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12/5/2011 - 12/11/2011

Change in Phosphorus and Potassium Contents of Cornstalks Over Time

By Antonio P. Mallarino and Ryan R. Oltmans, Department of Agronomy

Questions about phosphorus and potassium concentrations in cornstalks
Increasing corn yield and harvest of cornstalks for feed, bedding or bioenergy production are generating numerous questions about phosphorus (P) and potassium (K) concentrations in cornstalks and removal. The functions of P and K in plants influence their accumulation in different plant parts, and also may influence the magnitude of losses from live tissue or residue. A larger proportion of the total plant P is in grain compared with K, and most of the absorbed P is incorporated into the plant organic matter but K is in the inorganic soluble ion form. Therefore, P and K amounts removed by cornstalk harvest, concentrations and recycling to the soil may vary and over time be affected by rainfall and other factors. Also, high temporal soil-test variation, mainly for K, points to a potentially important effect of P and K recycling.

Summary of ongoing research
A study has been investigating P and K uptake by corn and soybean, removal with grain harvest, and recycling. This article summarizes results for cornstalks from nine fields that were sampled in 2009 or 2010. Aboveground plant samples collected at the black layer stage were separated into grain and vegetative portions (cobs, stalks and leaves). At grain harvest time, residue was placed in mesh plastic bags on top of no-tilled ground, which were removed at about 45 day intervals until the following spring. All samples were analyzed for P and K concentration and accumulation.

Figure 1 shows the P concentrations and amounts in corn tissues (except grain) from the black layer stage until spring of the following year. On average across fields, the P levels decreased until the time of grain harvest, remained the same until late fall, decreased again until mid winter, and thereafter remained about the same. The amount of P in residue at grain harvest was 69 percent of the amount at the black layer stage, and by April the amount remaining in residue had decreased to about one-half (53 percent).

Figure 1. Phosphorus concentration and amount in corn vegetative tissue at black layer and residue as a function of time. Solid boxes encompass lower and upper 25 percent percentiles, whiskers indicate minimum and maximum values, and black dots joined by a line represent average values.


Figure 2 shows that the trends over time for both K concentration and amount differed from trends observed for P. The K levels decreased sharply from the black layer stage until the time of grain harvest, during the fall, during early winter and in spring. Only during late winter there was no significant K loss. The amount of K remaining in residue at grain harvest time was 69 percent of the amount at the black layer stage (proportionally similar to the value observed for P), by late fall the amount remaining in residue was 59 percent, and by April had decreased to38 percent.

Figure 2. Potassium concentration and amount in corn vegetative tissue at black layer and residue as a function of time. Solid boxes encompass lower and upper 25 percent percentiles, whiskers indicate minimum and maximum values, and black dots joined by a line represent average values.


The magnitude and distribution of tissue P and K loss increased with increasing precipitation, but this was more consistent for K than for P. Moreover, increasing rainfall from black layer to late fall increased significantly the K loss but not the P loss (not shown). There was large variation in the P and K concentrations and amounts in cornstalks across the sites, but mainly from the black layer stage to late fall. This high variation resulted from differences in soil-test levels, fertilizer rates, hybrids, rainfall and other growth conditions. Contamination with soil and loss of leaves due to wind, which can affect P and K residue concentrations, were not factors influencing the results in this study but may become relevant when working with farm equipment.

The bottom line
Nutrient loss between the black layer and harvest was large for both P and K, loss from cornstalks between harvest and late fall were large only for K, and the overall proportional loss by spring was greater for K. The average P and K concentrations and removal shown in this article, related publications and articles can be used as a general guideline, but due to large variation across fields and years sampling and analysis of harvested cornstalks is recommended to adequately estimate values.

Additional information
More information from this study, including P and K removal with grain and results of a similar study with soybean, are available in articles written by Mallarino and collaborators in proceedings of the 2011 ISU Integrated Crop Management Conference and the North-Central Extension-Industry Soil Fertility Conference.

The study summarized in this article has been supported in part by the International Plant Nutrition Institute and the Iowa Soybean Association.


Antonio P. Mallarino is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management. Ryan R. Oltmans is a graduate research assistant.

CCA Exam Review Course Offered by ISU Extension

By Brent Pringnitz, Agriculture and Natural Resources Extension

Individuals planning to take the Certified Crop Adviser (CCA) exam can attend a two-day Iowa State University (ISU) Extension and Outreach course to prepare for taking the certification exam. During the CCA Exam Review Course, ISU Extension specialists will cover the performance objectives for the exam and focus on areas where students may need additional assistance. Interactive quizzes and a final practice examination will be included in the course.

