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Seedbed by Keith Reid The Lynch File by Pat Lynch January 2005 Probing the mysteries of what's happening under the snow : There is much unknown Understanding the processes that happen during the winter season will be necessary to account fully for nutrient flows and predict optimum fertilizer rates more accurately by KEITH REID In last month's column, I discussed the physical changes that happen to soils during the winter months. There are also chemical and biological transformations occurring, although at a much slower rate than during the warmer months. These have not been investigated as thoroughly as the physical transformations for two reasons. First, the common assumption is that the rate of any reaction is slow enough that it doesn't have any significant effect. Second, it is extremely difficult to study what is happening in frozen or snow covered soil, and also difficult to determine if any analyses reflect what was happening in the frozen state or the impact of thawing. Two factors cause a slowing of chemical and biological processes in the soil during winter. The first is simply cold temperatures. As a general rule, the rate of any reaction is cut in half for every 10-degree reduction in temperature, so the rate of a reaction in a soil at 2 C will be about one per cent of the same reaction in a soil at 22 C. The second factor is the lack of liquid water because the water gets tied up in ice. It is a common misconception that all the soil processes stop when the soil is frozen. This is not strictly true since there is still some liquid water held within the small pores, even in a soil that appears to be rock hard. It is correct, however, to say that things happen very slowly in a frozen soil. The freezing of most of the soil water has the effect of increasing the concentration of ions in the water that remains. This will increase the reactions of these ions with soil minerals, so potassium and ammonium, in particular, can be "fixed" within clay minerals. This reaction is reversible, so these nutrients will be available to plants eventually, but not as quickly as if they had remained on the surfaces of the minerals. Soil organisms mediate many of the processes in the soil, and these slow down significantly as the weather gets colder. The larger organisms go dormant first, so earthworms and insects will enter a dormant state long before the soil begins to freeze. Dew worms burrow deep into the soil, below any frost, and remain there until spring. Bacteria and fungi also slow down, so the breakdown of crop residue comes to an almost complete halt during the winter. Not all processes slow at the same rate, however, so mineralization can be happening at a slightly higher rate than nitrification, leading to an accumulation of ammonium in the soil. This is quickly re-assimilated into microbes, or converted to nitrate, in early spring. In soils that do freeze, many of the soil organisms will also freeze. This results in large declines in populations of bacteria, as the cell walls are pierced by ice crystals forming in the cellular fluids. The long-term impact on populations is small, as the remaining bacteria multiply rapidly when the soil warms up, but it does have the effect of releasing soluble carbohydrates and nitrogen compounds into the soil solution. There are lots of mysteries about what happens in the soil during winter and early spring. Recently, measurement of gas emissions in early spring showed a huge flush of nitrous oxide during thaw -- up to 40 per cent of the total yearly release in a few days. It is not yet clear whether this is the result of the sudden release of gas that had been accumulating slowly over winter below a frozen layer, or if there is a sudden flush of denitrification by bacteria that are taking advantage of a smorgasbord of cellular fluids that have leaked out of dead cells. Winter is a unique aspect of soil management in this part of North America. You don't have to travel very far south to find agricultural areas where frozen soils are rare, or don't last more than a few days at a time. Understanding the processes that happen during this season will be necessary to account fully for nutrient flows and predict optimum fertilizer rates more accurately. It will be a puzzle to occupy soil scientists for years to come. BF Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture and Food, based in Stratford. Email keith.reid@omaf.gov.on.ca © copyright 2005 AgMedia Inc.. top January 2005 Make your motto "rotation, rotation, rotation" You must rotate crops, tillage, and pesticides. But if you understand the principle of rotations and the limitations of poor rotations, you can get better yields from a less than ideal rotation by PAT LYNCH The three most important things for a successful business are location, location, location. For successful crops, they are rotation, rotation, rotation. You must rotate crops, tillage, and pesticides. The ideal rotation has the same crop in the same field only once every seven years. A basic rotation is cereal followed by three years of forages, beans and corn. This rotation could be varied by growing some type of "truck crop," such as turnips, somewhere in the rotation. Following that type of rotation is completely unrealistic in today's tough economic environment. In most of Ontario's cash crop belt, a more common rotation is corn, beans, wheat. This one year in three for each crop is a far cry from the one year in seven. It partly explains why some fields produce close to 200 bu/ac of corn while the provincial average is closer to 130. The same comparison can be drawn for the average yield of beans and cereals. Knowing what you should do and what you must do is different. But if you understand the principle of rotations and the limitations of poor rotations, you can get better yields from a less than ideal rotation. You must make the rotation fit the crop and the soil rather than following a set rotation on every acre. You must know what factors are holding back yield and whether rotating will help these yield reducers. In some cases, you can overcome a poor rotation with management. One reason that continuous corn or beans yield less is because of root diseases. We have made some small strides in using fungicide seed treatments to gain control over some soil-borne diseases. In soys, the main diseases are Phytophthora and Rhizoctonia. Seed treatment gives some protection, but the protection can be counted in days when we need weeks. These diseases tend to be worse on heavy land. This means that rotating beans with corn or wheat is more critical on heavy land than on light land. You also can get soy varieties with genetic resistance to one of these diseases, but not to both. The key here is to know what disease is holding back yields. You can grow continuous corn on a silt soil with much less yield loss than on a heavier soil. In fact, the world's highest corn yield came from continuous corn on a silt loam soil. On the other hand, if you have to grow corn after corn on heavier land, you can overcome some of the "yield drag" by applying extra nitrogen. If you are on medium-textured soils or heavier, using rootworm control helps yield. With the prevalent root diseases, growing wheat after wheat is a high-risk idea. The root diseases that affect wheat are in all soils and you need to have two to three years between wheat crops to let these soil diseases diminish. The end result is that the crop rotation must fit the soil and the crop, not just the farmer. I work with a producer who grows continuous corn on one field, while another field has a three-crop rotation and the next has two years of beans between corn and wheat. He needs corn and his continuous corn on his light soil yields, as well as first-year corn on his heavier soil. His light soil is so "rich" that soys will mould and the yield will be less than on his heavier soils. The rotation principle applies to tillage. It must fit the soil. Some land is going to yield best with mould board ploughing after wheat and corn, while other land can have beans and wheat successfully no-tilled with tillage before corn. There is land that will benefit from primary tillage every six to seven years. And there is land that must be notilled all the time, whether it is in cash crops or trees. It should never see any tillage and maybe should be put back into trees. Rotation also applies to pesticides. You should rotate through herbicide groups. In the future, there will be a need to rotate both insecticides and fungicides. In the horticulture crops, insect and disease resistance is a fact. In these crops, you either rotate or perish. Producers purchase many crop inputs because marketers promote them. The features and benefits are repeated so many times through a growing season that producers eventually buy the product. To bad we couldn't get some marketers interested in selling rotations. BF Pat Lynch CCA (ON) is head agronomist for Cargill in Ontario. © copyright 2005AgMedia Inc. . top back