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3/29/2010 - 4/4/2010

Monitor Soil Moisture This Spring

Mahdi Al-Kaisi, Department of Agronomy

Current wet soil conditions present challenges to early season fieldwork. Whether it is planting beans or corn, cultivating or rotary hoeing – there is high potential to cause significant soil structure damage. Make a risk assessment before rushing out to the field on the first few days of dry weather to work the ground. When soil moisture is at or exceeds field capacity, there is an increased potential for soil compaction, particularly at topsoil depths.

The general rule is to minimize traffic on the field that would increase soil compaction.

Field operations will not achieve their objectives if the soil moisture conditions are not suitable. The decision to perform any field operation this season should be justified and weighed against disadvantages or damage to the soil structure. Soil compaction is a particular concern because the impact is significant when the soil's physical and chemical properties, such as infiltration rate, bulk density and nutrient availability, are altered.

The impact of wet conditions on soil physical and chemical properties is also a function of soil texture. Well-drained, medium-textured soils (loams, clay loams, silt loams, and silty clay loams) are much less affected than fine-textured soils (silty clay and clay) where saturated conditions are likely to exist due to poor drainage. The wet soil conditions of poorly drained soils may persist longer, depending on drainage availability, causing significantly increasing potential soil compaction and sidewall compaction with tillage operations, which may lead to poor seed germination.  If planting and other field operations are rushed, poor seed germination and root development may result.

Check soil moisture
It is important to check for proper soil moisture conditions prior to implementing any field operation. Most of Iowa's soils have medium textures. For these soils, a simple method of checking soil moisture is the "feel" method. Probing the top 3-4 feet with a hand soil probe to assess the field's soil moisture conditions is time well spent.

Check the soil moisture status by pushing a ribbon of soil from between the thumb and index finger. If it breaks off within one or two inches, the potential for creating compaction is less. However, if the ribbon stretches out to four or five inches, it is still too wet and plastic. The chances are good that being in the field under these conditions may cause more problems than it will solve.

Another method is to make a ball of soil two inches in diameter and toss it through the air. If it hangs together until impact, it has a lot of cohesiveness, is still fairly plastic and probably is too wet to work the ground.

Right now is also a good time to evaluate fields to see whether drainage systems are working properly, especially for poorly drained and fine-textured soils. Keeping records of field moisture conditions can help in planning drainage system improvements, if needed. Keeping field records on soil moisture conditions is as important as recording information about soil fertility, insects and management practices. These records help explain yield variability across the field.

However, under the current prevailing wet conditions, the best choice producers can make is to stay away from the field and avoid traffic on wet soil to reduce soil compaction.


Mahdi Al-Kaisi is an associate professor in agronomy with research and extension responsibilities in soil management and environmental soil science. He can be reached at or (515) 294-8304.

How and Why to Avoid Soil Compaction

Mahdi Al-Kaisi, Department of Agronomy

This spring most Iowa soils have plenty of moisture in the profile and in some areas may exceed field capacity, causing many producers to enter fields at less-than-ideal soil conditions. High soil moisture increases soil compaction caused by field traffic and machinery. Over the past decade the size of Iowa farms has increased, leading to larger and heavier equipment. However, equipment size is only one factor among many causes of the soil compaction problem.

Rushing to the field when the soil is wet can increase chances for severe soil compaction combined with the weight of equipment and traffic pattern in the field. Conducting field operations during wet field conditions compounds the amount of compaction occurring. Maximum soil compaction occurs when soil moisture is at or near field capacity because soil moisture works as a lubricant between soil particles under heavy pressure from field equipment.

Compaction near the surface, within the top three to six inches of the soil, is generally associated with the amount of surface pressure. Compaction below that is primarily associated with axle weight. For example, if soil a foot below the surface is at field capacity and the tractor's axle load is seven to eight tons or greater, compaction can occur at this depth, despite lower surface pressures.

Indications of soil compaction during and immediately following a normal rainfall include slow water infiltration, water ponding, high surface runoff and soil erosion. Additionally, soil compaction can be diagnosed by stunted plant growth, poor-root-system development and potential nutrient deficiencies (i.e., reduced potassium uptake). These soil compaction symptoms are a result of increased bulk densities that affect the ideal proportion of air and water in the soil.

The most efficient way to verify soil compaction is to use a tile probe, spade or penetrometer to determine a relative soil density. Soil moisture conditions can have a significant effect on penetration resistance. For example, in dry soil conditions soil penetration resistance is much higher than wet conditions because soil water acts as a lubricant for soil particles. Therefore, it is wise to determine soil compaction early in the season or compare observations and measurements from suspected areas with adjacent areas that have little chance of soil compaction due to traffic patterns.


Management decisions to minimize soil compaction
First, the most effective way to minimize soil compaction is to avoid field operations when soil moisture is at or near field capacity. Soil compaction will be less severe when soil tillage, fertilizer application and planting operations occur when the field is dry. Soil moisture can be determined using a hand ball test or observing a soil ribbon test.

