preparing it for planting. All slow-release organic fertilisers are suitable, as are the newer (expensive) inorganic fertilisers in a resin coating e.g. Osmacote and Chempak’s Summerlong.

Top dressings are applied on the surface around established plants. They are best applied under a mulch or watered in thoroughly to distribute the fertiliser equally.


Seaweed Available as a LIQUID feed or in a DRIED form.

A superb organic fertiliser and soil improver which provides useful sources of potassium, magnesium, nitrogen, phosphate and trace minerals. A real all-rounder. Will give you healthy plants that resist disease and pests.

Can be forked (or watered) in lightly any time or year, but optimum times are Spring and Autumn. (instructions on bottle or packet). It can also be dug into plants in pots. It can be used with other fertilisers and manure.

The dried seaweed is also an excellent additive to compost.

Bone Meal An organic fertiliser which supplies phosphate for good root development and some nitrogen for growth and dark green foliage. Bone meal acts slowly and can be applied under the roots when planting out or forked in before planting over a larger area. Water in if dry.

Should not be used on acid loving plants such as blueberries. Good for root development of roses, trees and woody shrubs.

Use 100-125 grams per sq. metre

Fish, blood and bone An organic all round fertiliser which supplies a good balance of phosphate for root development, nitrogen for growth and dark green foliage and potassium for flower/fruit production.

Fish, blood and bone is faster acting than just bone meal. It is best used to improve soil before planting out or when seed sowing. It is also good as a top dressing during growth and a good soil improver if land has not been cared for.

Use 50-200 grams per sq.metre.

Volcanic Rock Dust An organic fertiliser composed of slow release minerals and trace elements which replace the nutrients and microorganisms in the soil that have been depleted due to many years of horticulture. Rock dust will increase water retention of the soil, increases resistance to pests and diseases and encourages worms.

The best time to apply is over the winter but it can be used all year round. Avoid applying on a windy day. Rock dust can be used alongside other soil conditioners and manures. If using lime do not mix at the same time – leave a few weeks between each. In the first year fork in up to 2kg per sq. metre and then in following years 0.5 to 1kg per sq. metre. Can be added to compost heaps to accelerate decomposition – 3-5mm every 13cm of matter.

For seeds or potting on – use 1 part rock dust to 4 parts all-purpose compost.

Poultry Manure Pellets An organic fertiliser which contains nitrogen for green leafy growth but smaller amounts of phosphorus and potassium and is therefore not for all-round use.

The nutrients are released slowly once the soil warms up. Improves soil structure by adding humus and soil moisture retention. Because of its alkaline nature poultry manure should not be used on acid loving plants such as blueberries. Add poultry manure to your compost heap to help the acceleration of decomposition Use 150 – 200 grams per sq. metre

Calcified Seaweed This is a completely different product to Dried Seaweed! It is a natural alternative to LIME. It will change the pH level of the soil, making it more alkaline, similar to lime. It is not an irritant and will not burn young plants. Therefore, you can add it to lime-loving plants at any time of the year. Produced from naturally occurring beds of calcified, coraline algae, sometimes called ‘maerle’ and formed from dead seaweed, cold water corals and crushed up shells on the seabed. The feeding hairs of the roots of plants actually like to grow directly on the calcified seaweed granules, allowing for a much better uptake of essential and trace elements. It can simply be forked in! The application is 140g. per sq. metre about 7-10 days before planting out.

Garden Lime A naturally occurring rock and its use as a soil conditioner is acceptable by most Gardeners, wishing to be organic. It will reduce the acidity of soil by increasing the pH level. It supplies calcium which improves plant structure and helps to deter club root. Fork into the soil in the autumn in areas where you plan to grow brassicas and other alkaline loving plants such as spinach and onions, but NOT where you plan to grow potatoes. Young plants can be burnt if planted into soil that has recently been limed, so autumn liming in preparation for later sowing is preferable. DO NOT dig in at the same time as manure as they will counteract each other.

Use 05 to 1 kg per sq. metre, depending on the pH of soil.

Bio Char Soil Improver is 90% biochar activated with 10% seaweed, wormcasts and mycorrhizal fungi. It can be used in a number of ways.

You can add the soil improver at a rate of 10% by weight to your existing compost. Mix in well to ensure an even distribution. One of the many benefits is that the product has a high water holding capacity, so you will almost certainly need to irrigate less frequently than usual.

There is no set limit to the quantity of soil improver you can add to topsoil. As little as 250 grams per square metre will bring positive results. However, if you apply each year then the levels and benefits will build up over time.

Add about a heaped teaspoon to the hole when planting out plugs and small plants. With larger plants mix into the soil or compost at the base of the planting hole.


Growmore N:P:K = 7:7:7 An inorganic fertiliser which is made up of equal parts of nitrogen, phosphate and potash. It is an all-purpose fertiliser which promotes good root development, general growth, production of fruits and flowers and helps with resistance to frost and fungal diseases.

Growmore can be used at any time of the year.

Use 50-125 grams per sq. metre.

Sulphate of ammonia An inorganic quick acting fertiliser which contains a high amount of nitrogen which is used for rapid growth of leafy vegetables such as cabbages and lettuces.

Apply between April and September sprinkling evenly between rows. Can be used in the autumn to help leeks recover from leek moth.

Use 25 grams per sq. metre

Can also be used on compost heaps instead of Garotta to accelerate decomposition. Dissolve 1oz in 2 gallons of water and pour over compost after each 6” of matter.

Sulphate of potash An inorganic fertiliser which has a high potash content. Potash is essential for production of flowers and fruits and is therefore good for tomatoes, fruit bushes and fruit trees.

Can be used as either a base or top dressing and is best used late winter and early spring. It can be dissolved in water and used as a liquid feed (1oz in 2 gallons). Be careful not to put directly onto foliage.

Use 25 grams per sq. metre

Make your own: Wood ash supplies organic potash but is mostly calcium carbonate. Potassium, phosphorus and many trace elements are also present. This can be dug in as a base or top dressing or in larger quantities as a soil neutraliser instead of lime.

Superphosphate An inorganic fertiliser supplying a concentrated amount of phosphates to improve root development and help the quick establishment of young and/or recently moved plants.

Can be used all year round as a base dressing before planting or sowing as well as a top dressing for established plants.

Peas, onions and root crops benefit from this fertiliser. Water in well and be careful not to get onto foliage.

Superphosphate will kill your worms.

Use 50-125 grams per sq. metre

Rose fertiliser

Made to Tonk’s formula specifically for roses (use 50-125 g per sq m or 2-4 oz per sq yd)

Roehampton Garden Society 2015