The CCA Exam Review Course will be held Jan. 9 and 10 in room 150 of the Scheman Continuing Education Building on the ISU campus in Ames. Registration opens at 8 a.m. with classwork beginning at 8:30 a.m. and ending at 3:30 p.m. on Jan.10. Registration is limited to 60 participants and pre-registration is required. The program brochure and registration form are available on the Web at

The cost of the course is $250. Registration and fees must be received by midnight, Jan. 4. Registrations will not be accepted at the door for this program. The fees include lunches, breaks and a copy of the Crop Diagnostic Notebook.

Registrations can be completed online with a credit card (MasterCard or VISA only). Registrations may also be faxed with a credit card to (515) 294-1311 or be mailed along with a check or credit card information to: ISU ANR Program Services, 2101 Agronomy Hall, Ames, Iowa, 50011-1010. For more information, call (515) 294-6429 or email

Subsoil Moisture Levels a Concern for Northwest Iowa 2012 Crops

Paul Kassel, ISU Extension field agronomist

The northwest Iowa fall survey of subsoil moisture completed by Iowa State University in November shows well below average amount of subsoil moisture. Subsoil moisture levels are checked in the fall in many northwest Iowa counties through this survey.

The level of subsoil moisture in Dickinson, Clay and Pocahontas counties ranges from 1.4 inches to 5.0 inches of plant available moisture. Most of the area had a full profile of soil moisture in mid-summer. However, a large part of the area received little rain in late summer and fall.

Typical Iowa soils have the potential to hold from 10.0 to 11.0 inches of moisture in the top five feet of soil. The dry conditions late last summer and fall left the soil moisture reserve very low. Soil moisture readings from the 10 sites where Iowa State University personnel analyzed soil moisture in northwest Iowa had 1.4 to 5.6 inches of plant available moisture. The average of these 10 northwest Iowa sites is 4.1 inches of moisture as of Nov. 1, 2011.

These results confirm what is expected in terms of reserve soil moisture. The amount of rainfall has been very limited since mid-July in many locations. Rainfall amounts since mid-July have been six to nine inches below normal.

Some areas – like the sampling site near Rossie - show the results of some late season rainfall and the fact the corn and soybean crop did not use soil moisture past mid- September.  A frost event on Sept. 15 effectively brought the growing season to a close and the crop did not use any soil moisture after that date.
This level of subsoil moisture is very low when compared to levels of subsoil moisture the past few years. With less than 0.5 inch of rain in November, the area will be very dependent on rainfall next spring as snow contributes very little to subsoil moisture. (Moisture from snow often evaporates or runs off with snow melt. However, the snow may provide some insulating value – to minimize the depth of frost penetration.) Crop production will be very dependent on summer rainfall without a reserve of soil moisture going into the summer crop growth time period. 

Rainfall during March and April 2012 will also contribute to subsoil moisture. Typical rainfall for those months is three to five inches. We can expect about 80 percent of that rainfall to contribute to subsoil moisture reserves.





Paul Kassel is an ISU Extension and Outreach field agronomist serving northwest Iowa. He can be reached by calling 712-262-2264 or 712-260-3389 and by e-mail at

A 10-Year Summary of Testing for Nematodes that Feed on Corn in Iowa

By Greg Tylka, Department of Plant Pathology and Microbiology

The Iowa State University (ISU) Plant and Insect Diagnostic Clinic is the only facility in Iowa that extracts and identifies plant-parasitic nematodes from soil and root samples submitted by farmers and those who advise them. The results of testing for nematodes that feed on corn in Iowa from 2000 to 2010 were summarized and published in an article this week titled “Testing for Plant-parasitic Nematodes that Feed on Corn in Iowa 2000-2010,” by G.L. Tylka, A.J. Sisson, L.C. Jesse, J. Kennicker, and C.C. Marett in the online journal Plant Health Progress

The main findings of the summary give an overall sense of what is currently known about the extent of plant-parasitic nematodes affecting corn production in Iowa.

Sample numbers

  • From 2000 through 2010, the ISU Plant and Insect Diagnostic Clinic analyzed 331 samples associated with corn for plant-parasitic nematodes.
  • Soil cores and root samples are needed to thoroughly test for all possible nematode species that feed on corn. From 2000 through 2010, 124 samples had soil alone, 17 samples only had roots, and 190 samples had both soil cores and root samples.
  • On average, only 15 samples were submitted annually for testing for nematodes on corn from 2000 through 2004. Annual sample numbers increased threefold beginning in 2005, but still averaged less than 50 per year through 2010. 
  • Samples were received from only 53 of the 99 Iowa counties, mostly from northern, central and eastern Iowa (Figure 1).