Second, properly adjust tire size and air pressure. Larger tires with lower air pressure allow for better flotation and reduce load on the soil surface. Additionally, by using larger tires that are properly inflated increases the "footprint" on the soil.

Third, use the same wheel tracks to minimize the amount of land traveled across. Most damage occurs with the first pass of the implement. Using controled traffic patterns can be done effectively by using implements that are the same width for soil preparation, planting, row cultivation, spraying and harvesting.

Soil compaction can be a serious problem for Iowa farmers, but with proper farm management, compaction can be minimized. Remember to hold-off on soil tillage operations until soil conditions are drier than field capacity and look into the benefits of conservation tillage systems.


Top 10 Reasons to Avoid Soil Compaction
1. Causes nutrient deficiencies
2. Reduces crop productivity
3. Restricts root development
4. Reduces soil aeration
5. Decreases soil available water
6. Reduces infiltration rate
7. Increases bulk density
8. Increases sediment and nutrient losses
9. Increases surface runoff
10. Damages soil structure


Mahdi Al-Kaisi is an associate professor in agronomy with research and extension responsibilities in soil management and environmental soil science. He can be reached at or (515) 294-8304.

Renovating Pastures or Hay Meadows Damaged by Flooding

By Stephen K. Barnhart, Department of Agronomy

Spring flooding can damage pasture and hay meadows almost anywhere across the state of Iowa. While most forage will likely recover, other areas that are eroded or silt- or sand-covered will require some reseeding. The following considerations are guides to planning and management.
Fertilization. Fertilizer, particularly nitrogen, is the quickest way to increase grass production in pasture areas adjacent to the reseeded area. Research over the years has shown that, with good rainfall, a modest application of nitrogen (30-50 lb. N/A) on grass pastures in late spring can provide an economical increase in dry matter production and associated animal gain per acre. 

To get the full benefit from nitrogen there must also be suitable soil pH, phosphorus (P) and potassium (K) levels, and soil moisture. Partial benefit is achieved where other fertility levels are deficient. Benefits from summer-applied nitrogen will be less without adequate and timely rainfall. Areas to be reseeded should be soil sampled, and needed lime, P and K incorporated during any seedbed preparation.

Reseeding.  When deciding what to plant, consider what is compatible with the forage surrounding it. Ideally, this is something not very much different. For hay, plant something that will mature similarly, and if pasture, try to plant species that have similar palatability as the residual sod. The more difference, the less uniformly it will be grazed in the future. See ISU Extension bulletin PM - 1792, Selecting Forage Species for traits of various forage species and some suggested mixtures.

When designing your own mixture, consider using only two or three compatible and well adapted grasses and or legumes.

There are several methods for planting forage crops in the spring. The most costly is complete renovation, using a drill or broadcast seeder in a tilled seedbed. Refer to ISU Extension bulletin PM - 1008 Steps to Establish & Maintain Legume-Grass Pastures. Accumulated sand or silt may have to be spread to level the area before final seedbed preparation. 

A slightly less expensive approach is using a no-till forage drill and planting into a killed sod or undisturbed surface, if conditions are suitable. For more on this method refer to ISU Extension bulletin PM -1097, Sod-seeding and no-till pasture renovation.

As with most of the pasture improvement alternatives, there are some additional issues to consider when contemplating a change in pasture species. Among those questions: 

• Is seed of your selected forage species and varieties available?

• Are the weather patterns and soil moisture conditions suitable to provide for establishment and stand development? Mid- to late-spring can be a challenging time for establishing shallowly-placed, small-seeded forage grasses and legumes. If soil moisture is not adequate and if timely rainfall for the remainder of the summer is uncertain, it would be best to delay planting until mid- to late-August or the first week of September. Weeds may grow on the flood-killed areas before you can accomplish the new seeding. Mowing closely before no-till seeding may be adequate; mowing and tilling may be necessary for seedbed preparation in some cases. There is a risk of delaying too late into the autumn. Late emerging new seedlings may not have enough time to establish sufficiently to survive the winter.

• Will weed competition be sufficiently controlled to provide for the successful establishment and persistence of the newly introduced species?

• Will grazing animals be kept off the newly seeded areas until the new seedlings are established?

• If the seeding activity leads to a temporary decrease in forage production, are there sufficient forage resources to support the existing animals until the forage improvements are realized?


Stephen K. Barnhart is a professor of agronomy with extension, teaching, and research responsibilities in forage production and management.

Corn Nematodes and Soybean Cyst Nematode: Similar But Very Different

By Greg Tylka, Department of Plant Pathology

Corn nematodes and the soybean cyst nematode (SCN) are microscopic, plant-parasitic worms that live in the soil and feed on plant roots. Iowa crop producers and agribusiness professionals generally are aware of the soybean cyst nematode and its biology, scouting and management.  But many people want to learn more about the biology, scouting and management of corn nematodes.

