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Boron Role and Deficiency
Boron is absorbed by plants as the H3BO3 ion. Boron is essential in carbohydrate metabolism and in auxin regulation. The amount of available boron is highly correlated to soil organic matter. Boron deficiency can be shown in heart rot of sugarbeets, head drop in sunflower, and yellows in alfalfa. Deficiency can result in growing point death in flax, mustard and bean. Deficiency of Boron has been observed in North Dakota in sugarbeets under dry soil conditions. North Dakota soils are generally high in boron, owing to their origin from shale derived sediments high in boron. Boron deficiencies observed in North Dakota are mostly environmentally induced and not caused by low soil levels.
Boron deficiencies can be verified by soil and plant analysis, however soil boron levels necessary for adequate nutrition need to be higher in dry surface soils than in soils which have adequate moisture.
Iron
Iron is absorbed by plant roots as Fe3 (ferrous iron) and then rapidly reduced to Fe2 (ferric iron) once inside the plant. Iron nutrition is decreased with pH greater than 7.0 which can cause precipitation of insoluble iron compounds. Wet soil conditions, and high soil bicarbonate (HCO3) levels associated with high soil carbonate levels are also important factors in decreasing iron availability to plants.
Iron is a part of many enzymes and is required for photosynthesis. Iron deficiencies appear first as yellowing of younger leaves. The yellowing is mostly interveinal, but can also affect the entire leaf. Different crops have varying tolerance for iron availability. Varieties within each crop vary in the tolerance to iron availability. Broadleaf plants tend to increase availability by releasing hydrogen ions and phenolic acids from the roots, decreasing pH levels surrounding the immediate root zone, and also by reducing Fe+3 to Fe+2 which is about a million times more soluble, a reaction possible only in a low root pH environment. Grass plants tend to release organic chelating compounds called phytosiderophores, which increase the availability of iron in the nearby soil. Efficient crops and varieties have greater ability to modify the root environment than less tolerant crops and varieties.
Iron deficiencies in North Dakota are caused by high soil pH, accompanied by high soil bicarbonate levels which tend to buffer the efforts of plants to acidify their root environment. Iron deficiencies also appear to be aggravated by any type of environment which restricts root growth, such as low soil temperature and the presence of high soil salt levels.
Deficiencies of iron can sometimes be verified with plant analysis, but it is not conclusive. Iron soil tests are not reliable because most iron deficiencies are pH related. Although present in soil, iron is therefore often low in availability to plants in North Dakota. It is also difficult to take a plant sample without contaminating it in some way with soil or high iron water. The plant tissue must be carefully washed with deionized water to remove soil, then handled in such a way to prevent contamination with iron compounds during processing.
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