Nitrogen is usually the element limiting plant growth and used in largest quantities.
Nitrogen is not found in large amounts in the soil except in the organic form; there is essentially none in soil minerals.
The source for all nitrogen is the air.
80% of the air we breathe is N2, there is an unlimited supply.
It has been calculated that there is over 35,000 tons of N over every acre of land area.
- component of all proteins, enzymes, and chlorophyll.
- required in largest amount by most plants.
- regulates the use of K, P etc.
- has quickest and most pronounced effect on plants.
- very mobile (anion).
But remember plants can only use N in the form of NO3 - or NH4+.
- Sources of N in the soil:
- Inorganic NH4+ NO3-
- Organic - 89-98% of total N.
Organic nitrogen becomes available as the organic matter decays.
- Sources of N (additional)
- Legumes: depends on the amount and type
of vegetative growth Symbiotic organisms: fix N2. Legumes
will fix between 40--300 lb6 N/A
Nonsymbiotic organisms: fix 10-20 lbs N/A.
- Plant residue: C/N ratio effects release of N.
- Animal residue (wastes): type of wastes controls N amount.
- Rain and Snow: electrical discharges from lighting generate heat that cause atmospheric O2 to react with N to form several different N oxides. These are brought to the soil by snow and rain. 5 lbs N/A/Y.
- Industrial Wastes: heavy metals are of concern.
- Legumes: depends on the amount and type of vegetative growth Symbiotic organisms: fix N2. Legumes will fix between 40--300 lb6 N/A
- Nitrogen cycle
- Plant Uptake
- Plant Loss
- Exchangeable NH4+
Soil organic matter, manure, etc.. is broken down into proteins, amino acids, and amines. This material is converted, by specific bacteria, to NH4+. This is ammonification..
- microbial oxidation of ammonium to NO2-3 occurs under aerobic conditions. This produces hydrogen
ions and the anion form of N which is more subject to leaching
- Factors affecting Nitrification
- NH4+ must be present
- Aeration: nitrification requires O2
- Temperature : nitrification occurs only above freezing and the rate increases up to about 85F. This is why fall applications may be delayed until the soil temperature stays below 50F
- Moisture: nitrifying bacteria are active in moist soils but inactive in waterlogged soils
- Neutral pH: nitrification rates are low in acid soils. This is due to bacteria preferring more neutral pH.
- C:N ratio
- Use of N serve
- Fumigation prevents nitrification because bacteria are killed
- microbial oxidation of ammonium to NO2-3 occurs under aerobic conditions. This produces hydrogen ions and the anion form of N which is more subject to leaching
- reduction of NO3 to N2, NO, and N2O
- gaseous loss of N from soils occurs under reducing conditions
- facultative anaerobic
- wet anaerobic reducing condition; like to have a large amount of plant residue available.
- Nitrogen Fixation
- conversion of atmospheric N2-4 +in cells
- occurs under aerobic conditions
- represents only major input of N to many soils
- N2 + 6H +6e 2NH3
- Symbiotic fixation by Rhizobium
- Non-symbiotic by free living organisms bacteria, blue green algae
- Urea: additional source added from animal wastes or synthetics
- anhydrous ammonia
- Ammoniumium applied to soils above ph 7. Loss of N as a gas can occur because of surface application of ammonium
- conversion of organic N to mineral form NH4+
- heterogeneous organisms release NH4+
- Brief review of the C:N ratio idea
- At any given time no more than 1-2% of the
total amount of N in the residue is available to the plant.
- Only about 2-3% of the immobilized N is mineralized each year
- At any given time no more than 1-2% of the total amount of N in the residue is available to the plant.
- conversion of mineral N (nitrate and ammonium) to the organic form
- N is not available to plants in the organic form
- remember that NO3-
- Fertilizer Production
- Fertilizer Addition
- Plant Uptake
- component of:
- role in plant maturation - (flowering, fruiting, root development, straw strength)
- The element P is in every living cell, both plant and animal. The light energy captured by photosynthesis would not support any of the necessary plant functions if compounds containing P were not present. P works as an "energy storage and transfer" structure.
- component of:
- Crop Demands
Only N and K are taken up in larger amounts by the plants.
P is required for N fixation to take place. The microbes in the system require that some P be present.
- Forms of P in soil
Inorganic ( phosphate anion)
Very little of total P is plant available at any given time.
- Plant Uptake
plants take up P from soil water. Because only small amounts of P are found in the soil water at any one time the P must be constantly replenished by soil particles.
soil organic matter contains P. As with other nutrients the organic matter must first be broken down before the plant can use the P.
* discuss the sewage plant problem with P. Very high rates.
- Affect of pH on Phosphorus form
HOH + HPO4-2
HOH + PO4-3
- P Fixation
formation of "fixed" or unavailable P compounds
- By Fe, Al, (acid conditions) : FePO4
- By clays : Fe and Al associated with clay
- Enough P must be added to overcome the fixation capacity and the plant needs. The soils fixation capacity may be slowly satisfied over a number of years. Then the P recommendation may be slowly decreased.
- By Fe, Al, (acid conditions) : FePO4 ,Al(PO4), MnPO4
- What happens to Phosphorous when it is added to
the P will convert to the most stable form the soil will permit. The form is related to the soil pH
These stable forms are less available to the plant.
because of the quick reaction, the mobility of P in the soil is greatly reduced. Rate of movement may not exceed 1 inch per year.
- Forms of P in the soil
Relatively unavailable <-->Labile <-->Soil Solution P
Many soils will have very high labile amounts. Some of this P may be mined.
The basic management strategy is to build soil test levels to a point where plant needs are generally met by the soil. Once this point is reached, additional P is applied in quantities necessary to maintain the soil at that level. Adjust the soil pH to proper levels with liming. This will help reduce fixation by Fe and Al. Increase or maintain organic matter levels to maintain goad release of P In many cases a least of part of the maintenance P requirement should be applied in a concentrated zone near the seed at planting. This is called starter fertilizer and assures sufficient P for early growth. Most annual crops have very high requirements for P early in their growth because of a limited root system.