Soybean crop nutrition

AGRONOMY AND PRODUCTS

Soybean crop nutrition

More than 330 million tonnes of soybean are grown globally every year and processed to yield oil and meal, a major source of animal protein. Its cultivation in Brazil, the US and other countries requires large applications of potash and phosphate. The nutrient needs of this major oilseed crop are reviewed.

PHOTO: UNITED SOYBEAN BOARD

Soybean (Glycine max.) is a bushy, green legume species native to East Asia. Now widely grown in the Americas, it produces an edible bean prized as a source of high-protein meal and oil.

The pod-producing plant is related to clover, peas and alfalfa. It is typically planted in the late spring, each plant producing up to 80 pods in the summer on flowering. Individual pods contain 2-4 pea-sized beans which are rich in commercially-valuable protein and oil.

Crop nutrition – redefining soybean success

Until relatively recently, almost 80 percent growers in the top five US soybean-growing states were not applying either phosphorus or potassium to support their soybean crop, according to USDA research. That was largely because, prior to 2015, only sparse research was available on the importance of crop nutrition for soybeans – particularly the importance of micronutrients.

That is no longer true, however, thanks to more plentiful information on how modern soybean varieties – which are capable of producing far greater seed yields – are acquiring and using nutrients. This includes pioneering doctoral research work published by Dr Ross Bender, now Mosaic’s director for new product development, and colleagues at the University of Illinois. Indeed, advances in plant genetics and agronomy have allowed farmers in Brazil and the US to achieve record soybean yields in recent years.

These record-breaking soybean harvests are being accompanied by the ever high uptake and removal of secondary nutrients and micronutrients such as sulphur, magnesium, boron and zinc. This is making the need for season-long macronutrient and micronutrient availability greater today than ever before. Fortunately, fertilizer producers have responded to this challenge by developing innovative, new fertilizer products and designing sophisticated, tailored fertilizer programmes for soybean.

Nutrient uptake and plant growth

Soybeans are known for their efficient use of residual soil nutrients, although modern, high-yielding varieties require more careful nutrient management and higher nutrient inputs.

Proper and well planned crop nutrition is known to be one of most effective ways of influencing both soybean yield and quality. Fertilizer management, and the alleviation of soil acidity, for example, generally have commercially valuable and positive effects on the oil and protein levels of soybeans. Maintaining soil fertility also protects soybean plants from environmental stresses such as weather, disease and nematodes.

In soybean production, soil pH has been singled out for its influence on soil fertility and plant growth. Soybeans thrive at soil pH between 6.0 and 6.8, and both nutrient uptake and yield are maximised at this pH range.

Soybean plants remove nutrients from the soil in large quantities from emergence until the point of maximum accumulation is reached at the pod-filling stage around 75 days later. After this point, plants mobilise their accumulated internal stores of nutrients from vegetative parts to the grain.

Nutrient uptake is highest at 45 days after plant emergence during the start of soybean flowering. Soybean plants will have taken up around half of their total nutrient requirement at this point. The following 30-day interval between flowering and pod-filling is the critical period for soybean crop quality and yield. Several factors such as drought, nutrient deficiency, pest attacks and diseases may dramatically reduce yields during this stage in the plant’s growth cycle.

Nitrogen-fixing

Soybean is a nitrogen-fixing crop with an ability to fix as much as 175 kilograms of nitrogen per hectare. It therefore requires little, if any, mineral nitrogen, although minor application to seedbeds is commonly advised. High soil nitrogen levels may even be counterproductive as they can cause excess vegetative growth, reduce nitrogen fixation, increase disease pressure and delay plant maturity.

Fig. 1: Typical soybean macro nutrient removal rates, Brazil (kg/t)

Soybean plants are able to satisfy their large nitrogen requirement (Figure 1) by fixing the majority of this from the air. They do this via nodules formed on their root system by Rhizobium bacteria. Soybean seeds require inoculation with this bacterium to promote nodule formation and, as a consequence, ensure good nitrogen supply.

