Summer, 1990 (v2n4)
MacRae, R.J. and G.R. Mehuys
Advances in Soil Science, Volume 3:71-94. 1985
Reviewer's note: Green manures are crops which are turned into
the soil in order to improve the growth of subsequent crops. This
article reviews the non-nutrient benefits of green manures, i.e.
their potential to improve soil organic matter levels, reduce
soil compaction and soil erosion, and enhance crop yields. To
narrow the scope of the analysis, the authors focused on literature
relevant to field crop systems (mainly corn) in temperate regions.
The information presented in the article partially explains the
recent resurgence of interest in using green manures and cover
crops, and also highlights the need for more and better research
in this area. The review is a good introduction for those who
want to find out more about the reasons for using green manures.
Historical Background
Earliest references to the use of green manures are from China,
dating back to the 12th century B.C. Ancient records from Greece
and southern Europe show that lupines and faba beans were favored
in the Mediterranean region. A seminal work by Pieters (1927),
suggests that the use of green manures occurred much later in
northern Europe, and that colonists brought the practice with
them to North America. The use of green manures on this continent
reached a peak in the beginning of the 20th century, but has declined
since the introduction of synthetic inorganic fertilizers. Historically,
farmers used green manures as a way of improving soil fertility
and increasing crop yields. Improvements in soil tilth that accrued
through regular additions of organic matter may not have been
as apparent.
Effect of Organic Matter on Soil Physical Properties
Research during this century has established that organic matter
affects some soil physical properties more than others. This review
focuses on aggregate distribution and stability, bulk density,
moisture retention, and water movement.
Soil Aggregates. Organic matter plays more of a role in
aggregate stability than in aggregate formation. It is, in fact,
the primary stabilizing agent for aggregates in temperate-area
soils. This stabilization process is accomplished mainly through
the by-products of organic matter decomposition (microbial gums
and mucilages).
Bulk Density. "With few exceptions, organic matter
decreases the bulk density of soil." This effect can occur
either directly by "diluting" the soil with a less dense
material, or indirectly through greater aggregate stability. Indirect
effects seem to be the most important and are not dependent on
soil textural class.
Moisture Content and Availability. Soil organic matter has a variable effect on available water in soil. It is generally a positive relationship, but whether or not the effect is significant depends on other soil properties, notably texture. One experiment, for example, found organic matter to influence available water only in soils of medium to low clay content (13 to 20 percent); other
researchers concluded that coarse silt, not organic carbon, was
the primary factor determining available water in mineral soils.
It is important to distinguish between water retention and
water availability. Some sources of organic matter (like
peat moss) have high water retention (i.e. they increase soil
water holding capacity), but a portion of this water may be held
so tightly that it is not available to plants. Increased soil
aggregation can have a similar effect.
Water Movement. Organic matter has a strong, positive effect
on infiltration of water into soils. This effect is due mainly
to a decrease in bulk density, and improvements in aggregation
and structure.
Effect of Green Manures on Soil Organic Matter
The maintenance or accumulation of organic matter in soils depends on a number of factors including: characteristics of the added material, soil and climatic factors as they affect microbial activity, and agricultural practices. All other factors being equal, it is generally accepted that low-nitrogen green manures (1.5 percent N or less) can be effective in improving soil organic matter levels. High-nitrogen materials such as legumes, on the other hand, cannot maintain organic matter levels because they decompose too rapidly. Varied reports from the literature suggest that each plant species should be considered on its own merits and in the context of the farming systems in which it will be used. Table 1 presents a sample of how diverse the literature is on this subject. Any interpretation of these varied results should also take into account soil type, soil nitrogen levels,
native soil organic matter levels and microbial activity.
Study |
|
| ||||
| Potting Studies | ||||||
| Prince (1941) | Decrease | |||||
| De Haan (1977) | Increase | |||||
| Increase | ||||||
| Field Studies | ||||||
| Poyser (1957) | Decrease | |||||
| Mann (1959) | Increase | |||||
| continued from Mann (1959) | ||||||
| Charter et al. (1970) | ||||||
| Increase | ||||||
| Decrease | ||||||
Effect of Green Manures on Soil Physical Condition and Crop Performance
The authors next address two practical questions: Are green manures,
used in a field situation, capable of improving soil physical
properties in the same way that other forms of organic matter
do? And the related question: Can the use of green manures improve
crop performance?
Green Manures and Soil Physical Properties. The same characteristics
evaluated in the first section of this paper are also evaluated
here: aggregate distribution and stability, bulk density, moisture
content and availability, and water movement. Though limited,
the literature on green manure effects is generally consistent
with that on the effects of organic matter per se.
Green Manures and Crop Performance. It is an accepted fact
that improvements in soil physical condition create the potential
for increased crop growth. It is, however, difficult to assess
this relationship quantitatively, and to determine the degree
of improvement necessary to effect a significant increase in crop
yield. Researchers who have looked at corn production report that
increased corn yields are associated with improvements in the
physical condition of the soil. Due to limitations in experimental
design, they were unable to differentiate the effect of enhanced
soil fertility from the effects of an improved physical environment.
One definitive point can be made: The benefits of green manuring
on crop yield are most apparent during dry periods, particularly
in rainfed production systems.
Conclusions
The authors' conclusions focus on the need for more creative research
into the effects of green manures and soil organic matter. The
number and complexity of factors involved necessitates
the use of a new approach to research, one that is scientifically-based,
and also holistic. Rather than following the traditional approach
of controlling all but the few factors to be studied, the authors
suggest that the interactions among many factors can be
established by systematically holding one factor constant and
measuring the effect of the green manure upon the others. These
experiments must be conducted under different climatic conditions
and over a long period of time. Though more expensive, this "holistic
approach" is probably the only way to fully understand the
benefits and challenges of green manuring.
References
Chater, M. and J.K.R. Gasser. 1970. Effects of green manuring,
farmyard manure and straw on the organic matter of soil and of
green manuring on available nitrogen. J. Soil Sci. 21:127-137.
De Haan, S. 1977. Humus, its formation, its relation with the
mineral part of the soil, and its significance for soil productivity.
In: Soil Organic Matter Studies. VoL II Int. Atomic Energy
Agency, Vienna, pp.21-30.
Mann, H.H. 1959. Field studies in green manuring, II. Emp.
J. Exp. Agr. 27:243-251.
Pieters, A.J. 1927. Green Manuring. John Wiley & Sons, New
York, NY.
Poyser, E.A., R.A. Hedlin and A.O. Ridley. 1957. The effect of
farm and green manures on the fertility of Blackearth-Meadow clay
soils. Can. J. Soil Sci. 37:48-56.
Prince, A.L., S.J. Toth, A.W. Blair and F.E. Bear. 1941. Forty-
year studies of nitrogen fertilizers. Soil Sci. 52:247-261.
Contributed by Dave Chaney
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