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 | | Apr 2012 | Volume 37 | Issue 6 |
Early-Planted Corn & Cold Weather Should producers be concerned about the health of their newly planted and/or newly emerged crops? Only time will tell whether cold temperatures and frost will cause permanent damage or death of early-planted corn. Recovery of damaged plants will usually be evident within 5 to 7 days following such events. Newly Planted Corn One of the risks that newly planted corn faces is that of imbibitional chilling injury due to cold soil temperatures during the initial 24 to 36 hours after seeding when the kernels imbibe water and begin the germination process. Kernels naturally swell or expand in response to the imbibition of water. If the cell tissues of the kernel are too cold, they become less elastic and may rupture during the swelling process. Symptoms of imbibitional chilling injury include swollen kernels that fail to germinate or arrested growth of the radicle root and/or coleoptile following the start of germination. Chilling injury during emergence can also occur, often causing stunting or death of the seminal root system, deformed elongation of the mesocotyl (the so- called "corkscrew" symptom) and either delayed emergence or complete emergence failure (i.e., leafing out underground). It is not clear how low soil temperatures need to be for imbibitional chilling or subsequent chilling injury to occur. Some sources simply implicate temperatures less than 50F. Others suggest the threshold soil temperature is 41F. Daily minimum soil temperatures at the 4-inch depth have certainly dropped into the mid- to high-40's in recent days, with some growers reporting temperatures as low as 40F at seed depth.
 Newly Emerged Corn Damage from exposure of above-ground plant tissue to frost can range from minor leaf injury to complete death of all exposed leaf tissue. That's the bad news. The good news is that the all-important growing point region of a young corn plant remains below the soil surface, safe from exposure to frost, until the V4 to V6 stages of development. That means that the above-ground plant tissue you see in fields younger than about V4 is composed primarily of leaves and rolled up leaf tissue in the whorl, but does not include stalk tissue or the growing point. As long as temperatures are not lethally cold, "simple" frost injury usually does not literally kill such young corn plants. Damaged plants will begin to show recovery from the whorl within 5 to 7 days, depending on temperatures following the frost event. Disclaimer: Repeated frost events that re-inflict damage to recovering corn plants can cause permanent stunting or death. When folks worry about the effects of cold weather on corn, they often fail to distinguish between simple frost events and lethal cold temperatures. Frost can occur at temperatures easily up to the high 30's F, but lethal cold temperatures for corn are generally thought to be 28F or colder. Whether such cold temperatures "penetrate" the upper inch of soil near the growing point region of corn seedlings is not clear, but may be possible in fields where soils are excessively dry and free from surface residue. Excerpted from Corny News Network, written by R.L. (Bob) Nielsen, Agronomy Dept., Purdue Univ.
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DOUBLE-CHECK SOIL PROBE SPRAYS
-Fred Vocasek, Senior Lab Agronomist, 22 December 2011
 Wet or clayey soil can be hard to sample because it sticks to the sides of the soil probe tube, making it hard to remove the soil core. We have always recommended spraying the inside of the tube with WD-40, as needed. This kept the soil from jamming up inside the tube and also let the core slip out smoothly. It didn’t seem to affect any of the routine soil test values, including organic matter. There are alternatives to WD-40, but one of our Laboratory customers just alerted us to a potential problem. They took about 40 soil samples and sent them in for analysis. The results for phosphorus and some other results didn’t correspond with the field history results. They repulled and resubmitted the samples, with better results. After doing some detective work, the company agronomist found that the samplers had used a commercial product that was promoted as being the same as WD-40. It was - if it was being used as a machinery or equipment lubricant. The agronomist quizzed the factory rep, finding out that the product apparently contained phosphorus and metals which may possibly have affected the soil sample. Spray lubricants are mixtures of light-weight petroleum compounds, surfactants, and other ingredients. To date, we are not aware of contamination problems from WD-40. A 1995 study by Univ. of Wyoming compared WD-40, PAM vegetable spray, Dove dishwashing liquid, motor oil, and silicone spray. They found low nutrient levels in each of the materials. There were slight changes in some soil test levels, but were thought to result from complexing or chelating properties of the lubricant mixtures. Overall, WD-40 seemed to perform well in this study. The final thought was that a sampler would have to use enough WD-40 to physically dilute the soil sample to have a serious effect on soil test results. We would advise caution when using an alternative product as a soil probe lubricant. |
Soil Nitrate Accumulates During Drought, Hot Weather
-Fred Vocasek, Senior Lab Agronomist, 12 August 2011
The severe drought conditions and hot weather of summer 2011 may have an unexpected benefit - more soil nitrate. Soil nitrate levels of 100 lb N per acre or more are not uncommon following a summer like 2011. There are three main reasons that soil nitrates may accumulate: 1) reduced crop uptake, 2) less leaching, and 3) more mineralization.
