Fourteen days

Describe the influences of planting depth on seedling growth of plants with differing growth patterns.

Two important factors that must be considered when growing crop plants are having a stand of uniform plants that develop at a similar rate and having the appropriate plant population. Uniform stands are important for management of the crop because growth requirements of the crop vary at different stages in the crops life cycle. Crop stands that are uneven in their development become very difficult to manage. Plant population is the number of plants in a given area (usually measured in acres, but also in square feet for smaller areas). Each crop species has a plant population at which it produces the highest yield of wanted quality.  With many agronomic crops, such as corn, soybean, wheat, and sunflower, the goal is to have a plant population that produces the highest yield.  For many horticultural crops, such as carrots, tomatoes, peppers, and watermelons, the size and quality of the vegetable or fruits are important considerations when determining optimum plant populations.

Crop plants must be planted at the proper depth to achieve a uniform stand of the intended population. Optimum planting depth is generally related to the seed size. Large seeds will emerge from greater depths because they have larger stored food reserves that can be utilized for germination. For example, many flower seeds are quite small and cannot survive plantings much greater than 1/4- to 1/2-inch. In contrast, hybrid corn seedlings can emerge from depths greater than 4 inches. Although large seeded crops can emerge from relatively deep plantings, it is better to plant them shallower. The recommendation for corn is 1 1/2 to 2 inches. Seed size, type of emergence, and the type of soil in which the seeds are planted determine the optimum depth of planting.

The seed must be able to imbibe enough water to germinate before the soils surrounding it dries out. Seeds require as much as 50% of their weight in water for germination. Thus, large seeds require a considerable amount of water to germinate. Planting deeper than 1 inch insures that large seeds are completly covered with soil and take in enough water for germination to proceed. Firming of the soil after seed placement provides good seed to soil contact, which results in water movement from the soil into the seed.

Plant species exhibit different types of emergence. In some crops, the cotyledons remain in the soil as either the mesocotyl or epicotyl elongates and pushes to the surface. In others, the hypocotyl elongates pulling the cotyledons to the soil surface. When comparing crops with similar seed size, those that must pull their cotyledons through the soil for emergence should be planted shallower than those in which the cotyledons remain in the soil.

Depth of planting is also influenced by soil type. Deeper plantings are possible in lighter soils than in heavy soils. The surface dries out quickly in sandy soils making deeper plantings necessary for placing the seed in moist soil. In cold, wet soils, shallower plantings are required because the oxygen concentration and temperature at greater depths may be too low for germination, especially in early spring.

In the following demonstration you will be planting seeds of three different crop species - corn, bean, and pea - at three different depths.  These three crops were chosen because of the different types of emergence they exhibit. Corn is a monocotyledon in which the mesocotyl elongates during germination and the cotyledon, which serves as the food reserve for the germinating seed, remains below the soil surface during germination.  Beans are dicotyledons that pull their cotyledons above the soil surface due to elongation of the hypocotyl.  Many other broadleaf crops like soybeans, sunflowers, and pumpkins also exhibit this type of emergence.  The cotyledons of pea remain below the soil surface as the epicotyl elongates during germination and emergence. Before you perform this demonstartion you should study the seedling anatomy of corn, bean, and pea.
 

• Sweet corn seeds
• Bean seeds
• Pea seeds
• Nine (9) six-inch diameter (1 gallon) pots
• Nine (9) gallons (1.2 cu. ft., 34 L.) sand
• Ruler
• Pot stakes and a permanent marker

1. Fill the pots with sand, water throughly and let drain (you may need to put paper towels in the bottom of the pot to keep the sand from flowing out).  Plant four seeds of each of the three species (sweet corn, bean, and pea) in pots at 1, 2, and 4-inch depths.  Cover the seeds with additional sand so they are planted at the correct depth. You will end up with three pots for each species, one for each planting depth.  Label each pot accordingly using the pot stakes and marker.  Water the plants as needed for the fourteen-day period.
2. Record the time required for plants in each of the pots to emerge.
3. At the end of the fourteen-day period, measure the average height of the plants in each pot.
4. Remove the sand and seedlings from the pot without disrupting the roots. Carefully wash the sand away from the roots.
5. Note the differences in anatomy between the three species and the three planting depths.

The deeper the seeds are placed the more time it will take for them to emerge from the soil.  At the end of the fourteen days, plants seeded at 1 inch should be taller than the plants seeded at 3 inches.  Carefully view the anatomy of the three different species.  The sweet corn seed will remain below ground and you should be able to see differences in the length of the mesocotyl, the area of elongation between the root and growing point.  The cotyledons of bean will emerge above the soil surface and you will find differences in the length of the hypocotyl for the three different planting depths.  This area between the root and the cotyledons will be of greatest length in the deeper plantings.  The pea cotyledons will remain below ground and their will be differences in the length of the epicotyl area between the seed and the soil surface.

Planting various other crop or garden seeds and testing the student's ability to identify the different types of emergence could be used to expand this demonstration.