Nitrogen (N)
The addition of nitrogenous fertiliser is not recommended except when plants fail to nodulate.
Soybean seed should be inoculated with its own specific strain of rhizobium before planting, particularly in areas where the legume crop has not been previously planted.
Nodulated plants should not be lacking in nitrogen, and will rarely respond economically to nitrogen application at any growth stage.
Phosphorus (P)
The Mycorrhiza fungi in soils play an important role in plant uptake of phorphorus.
The symbiotic relationship of the fungi and the plant root is known as VAM (vesicular-arbuscular mycorrhiza).
VAM fungi colonise plant roots and grow out into the soil, effectively acting like an extension of the root system. Nutrients, especially P and Zn, have been shown to become more available to the plant due to VAM. The fungus relies on live plant roots for its food and during a long, bare fallow, mycorrhizae will die out in the soil.
Soybeans have a medium dependancy on VAM resulting in potential losses of 40 - 60% if mycorrhiza are not present in the soil. In low VAM situations, such as after fallows of 12 months or more, large responses can be expected from phosphate fertiliser, particularly where soil bicarbonate P-levels are below 20 mg/kg. In high VAM situations, such as when double-cropping or after short fallows of less than six months, responses to phosphate fertiliser are only likely if the soil bicarbonate P is below 10 mg/kg.
Potassium (K)
Like most crops, soybeans require large amounts of K to achieve good yields and so soil K-status is important. Critical soil test values for K in soybeans are being developed, but at this stage soils that have low exchangeable K-levels prior to planting (i.e. <0.3 meq/100g) are likely to respond to K-fertiliser application. In coastal areas with sugarcanedominant cropping systems, K is usually one of the main nutrients required for good soybean growth during a fallow.
Soybeans are capable of accumulating large amounts of potassium in the plant material (100-150 kg K/ha), and if the soil K-status is good, this accumulation may be well in excess of the minimal requirements for growth and yield. Regardless of how much K is in the plant material though, soybeans are a crop that removes a lot of K in harvested grains (on average, 20 kg K/t). Therefore, in high yielding irrigated crops removal rates can be high and the resultant K-fertiliser rate to maintain soil K-status can be significant.
The main K fertilisers are generally muriate of potash (KCl) and sulfate of potash (K2SO4), with both obtained from naturally occurring mineral deposits, although animal manure can also contain significant amounts of potassium (1-2% on a dry-weight basis) if they have not been leached too heavily. The KCl form of K fertiliser is a lot cheaper per kg of K applied, but K2SO4 has advantages of also containing 18% S and not containing chlorides.
The key issue with the chlorides in KCl is not the amount applied (in many areas, more chloride falls naturally in rainfall each year than is applied in KCl fertiliser). Rather, the ready solubility and high resulting concentration of dissolved ions from a KCl band result in a ´salty´ solution that can cause poor crop establishment (if too close to the seed) or cause damage to plant roots for a short period until the fertiliser dissolves and moves through the soil.
Sulfur (S)
Consider applying sulfur fertilisers when double-cropping because the available sulfur levels can be severely depleted. Using single superphosphate to apply the above phosphorus rates will alleviate any potential sulfur deficiency.
Alternatively, use a starter fertiliser containing zinc or sulfur such as Phosul, Phozinc and Starter Z. Low rates of sulfate of ammonia can also be used to correct a sulfur deficiency in situations where soil P-levels are already considered adequate.
Zinc (Zn)
A foliar spray of 1-2 kg of zinc sulfate heptahydrate plus 1 kg of urea in a minimum of 50 L of water/ha plus a wetting agent will correct a mild zinc deficiency.
Apply one to two sprays within four weeks of emergence. Broadacre zinc or Teprosyn Zn (Phosyn) as a seed treatment is a cost-effective method of applying zinc in situations where soil P-levels are adequate but zinc levels are deficient.
Agrichem recommend that broadacre zinc be applied at a rate of 5 L product/t of seed. This would supply in excess of the crop´s zinc requirements. Apply the broadacre-zinc treatment to seed first and allow to dry before applying the inoculant. Other seed treatments such as Teprosyn Zn (Phosyn) are also readily available. Read the label carefully before use. For further information on products and management issues, check with Incitec and Phosyn.
Molybdenum (Mo)
Molybdenum plays an important part in nitrogen fixing and so is an essential nutrient. Soils most commonly Mo-deficient are light textured (sandy) and acidic. These soils are particularly common in coastal sugarcane areas.
Apply 50 g of Mo/ha for two consecutive soybean crops. Mo is usually applied as a seed dressing of molybdenum trioxide (60% Mo) or Mo superphosphate (commonly 0.025% Mo). Do not use sodium molybdate or ammonium molybdate as seed dressing because they are toxic to inoculant rhizobia.
Nutrition for organic soybeans
While organic certification organisations generally do not permit the use of processed inorganic fertilisers, the information above is still relevant to organic growers to understand the crop’s nutritional requirements.
Both organic and conventional farming results in a net outflow of essential plant nutrients from the farm. It is therefore important that managers address this by budgeting to replace these nutrients. With organic fertilisers, this can be more costly and more difficult. Check for the types of naturally occurring fertilisers allowable under the standards of each organisation. Submit a soil test to a reputable laboratory and use trained personnel to interpret results.
Soil organic matter
Organic matter levels in cropping soils are often depleted. Improve soil organic matter levels by applying manures and composts and incorporating green manure crops and crop stubble. This will help retain moisture in the soil profile and encourage more soil microbial activity and diversity, all resulting in improved nutrient availability.
Nutrient deficiencies can cause economic-yield loss well before the more obvious deficiency symptoms in the plant become evident. Nutrient imbalance can occur in lighter acid-soils that are low in organic matter and have a history of superphosphate use.
Nitrogen (N)
See conventional soybean information for nitrogen.
Phosphorus (P)
Where soil levels of plant-available P are low, P in the form of soft-rock phosphate (e.g. guano), and reactive-rock phosphate are acceptable for organic standards. These products are obtainable in most districts. VAM is known to improve availability of soil P to the crop, as in conventional soybeans.
Potassium (K)
See conventional soybean information for potassium. Growers should check the suitability of the different products for their management objectives.
Sulfur (S), zinc (Zn), and other trace elements
If the soil is deficient in sulfur the cheapest form for agricultural-use is gypsum. Organic certification bodies will accept mined natural gypsum, as opposed to phosphogypsum which is a manufacturing by-product. Gypsum is available in most areas.
Zn availability is often a limiting factor in soybean production, especially in soils with pH 7.0 or higher. Some products (e.g. zinc sulfate heptahydrate as a foliar spray, zinc sulfate monohydrate for soil application) are permitted by some certifier organisations.
One to two foliar sprays will be needed within four weeks of emergence. In acidic soils, lower than pH 5-6, manganese toxicity and iron chlorosis (yellowing) can affect crop yields. Check with individual certification organisations on the types and forms of trace elements permissible under their standards. Growers should consider liming low pH soils to avoid nutrient imbalance problems.
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