There are some similarities but also many major differences in various aspects of corn nematodes and the soybean cyst nematode. Keeping these similarities and differences in mind is important when trying to determine the most economical and effective way to manage these pests.

general aspects of nematodes

biology of nematodes chart

symptoms and scouting nematodes

nematode management chart

The information above is for most commonly found corn nematode species. But some of the generalizations are not true for the needle and sting nematodes. Following are specific exceptions to the generalizations listed in the table for needle and sting nematodes.
• only found in soils that are more than 70 percent sand
• sample spring or fall, not mid season
• extremely high damage potential; damage threshold is one per 100 cubic centimeters soil
• narrow host range
• nonhost crops for management include alfalfa and soybean


A printer friendly version of the article and charts can be downloaded.


Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

needle nematode damage

Symptoms of needle nematode damage to corn roots.

 swollen soybean cyst nematode female
Swollen soybean cyst nematode females on soybean root.

Evaluating Hay and Pasture Stands for Winter Injury

By Stephen K. Barnhart, Department of Agronomy

A grower has little control over climatic factors that influence winter injury, but understanding some management practices and winter weather characteristics may help to determine where winter injury risks are more likely.

Injury is more likely to occur on species and varieties with low inherent cold hardiness, such as alfalfa, birdsfoot trefoil, orchardgrass and the ryegrasses. Winterhardy species and varieties are less frequently injured.

Old alfalfa stands and plants weakened by disease are often more susceptible to winter injury. Young stands are less susceptible to winter injury.

Winter-Injury of alfalfa is less severe where a grass is present in the mixture.

Injury occurs more frequently where four summer-cuts were made or where a late fall cutting or grazing was practiced. Fall cutting or grazing may not allow accumulation of adequate carbohydrate reserves for the winter or leave stubble to catch snow.

Injury may be more severe where a good soil fertility program is not being followed.

Plants in areas with heavy ice cover, ponded and re-frozen ice, and where snow was absent during periods of extreme cold air temperatures are more vulnerable to winter injury.

Stand Evaluation
When evaluating winter injury, consider both the number of plants per square foot and the age of the stand. Crown and root diseases also have a major effect on stand reduction of legumes, so plants should be checked for dead, dying, or diseased tissue. Winter-injured plants are often slow to recover in spring, so a quick decision to destroy a winter injured stand is not recommended.

Wait until the spring regrowth is about three to four inches high. Select random stand count sites. Check at least one 1-square-foot site for every five to ten acres. Dig up all of the plants in the 1-square-foot area. Pick at the crown and buds with a knife to determine if the tissue is still alive. Then count the number of live plants per square foot. Use Table 1 to begin your rating of the stand. Next, split the taproots and evaluate their general health. The core of healthy taproots is firm and creamy-white. Damaged or dying taproots are yellowish-brown to chocolate-brown in color and watery or dry and fibrous in texture. Only healthy plants will contribute significantly to yield, so if any of the taproots are more than 50 percent diseased, reduce your initial stand count accordingly.

Table 1  winter injury

Plan your management this season, based on your stand evaluation.

If stands are winter-injured, but will be harvested this season, allow plants to mature longer before cutting. Allowing plants to develop to early, mid or even full bloom in a growth cycle will help the plants restore carbohydrates and vigor needed for subsequent production. It is best to allow plants in severely injured stands to go to nearly full bloom in first cut, and to early flower in subsequent cuttings. This gives weakened plants a chance to regain some vigor. Stands with less injury could be harvested somewhat earlier depending on the extent of the injury. If stands were only mildly injured allow at least one growth cycle during the season to go to 10 to 25 percent bloom. Most producers will choose second or third cutting to take advantage of the generally larger yield of the first cutting.
Increase cutting height. As the maturing stems are flowering, new shoots may be growing at the base of the plants. It is important to not remove these shoots as it will further weaken the plant to have to produce new ones.
Maintain good fertilizer and insect management. It is particularly important that winter injured stands have adequate fertility. Soil test and apply needed fertilizer prior to first cutting if possible. And, be particularly vigilant in your insect scouting and management during the growing season following winter injury.

Evaluate other legumes similar to alfalfa. The ability of red clover, white clover and birdsfoot trefoil to reseed may compensate for some stand loss. Sod-forming grasses, such as smooth bromegrass may spread and fill in for thin stands. But, bunch-type grasses, such as orchardgrass and timothy will not. For a legume in a legume-grass mix, consider the “marginal’ values given in Table 1 as ’“good’. While nitrogen fertilizer may help in the recovery of severely injured grass pastures, avoid using excessive nitrogen rates and be ready to manage weeds in these less competitive stands.

Reseeding in hayfields or pastures might be a viable option. Reseeding more alfalfa into or immediately after a 2-year old or older stand is not recommended. Overseedng or drilling grasses or red clover into thin or winter damaged stands should be done from now through April. Delaying seeding until later in the spring increases the risk of plant competition and seedling loss to increasingly dry and hot soil surface conditions of early summer.


Iowa State University Extension publications for further information

Evaluation for winter injury, PM 1365 
Selecting forage species, PM 1792 
Establishing new forage stands, PM 1008
Interseeding and no-till renovation, PM 1097


Stephen K. Barnhart is a professor of agronomy with extension, teaching, and research responsibilities in forage production and management.

This article was published originally on 4/5/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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