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Nitrogen is one of the primary nutrients required by plants and is thus termed a primary macronutrient. Plants need nitrogen to produce DNA, proteins and chlorophyll; all of which are vital to plant growth and indeed, the life of plants. Some plants, often referred to as nitrogen fixers, can form a beneficial association with nitrogen fixing bacteria. These bacteria can take nitrogen from the air and turn it into a form that plants can use so they don't have to rely on the nitrogen that's already in the soil. In return, plants provide sugars for the bacteria to feed on. Legumes are the most well known class of plant that can form this association with nitrogen fixing bacteria.

Factors that affect the nitrogen availability include:

Nitrogen Deficiency

Nitrogen deficiency is most likely to occur in high rainfall areas, if the soil is sandy and doesn't contain much organic matter or when soil is poorly managed such as when crops are frequently harvested without any organic matter being returned to the soil to compensate for what is removed. Having said that, some crops and ornamentals have a very high nitrogen requirement (these plants are often referred to as hungry plants) and may suffer from nitrogen deficiency despite good soil management practices. In such situations, you may need to provide supplemental nitrogen or space plants further apart than their size would indicate. If you chose to do the later, you can intersperse plants with other species that have a low nitrogen requirement.

Symptoms of Nitrogen Deficiency

The most obvious symptom of nitrogen deficiency is of older leaves. Nitrogen deficient plants also grow more slowly and are usually stunted. Many plant species will mature earlier than normal if they don't have enough nitrogen and will produce a lower yield (of flowers, fruit, seeds or foliage). Plant yields are also often of reduced quality. In very severe cases, leaves of nitrogen deficient plants will begin to die but in most cases, gardeners and farmers should detect the deficiency before it becomes that bad.

Treating Nitrogen Deficiency

Always check that the soil pH is appropriate for your plant before doing anything else. Nitrogen starts to become less easily uptaken by plants if the soil pH is below 6 or greater than 8. If the pH isn't between 5.5 and 8.5 then this is likely the primary cause of the symptoms your plant is displaying (except for certain special plants that like extreme pHs - eg blueberries and azaleas which like very acidic soils). Also make sure that your plant has adequate water.

If there are no obvious reasons why your plant isn't able to uptake enough nitrogen then nitrogen deficiency in the soil may be suspected. In this case, the first treatment should be the addition of compost, other green organic matter and a layer of mulch. In particular, legume foliage and roots contains good levels of nitrogen. You may also apply a balanced fertiliser such as blood and bone or fish meal or a generic synthetic fertiliser. These will release a variety of nutrients slowly and will help to correct the problem with little risk that you will accidentally throw out the balance of another nutrient. An application of liquid fish emulsion is a good choice if your plants are severely deficient and so you need to boost nitrogen levels quickly.

If soil tests confirm that nitrogen deficiency is indeed the culprit and is the only nutrient problem, you may consider using a high nitrogen fertiliser. Aged chicken manure is high in nitrogen (in fact all poultry manures are - they need to be aged or composted to prevent them from burning your plants) as is dried blood or blood meal or your can apply a synthetic urea, ammonium or nitrate based fertiliser provided you strictly follow the instructions.

Nitrogen Toxicity

Nitrogen toxicity can occur through a number of mechanisms but is usually a result of the application of excessive amounts of fertiliser. Toxicity symptoms may be a result of plants having uptaken too much nitrogen in any form or they may result from excess levels of ammonium which is a nitrogen containing ion (NH4+).

Symptoms of Nitrogen Toxicity

Plants suffering from nitrogen toxicity may be stunted but are more often very tall and spindly. Foliage will be a deep green colour but plants will be more likely to succumb to diseases and pest attacks and will be more susceptible to frost damage. Often if a plant fails to flower and/or fruit, it is because it has absorbed too much nitrogen.

Treating Nitrogen Toxicity

Adding lots of brown organic matter (such as autumn leaves and straw - these things have a high carbon it nitrogen ratio) to the soil (by digging it into the soil and by adding it as mulch) can alleviate symptoms of nitrogen toxicity because lots of nitrogen is required by the organisms that break down high carbon organic matter. Nitrogen is relatively easily flushed from the soil so frequent watering will help remove any excess. Be aware though that this will flush out other nutrients too and too much water can promote fungal diseases and cause roots to rot. Provided you don't go overboard, applications of potassium can help improve flowering and fruit set and potassium and dilute seaweed solution can both increase a plant's resistance to pest (including disease) and frost damage.

If you do nothing, high levels of soil nitrogen as a result of excessive fertiliser application will usually drop down to healthy levels in a few months but your plants may not recover if symptoms are severe.

Symptoms of Ammonium Toxicity

While you won't see this unless you did your plants up, the roots of plants that have been exposed to too much ammonium will turn brown and the tips of the roots will usually die. The result of this is that plant growth is decreased. Plants will often also develop spots on the stems and leaves. The new leaves of some plant species may be stunted and yellow.

Treating Ammonium Toxicity

Once again, if you do nothing, the amount of ammonium in the soil will reduce down to a normal level by itself though plants may not survive if ammonium levels are too high for too long so it is a good idea to take some corrective action if toxicity symptoms are apparent. Ammonium toxicity is more common on acidic soil so make sure the pH is appropriate for your plant. As with 'normal' nitrogen toxicity, adding potassium can help resolve symptoms but be sure you don't increase soil potassium levels above desirable levels (this can cause deficiencies in other nutrients). Again, adding lots of brown organic matter can also help treat ammonium toxicity.

Ammonium toxicity is common in plants grown in soil-less media so be especially careful when adding ammonium to hydroponic systems.

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