Seed Germination in Soil

Seed Depth

A number of studies have discussed the relationship between seed planting at various levels and germination success. In Koger, Reddy, and Poston’s study, Texasweed was measured in different planting levels with specific interest in finding where the greatest amount of seedling emergence may be found. The Texasweed is from the Euphorbiaceae family and is an erect herb whose height usually maxes out at 300 cm and produces leaves some 3 to 15 cm long in alternate and serrated patterns. It is a weed that warrants study as it is in increasing prevalence in domestic and commercial locations, specifically in rice and soybean fields. By analyzing its growing conditions, researchers may gain clues for how to overcome the adversity that Texas weeds poses for consumers and the environment. In their study, the researchers found that at once cm depth the majority of Texas weed, around 76%, sprouts. Seeds placed at 7.5 cm usually only had 7% emergence.  Performance indicates that by 10 cm, the seedling had all together a germination rate of 0. Since the best ratio of planting is to be found just near the surface, all seedlings below that have a steadily and then ultimately decreasing rate of success. Such a high rate of emergence at the top of the soil is likely why the Texasweed is such a weed, for the uppermost layers of soil are usually are the most available and ready for seeing as they are the most easily exposed by the activities of nature, animals, and people. Such an assertion is consistent with research done on the Campsis Radicans, one of the top ten most troublesome weeds in cotton in soybean, whose germination is also found to be most successful at shallow planting depths.

The Campsis Radicans, also known as the trumpet creeper, is a common perennial vine of the Mississippi Delta as well as the north central United States that can grow on most anything from field land plants, fence rows, and river banks. The plant is a deciduous, woody dicot and shrubby vine that may grow several meters in length.  Trumpetcreeper usually produces fruit capsules with numerous seeds inside that are protected by an outer thin, papering wing and flat round cotyledons. These get dispersed largely through the wind as the mature seeds break through the capsules and are carried on through later months of the year. When measured for their germination capacities in different soils, the emergence rate was found to be succinctly contained in all of just 4 cm of depth with 70% of the plants germinating at 0 cm, 10% or so at 1, and 0 by 4 %.  Again, the weed shows a strong ability to plant itself in even 0 cm depth in soils thereby showing how it is easily construed as a weed with too much and unwanted fecundity wherever it is found. This principle of topper most soil germination is consistent across numerous other species of plant than weed however.

In Bello, Hatterman-Valenti, and Owen studies for instance, favorable germinations in shallow depths were also found in Eriocloa Villosa, a variety of plant that goes by the common name Wooly Cup Grass. Wooly Cup Grass is an annual or perennial, depending on where it lives. Not usually seen as a weed, Wooly Cup Grass forms a loose tuft of leaves 2 to 3 feet tall in a green or light reddish green to make branch axils. In Bello, Hatterman-Valenti, and Owens’ study, the Wooly Cup Grass had optimal emergence from 1 to 4 cm yet the plant also was recorded to come up from as much as 15 cm deep.   Their studies also however measured seedling proclivities in the field with the finding that tillage usually affected the seed’s vertical distribution in the soil. Data in their research finds that total seedling emergence occurred from .5 to 9 cm deep however the maximum effect was found at 2 cm.

In a species of vine that is not typically considered a pest, except when found in soybean crops, Brunnichia Ovata, also called Redvine, has been indicated to be shaped by the soil depth it is found within. Redvine is common to the Southeastern United States and can typically be located across poorly drained areas where soil is under cultivation. Riverbanks, swampy forests, and general lowlands are all such categories. Although the Redvine seed is usually not a highly viable plant, with exhumed seeds having only a 3% viability rate after no more than 2.5 years, research on the Redvine seed’s depth germination ratio has been reported.  According to Shaw, Mack, and Smith, the Redvine seed has an optimal emergence rate of 74% in just .5cm of soil. The plant again shows a considerable decline at depths below 5 cm with next to none reported as germinating.  Perhaps since the Redvine does not bloom at 0 cm deep, long many other weeds have been shown to, it is not specifically classified by most as a weed as with a minimal soil depth of .5 it is still dealing with the competitions that other non-weed plants face.

