Nitrogen Fixation

Part of Crop Rotation

Nitrogen is the nutrient most commonly limiting plant growth. The atmosphere is 78% nitrogen gas, but plants cannot use atmospheric nitrogen directly — it must first be converted to ammonia or nitrate. Certain bacteria, living in symbiotic nodules on legume roots, perform this conversion, effectively manufacturing free fertilizer from air. Understanding how to maximize this biological process is one of the most valuable agricultural skills available without industrial inputs.

How Biological Nitrogen Fixation Works

The process requires three elements working together:

1. Legume host plant: Bean, pea, clover, vetch, lupin, soybean, alfalfa, and many other members of the Fabaceae family.
2. Rhizobium bacteria: Soil-dwelling bacteria that infect legume roots and are housed in root nodules.
3. Anaerobic conditions inside nodules: The enzyme nitrogenase that fixes nitrogen is irreversibly destroyed by oxygen. Nodules maintain an oxygen-free interior using leghemoglobin (a protein that gives healthy nodules a pink-red color) to bind and regulate oxygen.

The plant provides carbohydrates (sugars from photosynthesis) to the bacteria; the bacteria provide fixed nitrogen (as ammonium) to the plant. It is a genuine mutualistic relationship — both partners benefit.

Confirming Active Fixation

Dig up a legume plant and inspect the roots. Squeeze a nodule between two fingers:

Nodule Appearance	Interior Color	Fixation Status
Plump, firm, numerous	Pink to red	Actively fixing nitrogen
Small, sparse, hard	White or gray	Fixing poorly or not at all
Absent	—	No Rhizobium in soil; inoculation needed

Pink coloration comes from leghemoglobin — the same pigment as hemoglobin, serving an analogous function in regulating oxygen. Pink = active and productive.

Rhizobium Inoculants

Different legume species require specific Rhizobium strains. Strains are not interchangeable:

Legume Group	Rhizobium Species/Strain	Commonly Fixes (kg N/ha/yr)
Clovers	Rhizobium leguminosarum bv. trifolii	100–200
Peas, vetches	Rhizobium leguminosarum bv. viceae	80–150
Soybeans	Bradyrhizobium japonicum	100–200
Beans (Phaseolus)	Rhizobium phaseoli	50–100
Lucerne (alfalfa)	Sinorhizobium meliloti	100–250
Lupins	Bradyrhizobium sp.	100–180
Groundnuts	Bradyrhizobium sp.	100–200

Rhizobium bacteria survive in soil for years after a legume crop has been grown. If a field has grown clover within the past 3–5 years, appropriate bacteria are likely already present. If the species is new to a field, or if the soil has been fumigated, flooded, or highly acid (below pH 5.5), inoculation is necessary.

How to Inoculate Seed

Purchased inoculant (dried peat culture with live bacteria):

1. Mix 1 tablespoon of inoculant per 500 g of seed with enough water to form a slurry coating.
2. Allow to dry in shade for 30–60 minutes. Do not dry in direct sun — UV destroys the bacteria.
3. Sow immediately after drying.
4. Do not mix inoculant with fungicide-treated seed — fungicide kills the bacteria.

Homemade inoculant (soil transfer method): Where commercial inoculant is unavailable, soil from a field that has recently grown the same legume species can transfer appropriate bacteria:

• Collect 1–2 kg of soil from around the roots of healthy, well-nodulated plants.
• Mix into the seed furrow at sowing time, applying approximately 200 g of inoculant soil per 10 m row.
• This works reliably when the source soil has been growing the correct legume for 2+ years.
• Do not use soil from diseased fields or those with heavy pesticide history.

Conditions for Maximum Fixation

Several soil and management factors affect how much nitrogen is fixed.

pH

Rhizobium are sensitive to soil acidity. Most strains cease to function effectively below pH 5.5.

Soil pH	Effect on Fixation
6.5–7.0	Optimal
6.0–6.5	Good; most strains active
5.5–6.0	Reduced; clovers and alfalfa affected most
< 5.5	Very poor; most Rhizobium inactive
> 7.5	Reduced (alkaline stress); lupins affected

Lime acid soils to at least pH 6.0 before establishing a legume-based rotation. A single lime application (2–4 tonnes/ha agricultural lime depending on starting pH and soil type) can transform a field from poor legume performance to excellent.

Phosphorus and Molybdenum

Nitrogen fixation is energetically expensive — the plant must supply ample carbohydrates to its bacterial partners. Phosphorus deficiency directly limits photosynthesis and therefore the energy available for fixation. Molybdenum (Mo), though required in tiny amounts, is an essential cofactor of the nitrogenase enzyme.

