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Medwyn Williams

Hello. I'm Medwyn Williams – eleven times Gold medal winner at the Chelsea Flower Show, Past Chairman of the Royal Horticultural Society Fruit Vegetable and Herb Committee and President of the National Vegetable Society.

Soil Analysis – Nitrogen, Phosphorous, Potassium


All plants require the following major or macro nutrients to be freely available in the soil and ready to be absorbed them, Nitrogen, Phosphorous and Potassium which are commonly known as the NPK. The minor or micro nutrients are also important in much smaller degrees and the need therefore to have your soil analysed is vitally important.

The soil in our garden is probably the most important element that we have towards producing a perfect vegetable, yet, most of the time we have no idea of how or why it works so well. Many gardeners don’t bother to have their soil analysed, merely adding some manure and a handful of this and that every Spring time without bothering to find out what the soil needs in order to produce the quality vegetables that we require.

All plants require the following major or macro nutrients to be freely available in the soil and ready to be absorbed them, Nitrogen, Phosphorous and Potassium which are commonly known as the NPK. The minor or micro nutrients are also important in much smaller degrees and the need therefore to have your soil analysed is vitally important. When you do have your soil analysed it is also important to inform the analyst what crops you intend to grow, I have usually two tests carried out, one for the leeks and the other for the onions, both of which are grown in separate tunnels.


If the macro nutrients are important, what role do they therefore take in the plants life?, well Nitrogen is in effect the builder, it is important for the production of green tissue giving plants a healthy deep green colour. It also promotes stem and leaf growth and increases the protein content of edible plants such as Cabbage, lettuce cauliflower etc. All growing plants require some levels of Nitrogen because without it the nitrogen deficiency will soon show up as the plants become stunted and yellow looking with the leaves at the bottom of the plants eventually drying up and withering. Too much Nitrogen is also undesirable as too much stem and leaf growth will lead to soft tissue which leaves the plants vulnerable to disease and far more susceptible to frost damage. It also the most likely of the three major nutrients to be depleted from the soil as it will leach out over the Winter months when we have prolonged and heavy showers of rain.

The Nitrogen can be organic or inorganic and even though the green or organic lobby will always try and use sources of organic nitrogen the fact is that the plant wont know any difference and will freely accept and thrive on both organic and inorganic Nitrogen. Dried blood and Hoof and Horn are two sources of organic Nitrogen with a 12% level, however dried blood is the quickest acting and therefore is taken up faster by the plant whilst the hoof and horn will be slower and could take a long while for the material to break down and release the Nitrogen. Inorganic sources are Sulphate of Ammonia and Nitrate of soda, both of which are fairly quick acting with a % nitrogen of around 15 whilst Urea will have as much as 46%, this should therefore be used sparingly.


Phosphorous or phosphates as we commonly call them are in effect the root builders and are essential for the development of a strong and healthy root system. This item is also important for young plants or seedlings as they have a rapid intake of Phosphorous so it”s an important element when preparing your seed bed as well as making up your own seed compost. An important point to remember for those of us growing the long roots of carrots and parsnips as well as potatoes, is their need for an adequate supply of phosphates as their ultimate yield will most certainly decrease if this element is severely lacking. A deficiency of phosphate may be indicated in plants when the foliage becomes blue/grey shade of green. An organic source is Bone Meal which is slow acting with 15% phosphate and can remain in the soil for over a year. Continual use over many years can lead to an excess of phosphates in the ground which can lock up some of the trace elements. If you have been using a lot of Bone meal it is definitely advisable to have a soil test. An inorganic source is Superphosphate at 18% or Triple Superphosphate at 45%.


Potassium or Potash is the element that develops fruit and flowers and toughens up the plants making them much more resistant to diseases, it can also be used to counter balance any effect should you have over used the Nitrogen element. Potash plays an important part in the formation of sugars and starches which can be stored by the plant in swollen roots such as Turnips and Swedes. A deficiency of Potash shows first as yellowing of the leaf margins which may later turn brown and scorched looking. Another important point to remember with Potash is that if you apply too much to the plants it can cause them to absorb too much water which in turn makes it less resistant to frost. An organic source of Potash can be wood ash at 10% but the most often used is the inorganic Sulphate of Potash at 48%. Muriate of Potash is excellent for potatoes at 60% with Potassium Nitrate giving you 46% as well as 13% Nitrogen.

Farmyard Manure

Gardeners also seem to think that manuring the ground alone with farmyard manure is sufficient, well it isn’t; farm yard manure should always be supplemented with a balanced base dressing of the above elements as it generally contains very little plant food, probably 1.0%N 0.5%P 1.0%K.


If you do have your soil analysed, the formula below may be of help to you should the result in you report be shown as percentage deficiencies. You may also want to use fertilisers that are different from the ones that are recommended by the analyst, in that case it will be necessary for you to calculate the amounts that you require as shown below:

Each 1% deficiency represents the need for 6.66 ounces of plant food, (not actual fertiliser) on each 100 square yards of soil and the quantity of fertiliser required is in turn decided by how much plant food the fertiliser contains (i.e. how concentrated it is)

For example if the test result showed you had a 1% phosphate deficiency, and your alternative choice of fertiliser contained 20% of phosphorous plant food, the fertiliser would need to be applied at the rate of 6.66 x 100 which equals 33.3 ounces per 100

square yds or 0.33 ounces per square yard.

If a different fertiliser, containing say only 5% phosphorous were used, the calculation would become: 6.66 x 100 which equals 133.2 ounces per 100 square yds (or 1.33 ounces per square yd)


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