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 daylength 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 altereffective 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.