Growing Legume Crops for Nitrogen

By John Hollinger, MAFRI Provincial Organic Specialist (BDS)

Organic farmers have long recognized the value of including legume crops in crop rotations due to the ability of legumes to improve the physical, chemical and biological characteristics of the soil. Although annual grasses do a good job of scavenging available soil nitrogen, legumes play an important role in contributing nitrogen (N) to the soil. Legumes have a symbiotic relationship with naturally occurring rhizobia in the soil. Rhizobia are soil bacteria that fix nitrogen after becoming
established inside the root nodules of legumes.

Sweet clover, red clover, field peas and alfalfa have been the most widely used legumes in Manitoba, but there are a number of others that can provide nitrogen for succeeding crops:

Legume	Pros	Cons
Alfalfa	seed readily available deep rooted, breaks up compacted soil layers highest-rated N supplier (over 200 lb/acre)	seed can be expensive usually grown for more than 1 year
Sweet Clover	deep-rooted, breaks up compacted soil layers drought tolerant winter hardy inexpensive seed highly rated N supplier	slow to establish usually grown for 2 years volunteers readily
Red clover	seed readily available nitrogen release matches well with grain crops nitrogen release well documented option to take a seed crop if left to grow for the next year	inconsistent stands in many years
Kura clover	winter hardy competitive in pasture mixes moderate tolerance to flooding and drought	slow to establish more suitable for pastures
Field Peas	seed readily available easily planted and quick to establish	seed can be expensive large seed, needs good soil moisture to establish research suggests that there is not a lot of N transferred to the following crop
Fababeans	tolerant of early spring frost – can be seeded early will fix up to 90% of plant’s N requirements similar to alfalfa as an N supplier	not drought tolerant poor competitor with weeds seed can be expensive
Hairy Vetch	creates a dense mat once established	seed expensive, sometimes hard to find slow to establish, may allow weeds to gain ground
Chickling Vetch	more tolerant of dry soil conditions	seed not readily available in MB seed is poisonous to livestock

From the December 2010 edition of CROPS E-NEWS (subscribe)

Manitoba Agriculture, Food and Rural Initiatives

Growing Legume Crops for Nitrogen