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What are some strategies for fertility and weed management?

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An interesting part of my position with Marbleseed as a crop specialist is the variety of questions that come my way. The questions range widely from specific crop rotations to determining what to plant where, given the wide variations in climate and soil across the Midwest. It is these two facts, climate and soil types that are the first pieces of information I usually seek out when answering the inquiries that come in.

However, a majority of questions most often deal with fertility and weed management in organic field crops. So, let me try to give some general suggested management practices that can go a long way in dealing with these two concerns.

In organic field crop management, soil types are an important consideration. However, ambient soil temperatures and soil moisture are the two factors that dictate when to do seedbed preparation and when to actually begin planting or seeding. They are the two telltale signs of when to be in the field. Soil temperatures early in the growing season directly impact soil fertility availability because of the relationship of soil microbial activity to temperatures. And most of us understand that it is microbial activity that provides the nutrients for plants.

What does this have to do with weed management? Weather factors including air temperatures, wind conditions, and precipitation events impact weed management very directly, as moisture and soil and air temperatures determine the time and amount of weed seed germination.

From mid-April on, a good indicator of soil temperatures in the seed zone is to take note of the air temperatures. Soil temperatures will lag behind air temperatures by several hours with 8 a.m. usually being the coolest part of the day for soils. 5 p.m. is when soil temps will be at their maximum. You can use an ordinary meat thermometer as a reliable tool as well.

These soil temperatures directly impact seed germination because they determine growing degree units and days which, in turn, determine the rate of maturity for the growing plants. This applies to the planted crop as well as the weed seed banks in the soil.

For small grains like wheat, oats, or barley, seed these crops as soon as the fields are dry enough. This means soil temperatures are in the 38- to 45-degree range. Most small grains will germinate at colder soil temperatures than foxtail grasses and most other broadleaf weeds, except for lambsquarters.

Consider small grains very much like you would consider planting lawn seed. They love cool, wet conditions and a much firmer seed bed than row crops like corn or soybeans. And, like lawn seed, they can germinate at cooler temperatures. By planting these crops early there is a better chance to get the grain mature and harvested before the grasses and broadleaf weeds that do grow set seed.

One additional reason for seeding these small grains earlier is the desire to have them in the pollinating and seed-setting stages of growth ahead of the hotter midsummer temperatures that can significantly negatively impact this process and cut yield potential. Dried field peas are especially sensitive to temperatures at pollination. So, get them planted as early as possible, even above frost if field conditions allow.

Best weed management in row crops like corn and soybeans is nearly opposite to that of small grain. Plant later to allow the seed bed to warm so, when the crop is planted, it will germinate and emerge quickly to compete with the existing weed seed bank, which also likes warmer soils. Here again, pay close attention to soil temperatures. These temperatures should stay above 50 degrees 24-7 for at least three to four days; something that rarely occurs in western Minnesota before May 15. A great indicator that this is happening is that you will begin to see foxtail grasses and some broadleaf weeds emerging when it is time to prepare the seedbed.

This also means being able to tine weed, rotary hoe, and then cultivate after planting as quickly as possible. It is important to remember that any weed that is green and above the ground can only really be eliminated with a shovel on a shank. Rotary hoeing and tine weeding decrease in effectiveness with each passing day post planting.

Several other things about row crop weed management: An initial pass through the intended corn and soybean fields on a hot breezy afternoon in late April to create a stale seedbed is an excellent way to manage weeds. This creates ideal conditions for weeds to germinate that can then be tilled out just ahead of planting the corn and soybeans later in May. And, finally, higher seeding rates for row crops can help suppress weeds as well as lessen the impact of plant stand loss from any of the mechanical weeding equipment.

Feel free to call me with questions, especially now that field work is starting up again. If I do not have the answers, I know we can find someone to help.

Posted: May 2020
Answer By: Carmen Fernholz