1. Reduced crop uptake
Crops typically take up most of their nitrogen requirement during the vegetative growth stage. The uptake rate drops off quickly when the plant reaches the reproductive stages. If drought conditions hurt yield potential before pollination or bloom, some fraction of the soil nitrate is taken up and the rest stays behind to accumulate in the soil.
Nitrate accumulations are common for dryland or non-irrigated crops, but nitrate can also accumulate under irrigated crops with normal or above-normal yields. This can occur because of accelerated nitrogen mineralization caused by the hot weather. The amount of nitrogen mineralized can exceed the crop nitrogen removal, so the excess nitrate accumulates in the soil.
2. Less leaching
Nitrate can be transported or leached from an upper soil zone to a lower soil zone, depending on the amount of water that is able to percolate through the profile. Excess soil water will drain to lower depths, even if it is only a slight amount of excess. The leaching depth and amount of nitrate transported is affect by many factors. Factors which change constantly.
Higher evaporation rates and increased crop water demand during drought prevent excess water from accumulating in the upper root zone. For example, the total evapotranspiration vs. rainfall data from the Lubbock area (April-July) shows the net water loss to the atmosphere during summer 2011 was at least ten inches greater than the four previous years1. There was little or no opportunity for water to accumulate in the profile.
Nitrate remains in the upper soil profile simply because little or no leaching occurs during hot, dry conditions. There could be the same total amount of nitrate in the root zone, but a larger fraction of the total will be found nearer the surface during drought years. This accumulation pattern tends to skew soil sample histories, especially if one depends only on surface sample results for making nitrogen fertilizer recommendations. Collecting subsoil nitrate samples takes more time and energy, but will give a better idea of overall nitrogen fertility through the profile.
3. More mineralization
Total accumulations
Mineralization of organic nitrogen to nitrate is most rapid at high soil temperatures and with adequate soil moisture. Part of the nitrate accumulated during the growing season comes from the conversion of organic nitrogen found in the crop residue. In the case of wheat, about 5 to 7 lb N can be mineralized for every ten bushels of yield if the nitrogen in the straw is completely mineralized.
Much nitrate can accumulate from organic matter mineralization, even under what seem to be bad conditions. Nitrogen credits of 10 to 20 pounds of nitrogen per percent of organic matter are not uncommon under normal conditions. These are average numbers, but the actual nitrogen credit can be much higher under drought conditions. These credits have been documented in the Great Plains and other areas. 1
Note: Weather data from other drought-affected areas may be different than the Lubbock data, but the general trends would be similar.
Table 2 shows the nitrate that accumulated during the uncropped, fallow period at Alliance, Nebraska, for twelve different years. Soil samples were taken in the fall, either prior to or just after wheat seeding. In seven years out of twelve, the soil mineralized at least 70 lb N/ac, even when no nitrogen was applied (column a). Note that the highest amount of nitrate accumulated was during the 1980 season - one with widespread drought conditions very similar to those that occurred in 2011.
