Food for Thought - Nitrogen

Plants, like us, need nutrients to live and grow and there are thirteen essential mineral nutrients needed by all plants.  Six of these are needed in large amounts and are called macronutrients, and the remaining seven are needed in small amounts and are called micronutrients. Of all the thirteen nutrients, the one plants need in the greatest abundance, is Nitrogen. 

Nitrogen is used to make amino acids - the building blocks of proteins, as well as genetic components of the cell, the DNA and RNA.  It also increases the amount of chlorophyll and thus the green colour of plants.

Nitrogen deficiency in plants can typically be seen by the stunting of stems above ground and of the roots below. There is typically a yellowing of the entire leaf-blade with the older leaves succumbing first.  In contrast, an excess of Nitrogen will result in excessive top-growth of soft shoots that have less of the woody lignin plants need for rigidity and contain less natural phenol-based compounds, making them at greater risk of attack by pests and diseases, and also more susceptible to wind damage.

The air we breathe is about 78% nitrogen gas, but that form of nitrogen is almost inert and not able to be used directly by plants.  Lightning from thunderstorms is one way to get this atmospheric nitrogen into a form usable by plants, but generally, plants get their nitrogen from minerals in the soil; usually in the form of ammonium ions (NH4+) or nitrate ions (NO3-). 

Different fertilisers have different amounts of nitrogen, in different forms, and released over different time scales.  Whenever you buy fertilizer, be it organically derived such as Bonemeal, or Chicken Manure, or Blood, Fish, and Bone, or an inorganically derived fertilizer like Growmore, or Miracle Grow, or Tomorite (other fertilisers are available!) then the container will have a value for the N-P-K nutrients.  This is the amount of Nitrogen, Phosphorus, and Potassium expressed as a percentage.  Thus a bottle of Tomorite has an N-P-K value of 4-3-8 meaning that 4% of the liquid is Nitrogen.  Adding 20mls to a gallon of water and adding this to a growbag of tomatoes will give the plants a total of about 0.8g of Nitrogen, but this Nitrogen will be immediately available to the plant.

In contrast, a single handful of dried Chicken Manure (N-P-K = 4.5-3.5-2.5) contains about 2.2g of Nitrogen; almost three times the amount in 20 ml of Tomorite, but in this case, the Nitrogen is not immediately available to the plant and decomposition and the action of bacteria and fungi will slowly make this Nitrogen available.

So it is important to consider both the amount of nitrogen and how it will be available to the plant.  It is also worth considering what else that nitrogen source is doing to your soil.  Nitrogen as nitrate or ammonium ions are rapidly washed out of the soil by rainfall and indiscriminate agricultural use of inorganic nitrogen fertiliser has led to nitrates in rivers and water supplies, and ‘green’ lakes with algal blooms.  Slower release forms of nitrogen fertiliser can reduce this loss, but garden compost and rotted horse manure will also add slowly composting plant that holds moisture in the soil, provides food for beneficial bacteria, fungi and earthworms, and will help improve soil structure in the long term.

Garden compost is about 1% Nitrogen, horse manure is about 0.7%, so a wheelbarrow load of either will have a total of 140-200g of nitrogen, but only about 5g of that will be immediately available to the plants, with the rest taking up to 4 or 5 years to be released to the plant, but in that time it will have been home to thousands of beneficial animals and billions of good bacteria and fungi.