Nutrient	Effect if Deficient	Source
Phosphorus	Fixation severely reduced; stunted plants	Rock phosphate, bone meal, compost
Molybdenum	Fixation fails even with good Rhizobium	Often corrected by raising pH (Mo becomes more available above pH 6.5); apply trace molybdate if needed
Calcium	Nodule formation impaired	Agricultural lime, shell grit

Nitrogen Fertilizer — a Counterintuitive Problem

Adding external nitrogen fertilizer (manure, compost, or mineral nitrogen) to legume crops suppresses biological fixation. When plant-available nitrogen is abundant in the soil, the plant “turns off” the symbiosis — it costs carbohydrates to feed the bacteria, and if nitrogen is freely available, why pay for it?

Do not apply nitrogen-rich manure or compost to legume crops — it reduces the fixation you are trying to achieve. Apply any organic amendments in the crop preceding the legume, not under or into the legume year.

Major Nitrogen-Fixing Crops and Rotation Uses

Red Clover (Trifolium pratense)

• Perennial grown as a 1–2 year ley
• Fixes 100–200 kg N/ha/year
• High biomass production (3–6 tonnes dry matter/ha)
• Plow in while green for maximum nitrogen release; or cut for hay (removes nitrogen from field)
• Susceptible to clover rot (Sclerotinia) — do not grow in same field more than once every 5–6 years

White Clover (Trifolium repens)

• Perennial; lower-growing than red clover; good under fruit trees and in grass/clover leys
• Fixes 80–150 kg N/ha/year
• Tolerates closer mowing than red clover; suitable for grazed pasture

Hairy Vetch (Vicia villosa)

• Winter annual; extremely cold-hardy (survives to -20°C)
• Excellent overwinter cover crop in cold climates; fixes 100–200 kg N/ha
• Very high biomass; decomposes quickly when killed in spring
• Plant August–October for overwinter cover; roll or mow in April for spring planting

Field Peas (Pisum sativum)

• Dual-purpose: edible crop + nitrogen fixation
• Harvest peas for food; plow vines and roots in immediately after
• The nitrogen is held in the roots and vines; removing only the pods leaves most fixed nitrogen in the field
• Fix approximately 80–150 kg N/ha in a single season

Cowpeas (Vigna unguiculata)

• Tropical and subtropical; heat-tolerant, drought-tolerant
• Fixes 80–200 kg N/ha in a single hot season
• Edible seed, leaf, and pod; can be used as a green manure or harvested for food
• Quick-maturing (50–90 days); useful for filling gaps between main crops

Lupin (Lupinus spp.)

• Acid-soil tolerant (fixes at pH 5.0–6.0 where clovers fail)
• Very deep taproot (60–120 cm) improves drainage and subsoil nutrition
• Fixes 100–180 kg N/ha
• White lupin (L. albus) is the most productive; narrow-leaved lupin (L. angustifolius) is most acid-tolerant

Green Manure vs. Removing the Crop

The decision of whether to incorporate the legume or harvest and remove it significantly affects how much nitrogen remains in the field.

Action	Nitrogen Retained in Field
Plow or dig in entire plant (green manure)	80–100% of fixed N
Graze in situ (animals eat foliage, dung stays)	70–90% of fixed N
Cut for hay (remove all aboveground biomass)	20–30% of fixed N (root N only remains)
Harvest grain/seed (remove seeds, leave straw)	40–60% of fixed N

If you need fodder, compromise: cut one hay crop in early summer, allow regrowth, then plow in the second cut as a green manure in late summer. This gives you one cut of hay while retaining significant nitrogen from the second growth and root residues.

Nitrogen Fixation Summary

Legumes fix atmospheric nitrogen through symbiotic Rhizobium bacteria housed in root nodules. Pink-red nodule interiors indicate active fixation. Each legume group requires a specific Rhizobium strain — inoculate seed whenever the species is new to a field or the soil is acidic or fumigated. Maintain soil pH above 6.0 and adequate phosphorus for maximum fixation. Do not apply nitrogen fertilizers to legume crops — it suppresses the symbiosis. Incorporate entire green plants as green manure for maximum nitrogen retention (80–100%); cutting for hay retains only 20–30%. Choose species to match climate: vetches and clovers for temperate zones, cowpeas and groundnuts for tropics, lupins for acid or sandy soils.