Doctoral Dissertations

Date of Award

8-1974

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

William L. Parks

Abstract

Yields of soybeans often differ from year to year even when planted on soils of the same productivity and with equal fertility levels. This study was designed to investigate the effect of microclimate on soybean yields. Attempts were made to relate certain growth and yield characteristics with selected climatic factors.

York soybeans were planted in five different row arrangements on a Sequatchie loam at Knoxville, Tennessee. Beginning in 1971, a three-year study on the microclimate within and around these row arrangements was conducted during the months of June, July, August, and September. In 1973 soybean yields varied significantly with differences in row spacings but only small nonsignificant differences were found in 1972. Of the five row arrangements tested, the 24-48-inch paired row arrangement yielded five bushels per acre more than conventional 36-inch row spacings in 1973. The 72-inch middles contained in the 36-72-inch paired row arrangement and the 72-inch row arrangement were determined to be too wide to permit continuous cover to develop in these plots. The 18-inch row arrangements lodged during each year of the study and yielded slightly less than the 36-inch row spacings.

Within the soybean rows, isothermal layers were found to follow the contours of the soybean canopies in the arrangements which were not in continuous cover. Within the canopies that had attained continuous cover, temperature isotherms were largely parallel to the soil surface in the first 12 inches above the soil. The temperature immediately below the leaves of the outer canopy was approximately one degree higher than the air temperature surrounding the outer periphery of the plant. Soil temperature was generally coolest at the base of the plant during the day. Soil moisture was suspected to be strongly correlated with the thermal load received in the row middles. Net radiation studies indicated no differences in reflected energy from the soybean canopies of the five row arrangements after July 23 in 1973.

Approximately 90 percent of the light received at the plant surface was attenuated after it had passed 12 inches inside the canopy. Values of 0.5 to 1.5 percent of incoming radiation were recorded on the soil surface after the plots had attained continuous cover. In 1973, the 72-inch and 36-72-inch paired row arrangements exhibited considerable branch lodging which lowered the seed quality and size in the lower branches of those plants. Branch lodging caused a decrease in leaf density in the upper position of these plants and allowed slightly more light to reach the inner leaves in those canopies.

Undulations in the canopy surface associated with different row spacings provided different surface areas which were exposed to incoming radiation. The canopy of a 24-48-inch row arrangement had 73 percent more surface area than the conventional 36-inch row arrangement. Measurements of leaf area and leaf density showed that in most instances the 36-inch row plants had an equal or greater number of leaflets than 24-48-inch row plants. Likewise the average leaf area index for the 36-inch row plants was equal to or greater than that of plants in the 24-48-inch row arrangements. When the soybeans taken from plants of these five arrangements were counted and weighed, it was concluded that the undulations produced in row arrangements with middles greater than 36 inches had enhanced the photosynthetic response of these plants since each plant produced more seed with a higher seed weight.

Twenty to forty soybean plants from each row arrangement were stripped of leaflets and pods. The number and type of branches, branch direction, and the number of pods per branch were recorded. Filling of pod ovules, the number of seed and seed weight per plant were also measured. Plants taken from the 24-48-inch row arrangements produced more basal branches which contributed heavily to seed production. These plants did not lodge in either north-south or east-west plantings. They were planted far enough apart to resist entanglement, expose a large canopy area, and yet were close enough between rows to attain continuous cover by early August. The 36-inch row spacings of soybeans contained only a few plants which lodged or exhibited branch lodging. Seed yields in the 24-48-inch row arrangements were as high as 186 grams per plant and produced an average of 108 grams per plant, whereas, the conventional 36-inch rows averaged only 89 grams per plant.

As spacings between rows of soybeans were narrowed, the number of lower branches, the number of pods with aborted seed, and the number of seed per pod decreased.

In 1972 and 1973 CO2 measurements taken within the different row arrangements revealed a wide range of CO2 concentrations between determinations made within the plant canopy but above the 24-inch level above the soil surface and the ambient readings taken at seven feet above the soil surface. These CO2 sinks generally ranged from 30 to almost 50 ppm less than ambient and were greater in the 24-48-inch and 18-inch row arrangements. Zero displacement measurements indicated that wind penetration was deeper into the 24-48-inch canopies than into the 36-inch or 18-inch row arrangements.

When the hours of sunshine recorded at the McGhee Tyson National Weather Station were correlated with the yields of soybeans harvested from 11 planting arrangements over the period 1971-1973, a linear response in yield with increasing hours of sunshine was obtained. Arrangements with row middles greater than 36 inches but not greater than 72 inches produced yield functions with slopes ranging from 0.27 to 0.41. Spacings of rows less than 36 inches produced yield functions with slopes of 0.13 or less. Greater canopy surface area was observed to create a greater photonastic response from soybean leaves of those canopy peripheries.

Plant lodging in the narrower row spacings limited soybean seed yields in these arrangements. The 12 and 24-inch row arrangements produced well during years with 60 to 70 percent of the possible sunshine being received in the field. During years with more than 70 percent of the possible sunshine, the row arrangements with wider middles that formed complete canopies generally produced the best yields.

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