Planting in cold soils: how early is too early?

When it comes to 'in-field' cropping activities, nothing rivals the challenge of planting a crop that will have the foundation for top-end yield potential. To put Spring planting into perspective, it really is a complex exercise. There are so many factors – most revolving around field conditions and planter set-up – all of which dovetail into one another to either make or break the ultimate goal of achieving a plant stand that germinates and emerges quickly and uniformly. If all factors are optimized, the resulting effort in the field is reflected with a well-established crop. But if one small element is sacrificed, the negative impact on stand establishment can be very costly to both yield and profitability.

Much is made of the planting process as it relates to field readiness and equipment set-up. In recent years, we’ve witnessed a rapid uptake in planter upgrade technologies, whether manufacturer-developed, or available as after-market upgrades. As a result, the vast majority of Ontario corn and soybean fields are planted properly, with all elements in place and under control, and the desired full stand is achieved. Growers are very good at making the best of the conditions and variables that lie within their control.

When the planting is done and the planter leaves the field, that particular element of control leaves with him. It’s now up to Mother Nature to establish the crop. Most of the time she co-operates, however it’s nature’s way to throw a curve ball at our crops every once in a while. Proper stand establishment is about more than how nicely a field will dress with tillage or how well the planter performs.

It’s vitally important to understand what happens to a seed after planting and the management decisions growers can make to give their new plantings the best opportunity not just to grow, but to thrive and prosper.

The Early Planting Trend

There is no denying the trend towards earlier planting of both corn and soybeans in Ontario. A higher percentage of both crops are planted earlier every year. There are several factors driving this trend:

• The need to get started early and spread workload is undeniable as many farm operations have expanded rapidly in recent years.
• Improved genetics offer growers better cold tolerance and spring vigour characteristics and increased yield potential to reward early planting.
• Advancements in seed treatments have also enabled growers to plant earlier into higher residue situations which tend to have higher insect pest pressure.
• Better farming practices & equipment advancements have also facilitated earlier planting. With our improved equipment and expanding knowledge, we do a better job planting into seedbeds which are better prepared year after year, which allows for quicker germination and stand establishment.

Planting Date: how early is too early? 

Many industry sources cite April 20 – May 5 as the 'sweet spot' for planting dates to optimize yield (this may be a few days earlier for longer-season areas of Ontario, and vice versa for short-season regions). Planting earlier than this window can be successful if conditions are good, and most importantly if trend-line temperatures are above normal. If this is the case, the opportunity for increased yields can be taken advantage of with early planting.

As for soybeans, recent research from both private and public sources suggests that top yields are achieved when planting as soon as soils are fit, in combination with full-season variety selection. Basically, once the calendar reaches May 1, it’s time to plant if soil conditions are suitable. Many industry sources suggest a loss in yield potential of 1% (or 0.5 bu/ac) per day beyond May 20th , with the losses becoming greater as mid-June approaches.

Soil Conditions and Germination Rates

There are two key critical factors that influence germination rate of the seed:

• Seed-to-Soil Contact
  Regardless of soil moisture levels, seed-to-soil contact rules the day when it comes to achieving quick, even germination of a crop stand. Seeds with good soil contact can and will find soil water, even in a dry seedbed. Seeds with minimal soil contact generally have to wait for moisture to come to them, particularly in dry conditions.
• Soil Temperature
  Perhaps best considered as soil water temperature, the temperature of the seed’s first drink of water is absolutely critical to the next two to four weeks of plant development. Generally, soybeans are more sensitive than corn when it comes to this factor.

Corn Seedling Development begins with germination of the seed at a minimum soil temperature of 46º F, therefore most Certified Crop Advisors will recommend planting corn at minimum soil temperatures of 50º F. It’s important to consider when planting early, as it doesn’t take long to lose a few degrees from the required soil temperatures. Once the kernel has absorbed 30% of its weight in water, germination and emergence of the radicle root from the bottom (tip) of the seed corn kernel occurs.

While germination and radicle root (downward growth) begin at 46º F, development of the sprout components, the mesocoytl and coleoptile (upward growth) do not begin until soil temperatures reach 60º F. In past seasons, we’ve observed corn fields that experienced 10 to 14 days of cool temperatures after planting, and found seeds germinated with plenty of radicle root development, but no upward sprouting activity. Many producers will assume a problem with seed quality in this case.

However, the reason this is occurring is because soil temperatures hovered above 46º F for germination, but never reached 60º F to achieve sprouting and emergence.

What happens if soil water temperature changes dramatically and rapidly? First and foremost, a soil water temperature reduction of as much as 27º F does not affect germination rate. This confirms the importance of temperature in the first water available to the seed. Such temperature swings will not distort the rate or visual appearance of the germination process.

However, this same temperature swing dramatically impacts the development of the mesocotyl and coleoptile. A rapid drop in soil temperature, such as those induced by near-frost conditions within two weeks of planting, can result in distorted development of the seedling. This condition is referred to as Cold Water Imbibational Injury or Chilling Injury depicted in the photo below.

Symptoms of this injury are corkscrewing of the mesocotyl and coleoptile, leafing out underground, and generally a mottled or twisting of the sprouted seedling as it approaches the soil surface.

Any delays in mesocotyl and coleoptile development result in delayed emergence. This leaves the corn seed and seedling prone to attack from pests such as millipedes, fungal seedling disease infections, and can also lead to a loss of uniformity in stand establishment.

Soybeans

Soybean Seedling Development by its very nature is totally different from corn. A soybean seed requires a soil water temperature of 55º F to 60º F and must uptake 50% of its weight in water to begin germination. This highlights the critical nature of the temperature of the first water imbibed by the seed. Ideally, the soybean seed will absorb this water within six hours of planting, at which point the seed germinates and the radicle root emerges within two days.

We stated earlier that dramatic swings in soil temperature do not affect the germination rate of seed corn. However, the same temperature swing wreaks havoc on soybean seed germination rates. A rapid drop in soil temperature stalls or ceases germination, and the process will not resume until soil temperatures sustain the critical 55º F to 60º F range or better.

Delayed germination can substantially reduce stand development depending on field conditions. These delays provide insect pests such as seed corn maggot and wireworm the opportunity to consume and destroy germinating seeds. Likewise, fungal pests such as Pythium, Phytopthora, and Rhizoctonia take advantage of this opportunity to infect seedlings as well, resulting in plant mortality and reduced stand populations and plant vigour.