When an extra 40 lb N/ac was applied to the preceding wheat crop, the soil nitrate exceeded 70 lb N/ac in nine of twelve years (column b). However the amount of extra nitrate found in the soil test could not be entirely attributed to carryover fertilizer alone.
In another set of experiments, a Weld silt loam (Akron, Colorado) had a three-year average nitrate accumulation of 106 lb N/ac with no N applied and a Holdrege silt loam (North Platte, Nebraska) had a five-year average accumulation of 110 lb N/ac with no nitrogen applied.
Timing of accumulations
Much of the nitrate found in these situations accumulated in July and August, months that typically have the highest temperatures and lowest rainfall. Table 3 illustrates the pattern of nitrate accumulation during the fallow season for two of the Alliance sites. The Duroc soil was broken out of native sod in 1970, the Alliance soil was broken out of wheatgrass pasture in 1969. Nitrate mineralization was slow right after harvest and during early spring, but preceded rapidly during late July and August. In fact, the researchers found that soil samples taken in July might only contain 50% to 60% of the nitrate found at wheat seeding time.
Temperature has a huge effect on the microbes responsible for mineralizing organic nitrogen to nitrate. Figures 1 and 2 shows the effect of temperature on nitrification. Nitrate accumulated faster in warm soils than in cool soils. Figure 1 shows that nitrate accumulated to a concentration of 50 ppm about three weeks sooner at a summer-time 86/F soil temperature than at a spring-time 61/F soil temperature.
Figure 2 contains the same data as Figure 1, except the calendar days were converted to degree days. Nitrate accumulated to 50 ppm at roughly 450 degree days for all soils at all temperatures. Degree days are a more accurate way of helping define the rate of biological processes, including plant growth, insect development, and microbial processes. Degree days accumulate more rapidly in hot years than in normal years, so the rate and extent of nitrification increases as well because the microbes are more active.
The increased microbial activity helps account for the soil nitrate accumulations found after irrigated corn harvest, even during years when yields are high. Degree days accumulate quickly when air temperatures are high. Corn matures earlier, reaches senescence earlier, loses leaf canopy earlier, and quits taking up soil nitrate earlier in these situations. Leaf loss allows sunlight to penetrate and warms the underlying soils earlier than normal. This extends the time for optimum microbial activity, including the nitrification process. Thus, soil nitrates accumulate, especially if there is no rain or irrigation to move the nitrates deeper into the profile.
Summary
Soil nitrates often accumulate under high temperatures and drought conditions. Soil nitrate accumulations may be due in part to lower yields, less nitrogen uptake, and less water percolating through the profile. Don't dismiss the nitrate contribution from soil organic matter mineralization in these situations - whether under irrigated or non-irrigated conditions. The nitrification rate is related to the rate of degree day accumulation, both of which are more rapid under higher temperatures.
Soils are "alive" and their activity changes with changing weather. Not all of the changes in the nitrate soil test are due strictly to our fertilizer applications or lack of applications. In the words of a famous agronomist, "Man, you ain't raisin' those crops on styrofoam - you're raisin' ‘em on soil".
References:
Honneycutt, Cw.W., L.G. Potaro, and W.A. Halteman. 1991. Predicting nitrate formation from soil, fertilizer, crop residue, and sludge with thermal units. Journ. of Env. Quality. 20:850-856.
Lamb, J.A., G.A. Peterson, and C.R. Fenster. 1985. Fallow nitrate accumulation in a wheat-fallow rotation as affected by tillage system. Agron. Journ. 49:1441-1446.
Smika, D.E., A.L. Black, and B.W. Greb. 1969. Soil nitrate, soil water, and grain yields in a wheat-fallow rotation in the Great Plains as influenced by straw mulch. Agron. Journ. 61:785-787.
Texas AgriLife Extension. 2011. TexasET Network. Lubbock Weather Station data, http://texaset.tamu.edu/ accessed 08/11/11.
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