PLANT NUTRITION 101-2 Nitrogen (N)
By: Riley Holly
Plant tissue typically contains 1-5% nitrogen, which is the most prevalent of the primary nutrients. Plant uptake is by nitrogen ions in the water uptake. Nitrogen is a constituent of proteins, amino acids, nucleic acids, chlorophyll, and coenzymes in plants. The two forms of nitrogen that plants utilize are NO3¯ (nitrate) & NH4+ (ammonium}. Nitrate is the primary source, as the ammonium ion is often bound to clay particles whose negative surface charge reduces its availability, although NH4+ some is taken up in the water. Ammonium is converted to nitrate by soil microorganisms and oxygen. Organic minerals must be converted to ammonium by microorganisms and then to nitrate over a period of weeks to months. If incomplete compost is added to the soil, the microorganisms will deplete the nitrogen. As the compost (2% N) is completed, the microorganisms die off and nitrogen is released. Other organic sources are blood meal (11% N), soybean meal (7% N), cottonseed meal (6% N), & fish meal (6% N).
Nitrogen is also mobile in the plant. The nitrogen will move from older leaves to newer leaves (translocation). This explains one of the symptoms of nitrogen deficiency, older leaves yellowing (chlorosis) while the newer leaves are green. Other symptoms of nitrogen deficiency are slow growth, tip and margin necrosis, green veins in yellow leaves, early leaf drop, and small leaves & shoots. Sometimes purple discoloration (due to an accumulation of anthocyanins) is also an indicator. An excess of nitrogen can delay flowering and fruiting, lead to dark green leaves, diminished fruit flavor, a delay in fruit maturity, and a reduction in fruit color.
Of course we don’t want to wait for symptoms to occur before modifying our nutrition management. Tissue analysis is expensive, especially for the backyard gardener who would have to have several analyses made for the different tree genus/species and soil micro-climates. Each plant will have a different requirement even within a species. For example, avocados range from 1 lb. of actual nitrogen the first year to 6 lbs. when mature.
Because nitrogen is highly leachable, applications should be spread out in the growing season, but not in the flowering period. It is recommended that avocados receive applications in August and November. This would also be appropriate for stone and some fruit trees.
The nitrate form is highly mobile in the soil and tends to leach out of the root zone during high precipitation, or excess irrigation, especially in coarse (e.g. sandy), and shallow soils. The soil can gain nitrogen through mineralization (from organic material), nitrification (ammonium to nitrate), biological fixation (nodules on root hairs of legumes), precipitation, applications of manure and fertilizers, and irrigation water. Losses can be attributed to sorption at exchange sites (clay - ammonium ions), immobilization (inorganic to organic at high C:N ratio [>30]), denitrification (removal of oxygen in nitrate, loss in decomposable organic soils), volatilization (ammonium ion converting to ammonia gas at high pH), plant uptake, and leaching.
Some inorganic sources of nitrogen are:
*ammonium nitrate (32-0-0)
*ammonium phosphate sulfate (16-20-0)
*ammonium sulfate (21-0-0-24S)
Remember it is most important to maintain the proper balance of nitrogen, neither a deficiency nor surplus. Keep a record of when and how much is applied and note whether symptoms of excess or deficiencies are apparent.