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6/28/2010 - 7/4/2010

Is Tissue Testing Useful in Identifying Corn and Soybean Fields Responsive to Phosphorus and Potassium Fertilizers?

By Antonio P. Mallarino, Professor, Department of Agronomy

Producers often ask questions about tissue testing to decide "emergency" in-season phosphorus (P) and potassium (K) fertilization for corn and soybean. Previous articles(June 2010 and July 2008) have discussed the viability of post-emergence application of P and K fertilizers. A large application of granulated P or K fertilizer to soil during the very early crop growth stages may result in some grain yield increase although an economic benefit for the current crop is not likely. The probability of an economic response to foliar fertilization is likely with insufficient preplant fertilization or when soil and climate factors (other than drought) limit nutrient uptake. Traditionally, farmers and crop consultants have used soil sampling and testing of visually affected and seemingly unaffected field areas to determine if there is a nutrient deficiency. This is an effective practice when crop growth is limited by low soil nutrient supply, but will not be effective when soil or climatic factors other than low soil nutrient levels limit early nutrient uptake.

Is tissue testing helpful for identifying fields responsive to foliar fertilization?
No simple and reliable tissue test exists to identify the conditions that increase the chance of corn or soybean response to P and K fertilization. In spite of many field trials in Iowa, we have not been able to identify a useful critical or optimal P or K concentration in plant tissue. Figures 1 and 2 show a very poor relationship between the nutrient concentration in young plants or leaves and the yield response to fertilization across several fields and years. The reason is that many factors other than nutrient supply affect plant growth and influence the tissue nutrient concentrations due to nutrient uptake and also dilution and concentration of nutrients in the dry matter. Relationships (not shown) are acceptable for different fertilizer treatments or soil nutrient levels in a specific field and year, but a diagnostic tool should work reliably across fields and years. Attempts to overcome this problem by using nutrient ratios have not been successful and often suggest higher fertilizer rates than needed.

Some universities suggest as a general guideline a high tissue test level at which there seldom is yield response to fertilization. The problem of using a "safe" but too high critical nutrient concentration is that such a level encourages farmers to apply fertilizers when the probability of an economic yield increase is very small or inexistent.

Use of soil testing and fertilization before planting is the most effective way of assuring adequate P and K supply for corn and soybean. A practical and useful way of using tissue testing is to use test results in conjunction with in-season soil testing, comparing field areas with apparent deficiency symptoms or poor growth with nearby seemingly unaffected areas. This strategy may not solve the problem for this years' crop, but will provide clues to improve fertilizer or soil management for next year.

figure 1

Figure 1. Relationships between relative corn yield response to P and K fertilization and the nutrient concentration of small plants or leaves (at V5-V6 or silking) across several Iowa field trials. Relative yield represents the yield without fertilization expressed as the percentage of the maximum yield achieved with fertilization.

figure 2

Figure 2. Relationships between relative soybean yield response to P and K fertilization and the nutrient concentration of small plants or leaves (at V5-V6 or R2-R3 stages) across several Iowa field trials.


Antonio P. Mallarino is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.

Could 2010 Turn out Like 1983?

The two years 2010 and 1983 have a lot in common so far, at least at Ames, Iowa. I haven’t analyzed other locations, but Ames was not typical of the region either year. This year the growing degree day (GDD) accumulation has climbed from normal in mid-May to about 100 GDD above normal at the last week of June. In 1983 the GDD accumulation was about 200 GDD behind usual by mid-June then it rapidly increased to slightly above normal by mid-August.

The 1983 heat in July was accompanied by greatly diminished rain after the first week of July until the last week of August. The temperature stress, hours of temperature exceeding 86 F, reached 260 by the last week of August. Normal temperature stress is 65 for the period.

Are the years 1983 and 2010 alike to date? There are three considerations. First, neither year had significant periods of abnormally high temperature in the spring. Second, both years had above normal precipitation reaching to more than 16 inches between January and the end of June (normal is less than 14 inches) and both years had above usual moisture during the previous fall season. Third, a winter El Nino condition rapidly dissipated during May and June. 

