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Growing Soybean
crop nutrition advice
Everything you need to know about soybean fertilisation, best practice, suitable products, field trials and more.
Crop nutrition for growing soybean (Glycine max)
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Soybean is an annual crop belonging to the family Fabaceae (Leguminosae).
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Soybean plants have one or more stems where the trefoils develop. Flowers form in the nodes, and these flowers will generate the pods which contain the grains or seeds.
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Plants have a pivot root system with a depth of up to 1.80 m, although 60 to 70% is at 0.15 m depth. Roots possess nodules where biological nitrogen fixation occurs.
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The general content of the grains is oil: 17% to 22%, protein: 38% to 45%, and carbohydrates: 30% to 35%.
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The crop development stages are defined as the vegetative stage (described as VE to Vn) and the reproductive stage (described as R1 to R8).
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Soybean requires well-drained loamy soil for optimum growth. In compacted soils, the hypocotyl can break easily during emergence, if under pressure, and the root system will not be deep.
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The crop development will be better between 20°C and 35°C. Temperatures below 15°C can cause impaired germination and emergence, with a potential increase in diseases. Temperatures above 40°C have an adverse effect on the growth rate, cause disturbances in flowering, and decrease the plants' capacity to retain pods. These problems are accentuated further when combined with water deficits.
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To achieve high soybean yield, water demand is around 700 mm of rainfall. The ideal distribution of rainfall should be 200 to 250 mm (35%) during the vegetative growth phase, and 450 to 500 mm (65%) during the reproductive phase.
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Lack of water during flowering (dry spells) dramatically reduces yield, especially in sandy soils and in areas where the root system is superficial due to calcium deficiency or aluminium toxicity deeper in the soil. In the Brazilian Cerrado, there are reports of losses ranging from 10% to 70% depending on the number of days with a lack of rain.
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Dry weather is necessary for ripening.
Young soybean plants
Soybean crop rows
Nutrient requirements
Requirements in temperate region, Mollisols, USA
| Part of plant | N kg/t | P kg/t | K kg/t | Ca kg/t | Mg kg/t | S kg/t | B g/t | Cu g/t | Fe g/t | Mn g/t | Zn g/t |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Grains (offtake) | 59 | 11.5 | 2.6 | 11.6 | 2.6 | 3.4 | 32 | 11.6 | 72 | 28 | 42 |
| Other parts | 23 | 3.2 | 29 | 31.6 | 12.6 | 2.3 | 64 | 7.2 | 181 | 92 | 59 |
| Total (uptake) | 82 | 14.7 | 52 | 34.5 | 15.2 | 5.7 | 97 | 18.8 | 253 | 120 | 101 |
| % offtake | 72% | 78% | 44% | 8% | 17% | 60% | 33% | 61% | 28% | 23% | 41% |
Source: Bender et al., 2015, USA. Average for 3 varieties and yield 3.4 t/ha.
Requirements in tropical region, Oxisols and Ultisols, Brazil
| Part of plant | N kg/t | P kg/t | K kg/t | Ca kg/t | Mg kg/t | S kg/t | B g/t | Cu g/t | Fe g/t | Mn g/t | Zn g/t |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Grains (offtake) | 54 | 4.8 | 18 | 2.8 | 2.5 | 2.8 | 31 | 11.5 | 65 | 39 | 41 |
| Other parts | 24 | 2 | 30 | 1.3 | 8.2 | 1.4 | 51 | 8.3 | 310 | 159 | 34 |
| Total (uptake) | 78 | 6.8 | 48 | 22.1 | 10.7 | 4.2 | 82 | 19.8 | 375 | 198 | 75 |
| % offtake | 69% | 71% | 37% | 13% | 24% | 66% | 38% | 58% | 17% | 20% | 55% |
Source: Embrapa (2020), Brazil. Average for 5 varieties and yield 3.4 t/ha
Dynamics of soybean nutrient uptake over a crop season
Soybean is been grown in the most varied types of soil in the world, for example, Molisols and Alfisols (neutral to alkaline, with high fertility) to Oxisols and Ultisols (acidic, with low fertility). It is important to correctly manage the fertility of each type of soil to achieve profitable production for the producer.
