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Care of the Soil

About NPK Numbers

North Bournemouth Allotment Society Limited

Cornelia Road, Wallisdown, Bournemouth, Dorset, BH10 4FG

Fertiliser & Manure – NPK numbers & Trace elements

Plants need to extract chemical nutrients from the soil in order to live and thrive.  Generally, the levels of available nutrients will decline over time unless they are replaced.   Ideally, the individual requirement of plants needs to be matched with the type of fertiliser used.   Unlike humans who normally get all the complex molecules they need to survive ready made from food, plants need to build these molecules from available minerals and elements in the soil.

Fertilisers have “NPK” numbers - what do they mean?

NPK Values show the relative amounts, by percentage, of Nitrogen (N), Phosphorus (P) and Potassium (Potash) (K) in a fertiliser.   Since two ‘P’ letters would be confusing Potassium uses the letter K in the periodic chart derived from Kalium, the mediaeval Latin for potash.


What does each chemical contribute to a healthy plant?

Nitrogen (N) promotes strong plant growth and is particularly important to leafy green vegetables.   Nitrogen gives plants their green colour, plants lacking nitrogen often look yellow, this is particularly noticeable on older leaves.   Too much Nitrogen in the soil will promote quicker growth, but often means you end up with weak, rather spindly looking plants that are more susceptible to diseases.

Although Nitrogen is abundant available as a gas, comprising approximately 80% of the earth atmosphere, few plants are able to make use of it in this form.  The exception are legumes group of plants (Peas and Beans) that are able to fix their own Nitrogen in the soil.   These plants use a common bacteria called Rhizobium to convert Nitrogen from the air and store it in nodule on the plant roots.

Nitrogen is found in all living cells and is an essential part of all enzymes, proteins and the metabolic process.

Phosphorus (P) is critical to flowers and root development, in simple terms it helps transfer energy from one part of the plant to another.   Phosphorous is taken up by the plant root system, as it moves slowly through the soil it needs to be incorporated in the soil so that it is directly available to the plant roots.

Potassium (K) helps regulate a plants metabolism, it is critical to the stress tolerance and overall health of a plant, it is in second place after Nitrogen in importance to the plant.   Potassium plays a major part in the plants ability to take up water through its roots.   It affects the plants general health, shape, colour size and taste and is often through of as the “Quality Nutrient”


Do all plants have the same needs?

No, all plants do not have the same needs when it comes to fertilisers.   It is important for plant health to provide them with right type of fertiliser.   


Are the NPK requirements all that matter for healthy plants?

Other factors like the availability of trace minerals and elements are also important.   These can be divided into two categories:

Secondary Nutrients.

Calcium (Ca) – essential for a healthy cell wall structure and plant strength, it helps distribution of other nutrients in the plant.

Magnesium (Mg) – essential for chlorophyll, which is required for photosynthesis.  (the conversion of light energy, normally from the sun, into chemical energy.) Seed germination and the production of fruit and nuts.

Sulphur (S) – Plants use Sulphur as part of the process that produces protein, amino acids, enzymes and vitamins.   It plays a vital part in disease resistance, resistance to cold, seed formation and general growth, it also helps to give Broccoli and Onions their flavour.    It is essential to legumes as part of their nitrogen fixing process.   Sulphur is very slow in breaking down in the soil into a form useable by plants.   

A quick fix can be applied in the form of Epsom Salts, which adds both Sulphur and Magnesium. Epsom salts is hydrated Magnesium Sulphate (about 10 percent magnesium and 13 percent sulphur).  

Epsom Salts can be diluted with water and applied as a foliar spray, using one to two tablespoons of salts per gallon of water, up to four applications at 10 to 14 day intervals are normally applied in one growing season, but a greater number of applications will not do harm, but do not use more than 1 tablespoon per gallon.   Foliar spraying seems more effective than watering the soil or applying Epsom Salts directly to the soil especially prior to planting out.

Micronutrients or trace elements.  These are needed in very small amounts and include Boron, Carbon, Cobalt, Copper, Iron, Sodium, and Zinc.

Oxygen and Hydrogen are also import.   It is very important to have a well aerated soil that is capable of supporting beneficial microbes and earthworms.

Digging in good compost or manure is an excellent way of maintaining most levels of secondary and micro nutrients need in the soil, it also helps aerate the soil.

Boron (B) – Plays a part in the production of carbohydrates and sugar, essential for fruit and seed development.  It also plays a part in cell wall structure and cell division particularly at the growing tip of a plant.

Copper (Cu) – Plays a part in plant reproduction.

Chloride (CL) – Helps with plant metabolism.

Iron (Fe) – Plays a part in the formation of Chlorophyll.

Manganese (Mn) – Breaks down Nitrogen and carbohydrates.

Molybdenum (Mo) – Helps breakdown Nitrogen.

Zinc (Zn) – Controls consumption of sugars and helps regulate growth in a plant.

See next navigation bar for the next article giving an approximate breakdown of the NPK numbers in common manures and fertilisers.