The May through June 2010 switch from El Nino to La Nina, as assessed by 90-day Southern Oscillation Index values, does present an historical probability of an increased incidence of stressfully hot days during the 2010 season. The statistical chance that it could result in a U.S. corn yield below the trend (below 160 BPA) is near 70 percent as opposed to a 50/50 chance on an average year. 



Elwynn Taylor is Extension Climatologist and can be reached at or by calling (515) 294-1923.

Is In-Season Fertilization for Soybean Effective

By Antonio P. Mallarino, Department of Agronomy

Questions often arise in June about "emergency or catch-up" fertilization for soybean. Most producers are not worried about this because they typically apply adequate amounts of phosphorus (P) and potassium (K) fertilizers before planting soybean or apply sufficiently large amounts before the previous year’s corn for both the corn and soybean crops. Producers asking questions wonder, however, if fertilizer rates were inadequate or if late planting dates, replanting and cold or excessively wet conditions have altered the crop nutrient uptake and fertilization need. Others wonder if the soil nutrient levels might limit high yield potential when growing conditions are good.

Is applying dry, granulated fertilizer a viable post-emerge option?
The short answer to this question is probably not. Typically this is not a good option for two main reasons. One, both P and K (but especially P) are needed at early growth stages to enhance plant cell multiplication when the number of nodes, leaves, and potential seed numbers are largely determined. Two, the application of fertilizer to the soil surface or banded/injected between the rows will be of low efficiency, mainly due to the common low or infrequent summer rainfall (perhaps not the case this year). There is only one situation in which an in-season application of granulated P and K fertilizer might be considered. That is when the soil tests are very low, and for certain the producer will have to apply a high fertilizer rate for next year’s corn crop to assure adequate P and K levels. Also, perhaps if the plan is to apply a greater than optimum rate to quickly buildup soil P and K test levels. In this scenario, an in-season application when the soybean plant is still small (only three or four leaves) may not be very efficient at increasing grain yield but will increase soil P and K levels and decrease the rate needed before the next corn crop.

What about foliar fertilization?
Some producers ask if foliar fertilization could help improve soybean growth and grain yield. It may, but the potential is quite low in fields that have been well fertilized or where growth is limited by factors other than nutrient supply.

Prior to the 1990s hundreds of field experiments conducted across de Midwest focused on foliar fertilization at late soybean reproductive stages (R4 to R7) using fertilizers that included nitrogen (N), P, K, sulfur (S), and other nutrients. Researchers theorized that if nutrients were applied to the foliage at this time, leaf senescence and “seed starving” could be alleviated and grain yields increased. A few early Iowa trials suggested that spraying a nutrient mixture in a ratio 10-2.3-3.6-0.5 N-P2O5-K2O-S between the R5 and R6 growth stages could increase yield by seven to eight bushels per acre. However, many subsequent trials in Iowa and across the Midwest and Southern states from the late 1970s to the early 2000s, showed inconsistent results, with an equal frequency of yield increases and decreases. More recent work in the Midwest under rain-fed conditions showed similar results, and often yield decreased when N sources were sprayed alone or in a mixture. The more positive results were observed under very high yield conditions with irrigation in Kansas. Therefore, these results have discouraged further research and adoption of foliar fertilization of soybean at late reproductive stages.

Other researchers thought that small amounts of nutrients sprayed onto soybean foliage at early growth stages could supplement inadequate pre-plant fertilization, increase nutrient supply even with presumably adequate pre-plant fertilization especially when soil conditions limit nutrient uptake. About 100 replicated field trials were conducted in Iowa from 1994 until the early 2000s to evaluate these possibilities. Applications included spraying foliar fertilizers with or without mixing with glyphosate herbicide at the V5 to R3 growth stages. The products tested (not all products were included in all trials) included the low-salt fluid fertilizer 3-18-18 (N-P2O5-K2O) and 10-10-10 (N-P2O5-K2O) both with or without S and with or without the micronutrients boron (B), iron (Fe), and zinc (Zn); and also 8-0-8 (N-P2O5-K2O). Product rates ranged from two to six gallons per acre applied once or twice (spaced 8 to 10 days). The fields were managed with no-till, ridge-till, or chisel-plow tillage. These results were summarized before and are only briefly, summarized here.

