Soil fertility, pH, texture and nematodes

In a recent Field Crops CAT Alert newsletter, Dr. Warncke submitted an article entitled, “Effects of cutting back on potassium.” After reading that article, I got motivated to write this one. Since virtually all plant-parasitic nematodes spend some time in the soil and it is the medium in which plants grow, I thought I would address the influence various edaphic factors have on nematodes and their impacts on plants.

Of the three major nutrients, potassium seems to be the most critical in reducing the expression of symptoms due to nematode feeding. Potassium plays many roles in the plant, but two important ones are its effects on water relations and on cell walls. With adequate K, cell walls are thicker and provide more tissue stability. This impact on cell growth normally improves resistance to lodging, pests and disease. Potassium is important to minimize stress in plants. Feeding by plant-parasitic nematodes definitely results in stress.

To achieve soybean yields in soybean cyst nematode-infested fields somewhat close or equal to those of non-infested locations, adequate potassium levels are required. In soybean cyst nematode-infested sites, failure to maintain potassium levels will result in more serious symptoms and yield loss. The K levels will have no impact on soybean cyst nematode population densities but will aid in reducing symptoms.

The same is essentially true for root-knot nematodes on their hosts. However, with certain species of these nematodes, population densities of root-knot nematodes will be lower on K-deficit plants than those receiving adequate K levels. Potassium is required for turgor buildup in plants and for long distance transport of water. Since root-knot nematodes disrupt the xylem and phloem of their hosts, the transport of water and nutrients is compromised. Therefore, root-knot infested plants often wilt. Because infested plants do not grow well, the poor growth also deprives the nematodes of nutrition. If the host suffers, the nematodes often suffer along with it.

The bottom line appears to be, if you consider cutting back on your potassium, be sure fields are not infested with damaging nematodes. Even in the absence of nematodes, Dr. Warncke explains yield loss will occur if you reduce or fail to apply K in fields where it is below the critical level. If damaging numbers of nematodes are present, especially soybean cyst nematodes, yield loss can be severe.

Plant-parasitic nematodes can survive in soils over wide ranges of pH. Some nematodes can actually be used as biological indicators of low pH soils. According to Jon Dahl (personal communication) at MSU’s Soils Testing Laboratory, soils in Michigan range in pH from 3 to 10. About 20 percent of all soils have a pH of 8.0 or above but many of these are from urban sites. Roughly, 10 percent of all agricultural sites have a pH greater than 8.0 and these alkaline soils are more common in the thumb. About 10 to 20 percent of all samples tested have a soil pH less than 5. Metals become more mobile in acidic soils potentially leading to heavy metal toxicity. If pathogens are also present, severe problems can result.

Most data support the conclusion that soybean yield increases as soil pH increases. Exceptions occur when soil pH exceeds 7.4 probably due to low availability of iron. However, in studies conducted in Iowa and Wisconsin, it was demonstrated that soybean cyst nematode population densities are often highest in soils of pH 7.0 or higher when compared with areas of soil pH 5.9-6.5. The optimal pH range for soybeans is 6.0-6.5. In addition, the yield advantage of soybean cyst nematode-resistant varieties was greatest in high pH (7.1-8.0) soils (soybean cyst nematode-resistant varieties averaged 9.6 bushels higher over four years than soybean cyst nematode-susceptible ones) than in soils of pH 5.8-6.4 (yield advantage of 1.4 bushels) and soils at pH 6.5-7.0 (yield advantage of 5.6 bushels). The yield advantage of soybean cyst nematode-resistant soybean varieties over susceptible ones generally increases with rising soybean cyst nematode population densities and soil pH seems to play an integral role (Grau, Kurtzwell and Tylka, The Yields II Project).

Soil texture also affects nematode population densities. As a generalization, sandy soils are nematode-loving soils. However, nematodes exist in soils of all textures some even being favored by a more fine texture.

In the early 1990s, when we first began observing symptoms of soybean cyst nematode feeding around Michigan, growers typically noted symptoms were confined to sandy spots or ridges in fields or just their sandier fields. The foundation of this observation is based in the fact that soybean cyst nematodes reach damaging levels more rapidly in sandier locations than sites with fine-textured soils. If there is an early fallacy that was eventually exposed, it is growers who farmed fields with fine-textured soils thought they would be spared from soybean cyst nematode infestations. Not to be.

A study was conducted on campus in the mid-90’s to investigate the role of soil texture on soybean cyst nematode population densities. Not surprising, it was learned that the soybean cyst nematode populations used in this study increased much more rapidly in sand (ca. 90 percent sand) than in loamy sand (83 percent sand) or sandy loam (76 percent sand). For example, in one plot during the first year of the study, the soybean cyst nematode population density went from 250 eggs per 100 cm3 at planting to 90,000 eggs per 100 cm3 at harvest in sand on an soybean cyst nematode-susceptible soybean variety. By the third to fourth year of the study, the soybean cyst nematode population densities were roughly equal over the three soil types. It was also learned in this study that soybean cyst nematodes appeared to survive the winter better in the more fine-textured as opposed to the more coarse-textured soil.

In conclusion, control of plant-parasitic nematodes typically requires an integrated approach. However, there are often no or very few chemical options. In field crops, due to the cost of nematicides, their use is not economically justified. Therefore, cultural tactics should often be the first line of defense for management of plant-parasitic nematodes. The importance of maintaining proper fertility and optimizing soil pH has been emphasized here to reduce the impact of nematodes, particularly soybean cyst nematodes. Whether using conventional fertilizers or composts, manures or municipal sludge, it is critical to keep soil as healthy as possible to mitigate the impact of pathogens and pests.

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