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Since 2008 I have served as the Precision Nutrient Management Extension Specialist for Oklahoma State University. I work in Wheat, Corn, Sorghum, Cotton, Soybean, Canola, Sweet Sorghum, Sesame, Pasture/Hay. My work focuses on providing information and tools to producers that will lead to improved nutrient management practices and increased profitability of Oklahoma production agriculture
This article is written by Dr. George Rehm, University of Minnesota, Soil Fertility Specialist (retired).
See more of Dr. Rehm’s blogs at http://www.agbuzz.com
During the past two or three years, there has been an increase in the promotion for the use of tissue testing/plant analysis as a management tool in development of fertilizer programs. At times, if you read all of the advertising literature, you might get the idea that the practice of plant analysis/tissue testing is so important that you can’t make a profit without it. So, is this really a new and exciting management tool to be used by every crop producer? A close examination of the facts without all of the advertisement leads to the answer: not really.
There are problems with placing dependence on the use of this management practice. Some of the problems and pitfalls have been identified by Dr. Dan Kaiser, Associate Professor and Extension Soil Scientist at the University of Minnesota. These are briefly described in the paragraphs that follow.
STAGE OF GROWTH at sampling is a major consideration. With corn, for example, it’s impossible to compare analysis of plants sampled at the V5 growth stage with analysis of plants sampled at some later growth stage. As the corn plants grow, nutrient concentration is diluted and concentrations, therefore, decrease. If all other factors are equal, a concentration of nitrogen, for example, may be higher and adequate at V5. The concentration percentage will be lower at V10 and still be adequate. This concept has been verified by substantial research conducted by faculty at Land Grant universities.
In order for tissue testing/plant analysis to be meaningful, the results of analysis of the plant tissue must be compared to some standard. For corn and other crops, these standards have not been developed for every stage of growth. This is usually true for stages early in the growing season. That’s primarily because concentrations are rapidly changing at those times. So, what’s the point of analyzing corn plants at the V5 growth stage if there are no standards for nutrient concentrations at that growth stage? I don’t know. I don’t believe that there is general agreement among researchers knowledgeable about plant analysis as to what the adequate concentrations are in whole plant corn tissue at the V5 growth stage. With corn, accurate interpretation of plant analysis information is possible if plant samples (leaves) are collected at the time of silking.
At silking, however, it’s too late to apply nutrients that might correct a deficient situation. So, analysis of corn leaf tissue at silking cannot be used to predict rates of any nutrients needed during the growing season.
TIME OF DAY used for sampling can also affect concentration of nutrients in specific plant parts.
Research has shown that this is especially true for nitrogen. Nutrients may be more concentrated in plant tissue in the morning; but, as plants grow, the concentration can be diluted by dry matter added during the day that is the result of the normal growth process. This effect of time of sampling just adds to the variability that may be experienced with plant analysis.
HYBRID AND?OR VARIETY can also have a substantial influence on “critical levels” associated with plant analysis. Researchers are finding that the rate of nutrient accumulation is different among modern hybrids or varieties. Therefore, it’s reasonable to expect that nutrient concentration in any plant part at any stage of growth will vary with hybrid or variety. This is yet another source of variability in plant analysis.
It is known that nutrient concentration in plant tissue is affected by stage of growth at time of sampling, time of day used for sampling, and hybrid or variety. There are obviously other factors that contribute to variability in the results of plant analysis.
Many have used the results of plant analysis as an aid in the diagnosis of a problem in a field. Plant analysis was originally developed as a diagnostic tool. When combined with companion soil samples, this tool has helped to solve many problems. It is, however, a stretch to use this practice as a tool to predict the rate of any nutrient that should be applied to any crop. It is simply not a predictive tool that can be used with confidence. There are too may opportunities for error.
While there are several factors that can produce variability in the concentration of nutrients in plant tissue, there is only minimal variability in the laboratory procedures used in the analysis. The analytical procedures have been standardized among laboratories by using “standards” with known concentrations. If there problems with the laboratory analysis, the routine use of these “standards” will identify those problems.
Plant analysis/tissue testing is not something new. The concept has been around for many years. When used appropriately, it has value. However, the ability of this management practice to predict rates of nutrients needed for crop production is now and has been limited. Don’t expect any more than what this practice can deliver.
Dr. George Rehm,
University of Minnesota
Nutrient Management Specialist (retired)
This article is written by Dr. David Mengel, Kansas State University Soil Fertility Specialist.
One question that commonly comes up with continuous no-till operations is: “How deep should I sample soils for pH?” The next common question is: “How should the lime be applied if the soil is acidic and the field needs lime?”
Sampling depth in continuous no-till
First, sampling depth. Should two sets of samples be taken, at different depths?
Our standard recommendation for pH is to take one set of samples to a 6 inch depth. On continuous no-till fields where most or all of the nitrogen (N) is surface applied, we recommend taking a second sample to a 3-inch depth. We make the same recommendation for long-term pasture or grass hayfields, such as a bromegrass field that has been fertilized with urea annually for several years.
Nitrogen fertilizer is the primary driving force in lowering soil pH levels, so N application rates and methods must be considered when determining how deep to sample for pH. In no-till, the effects of N fertilizer on lowering pH are most pronounced in the area where the fertilizer is actually applied. In a tilled system, the applied N or acid produced through nitrification is mixed in through the action of tillage and distributed throughout the tilled area.
