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Crops: Seedbedby Keith ReidKeith Reid is a Soil Fertility Specialist for the Ontario agriculture ministry, based in Stratford.
Crops: The Lynch FilePat Lynch CCA (ON) is head agronomist for Cargill in Ontario.June/July 2000 What's wrong with my soybeans?Some tips to help you with do-it-yourself field diagnosis on yellowing plantsby KEITH REIDYour beautiful green field of soybeans has suddenly started to sprout yellow leaves, and you would like to sort out the cause. But field diagnosis is a complex task because many problems share similar symptoms. Even worse, we tend to look for the problems we are most familiar with, so a pathologist will see only disease, and an agronomist may see only nutrient deficiencies or herbicide damage. Careful observation, though, can help you sort out the possible causes, or at least help you describe the symptoms clearly to an experienced agronomist. It is important to remember that the symptoms you see are only the plant's reaction to what is happening to it, and may not be the root cause of the problem. A nutrient deficiency, for example, may be caused by a lack of nutrients in the soil, by poor root growth in a compacted soil, or by reduced root volume from a disease or insect feeding.The two main things you will need to look for are where on the plant the yellowing is occurring, and the pattern of yellowing on the leaves. Of course, having an accurate field history makes the job much easier as well. The following is a key to some of the most common causes of leaf discolouration, with some hints on how to confirm whether you have identified the problem correctly. It is not a complete list, but should be a starting point. Yellowing primarily on bottom leaves * Entire leaf is yellow. General yellowing of the leaves, gradually spreading from the bottom towards the top, usually indicates a nitrogen deficiency. The most common cause of N deficiency in soybeans is nodulation failure, so check the roots for nodules. If there are few or no nodules, an application of nitrogen fertilizer may be required. If nodules are developing, the plant will most likely grow out of it.Yellowing primarily on top leaves * Leaf blade is yellow while veins stay dark green. This distinctive pattern is diagnostic of manganese deficiency. You can confirm this by spraying some manganese sulphate solution on a small area. The plants should green up with 24 hours.Yellowing in middle of plant * Leaf blade is yellow while veins stay dark green. This indicates that the plant had suffered from manganese deficiency at an earlier stage, and has now grown out of it. This commonly occurs when there is a stretch of cool, dry weather, followed by a rainfall. The moisture encourages the mineralization of more manganese out of the organic matter in the soil.Entire plant shows yellowing * Whole leaf is yellow. Sulphur deficiency will cause a general yellowing over the whole plant, but it is uncommon in Southern Ontario. This diagnosis you would definitely want to have confirmed by tissue analysis.Stratford-based Keith Reid is Crop Fertility Specialist with the Ontario agriculture ministry. Email: keith.reid@omafra.gov.on.ca February 1, 2000 Micronutrient of the Month - ZincBy KEITH REIDThe last micronutrient in the alphabet is zinc, but that does not mean it is any less important than the others. Zinc is one of the most commonly used micronutrients, and while its reactions in the soil are not as complex as some other nutrients, there are still some fascinating twists.
In the plant:
Deficiency Symptoms: Legumes (including soybeans, alfalfa) - thin, short stems, pale green or bronzed foliage with yellow mottling. Tree fruits and strawberries - chlorosis of young leaves; blind bud, little leaf and rosetting (small basal leaves forming on short terminal and lateral shoots). Onions - stunted growth with twisted yellow-striped foliage Zinc toxicity is very uncommon in Ontario, but can occur on acid soils that have received high rates of zinc from industrial emissions. Soybeans are most likely to show toxicity, but there is a wide range of sensitivity among varieties.
In the soil: Zinc is much less soluble at high pH, so availability is reduced where soil pH is above 7.5. The areas most commonly showing deficiencies are eroded knolls where there is a combination of low organic matter, high sand content, and high soil pH. Deficiencies may also occur in organic (muck) soils. High levels of phosphorus can interfere with zinc uptake, although the exact mechanism is not known. In one study at Ridgetown, corn grown on a high P soil which also received a high rate of high P starter fertilizer showed zinc deficiency. In this case, adding zinc to the starter fertilizer did not increase corn yields, but reducing the starter rate did. The soil test for zinc is fairly reliable. It combines a measure of the amount of zinc in the soil with the soil pH to give an index of zinc availability.
Correcting deficiencies:
To get the biggest bang from zinc, ask for a pure zinc sulfate fertilizer, and make sure it is used up within a few days of blending to avoid caking problems. January 1, 2000
Micronutrient of the Month - ManganeseBy KEITH REIDManganese, this month's micronutrient, is unique in many ways. It is subject to many transformations in the soil, so that the amount of manganese in the soil has almost nothing to do with whether there is a deficiency or not. It is the one nutrient where foliar application is the best method, and the only micronutrient that should be used more frequently than it is.
In the plant:
Deficiency Symptoms:Manganese toxicity is not common, but it can occur in acid soils where the solubility of manganese is high. Specific symptoms occur in Red Delicious apples, where excess manganese causes raised "measles" under the bark. Otherwise, the symptoms are impossible to separate out from the other effects of low soil pH.