In keeping with the study of vines, there is the case of the Tall Morning Glory and its germination cycle that also happens optimally in the first few cm of top soil. Found throughout the Southeastern United States as well, the Tall Morning Glory bares characteristics like that of the red vine with a preferred adaptation in wet and hot environments. The Tall Morning Glory, Ipomoea Purpurea, is also considered a weed because it is highly successful in growing in most all manner of soil including soybean, cotton, and sunflower fields. Unfortunately, it is a bother for production values in such fields can be dropped by a third with excessive Tall Morning Glories.  Research reports on the Tall Morning Glory indicate an average yield of more than 90% in 0 cm of top soil, 75% in 4 Cm of top soil, and 50% at 10 cm of top soil.  Here then is a case where a weed is both top soil adapted to germinate in just the highest layer of soil and even at the lower ranges of soil that most other plants seldom manage to germinate in. The Tall Morning Glory’s ability to bloom in such a wide range of soil depths clearly shows why it is both such a weed and possibly why the plant has had such an effect on crops for the deeper the weed the more likely it is to crowd out the growing cycle of preferred plants.

In review of the literature discussed so far, almost all the seeds have shown a strong ability to grow in the upper layers of top soil. Several dozen more articles exist to testify to this principle including by the time they go down a few extra cm in the top soil, a production rate dozens of times less effective is observed with a total stop at around 6 to 8 cm reported in several kinds. There are exceptions to these rules, namely the Tall Morning Glory, yet the exception may well prove the rule in this case since there are so many examples to the contrary. After reviewing a number of seed depth experiments on germination, it is almost universally observable that plants do best on top of the soil rather than too far under it.  Such reports are semi-consistent with our F-Data wherein seeds after 4 weeks planted at 3cm depth had an average of 7.4 germinations or 5.6 germinations at 9cm.  In almost all cases, the further down that a seed went in the soil the sharper the decline, begetting the question of why cover it at all? To look for an explanation as to what may possibly make the top of the soil so successful, let’s turn to the matter of light reception, one of the critical benefits that seeds near the surface get more than those buried a few cm below.

Light Sensitivity

Light is one of the basic ingredients for most all forms of known life. Seeds in particular have a need for this resource as the photonic discharge of life is useful in making the heat for cracking open the shell and beginning the germination process.  Light, however, is not usually one of the principles that is associated with soil, the substrate that most naturally forming seeds germinate within, especially the deeper that one goes into said soil. Some speculation exists regarding the method that these seeds take their light ranging from a contact that is once met by the sun and then subsequently buried as well as light penetrating in to the soil’s surface and filling the seed with light therefrom. For the purposes of this study, the hypothesis regarding seeds receiving light while within the soil is analyzed as this is consistent with the overall goal of knowing the optimal manner and depth by which seeds germinate while within soil, the common medium that they grow in.

