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Crop Architecture and the Solar Corridor
Dr. Jerry Nelson, Plant Physiologist, University of Missouri, Columbia
Interest is increasing regarding the use of the solar corridor to optimize use of solar
radiation for crop production. The solar corridor is defined as the radiation density
incident to a crop canopy that is dependent on the solar angle from dawn to dusk
each day of the year and the effects of latitude. Cloud cover also shades the crop
and changes the radiation from direct, or point source, to diffuse from the sky. The
underlying physics and the probabilities of various types of cloud cover are
available to calculate the expected radiation density for nearly any geophysical
location on earth. In general, radiation during the growing season in the US is
highest on June 21 when day length is longest and radiation is most direct. In
addition, cloud cover generally decreases as on moves from the east to west in the
more humid areas of the country.
These interests and potentials are causing crop and soil scientists to re-examine
how physical and physiological principles developed for intensive crop production
in monocultures to intensive production in species using intercropping systems.
For example, planting 2-4 rows of corn with other crops such as soybean or melons
planted in the areas between the strips of corn may have advantages in solar energy
use than either crop in monoculture.
Much of the research on intercropping has focused on yield and land-use efficiency
(economic return from equal land areas) with little consideration of the basic
principles involved. The effect desired is higher yield per unit land area of at least
one of the crops to provide a higher production in the field of the intercropped
species compared with the yield in a monoculture. This equates to an increase in
land-use efficiency for crop production.
Crop yield depends on availability of water, nutrients and solar radiation causing
cropping systems to be designed to optimize the use of the three major inputs to
maximize yield. Usually this is for a monoculture and management practices based
on science have been developed. In most cases, when sufficient water and nutrients
are available, yield is a direct function of the capture and use of solar energy.
Most current major crop species have been selected to maximize use of solar
energy in monoculture by altering plant populations, optimizing row spacing,
improving the architecture of the canopy to capture and use the radiation
efficiently, reducing weed competition for light and reducing leaf diseases and
insect damage that affect the functional leaf area for radiation interception.
The above principles for monocultures are probably not the same for mixed
cropping systems including intercropping. Growing plants in a different
configuration likely change the relationships and interactions that alter effective
use of resources of water, nutrients and solar radiation. There are several
experiments that show more effective land use by intercropping, but the basic
principles are not understood. These principles include detailed studies on
optimization of row spacing and plant populations of the two component species,
e.g., are two rows of corn better than four or six? Or what proportion of the land
area should be in each crop? For example, with a low number of corn rows, will
solar radiation penetrate from the side more readily and increase yield by having
two ears instead of one?
Similarly, will four rows of soybean have higher yield or fix more nitrogen than if
there are two rows or six rows? Can a higher plant population be used? How will
the following crop respond to the areas that were managed differently? Is there a
minimum size for each strip that will allow for crop rotation using GPS to insure
the corn is planted in areas that were in soybeans the previous year? This would
allow the known “rotation benefits” achieved in monoculture to be expressed in the
intercropping system.
Already, there is evidence that use of intercropping in developing countries has
benefits, but wide-scale adoption in larger land areas has not been realized. Main
reasons are the challenges with mechanization, but agricultural engineers can
design planters for two crops, equipment for differential fertilizer and pesticide
application, and even harvest equipment.
Research on intercropping systems using a range of crops in a broad range of
environments is needed to understand the basic principles involved. Goals should
be to optimize use of solar energy within the constraints of other inputs. The
outcomes from optimizing solar energy capture and use should include
relationships with increased water-use efficiency, increased nutrient-use efficiency,
and economic return per land area.
Crop plants differ in optimum temperatures for photosynthesis, growth and yield
formation which may allow relay cropping when one crop is planted early in the
season and another is planted later. For example if corn is the tall crop it may be
feasible to grow a cool-season cover crop such as barley or winter peas into which
the corn is planted using no-till. The barley or peas in the inter-zone can be
harvested for grain before the corn canopy is large. After harvesting the barley
another cool-season vegetable crop could be planted such that if matures after the
corn harvest when there will be little shading as the days shorten and solar angle is
less favorable. The range of possibilities for inter- or relay cropping is wide and
needs to be evaluated.
The principle reason for intercepting solar radiation is to drive photosynthesis.
There is wide genetic variation for most of the characters associated with efficient
use of solar energy. In most cases the upper leaves are the most critical since they
intercept direct radiation whereas lower leaves are partially shaded and usually
older so contribute less to the plant. But with intercropping of a shorter species
allows light to be intercepted by the lower leaves of the taller crop to add more
sugars to the plant. This may be used to produce a second ear or a deeper root
system to explore more soil volume for water and nutrients.
Optimizing the canopy
There are several basic principles for describing optimum canopies for crops
grown in monoculture, but they may not be transferable to intercropping systems to
more effectively use the solar corridor. These principles have been worked out for
the major cereals such as wheat, rice and corn. It is easier to modifiy the canopy
architecture of monocots (grasses) than of dicots (broad-leafed plants). The first
principle is to capture the maximum amount of solar radiation by closing the
canopy rapidly and then use the radiation efficiency. This has led to selection for
early seed germination and seedling vigor for early planting to use the early
radiation and be at maximum interception during the highest radiation period in
June and July. Second was the need to overcome barrenness and pollination
problems (no ear formed or pollinated) with high populations.
The goal in monoculture is to have a large number of stalks with one ear per stalk.
When planted early the plants flower earlier, usually before there is drought stress
that reduces pollen longevity and abortion of early stages of kernel development.
Other grasses are more tolerant. Most major cereal grasses are now designed with
upright leaf angles to accommodate crowding and have more solar radiation reach
the lower leaves. Tassels and reproductive heads on grasses tend to shade the plant
with non-green tissues, so the tassel size of corn has been reduced and the rice
panicle now tips downward after pollination to be located below the uppermost
leaves. Plant height has been reduced to conserve sugars for growth of the grain
and to reduce lodging when higher levels of N fertilizer are used. This also reduces
lodging. Insect and disease resistance help retain functional leaf area.
The above principles apply to broad-leaved plants like soybean, potatoes, and
many vegetable crops. Unfortunately, some plants like soybean are solar-tropic and
tend to orient the leaf blades perpendicular to the sun. In most cases with soybean
and other broad-leafed plants the outer two inches of the canopy intercept nearly
90% of the radiation. Smaller leaf sizes, especially narrower leaf blades may aid
radiation penetration in these species. Increasing plant height will usually increase
penetration of solar energy.
Other features of importance for broad-leaved plants include shade tolerance,
optimize the key architecture Optimize the shape for a new ecosystem: Leaf
angles, Leaf length, tassel size and shape, plant height, stalk strength, insect and
disease resistance. Broadleafed plants, leaf size and orientation, shade tolerance,
photosynthesis of lower leaves, Internode length, yield components,
Resource use: solar angles (N-S or E-W) shading in the lower crop, water use per
crop in the mix, nutrient use in the mix, N-fixation, rotation effects (allelopathy)
insect populations and disease situations, random roughness of the canopy to
reduce CO2 gradient.
NSF research would optimally be conducted in all crops, in all states, in all
climatic conditions, in all watering configurations from organic to gmo situations
to create a national framework and data for local use by farmers. International use
and additional research is also critically important to the FEWS nexus as FEWS is
an international framework and frame of referencing the efficiency and
sustainability of agriculture and farming systems.