High potassium demand

Soybeans also require large amounts of potassium (Figure 1), especially during the period of rapid vegetative growth. Potassium has a major effect on both yield and quality of the crop and is therefore essential for healthy, high-yielding plants. For certain soils, potassium application rate has been shown to correlate with both yield and seed oil content.

As well as being vital for vegetative growth and pod and seed formation, potassium also:

• Reduces pre-harvest pod shedding
• Improves seed quality by keeping the numbers of shrivelled, shrunken, mouldy and off-colour beans to a minimum
• Promotes root nodulation and nitrogen fixation from Rhizobium bacteria
• Improves transpiration by reducing water loss from the leaf
• Helps minimise the effects of frost in prone areas.

Phosphorus benefits roots, yield and quality

As with many crops, phosphorus availability is important for good root development and crop establishment. Under certain soil conditions, applications rates have been shown to correlate with yield, seed oil content and seed protein content. Phosphorus is involved in:

• The development of roots
• The production of root nodes and hence nitrogen fixation ability
• The movement and uptake of other nutrients
• Plant growth and maturation
• Seed numbers, seed size and seed germination.
• Phosphorus, together with potassium, can also limit damage from several plant diseases.

Fig. 2: Typical soybean micronutrient removal rates, Brazil (g/t)

Other nutrients such as magnesium, sulphur and iron are also required during photosynthesis and maintain good growth.

Calcium has a direct influence on crop yield. It strengthens cell walls and is involved in pollen tube growth and pollen germination. It is also an essential nutrient for flower impregnation, flower bud fixation and pod formation. Deficiency causes the shedding of flowers and pods.

Sulphur helps optimise yield and quality and is involved in the formation of nitrogen-fixing nodules on soybean roots. Sulphur availability is directly linked to the quality of harvested seeds, as it promotes oil formation and helps makes oil easier to extract.

Micronutrients maximise yields

As growers aim for ever higher soybean harvests, the likelihood of nutrient deficiencies holding back yield improvements also increases. This means soybean farmers are now having to look beyond ‘the big three’ – nitrogen, phosphorus and potassium – in their fertilization plans. Indeed, micronutrients, particularly zinc, boron and manganese, are known to be yield-limiting. Iron, manganese and chlorine removal rates for soybean are notably high (Figure 2).

Boron is required for pollen tube growth and pollen germination, and also ensures good fruit set. Boron-deficient plants show poor pod fill and as a consequence produce small, poor-quality seeds. This element also promotes nitrogen-fixation and counteracts aluminium toxicity. Foliar applications of boron and manganese help to ensure consistently high yields, especially for intensive cultivation in poor soil conditions. Manganese is involved in chlorophyll formation and can helps increase seed protein content. It also improves disease tolerance. Zinc enhances photosynthesis.

Balanced fertilization

Nutrient management for soybean firstly requires soil testing of macro-nutrient and micronutrient levels. Soil pH is also an important consideration because of its influence on nutrient availability, as The Mosaic Company notes:

“As soil pH increases, the availability of phosphorus (P), zinc (Zn) and iron (Fe) decreases. Although variety selection can help manage iron deficiency in soybeans, fertilizer application is still needed to address the P and Zn deficiencies prevalent in high-pH soils.”

One of the main formulations offered by Mosaic for soybean is zinc-fortified MicroEssentials SZ (12-40-0-10S-1Zn). This combines 12 percent nitrogen, 40 percent phosphorus and 10 percent sulphur with one percent zinc (Fertilizer International 478, p 24). With a balanced mix of nutrients in every granule, MicroEssentials SZ is able to maximize soybean yields, according to Mosaic, by counteracting the influence of pH on P and Zn availability.

Soybean production and consumption

World production and consumption of soybean oil is second only to palm oil (Fertilizer International 479, p14). Soybean meal is also the world’s largest protein source for farm animals, being a major feed ingredient for chickens, pigs and cattle.

Soybean was one of the first crops to be cultivated agriculturally, having originally been grown for food in China nearly 6,000 years ago. In recent decades, soybean growing, processing and trading has turned into a multi-billion dollar global industry – and a cornerstone of world farming and agricultural trade – because of the millions of livestock it feeds across the planet.