Results showed that foliar fertilization increased yield in 15 to 30 percent of fields depending on the trial set and year, and about 15 percent of fields on average. The average response to the best treatment across all fields was 0.7 bushels per acre. Differences between treatments were not consistent across fields, but responses tended to be higher for the three gallons acre rate of 3-18-18. Adding S or micronutrients did not produce higher yield, and the highest rate of 10-10-10 (with or without S) and 8-0-8 fertilizers reduced yield in a few fields (some leaf burn was observed). Yield with the double applications were the same as single applications.

As expected, yield increases were observed in fields testing low in P and K, but sometimes also in fields testing Optimum or higher due to reasons difficult to identify. Soil-test results, tissue-test results, and climatic conditions did not support strong conclusions, but suggested that conditions in which a response to foliar fertilization was more likely included ridge-till and no-till fields and slow early plant growth and P or K uptake due to cool early temperatures and excessive rainfall. Therefore, conditions that inhibit root growth and/or nutrient uptake early during the growing season (except drought) increased the likelihood of a yield response.

Five field trials conducted in 2005 and 2006 that studied foliar fertilization and fungicide application alone or in a spray mixture produced even more disappointing results for foliar fertilization (Figure 1). Eight treatments were a non-treated control, four foliar fertilization treatments without fungicide (a single application of three gallons per acre of 3-18-18 at the V5 and R2 to R3 growth stages, a double 3-18-18 application at V5 and R2 to R3 stages, and 3.3 gal/acre of 28 percent UAN at the R2 to R3 stages), and three fungicide (Headline®) treatments at the R2 to R3 growth stages (alone and in combination with 3-18-18 or UAN fertilizer). On average the fungicide increased yield by 2.9 bushels per acre, although the responses were statistically significant only at three fields. The fungicide delayed leaf senescence at most fields, although clear disease control was observed only for Brown Spot in three fields. Spraying soybean with 3-18-18 fertilizer did not affect yield at four fields and increased it slightly at one field. Spraying with UAN did not affect yield at two fields, increased it slightly at one field, and decreased it at two fields. The UAN application caused moderate leaf burning and the 3-18-18 application caused no burning. Mixing the two fertilizers used in this project with the fungicide did not cause problems or an additional yield response compared to the products alone.

In-season fertilizer application for soybean seldom will be cost-effective in Iowa production systems. The exception might be when soil samples confirm that the soil tests very low or low and there was insufficient preplant fertilization. A large application of granulated fertilizer to soil during the very early growth stages may result in some yield increase and will begin to build up soil test levels that will have to be increased for the next crop anyway, but the economic benefit for this year's soybean is very doubtful. The probability of an economic response to foliar fertilization is small, but this practice may be justified when nutrient deficiency symptoms are obvious, with confirmed deficient-testing soil, or when soil or climatic factors (other than drought) limit nutrient uptake in late spring and early summer.


Figure 1. Effects of foliar fertilization and fungicide application on grain yield of soybean (average across five trials in Iowa).


Antonio Mallarino is a professor of agronomy, with research and extension responsibilities in soil fertility and nutrient management.  Mallarino can be reached at or by calling (515) 294-6200.

Forage and Cover Crop Considerations for Delayed Planting and Flooded Sites

Stephen K. Barnhart, Department of Agronomy

Extended periods of rainfall, flooding, hail, or all of the above have producers scrambling for replant or prevented planting options. Each choice has practical and economic implications, so should be approached with some thought. 

One consideration is the implication for crop insurance. Before changing crops or planting an ‘emergency’ forage crop, check with your crop insurance representative.

When considering crops for an annual forage, practical issues include:  Can I use, sell or rent the forage? Will the forage crop be harvested as silage or dry hay? Will it be grazed or simply be a ‘cover crop’? While many species may fit the criteria for these uses, the following are among the most practical, predictable and economical.

Dry hay options - Foxtail millet, Japanese millet,  Sudangrass (maybe), Teff, Oats         

Silage options- Foxtail millet, Japanese millet,  Sudangrass, Sorghum X Sudan Hyb, Hybrid pearl millet, Teff, Oats      

Grazing options - Foxtail millet, Japanese millet, Sudangrass, Sorghum X Sudan Hyb, Hybrid pearl millet, Teff, Oats

Seed supplies of some of these forage crops are in short supply in normal production years. As you consider these as options for your needs, check on seed availability.

