By Mahdi Al-Kaisi, Iowa State University Department of Agronomy
Dry conditions and lack of soil moisture availability, especially during planting time this spring, can create water stress resulting in delayed germination, a reduction in plant stands or may prevent seed germination.
When a corn seed absorbs 30% of its weight in water the germination process commences. For comparison, soybeans absorb half of their weight in water before they germinate. Therefore, the level of soil moisture in the soil seedbed at planting dictates this critical process.
For successful seed germination, ideally soil moisture should be at or close to field capacity. At field capacity the soil retains the maximum amount of moisture. Field capacity is influenced by soil texture; for example, fine-textured soils, such as clay or loam soils, have larger moisture holding field capacity than coarse-textured soils such as sandy-textured soils.
Soil texture and tillage influence available soil moisture
Dry conditions influence soil moisture availability differently depending on soil texture. Fine-textured soils have less available water than medium or coarse-textured soils. For example, loam soils that contain 20 to 37 percent clay have greater water available to the plant than clay soils that contain greater than 40 percent clay.
The other factor that affects water availability in dry conditions is the tillage intensity, especially at seeding depth. To understand how moisture moves within the seedbed under different moisture conditions, we need to understand the process by which water moves in the soil profile and the factors affecting this process.
Under dry conditions, water moves upward in the soil profile toward the soil surface where soil water evaporation takes place. Suction or tension is the force that moves water upward in the soil profile. This suction or tension is highly influenced by soil texture and moisture condition, where greater tension is associated with fine soil texture and dry soil.
Water moves from wet areas (areas of low tension) to drier areas (areas of high tension). The drier the soil surface, the greater the soil suction that moves water from the subsoil to the soil surface.
Soil texture and tillage affect seeding depth
Many factors affect the water movement process and dictate how deep seeds must be placed in the soil.
First, we need to consider soil texture. As I indicated above, the finer the soil texture, the greater the soil suction is to move water toward the soil surface than in coarse-textured soils. Therefore, seeding depth can be shallower in fine-textured soils than in sandy soils depending on how dry the soil actually is.
Generally, when the soil moisture condition at the seeding depth is much below field capacity, planting deeper than usual is advisable. Seed should be placed in soil that is at field capacity for optimum germination.
To determine if soil moisture is at field capacity, take a handful of soil from the proposed seeding depth. If the soil is at field capacity, it will leave a trace of moisture on the palm of your hand when you squeeze it. Or you should be able to form the soil into a ball, which, when thrown in the air, will not disintegrate.
The second factor that dictates seeding depth is the type of tillage system. Generally, conventional tillage alters the soil surface condition, resulting in faster soil evaporation throughout the tillage zone. This leads to significant soil moisture losses. In a dry year, these conditions are detrimental to moisture availability in many ways.
First, tillage increases water evaporation from the tillage zone. Second, tillage destroys soil structure and reduces water movement through capillary action. It does this by destroying the continuity of the capillary system responsible for moisture supply to the seedbed, and reduces water recharge to where the seeds are placed.
These conditions are completely opposite from what is found in a no-till system; in this system, the soil structure remains intact and moisture moves evenly to the soil surface. One reason for this is that in no-till the soil structure and the capillary system is intact and continuously supplies moisture to the seedbed.
The other reason is that the residue on no-till soil surfaces insulates the soil surface and reduces soil evaporation and also reduces or moderates soil temperature.
In dry conditions, seeding depth can and should differ depending on the soil texture, tillage system and residue cover. Knowing the texture of the soil in your field and its management requirements especially in dry conditions will dictate how deep seeds should be placed to have adequate available moisture for successful germination.