A review of leguminous fertility-building crops, with particular ...

A review of leguminous fertility-building crops, Defra project OF0316 Executive Summary
later years. Nitrogen transfer can occur through root excretion and decomposition, leaching
from leaves and in leaf-fall and animal excreta.
Estimates of N fixation by white clover span a wide range of values and in spite of the
abundant information, there is still uncertainty about the quantities fixed. Much of the
variation in N-fixation can be attributed to differences in the proportion of clover in mixed
swards. Almost all studies indicate that white clover obtains most of its N from the
atmosphere rather than from the soil. There is evidence that the proportion of N derived from
the atmosphere (%Ndfa) is less under grazing than where swards are mown.
Red clover may be more productive than white clover. As with white clover, a high
proportion of N in red clover is obtained by fixation. Also, as with white clover, there is
evidence of differences in the quantity of N fixed by different cultivars.
Data for subteranean clover are fewer, reflecting that it is not a common UK crop.
However, N fixation appears to be affected in a similar way as with white and red clover.
The crop obtains most of its N from fixation, and fixation is greater under a cutting than a
grazing regime.
Lucerne is more persistent than red clover and has a productive life of up to 4 years. Yield is
little affected by drought during British summers. Constraints on growth are poor drainage
and very acid subsoils. It is therefore restricted to deep, free-draining, calcareous soils. As
with the other legumes reviewed, there is a wide range of N fixation values. Lucerne seems
particularly susceptible to soil nutritional status, and inoculation is essential.
There is little information about the quantities of N fixed under UK conditions by trefoil and
by sainfoin. There is an indication that sanfoin is less effective at fixing N than red clover.
Peas and beans tend to be grown as a fertility boost, part way through the arable phase of a
rotation. N fixation varies, partly depending on the amount of soil mineral N present.
Generally, the proportion of the crop’s N derived from fixation is smaller than from red or
white clover. A large proportion of the fixed N is removed from the field in harvested seed,
though the haulm, roots and rhizodeposits usually provide a positive N balance to the soil.
Net benefits of soybeans to following crops are also generally small because of the removal
of N in harvested grain.
Factors that limit plant growth normally also limit fixation, either through direct effects on
nodule formation or function or indirectly through the host plant's ability to supply nutrients.
However, some forms of stress (e.g. excess soil water, drought, cold or heat) have a greater
effect on nodule function than on nitrate uptake so that plants relying on fixation may be
more severely affected than plants supplied with fertiliser-N. In these situations,
improvements in general growing conditions would be expected to benefit N-fixing plants
more than those relying on fertiliser. Fixation can also be affected by soil structure and by
disease. Where inoculation is used to introduce more effective rhizobia, the response will be
influenced by the effectiveness of the native rhizobia and their competitive ability.
Effectiveness can also be reduced, e.g. in soybean, where there is a temporal separation
between maximum nitrate assimilation and N-fixation activity.
Regional differences and year-to-year fluctuations in weather patterns will influence legume
growth and fixation activity through effects of temperature, moisture supply and radiation.
Written by S Cuttle, M Shepherd & G Goodlass Page 4