Sudangrass can be a multiple-cut, summer annual, for use as fresh cut forage, pasture or silage. It should be rotationally grazed for best use, and can be difficult to dry thoroughly for hay.  Varieties vary in height and leafiness. Sudangrass can be planted through early-July, with the first growth useable in about 50 days. At this late planting date you may get a second harvest or grazing. A hydrocyanic acid poisoning (Prussic acid) risk is minimal, but avoid pasturing severely drought stressed or very short (less than12 inches) growth/tiller regrowth and use caution if grazing soon after frost.

Hybrid Sorghum X Sudangrass is a multiple-cut, summer annual; used for fresh cut forage, pasture (rotation grazing is recommended) or silage. Varieties vary greatly in height, leafiness, and grain yield depending on the parent lines making up the hybrid. These hybrids can be planted through early-July, with the first growth useable in about 50 days. At this late planting date you may get a second harvest or grazing. There is a hydrocyanic acid poisoning (Prussic acid) risk if plants or tillers are grazed or green fed at short height (shorter than 24 inches) or during severe drought, and use caution if grazing soon after frost.

Sudangrass, and sorghum X sudangrass hybrids are better adapted than most species to drought, high temperature, and low soil pH than corn, but will yield less in seasons with cool August and September temperatures. Sudangrass and sorghum X sudangrass hybrids should be harvested at two to three feet of height (two to three cuttings for season). Harvesting at later maturity may increase yield but will result in very low forage quality. 

Short grain sorghum / forage soybean mixture planted through early summer. Harvestable within about 60 days. Requires good fertilization for production. Harvest at late vegetative or very-early head stage of the sorghum.

Foxtail Millet is a warm season, annual grass also called German, Siberian, or hay millet. Foxtail millet can be used as harvested or grazed forage. It can be planted through early July, and is useable  in about 50 days. Only one summer growth should be expected. Foxtail millet is the best of the 'millets' for an emergency hay crop, but can become a weedy grass if allowed to produce mature seed.

Japanese Millet is a summer annual grass, and produces a relatively coarse (stemmy) forage. It can be used as fresh cut forage, hay, silage or pasture. It can be plant through early July, and is useable in about 50 days. If the first crop is cut at vegetative growth stage, regrowth yields are more likely. Japanese millet is closely related to the grassy weed barnyard grass, so avoid allowing seed formation.

Hybrid Pearl Millet is a multiple-cut, warm-season annual, used for fresh cut forage, pasture (rotation grazing is recommended), or silage. It resembles sorghum X sudangrass hybrids in plant structure It can be planed through early July, and is useable in about 50 days. It has somewhat slower regrowth than sorghum X sudangrass hybrids, and has limited production in cool summer seasons. There is no risk of hydrocyanic acid (Prussic acid) poisoning with pear millet.

These annual millets have been of particular interest in recent years. Remember that these are warm-season crops and perform best in warm, sunny growing seasons. They have not performed up to expectation during cool, cloudy summers.

Teff , an annual forage grass that has been grown by a few growers in Iowa, is a warm-season grass. It can be planted from mid-May into July. Its seeds are very small and must be planted very shallowly. Teff seedlings are relatively non-competitive, but once established, can produce one or more harvests of grass hay.  Grazing should be delayed until later in the summer, when root systems are stronger. As with other annual forage crops, later planting dates limit the yield potential.

Oats can be planted in July as a cover crop and grazed about any time. It may produce seed heads at a short height. It can be cut and stored as dry hay or silage form late-vegetative through early milk stage. At dough stage, the stems decrease feeding value greatly. Other cereal grains may also fit this use, such as barley, spring wheat, or spring triticale, but their seed will likely be more expensive and in shorter supply than for oats.

While it is the farthest thing from our minds now, these annual forages can come under scrutiny later in the growing season for high nitrate risk if the season turns dry.



Stephen K. Barnhart is a professor of Agronomy and the Iowa State University Extension forage agronomist. He can be reached at 515-294-7835 or by emailing

This article was published originally on 7/5/2010 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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