Spring rains can come hard and fast, as Iowa farmers have experienced the last few weeks, causing substantial soil erosion at a time when soils are most vulnerable. Extensive tillage in a number of fields meant there was less crop residue cover to protect the soil and, of course, there's a lack of crop canopy this early in the season.
The soil profiles in most of Iowa are now filled to capacity with water. Therefore, the intensity and amount of rain received has exceeded the soil's capacity to filter water and minimize surface runoff even in fields with the most adequate conservation practices.
"The lack of residue on fields with conventional tillage can cause significant soil crusting problems and eventually will create corn and soybean germination problems and other issues," says Mahdi Al-Kaisi, an Iowa State University Extension soil management specialist.
Crop residue helps control soil crusting
Crusting is usually most noticeable in fields with high silt content, low organic matter and little surface residue, especially where excessive tillage occurred. Soil crusting can prevent emergence, reduce oxygen flow to the roots and set up a barrier to efficient water infiltration, he explains.
In addition, the sealing effects created by surface crusting can enhance soil erosion significantly due to the destruction of soil surface structure. "The role of surface crop residue is to minimize the intensity of the rain drops in destroying soil structure and reducing surface runoff. Also, crop residue will reduce water evaporation and dry the soil surface much slower than a bare soil surface where high air temperatures and wind can speed up soil evaporation causing crusting.
Soil crusting in general is a very thin layer, about one inch thick that acts as a barrier to plant germination and water movement. Emergence can be problematic with a dense surface crust because the seedling may not only be completely depleted of carbohydrate reserves before emergence, but also the hypocotyl is easily is easily broken when pushing against a solid crust. It is therefore recommended to monitor high-risk fields (for soil crusting) where corn or soybean emergence hasn't yet occurred.
When to bust the crust with rotary hoe
Combating soil crusting quickly is a real challenge for farmers but you need to do it to prevent a poor stand, says Al-Kaisi. The cooler the weather, the longer the seedling can survive, unless a seedling disease infects it. The warmer the weather, the faster the seedling grows and the sooner it runs out of energy. It is therefore important to deal with the crust soon after it forms, he adds.
Using a rotary hoe is one of the best ways to break the soil crust and enhance seed emergence, says Palle Pedersen, ISU Extension soybean agronomist. If done properly, rotary hoeing causes very little damage to the young plant and little disturbance of crop residue, thereby enhancing infiltration of water into the soil and preventing erosion. If cotyledons are damaged or ripped off, the plant will die. That's because during the cotyledon stage it can't use photosynthesis and it gets all its energy from the cotyledon.
Pedersen and Al-Kaisi emphasizes that there are lessons to learn from the erosion events caused by the rains this spring. You need to look at the degree of damage caused to fields that have better residue cover compared to the fields that had more intensive tillage and less crop residue cover to protect the soil.
How does erosion damage happen?
In a normal rainfall, raindrops that are 6 millimeters in diameter hit the ground at 20 miles per hour. The cumulative impact of millions of raindrops hitting the ground in a hard-hitting spring storm can be incredible, dislodging soil particles and "splashing" them up to 3 to 5 feet away.
The splashed particles clog soil pores, effectively sealing off the soil surface and leading to poor water infiltration. Instead of soaking into the soil, rain water collects and moves down slope in sheet or rill erosion, forming gullies and carrying soil particles. "An effective soil conservation plan that limits exposed soil and rain splash erosion also depends on observation and maintenance," says Al-Kaisi. "Spring is as good a time as any to develop a new and different strategy for addressing conservation planning."
Now is the time to evaluate your fields
What are the lessons of the rainfall events over the last several weeks? Heavy rain in such intensity that causes significant property and soil damage provides an opportunity to examine what can be done differently in the field to minimize, if not control, soil erosion. Some recommendations are:
* Look at the pattern of surface runoff and the placement of buffer strips on the field when directing surface runoff and minimizing sediment transport.
* Examine your choice of tillage and compare it to other fields in the area to evaluate the degree of damage caused by soil erosion in each conservation system.
* Evaluate the residue cover, the uniformity of residue distribution, and residue effectiveness in minimizing soil erosion.
* Document your field conditions with photos, if possible, and assess the water ponding on the surface for each tillage system.
* Evaluate your field fertility conditions, especially if nitrogen was applied in the fall. There can be substantial nitrogen, phosphorus and potassium loss due to leaching and surface water runoff.
* Evaluate your plant populations, the damage your field experienced and the alternatives for replanting.