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How to Improve your Soil – A beginner’s guide to soil additives

Every year, we take away the crops to feed ourselves and with them the nutrients that have gone into producing them. You don’t get something for nothing and if you don’t put anything back into your soil after a few years it will be exhausted, and you will produce very little.

Compost and manure are great, not only adding the basic nutrients but also the micro-nutrients, the vitamins of the vegetable world, but often these just can’t provide the optimum levels for our crops. This is where the fertilizer comes in.

The three main elements needed by plants are Nitrogen (N), Phosphorus (P) and Potassium (K), which is commonly known as Potash. Very simply put, plants need: nitrogen for the leaves, phosphorus for the roots and potash for the fruit.

Most fertilisers will contain the three elements (NPK) essential for growth.


Nitrogen is used by the plant to produce leafy growth and the formation of stems and branches. Plants most in need of nitrogen include grasses and leafy vegetables such as cabbage and spinach. Plants in the bean family have nodules on their roots where bacteria live which fix nitrogen from the air for use by the plant. They produce their own nitrogen fertiliser this way.

You can tell if your plants need nitrogen when their growth is stunted with weak stems and often they will have yellowed or discoloured leaves. Nitrogen fertilizers are quickly washed out of the soil by rain and need to be renewed annually.


Phosphorus is essential for seed germination and root development. Root veg such as carrots, swedes and turnips obviously need plentiful phosphorus to develop well. Without ample phosphorus you will see stunted growth and low fruit yields. Phosphates remain the soil for two or three years after application so the amount in a general fertilizer is probably enough. Add just before planting or top dress during growth periods.


Is called Kalium in Latin. It promotes flower and fruit production and is vital for maintaining growth and helping plants resist disease. Carrots, parsnips, potatoes, tomatoes and apples all need plenty of potassium to crop well. Plants that are short of potassium will have low resistance to disease, and poor fruit yield. Potash usually lasts for two or three years in the soil but for vegetable production, especially tomatoes and potatoes, extra will be required. It can be applied as a liquid feed (including one made from comfrey, a herb) or a specially prepared fertiliser (e.g. potato fertiliser).

Fertilisers are not substitutes for maintaining a healthy soil with organic matter (humus), but they boost production and feed crops.

Inorganic fertilisers we sell: Growmore and Potato fertiliser

These are balanced for different crops but just contain three main constituents (NPK) in different proportions. The allotment shop sells Growmore, which was originally known as National Growmore and made available to growers by the government in the war, and is balanced at 7-7-7, and our Potato Fertiliser supplies NPK 6-10-10. Prepare the soil before planting by applying 135g/sq.m as a base dressing and work in well with a fork. Growmore can also be used on established plants. Apply 70g per sq.m and work in well. You can apply every 4-6 weeks. It will also feed shrubs and trees in your garden during the growing season. For potatoes there is less nitrogen in the mix as leafy tops are less important than root development. Use our potato fertiliser at a rate of 30-60g per square metre for a bumper crop of potatoes and other root vegetables. Lightly fork into the soil.

Other inorganic fertilisers for sale in the allotment shop:

Sulphate of Ammonia contains 20% nitrogen, which boosts leafy plants like cabbages. Apply 35g per sq.m. or work into the soil 2 weeks before planting. Can also be used as a compost activator. It's also a handy weed killer - put half a teaspoon on the centre of a weed and see what happens.

Sulphate of potash is a supplement to your soil to remedy a shortage of potash. Apply 35g per sq.m.

Superphosphate (0-17-5) is a supplement to remedy shortage of phosphorus - for healthy vegetables, apply 70g/sq.m as a top dressing to the soil surface and work well into the soil. Water in well after application.

Magnesium deficiency

Symptoms: Yellowing between the leaf veins, sometimes with reddish brown tints and early leaf fall. Magnesium deficiency is common in tomatoes, apples, grape vines, raspberries, roses and rhododendrons. Cause: Magnesium is needed for healthy leaves and for plants to harness energy from the sun (photosynthesis). Soil shortages of magnesium are more common on light, sandy soils. Over-use of high-potassium fertilisers (such as tomato feed) can cause magnesium deficiency, as plants take up potassium in preference to magnesium.

Remedy: In the short term, apply Magnesium Sulphate (Epsom salts) as a foliar feed in summer. Dilute the salts at a rate of 20g of Epsom salts per litre of water (1/3oz per pint) plus a few drops of liquid detergent. Apply two or three times at fortnightly intervals, spraying in dull weather to avoid leaf scorch. In the long term apply to the soil around the roots either Dolomite limestone (calcium-magnesium carbonate) at 100g per sq m (4oz per sq yd) or Epsom salts (magnesium sulphate) at 30g per sq m (1oz per sq yd).