Where N sources such as urea or liquid UAN solutions are broadcast on the surface in no-till system, the pH effects of the acid formed by nitrification of the ammonium will be confined to the surface few inches of soil. Initially this may be just the top 1 to 2 inches but over time, and as N rates increase, the effect of acidity become more pronounced, and the pH drops at deeper depths. How deep and how quickly the acidity develops over time is primarily a function of N rate and soil CEC, or buffering capacity.
Where anhydrous ammonia is applied, or liquid UAN is knifed or coulter banded below the surface, an acid zone will develop deeper in the soil, usually 2-3 inches above the release point where the fertilizer is placed in the soil. So if the ammonia is injected 8 inches deep, there will be acid bands 5 to 8 inches below the soil surface. As with long-term surface applications, these bands will expand over time as more and more N fertilizer is placed in the same general area. The graphic below illustrates the effect of a high rate of ammonia placed in the same general area in the row middle on a high CEC soil for more than 20 years.
The actual depth of the acid zone in fields fertilized with ammonia gets tricky as application depth can vary depending on the tool used to apply the ammonia. Traditional shank applicators generally run 6 to 8 inches deep, so a sample for pH measurement could be taken at 3-6 inches or 5-8 inches deep, depending on how deep the shanks were run. The new low-disturbance applicators apply the ammonia 4-5 inches deep. A sweep plow or V-blade applies ammonia only 3-4 inches deep. So sampling depth for pH should really depend on where the acid-forming N fertilizer is put in the soil.
Liming application methods in continuous no-till
Now, where do you place the lime in continuous no-till? If you surface apply N, then surface apply the lime. That’s a simple but effective rule. But remember that surface-applied lime will likely only neutralize the acidity in the top 2-3 inches of soil. So if a producer hasn’t limed for 20 years of continuous no-till and has applied 100 to 150 pounds of N per year, there will probably be a 4-5 inch thick acid zone, and the bottom half of that zone may not be neutralized from surface-applied lime. So, if a producer is only able to neutralize the top 3 inches of a 5-inch deep surface zone of acid soil, would that suggest he needs to incorporate lime? Not really. Research has shown as long as the surface is in an appropriate range and the remainder of the acid soil is above pH 5, crops will do fine.
Liming benefits crop production in large part by reducing toxic aluminum, supplying calcium and magnesium, and enhancing the activity of some herbicides. Aluminum toxicity doesn’t occur until the soil pH is normally below 4.8. At that pH the Al in soil solution begins to increase dramatically as pH declines further. Aluminum is toxic to plant roots, and at worse the roots would not grow well in the remaining acid zone.
This implies that the acid zones from ammonia are probably not a major problem. We have monitored ammonia bands in the row middles of long-term no-till for many years and while the pH got very low, below 4.5, we never saw any adverse impacts on the crop that would justify liming and using tillage to incorporate the lime. In fact, some nutrients such as zinc, manganese, and iron can become more available at low pH, which can be an advantage at times.
Yield enhancement is not the only concern with low-pH soils, however. Herbicide effectiveness must also be considered. The most commonly used soil-applied herbicide impacted by pH is atrazine. As pH goes down, activity and hence performance goes down. So in acid soils weed control may be impacted. We do see that in corn and sorghum production.
Liming products for no-till
When choosing a liming product, is there any value to using dolomitic lime (which contains a large percentage of magnesium in addition to calcium) over a purely calcium-based lime product? On most of our soils in Kansas we are blessed with high magnesium content. So as long as we maintain a reasonable soil pH, there normally is enough magnesium present to supply the needs of a crop. Calcium content is normally significantly higher than magnesium, so calcium deficiency is very, very rare in Kansas. The soil pH would need to be below 4.5 before calcium deficiency would become an issue. Before calcium deficiency would occur, aluminum toxicity or manganese toxicity would be severely impacting crop growth. So producers really don’t have to worry about a deficiency of calcium or magnesium on most Kansas soils.
What about the use of pelletized lime as a pH management tool on no-till fields? The idea has been around for a while to use pel-lime in low doses to neutralize the acidity created from nitrogen and prevent acid zones from developing. There is no reason it won’t work, if you apply enough product each year. Pel-lime is a very high-quality product, normally having 1800 to 2000 pounds of effective calcium carbonate (ECC) per ton, and can be blended with fertilizers such as MAP or DAP or potash easily.
But it is costly. As an example, at a cost of $160 per ton and 1,800 lbs effective calcium carbonate (ECC) per ton, 100 pounds of ECC pel-lime costs $8.80. If it costs $25 per ton to buy, haul, and apply a 50% ECC limestone, that equates to $2.50 per 100 pounds ECC.
If you were applying 100 pounds of urea-based nitrogen, it would take approximately 180 pounds of ECC to neutralize the acidity produced by the N. This would require 200 pounds of 1,800 pound ECC pel-lime or 360 pounds of 50% ECC ag lime. The cost would be around $16 per acre with pel-lime or $4.50 per acre with ag lime. So technically, the pel-lime option is fine. But it would cost more than 3 times as much, at least in this example. You can use your own figures regarding costs and ECC of different lime products available to you to do a similar calculation. Deciding which product to use is a simple economic choice.
Applying N fertilizer to soil will cause the soil to become acidic over time. Placement of the applied N and the level of soil mixing done through tillage determine where the acid zones will develop. Make sure your soil testing program is focused on the area in the soil becoming acidic, and apply the lime accordingly.
Kansas State University
Professor Soil Fertility Specialist