In the soil: Aeration of the soil also plays a part. Manganese is more soluble in anaerobic (saturated) soils, than in well aerated soils. This shows up occasionally in winter wheat as healthy, green plants in the tire tracks from the fertilizer spreader, while the rest of the field is yellow and stunted from manganese deficiency. The compaction in the tracks was just enough to make enough manganese soluble to keep the plants healthy. The amount of organic matter also influences manganese availability. Soils very low in organic matter, especially sands, contain little manganese. At the opposite extreme, in muck soils, the organic matter ties up the manganese so tightly that it is unavailable to plants. It is not uncommon for cereals to show deficiencies in muck pockets within a field, especially in the first few years after tile drainage.
Correcting deficiencies: December 1999
Micronutrient of the Month - CopperBy Keith ReidThe second micronutrient, in alphabetical order, is copper. Copper was in the news a few years ago when it was discovered that large parts of the prairie wheat-growing area responded to copper application. It soon became apparent that this situation did not exist in Ontario, and interest died down, but it is still useful to keep copper in mind.In the plant: The main functions of copper are to act as catalysts for enzyme systems, including photosynthesis, and in solidifying cell walls. Deficiency symptoms show up as a lack of firmness in plant tissues, either at the growing point or in storage tissues. Limited translocation of copper will occur from young tissues, but stops once the tissue is fully mature.
Deficiency Symptoms: Excess copper can be a problem with livestock eating forages, particularly sheep. Sheep, unlike other livestock, cannot excrete copper from their liver. If there is too much copper in their diet it will build up and eventually they will die from cirrhosis of the liver. Since copper is routinely added to hog and poultry diets, care should be taken feeding forages from soil that has received hog or poultry manure. In the soil: Most of the copper in the soil solution is present as a positively charged ion, similar to calcium or magnesium, so it will be held on the soil colloids by cation exchange. A significant portion, however, is held in chelated form by the organic acids associated with humus. These chelates are thought to have a large impact on the availability of copper to plants. If there is too little organic matter in the soil, the copper will get tied up on the soil minerals. If there is too much, the chelates will bind the copper so tightly that plants can't pull it out of the soil. This means that deficiencies can occur in either very low organic matter soils, or in muck soils. It is rare that organic matter is so low that copper deficiency would occur, but the place to look for it is on sandy, eroded soils with high pH. Much more common is copper deficiency on muck soils, particularly when they are first drained. These soils developed in environments that were low in copper to start with, and then further deplete the supply by holding on to it.
Newly developed muck soils should receive high rates of copper (14 kg/ha of Cu) for the first three years. After that, and on mineral soils, copper requirements should be determined by tissue testing. Soil tests for copper are unreliable. If a deficiency has been confirmed, copper sulphate or copper chelates can be broadcast or foliar applied. Caution must be used with foliar copper sulphate, because it will burn foliage if the concentration of the spray is too high.
November 1999
Micronutrient of the month - Boronby Keith ReidMicronutrients spawn far more questions than any other fertilizer ingredient, far out of proportion to the number of micronutrient deficiencies that occur in the field. This doesn't mean that micros aren't important, but it does mean that a clear understanding of where micronutrients fit is essential for making profitable use of these materials. To this end, I will devote the next few columns to discussing the various micronutrients, starting with Boron.Boron, its exact role in the plant, its behaviour in the soil, is the least understood of all the nutrients. The impact of a lack of boron, however, is striking. In the plant, the main functions of boron appear to lie in cell division and elongation, and in cell-wall integrity. Deficiency symptoms show up most commonly at the growing point of the plant as reduced growth, and in storage tissues as cellular breakdown. Boron does not readily translocate within the plant, so symptoms can be quite localized. Deficiency symptoms can be identified as follows:
Rutabaga - hollow centre, brown watery areas (water core);The requirements for boron are very few and many plants are sensitive to excess boron. In general, the grass family (including cereals and corn), soybeans and edible beans have low boron requirements and can be injured by boron applications. Toxicity symptoms include bleaching of the leaf tissue and browning of the leaf margins. Adequate boron for one crop may be excessive for another. In the soil, boron exists in the solution as a weak acid, so weak, in fact, that most of it does not break down into separate ions. This means that most boron, unlike any other plant nutrient, does not carry any electrical charge in the soil. Since it is the electrical charges that are responsible for holding ions in the soil, boron is free to move with the soil water and is quite subject to leaching. This leads to low boron content in gravelly or sandy soils. The other pool of boron in the soil is found as part of the organic matter. Boron released from decaying organic matter is an important source for growing crops. In dry conditions, this breakdown slows down. Boron deficiency may therefore occur. It is rare, for example, to see boron deficiency in first-cut alfalfa, but much more common in second or third cuttings. Pinning boron down as a culprit in poor crop growth is frustrating because the problem comes and goes with the weather. This irregular supply also makes boron soil tests unreliable. Tissue analysis is a much better indicator of boron availability, and even that will vary from year to year.
If a boron deficiency has been identified, it can be corrected with either broadcast granular fertilizer or foliar fertilizer. Broadcast applications of 1.0 to 2.0 kg/ha of actual boron per year will overcome a boron deficiency in alfalfa. DO NOT apply boron in a band at seeding, because the increased concentration can lead, even in tolerant crops, to seedling injury.
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