Wooly and Stoller have dedicated some highly intriguing research into the nature of light and seed interactions in the soil with their spectrophotometric and biological measurements of light wavelengths on lettuce seeds. Controlling for temperature, with soil depth for variability, they used lettuce seeds in their experiments as these are known to be quite light-sensitive and therefore are optimal barometers for the effect that soil depth may have on seeds lights receptivity.  Although the lettuce seeds are known to be able to germinate in soils without light provided that they are given sufficient warmth, Wolly and Stoller maintained the soil temperatures so that such event could purely be deduced as the work of light reaching the seeds depending on the soil depth that it is placed within. Working with a Beckman DK-2A Spectro reflectometer, they worked in Drummer silty clay, and Broom-field sand to ensure that the seed depths were varied by field characteristics common and applicable to most seed kinds. The particulate sizes of the Drummer variety came in two sizes of .42 to .5 mm and .84 to 1 mm, a consistency derived through sanding.  The Broom-Field sand was a large grain size of .3 and .5 allowing for a comparison of soil effects on light transmission. Faithfully, the colors were shared for each as well with drummer soil being the far darker of the two varieties even going black when moistened whereas sand is dark yellowish brown. With such rigors in mind, the experiment began with seeds planted at 2mm or 6 mm below the surface of each soil variety inside of 12 muffin tips and then placed into sealed holding for darkness. Following the delivering of an amount of moisture, a 1,000-w metal halide lamp was given to the seeds in equal levels to each pan with the Spectro reflectometer there to determine how depth characteristics influenced the light’s receptivity. They found with their experiment that the smaller grains of both sand and silty were more amenable to light transmission yet the silt was a better conduit dry whereas the sand was better wet.  In both Drummer’s silt clay and the Broom-field sand, the effect of both soil particulate size and planting depth were further measured to find the germination rate of the lettuce seeds. When planted at depth of two mm, 63% of lettuce seeds germinated in the Brooms-field mixture whereas just 2% of the lettuce germinated in the same soil at 6 mm after six minutes exposure to the light.  Even though that is just a 4 mm difference, the top seeds are more successful by a factor rate of more than 30x.  Drummer’s silt of .42 to .5 mm particulate sizes had some of the most successful rates of germination at the greater depths with 17% becoming germinated at 6 minutes.  Such a degree however pales in comparison to the 2 mm depth in the same medium which reached 83%, a high germination ratio by most anyone’s standard.

In perspective of the Wooly and Stoller’s results on seed depth, soil type, and light effect, some very important and applicable conclusions can be drawn. First among these is that light has a varying effect inside of soil but that the intensity of the light in the soil is a direct result of how deep it is and what kind of soil that it is. In this experiment, the better substrate of the two soils was the Drummer’s soil, a variety of soil that was both characteristically darker as well as made of particles about a fourth the size of Broom-field sand. Most relevant however to their study was the effect that soil depth had on the germination rates with the 2mm distance being reported as substantially more effective at inducing such a phenomenon than at the 6mm depth. Although the 6mm depth was 3x deeper than the 2mm, its success rate was usually dozens of times lower than in the than the other. The only time when the germination ratio was reasonably close to the depth ratio was in the Drummer’s clay but only at the smallest variety.  This means that when planting seeds, light does have a considerable influence in the germination rate that the seeds will produce with lower seed depths gaining substantially less success. There is however the possibility of increasing this germination rate if the soil substrate is of a smaller size, possibly because this allows for the easier passage of light into deeper passage ways of the soil. That there is a way to improve seed germination rates even while at deeper levels of soil is critical finding for though many write off lower planting depths as a waste of seed, in some instance, it is critically necessary to plant at such levels to avoid problems associated with seeds planted to near to the surface such as consumption by birds or being washed away in the rain. Therefore, the experiment Wooly and Stoller conduct has several influential findings on soil depth germination with a leading kind being the soil variety.

Soil Texture

Clearly, soil texture is a key factor in the seeds germination and that its affect is varied by the depth of the seed’s planting. Benvenuti focuses in on these qualities in his soil experiments wherein 10 soil varieties were measured with seed depth characteristics of 0, 1, 2, 4, 8, and 12 cm as an independent factor. The germination and inhibition rate of these seeds were the study’s dependent variables. With quartz, sandy, and clay as the three main soil types, and variabilities broken down therein to distinguish 10 total kinds, Benevenuti reported that the clay soils were the most likely to give a decrease in seedling emergence for by just 4 cm deep the germination ratio was cut in half for his seedlings, jimsonweed (Datura Stramonium L.).  Such effects were less strong in the sandy soil and quartz sand with an emergence rate of 20% occurring even at 8 cm deep. Nevertheless, no seeds germinated at 12 cm deep regardless of the soil type with failed germination as the primary ascribed cause of loss.   Strangely, these findings are unlike the Wooley and Stoller who had much better results in clay than in the sandy variety. From this counter intuitive finding, one may perhaps be able to deduce that either the type of soils used were different and that soil variety is even more effective and subtle in affecting germination or the reverse, that the situation is so mixed as to be unreadable.

