By William L. McKibben
Tissue analysis is a great way to verify soil additive claims. I am sure everyone has heard many claims about using this or that product will release nutrients locked up in the soil. What better way to find out than to use a tissue analysis and plant weights to really see if this is true.
This guide to understanding tissue analysis (from an agronomist’s point of view) is probably a very different approach than that from which a plant physiologist might take.
The place to always start is with the established tissue guidelines. Keep in mind though that many of these guidelines were set up over 40 years ago. Therefore we may need to constantly tweak the numbers to achieve maximum quality and production. I believe that one of the most important steps in tissue analysis is being able to relate the data back to plant weight. Plants adjust their growth based on the least available nutrient and therefore will slow down growth and size to match their nutrient feed uptake. It is for this reason that when good and bad tissue samples are analyzed, they come back from the lab looking very close to the same.
When it comes to plants, bigger is generally better, with healthier and larger plants tending to provide bigger yields. Therefore when sampling tissues, it is imperative to collect the same number of leaves or plants and weigh them. Even though you may not be comparing tissue within a field, you will be comparing them to your healthy, high-yielding crop standard. Figure 87 shows a weight range of corn plants collected at the pre-6 leaf collar stage for whole plants and at the 7-leaf collar stage.
You can see there is wide range of weights within the various maturities of the corn plants collected at the pre-6 leaf collar stages. This range varies as much as 300 percent. This data was collected over one season and tissue analysis was preformed on each composite sample of 12 plants or leaves. The weight range narrows considerably when sampling only the leaves and this sampling probably would not be necessary unless you are making a comparison between good and bad plants.
The range of the nutrients varies about 200% for the whole plants, but narrows somewhat when just looking at the leaves. When examining the data for individual samples the heaviest plants usually do not have the highest nutrient values. The lighter samples have some of the highest nutrient numbers. Therefore comparing all these samples on an equal basis without adjusting for the weight for the whole plant samples is like comparing a 300-pound mini-horse to a 2,000-pound drafthorse. The horse blood analysis would be very comparable, but the level of work that could be done by each horse would be vastly different.
This adjustment by weight is really only practical for crops planted on an individual basis like corn. Wheat could be done, but it would be very time consuming. For turfgrass, tissue numbers could be adjusted by clipping weight. It would not be very practical to collect whole fields, but collecting sample areas and calculating that to the whole field, green or tee could be done without too much problem.
Why sample this early in the crop maturity anyway? Grass crops such as corn and wheat determine some of their yield very early. Corn, for instance, is determining the number of ear kernel rows around the 6-leaf collar stage. It would also be good to sample corn at the tassling stage—another critical time in which ear length is being determined.
Soybean yields are determined during the back third of the season so I tend to hold off tissue sampling until that time unless something is restricting the growth during the earlier vegetative stage. Tissue sampling is best done prior to the critical times in the crop lifecycle.
With a standard soil test the amount of exchangeable nutrients in the soil is known. A paste test tells what is in solution and now we know that the tissue test tells us how well the nutrients are being picked up out of the solution. Using a combination of testing tools such as the standard and paste soil test along with tissue analysis will go a long way toward understanding crop production issues.
Ultimately all crop production issues can come back to nutritional problems. Unfortunately these crop production problems, whether quality or quantity issues, are not always related to the level of nutrients in a soil. Diseases or root pruning caused by insects can easily restrict nutrient uptake too. Also remember that compaction is still the top reason for limiting nutrient uptake.
Figure 88 shows a seed treatment test that I was working on in 2007. Although there were large differences in weights and nutrient levels at 6-leaf collar stage, the yields were essentially the same at harvest with the exception of the outside 6 rows. The outside 6 rows received limited compaction. I have in the past tested the soil in the outside 15 feet and found it to have poorer fertility than the center of the field. The compaction, along with fertility shortages in the center of the field, neutralized the gains set up by the seed treatment. Many of the disease issues in crops are nutritionally related. I am certainly not qualified to discuss this topic in much depth, but the book Mineral Nutrition and Plant Disease written by a number of authors and edited by Datnoff, Elmer and Huber is certainly a book that should be in everyone’s personal library.
Personally I have seen that fungicide applications on soybeans have not yielded much response, especially where a foliar application of manganese was applied to improve plant nutrition.
Source: The Art of Balancing Soil Nutrients by William McKibbin