Posted: May 6, 2019
As planting season is approaching it is time to start thinking about planting depth. There are two reasons that planting depth is important for production crops. The first is to achieve good seed-to-soil contact, the second reason is to establish a strong nodal root system. Seed-to-soil contact is critical for the imbibition step in germination. Seeds absorb water from the soil to activate enzymes that will start the process of germination. Nodal roots are important because they are responsible for the uptake of most of the water and nutrients that a plant will need to take up in its lifetime. Good germination is the first step in plant growth, understanding how germination happens will explain why planting depth is important.
Seeds need three major components in order to germinate: oxygen for respiration, water for imbibition, and the right temperature. Oxygen is rarely a concern for seed germination unless there is flooded soil conditions that persist for a long enough period of time. Water is an important factor because of seed imbibition, which is a seed's or a plant's absorption of water; a process that causes swelling in some plant cells and organs and makes the seed grow. Imbibition is the first step of germination; it encourages seedlings to emerge out of the soil and establish themselves by activating enzymes and breaking the seed coat once the plant emerges from the soil surface. Having good seed-to-soil contact is important for imbibition because the seed needs to be pressed down into the soil where soil moisture conditions are consistent. In order for plants to begin the process of germination they need the proper temperature. Corn seed needs the soil temperature to be between 46°F and 86°F to germinate, it is recommended to plant corn when the soil temperature is at or above 50°F. Soybean seed needs the soil temperature to be between 50° F and 77° F in order to begin germination.
Corn seed should be planted at 1.5 to 2 inches. Planting shallower than 1.5 inches can lead to early-season root lodging due to shallow nodal root development or overall poor root development. Planting at 2 inches ensures that the seed will get adequate seed-to-soil contact and to allow proper moisture uptake through imbibition, it also provides enough time for the roots to develop before emergence happens. Corn seeds need to imbibe or absorb about 30% of its weight to germinate. Soil moisture should be taken into consideration at the time of planting because it is important for imbibition to take place. Soils deeper than 2 inches may take a longer time to warm up, which could lead to uneven emergence if corn seed is planted too deep.
It is recommended that soybean seeds be planted between 1 and 1.5 inches deep. When planting into fine textured soils or planting into a high residue field you should plant around 1 inch. When planting into course textured soils or very dry soils it is recommended to plant around 1.5 inches. Planting too deep can delay emergence and lead to uneven emergence. During the imbibition stage in soybeans, the seed will absorb 50% of its weight in moisture, which makes adequate seed-to-soil contact the most important part of soybean germination. This also means that soil moisture is an important factor to consider when planting soybeans. Plant deep enough for the seed to be placed in moist soil. Planting too shallow could reduce the amount of seed-to-soil contact, which would reduce the germination rate because of poor imbibition.
The proper planting depth for corn and soybeans has been determined through many years of farming and experiments. Seed-to-soil contact is important to think about when considering planting depth because it gets the seed in contact with moisture so it can start the first step of germination which is imbibition. When the shoots start growing towards the soil surface nodal root development becomes important for the overall structure of the plant as well as nutrient absorption. Planting too shallow could lead to root lodging or reduced germination. Planting too deep often causes uneven germination. Checking that you are planting at the proper depth could help increase yields through even emergence and a high germination rate.