Nematodes found

  • One or more species of plant-parasitic nematodes that feed on corn were found in 92 percent of the samples analyzed from 2000-2010. 
  • The nematodes most frequently found were spiral (present in 77 percent of samples submitted) and root-lesion nematodes (found in 51 percent of samples submitted).
  • Most species of plant-parasitic nematodes cause damage to corn only when numbers exceed a damage threshold. Overall, 15 percent of the samples from 2000 through 2010 had nematodes present in numbers exceeding the damage threshold.
  • No sample had more than one nematode species present at damaging levels.
  • The nematode most commonly found at damaging levels was the needle nematode (eight percent of all samples submitted). Almost all of the samples with needle nematode were from Muscatine County (Figure 2).
  • The dagger nematode was second most frequently present at damaging population densities (six percent of all samples submitted).  
  • Although spiral nematode was found in 243 of the 331 samples, only one percent of the samples had numbers exceeding the damage threshold.

Implications of the results
It was not surprising that most samples contained one or more species of nematodes that feed on corn. Most of these nematodes are likely native to Iowa and fed upon native plants before corn was cultivated as a crop. The nematodes are not specific to corn. They are very commonly found at low population densities not thought to be damaging to corn.

The high concentration of damaging populations of needle nematodes in Muscatine County is likely because needle nematode is damaging at very low population densities (basically at the detection level of one worm per 100 cc soil) and because of the high prevalence of sandy soils in that area of Iowa. (Needle nematode occurs only in soils with at least 49 percent sand).

One should not extrapolate the summarized results to counties from which no nematode samples were submitted for testing.

The total number of samples tested for plant-parasitic nematodes that feed on corn from 2000 to 2010 was extremely low considering there are more than 13 million acres of corn grown in the state. There were 77 samples submitted to the ISU Plant and Insect Diagnostic Clinic in 2011 for nematodes that feed on corn.

It is not likely that healthy-looking corn is being damaged by plant-parasitic nematodes. So, not every field in the state needs to be sampled for nematodes that feed on corn.  But significantly more cornfields showing symptoms of stress should be checked for plant-parasitic nematodes. The ideal sampling times and methods for nematodes that feed on corn were discussed in an earlier article in ICM News

Increased sampling for nematodes that feed on corn will lead to a better understanding of the importance of these native nematode pests in corn production in Iowa. And information from such samples will allow farmers and those who advise them make more informed decisions concerning the use of current and future nematode management products.

Figure 1. Number of samples submitted to the ISU Plant and Insect Diagnostic Clinic from corn fields to be tested for plant-parasitic nematodes from 2000 to 2010, by county.
From Tylka et al. 2011. Testing for Plant-parasitic Nematodes that Feed on Corn in Iowa 2000-2010. Plant Health Progress doi:10.1094/PHP-2011-1205-01-RS.

Figure 2. Number of damaging population densities of plant-parasitic nematodes on corn from samples submitted to the ISU Plant and Insect Diagnostic Clinic from 2000 to 2010, by county.
 From Tylka et al. 2011. Testing for Plant-parasitic Nematodes that Feed on Corn in Iowa 2000-2010. Plant Health Progress doi:10.1094/PHP-2011-1205-01-RS.


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

Weed Identification Guide Available Electronically

Daren Mueller, Department of Plant Pathology and Microbiology

The popular Weed Identification Field Guide is now available electronically as an e-book and, for the first time, downloadable on iPad. The publication is distributed by the Iowa Soybean Association (ISA) and Iowa State University (ISU) Extension and Outreach. Pioneer Hi-Bred, a DuPont business, has sponsored the capability for users to download the electronic versions for no charge.

The Weed Identification Field Guide, CSI 0003, authored in 2010 by Iowa State University Extension specialists, includes images and descriptions of 56 broadleaf weeds and 19 grass and grass-like weeds. The electronic publications, like the print version, include tools to aid in accurate weed identification, as well as weed lifecycle and herbicide management and stewardship information. They also include detailed diagrams, including 24 illustrations, and more than 250 zoomable, high-resolution photographs of weeds common to Iowa.

The industry has rapidly adopted electronic technology for use in the field. With this field guide available in print, as an e-book and for the iPad, it gives farmers more options to access information. Plus, the ability to zoom in on high resolution photos will make identifying weeds, the soybean's biggest competitor, that much easier.

This is the second collaborative ISA/ISU Extension field guide to go electronic, with the first being Soybean Diseases. By immediately identifying weeds, an effective management plan that is vital to maximizing crop production can be determined more accurately.

Field guides, print and electronic, can be found on the ISU Extension Online Store at or at

Funded by the soybean checkoff.


Daren Mueller is an extension specialist with responsibilities in the Iowa State University Integrated Pest Management program. Mueller can be reached at 515- 460-8000 or by email at

This article was published originally on 12/12/2011 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.