Intriguingly, the Benvenuti study also looked at the seed weight to calculate how this factor endured at different weights and soil types. Usually, when the jimsonweed seeds were very small, they almost had a complete inhibition of germinating even while at extremely shallow burial depths.  To this finding, the author offers the explanation that the seed’s inhibition is a species wide survival strategy that is practiced so that they do not unnecessarily germinate in soils they otherwise would not have as safe a fair to take their emergence within.  This conclusion, though unproven, is consistent with natural logic of self-preservation and is a reasonable guess as any for why smaller seeds do not fare as well in lower depths. Soil varieties had an especially strong effect on limiting the smaller seed within even the shallowest depths of clay soils therefore once again confirming the Benvenuti trend in clay’s inhibition of seed germination. The possible explanation for sandy soil success over clay is suggested in his research as due to the higher air permeability of sand.  This air permeability then permits the greater exchange of oxygen between the seed and the soil thereby giving it the necessary element for beginning germination.  The relationship between aeration and soil depth is not discussed in Benvenuti’s study however the earlier results suggests that there likely is some correlation between seed depth, seed aeration, and plant success.

Conclusion

Seeds show themselves to be best suited to the top most layer of soil in germination in a wide variety of studies that account for seed and soil varieties as well as environmental conditions. To take their emergence, seeds work in the top most layer of the soil where the best conditions are found including full aeration, plenty of sunlight, and possibly better access to water. Generally, plant weeds are those that show some of the best adaption across the stratification of soil layers with the top layer being the most favored. Their range and prolific abilities are certainly what make them weeds to many farmers and gardeners who see them as a sore to their usual seed cultivation methods. Most seeds it seems, have a very specific range of depth, usually within the first 3 to 4 cm of soil, to take their germination. Those that do no germinate likely go into dormancy for there they may wait until they are brought to the surface, typically through some process of nature or man like tilling. This is a survival trait of seeds yet since weeds reproduce so copiously, they may do even more germinating than regular seeds. Such a principle is just a speculation however yet perhaps further reading and research could find a definitive answer to such a question. Certainly, since there is such a fine line between weeds and not weeds, having a new way to further distinguish them may prove advantageous to all.

Bibliography

Benvenuti, Stefano. "Soil texture involvement in germination and emergence of buried weed seeds." Agronomy Journal 95, no. 1 (2003): 191-198.

Benvenuti, S., Macchia, M., & Miele, S. (2001). Quantitative analysis of emergence of seedlings from buried weed seeds with increasing soil depth. Weed Science, 49(4), 528-535.

Bello, Iliya A., Harlene Hatterman-Valenti, and Micheal DK Owen. "Factors affecting germination and seed production of Eriochloa villosa." Weed Science 48, no. 6 (2000): 749-754.

Chauhan, Bhagirath, Gill, Gurjeet, Preston, Christopher. Factors Affecting Seed Germination of Annual Sowthistle (Sonus Oleraceus) in Southern Australia. (Weed Science 54, 2006): 854-59

Chachalis, Demosthenis, and Krishna N. Reddy. "Factors affecting Campsis radicans seed germination and seedling emergence." Weed Science 48, no. 2 (2000): 212-216.Woolley, Joseph T., and Edward W. Stoller. "Light penetration and light-induced seed germination in soil." Plant Physiology 61, no. 4 (1978): 597-600.

Faked Data. Total Number of Seeds Germinated. Professor 2017.

Shaw, David R., Robert E. Mack, and Clyde A. Smith. "Redvine (Brunnichia ovata) germination and emergence." Weed Science (1991): 33-36.

Sing, Megh, et al., “Factors Affecting the Germination of Tall Morningglory (Ipomoea Purpurea)”. Weed Science 60, 2012: 64.

Koger, Clifford H., Krishna N. Reddy, and Daniel H. Poston. "Factors affecting seed germination, seedling emergence, and survival of texasweed (Caperonia palustris)." Weed Science 52, no. 6 (2004): 989-995.Benvenuti, S. (2003). Soil texture involvement in germination and emergence of buried weed seeds. Agronomy Journal, 95(1), 191-198.