13:54 - 14 February 2011
If you buy a packet of fertiliser in a garden centre then you will find a value for NPK. This is a ration of the content of essential plant nutrients in the mixture where:
N
N is for Nitrogen - which is what plants need to make healthy green leaves and to fuel daily growth, it's great for leafy vegetables and foliage plants. Despite the fact that the atmosphere contains a massive 78% of nitrogen, this is mainly unavailable to most plants. The exception are the legumes from the family Leguminosae which have a special relationship with bacteria that live in root nodules on the plant roots and actually ‘fix’ nitrogen from the air making it available to the plants.
Some heavy feeders such as Box (Buxus) plants quickly show a deficiency in Nitrogen and thus a need for feed, by turning a yellowy brown in colour. Other plants grow weakly and take on yellowing hues mid season. Nitrogen in the form of nitrates is quickly washed out of soils after heavy rain and sandy soils especially struggle to hold onto these nutrients. Leafy crops and foliage plants need a regular supply to meet their needs.
P
P is for Phosphorous - it's needed by all plants to encourage strong, vigorous root growth and also seed germination. Young plants that are still growing their root system and also fruiting plants that are making their seed all need a good supply of phosphorus. It is also very important for root crops, such as carrots, parsnips and Swedes. It is also needed by a lawn to build up the root system and enable it to withstand flood and drought better.
In many plants a deficiency in Phosphorus often shows as a purple tinge to the edges of leaves. Fertiliser containing phosphorus should be added to the planting hole when you first plant to encourage the development of the root system. It can also be applied as a top dressing and is not as easily leached from the soil as nitrates.
K
K stands for Potash (from the Latin Kalim for potassium) and it's this that supports the production of flowers. It's essential for fruiting crops too like tomatoes and peppers. It is vital for sustaining plant growth and also plays a major part in resisting plant diseases. It is also called potassium and is naturally found in wood ash from bonfires and woodburning stoves. Plants with a sweetness, such as carrots, parsnips and fruit need good supplies of potash to build sugars and starches. It isn’t easy to diagnose a deficiency in potash; plants will grow poorly with low fruit yields and may be ridden with disease or may become scorched in hot weather. Since all plant foods contain a level of potash, a liquid foliar feed of a soluble fertiliser should result in healthier plants.
NPK
NPK is the ratio of the proportion of these essential constituents present in a fertiliser. To enhance fruiting and flowering plants look for higher levels of K, for plants that need strong healthy foliage, such as leafy crops, look for a fertiliser with higher N content. A balanced fertiliser will contain equal levels of all three.
Feeding tips
Always water plants thoroughly before applying plant food or you risk scorching the roots and causing damage.
Where plants are ailing and need food quickly apply a foliar feed to the leaves.
Feed the soil as well as your plants by improving it with well-rotted farmyard manure, soil conditioner or well composted garden compost.
Don’t overfeed, follow the instructions on the packet and apply as directed. Overdosing your plants with fertiliser can damage plants and roots, it doesn’t get any more food to your plants any quicker and it will cost you more in the long run.
If you are not able to feed your plants on a regular basis then concentrate on improving the soil and use slow release fertilisers that provide your plants with a trickle feed as they grow over several months.
