Adjusting planting soil depth for better germination

Many producers wonder how this spring's lack of rainfall will affect planting and seed germination. Before performing any tillage operations, producers should inspect the top 6 inches for soil moisture status. If soil moisture is low, producers should minimize tillage unless it is absolutely necessary because each tillage pass can cause soil moisture loss of up to a 0.25 inch.

Although subsoil and topsoil moisture status in crop ground is normal in many areas, the National Weather Service has forecast a 60 percent chance of drought in the Corn Belt. Subsoil moisture deficits have been measured, particularly in northwestern and southwestern Iowa, at 5 to 7 inches below optimal levels. Optimal subsoil moisture is 10 inches of plant-available water in the top 5 feet of soil. When topsoil moisture is limited, producers need to use strategies that to help them compensate for this lack of moisture.

Monitor soil moisture by inspecting the top 6 inches for soil moisture status, with either the hand feel method or with gypsum blocks (see the April 24 ICM newsletter article How to evaluate soil moisture in the field). As mentioned, producers also can save soil moisture by minimizing tillage because each tillage pass releases as much as 0.25 inch of water.

Another option is to use soil conservation practices. No-till, mulch, ridge-till, and strip-till are all effective soil-moisture conservation practices. Avoid cultivation during persisting dry weather conditions and use alternatives such as herbicides for weed control.

Producers who use liquid starter fertilizer should reevaluate this strategy for this season. Application of liquid fertilizer in limited soil-moisture conditions may decrease seed germination by increasing salt concentration around the seeds, thereby minimizing moisture availability to the seeds.

Risk can be reduced with realistic yield goals that reflect production conditions because reasonable yield goals can minimize production inputs. Reduced planting population in seasons when limited soil moisture is anticipated can improve production by reducing competition for limited moisture.

This season, it is very important to control planting soil depth and seedbed. Good seed depth--1.5 to 2.0 inches or even deeper--is recommended in dry conditions to ensure good moisture availability for successful seed germination. Soil texture plays a role in soil moisture availability, especially in the top 6 inches where coarse-textured soils (e.g., sandy loam and loamy fine sand) tend to dry faster than medium-textured soils (e.g., loam, clay loam, silt loam, and silty clay loam).

Producers should realize that although planters have seed population monitors (so seed drop can be checked from the tractor cab), there is currently no monitor for checking seed depth and seed-to-soil contact. As soil conditions change in different locations or with the weather, it is important that operators check seed placement behind the planter for depth and seed-to-soil contact.

Producers also should calibrate their planters for penetration. Where surface soil is dry and hard, it may be difficult for seed openers to penetrate the surface to an adequate seed depth. Although the operator may have correctly set the depth adjustment, depth wheels may not be firmly in contact with the soil and the planter unit may be riding up on the seed opener. Additional down-pressure or weight is necessary for the seed opener to penetrate to planting depth.

Seed-to-soil contact is usually controlled by coverage and compaction of press wheels and covering discs. Many planters have an adjustable down-pressure spring to vary the amount of surface pressure and coverage for supplying adequate soil contact. Spring pressure may need to be increased in drier surface soil for adequate soil contact and to help bring moisture to the seed. Pressure may be decreased after surface soil moisture has been recharged by rainfall to avoid overcompacting soil around the seed.

Planning for dry weather may be one of the key management challenges producers face this season. Adequate water is especially important at planting time. Without sufficient topsoil moisture, seed germination could be impaired, resulting in low plant populations and low yield.

This article originally appeared on page 67 of the IC-484 (8) -- May 8, 2000 issue.

Updated 05/07/2000 - 1:00pm