Organic Fertilisers

Fish, blood and bone (NPK 5-5-5-) is the organic version of Growmore. The nitrogen is usually released over a slightly longer period. This helps to prevent overdosing but also means it is not effective as quickly as a chemical fertiliser, so you need to think ahead and apply a fortnight before it is required. Before planting or sowing: Apply 140g/m² and lightly fork or rake into the soil. As a top dressing: Apply 70g/m² around the plants and lightly fork into the soil. Water in well if rain does not occur within 24 hours of application.

Bone meal (sterilised) (NPK 3.5:20:0) is low in nitrogen, very high in slow-release phosphorus but has no potassium. It encourages root growth and is recommended for use when planting fruit trees and bushes. Before planting or sowing: Apply 140g/m² and lightly fork or rake into the soil. As a top dressing: Apply 70g/m² around the plants and lightly fork into the soil. Water in well if rain does not occur within 24 hours of application.

Organic Pelleted Chicken Manure (NPK 4.5-3.5-2.5) + trace elements for balanced major and minor nutrients. Work the nutritional pellets into the soil. Water the bed before applying pellets; the pellets break down when in contact with moisture. Apply several days before sowing seeds or planting out small plants. Apply in autumn when preparing winter beds or early in spring when preparing your beds for planting. Apply an additional feed 3-4 weeks after planting to ensure a healthy harvest. Work in 100 grams (~3½ handfuls of pellets) per square metre before sowing - apply at least 1 week before; 200 grams before planting hungry plants: brassicas, potatoes, sweet corn, and salad crops; 300 grams before planting shrubs, hedges and trees. Note that the application will make your soil more acid temporarily. A handful dissolved in a gallon of water makes an excellent liquid feed. Also ideal for a lawn feed at one handful per m2.

Maxicrop Organic Original Seaweed Extract (NPK 5:2:5) Rich pure liquid seaweed extract, suitable for use on all plants at every stage of life. Helps to stimulate germination, rooting and establishment from seed through to mature plant. Approved by the Soil Association for organic gardening, the nourishing liquid boosts flowering and fruiting and, if used regularly, can even improve resistance to pests, disease and stress. Ideal for use on all flowers, shrubs, trees and edibles in the home, garden or allotment. Method: Shake bottle before use. Mix 20ml product with 4.5 litres of water in a watering can. Apply solution around the base of plants every 7-14 days during the growing season.

Maxicrop Organic Tomato (NPK 4:2:6) This is seaweed extract with added potassium which helps stimulate tomato production. Application method: Shake well before use. Add required amount to water and stir well Apply using watering can - root drench only OUTDOORS - IN SOIL - Dilute 30 ml in 9 litres - Apply when second truss has set, feed every 7-14 days - GROWING BAGS/CONTAINERS - Dilute 30 ml in 9 litres - Apply when second truss has set Feed every 7 days UNDER GLASS -IN SOIL - Dilute 30 ml in 9 litres - Apply when second truss has set, feed every 7 days - GROWING BAG/CONTAINERS - Dilute 30 ml in 9 litres - Feed once per week and increase to twice weekly when second truss has set.

Other Soil Additives

Calcified Seaweed granules Like lime, calcified seaweed raises the pH of your soil. Don’t use it on acid-loving plants. Calcified seaweed works by breaking down heavy soils and encourages the development of beneficial micro-organisms. It improves the soil structure with nutrients and trace elements, helping plants to grow stronger root systems leading to healthier and more vigorous plant growth. Rich in minerals, Calcified Seaweed can also be used as a lawn dressing early in Spring. Use at 60-75g per square metre.


Plants don't take up nutrients well if the soil is too acid or too alkaline. Before adding nutrients to the soil, test the pH level using a kit. We sell them in the allotment shop, or any garden centre stocks them.

Use these additives to adjust the pH level of the soil. Read about the pH level here.

Use Iron Sulphate as a long-lasting plant tonic for acid-loving plants like heathers, rhododendrons, conifers, camellias and azaleas. Iron Sulphate helps to acidify (lower the pH of) the soil and helps prevent leaf yellowing. (My plot is alkaline and the RHS recommended Iron sulphate to acidify the soil.) The recommended application rate is 35gramms per square metre. Do not over apply: use the recommended rates only or you will kill your plants. Iron sulphate can be used as a lawn tonic and to kill moss. Application rate for green up of lawn: 0.5 to 1g per square metre.

Use Lime if your soil is acid to raise the pH of your soil. Go to the RHS website for specific advice on liming sandy, loam or clay soils.


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