Global soybean production grew almost ten-fold during the 50-year period between 1960 and 2010. World production has expanded by almost 20 percent in the last six years, increasing from 283 million tonnes in 2013/14 to reach 337 million tonnes in 2019/20 (Figure 3). Approximately 75 percent of that total is ultimately destined for animal feed.

Fig. 3: World soybean, meal and oil production, 2013/14-2019/20

Soybean can be successfully grown in tropical, subtropical and temperate climates. Global production is concentrated in seven main growing countries, Brazil, the US, Argentina, China, India, Paraguay and Canada (Figure 4). Of these, three countries in the Americas, Brazil, the US, and Argentina, collectively account for more than 80 percent of global output. Furthermore, the US and Brazil also dominate the soybean export market, being responsible for over four-fifths of world trade. Argentina has adopted a different approach, choosing to process much of its domestic soybean harvest at home, and export soybean meal and oil in large tonnages instead.

China is the world’s biggest soybean importer by a large margin, and the main destination of Brazilian exports. Thanks to its importance to global agriculture, this trading route even has a name: the Brazil-China soybean pipeline. The 97 million tonnes of soybeans imported into China in 2019/20 represents 60 percent of global trade in this commodity. By processing these imports, China has also become the world’s largest soybean meal and oil producer (Figure 4).

The EU is the biggest importing region for soybean meal globally (18 million tonnes in 2019/20), and also imports large tonnages of soybeans (16 million tonnes). Southeast Asian countries (Vietnam, Indonesia Thailand and the Philippines) collectively imported a further 16 million tonnes of soybean meal in 2019/20.

Fig. 4: World soybean, meal and oil production and exports, 2019/20 (milion tonnes)

Traditionally, in corn-soybean crop rotation systems, enough nutrients are provided at the start of the corn crop to cover both its needs and the needs of the subsequent soybean crop. But a series of University of Illinois trials commissioned by Mosaic suggest this approach may cause soil nutrient mining and leave insufficient nutrients for the soybeans.

“We think that growers are not adequately fertilizing phosphorus because they don’t know how much the corn is removing, and they’re not actually fertilizing their soybeans,” says Dr Fred Below, a lead scientist on the Illinois trials. “That’s what we’ve demonstrated in our trials over the past few years.”

He continues: “This leads [on] to the idea of balanced crop nutrition. For some reason, when the potassium is adequate, the plant seems to use the phosphorus better – it can squeeze another bushel or two out of it.”

Mosaic’s trials show that nutrient removal rates for soybean, especially for potassium, can be as high if not higher than for a standard corn crop.

“Compare a 60-bushel soybean crop [4 t/ha] to a 230-bushel corn crop [15.5 t/ha]: That corn crop is going to remove about 80 pounds of P2 O5 [36 kilos], while the soybean crop is going to remove about 40 pounds [18 kilos],” comments Dr Matt Clover, former research manager at Mosaic. “But when we look at potassium, that 230-bushel corn crop is going to remove about 58 pounds of K2 O [26 kilos], and that soybean crop is actually going to remove about 75 pounds of K2 O [34 kilos].”

Manganese is one of the most common micronutrient deficiencies in soybean, and a particular problem in high-pH soils. Soil alkalinity, in turn, may be caused by calcium, magnesium and iron soil imbalances. Manganese deficiency is most acute in high organic matter soils during cool spring months when soils are waterlogged, and symptoms often disappear as soils dry-out and soil temperature rises. Mosaic suggests several ways of correcting manganese deficiencies:

• Keep soil pH below 6.5 if liming is causing the deficiency
• Mix a soluble form such as manganese sulphate (MnSO4 ) with the starter fertilizer and apply in bands, as a high-phosphorus starter fertilizer helps move manganese into the plant
• Correct field deficiency symptoms with a foliar application.

Please see our previously published article (Fertilizer International 481, p33) for further information on fertilizer products for soybean and manufacturer recommendations.