The nutrient offtake is very similar in different regions, with the exception of P which is higher in grains grown on richer soils.
Dynamics of nutrient uptake
- Increment of uptake during pod and grain development
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- N, P, Ca and Mg: 50-55% of uptake until R4; 45-50% after
- S: 45% until R4; 55% after
- K2O: 50% until R2; 50% after
- There is possibly a higher response to split fertilisation in sandy soils (especially with K2O and S), with part applied in a band at sowing and the remainder as a top dressing 2-3 weeks later.
- Another possibility is to use fertilisers that provide prolonged availability of nutrients.
Source: Bender et al., 2015, USA. Average for 3 varieties and yield 3.4 t/ha.
Role of nutrients
Nitrogen
Nitrogen is the most important plant nutrient for soybean crops and it is the nutrient that is required in the largest quantity. 80 kg of N is needed to produce 1,000 kg of soybean. However, most of soybean’s N requirements are met by nitrogen from the air through the symbiotic fixation by rhizobium bacteria in the nodules on the plant’s roots. The biological N fixation can supply all the N required by the soybean plant, but due to price concerns, it is normal to apply some N fertiliser. It is important that this quantity does not surpass 20 kg N/ha.
Phosphorus
Phosphorus’ function in plants directly relates to energy storage. Phosphorus is essential for the absorption of nutrients from photosynthesis and has a role in the buildup of organic compounds with high energy value. For soybean, this means the oil.
Potassium
Potassium is important for the construction and transport of carbohydrates, and also assists in reducing diseases. A sufficient supply of potassium helps support healthy plants.
Calcium, magnesium and sulphur
Calcium, magnesium and sulphur are important nutrients for root growth, photosynthesis, and the formation of the grains, respectively.
Molybdenum
Molybdenum is a constituent of the enzyme nitrogenase which is found in nitrogen-fixing bacteria. Together with cobalt, it is essential for efficient nodulation and biological fixation of nitrogen. Supplying these micronutrients has become a basic practice on most soils where soybean is grown.
Manganese
Special attention should be given to manganese, as this is increasingly becoming one of the micronutrients that is most often deficient in soybean crops. The reduction in the uptake of manganese can be due to the use of Roundup-Ready (also known as RR, or glyphosate-tolerant) varieties, or due to excessive liming.
Boron
Boron has an important role in pollen germination and the growth of the pollen tube. The symptoms of boron deficiency may not always be apparent, leading it to be referred to as ‘hidden hunger’, and yet boron deficiency will have an adverse effect on grain production, and therefore yield.
Nutrient deficiencies
| Nutrient | Description | |
|---|---|---|
| Nitrogen | Total chlorosis of older leaves followed by necrosis. Nitrogen deficient crops will have a low protein content in the grains. Absence of molybdenum (Mo) will lead to nitrogen deficiency. |
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| Phosphorus | Plants with reduced growth, low insertion of pods, and older leaves with a bluish-green colour. | |
| Potassium | Internerval chlorosis, followed by necrosis on the edges and apex of old leaves, due to the formation of putrescine; the plants show the appearance of being scorched by fire, or damaged by herbicides. Small, wrinkled and deformed grains with low vigour and low germination rate. | |
| Calcium | The cell membrane becomes “porous”, breaking cell compartments, and losing the membranes' selectivity to only absorb desirable ions; growth points are affected, both roots and shoots; symptoms appear in the younger parts of the plant, atrophying the root system, killing the apical bud; there is a delay in the emergence of primary leaves and when they emerge they become cup-like in shape - this is called wrinkling. The petiole also collapses due to the disintegration of the cellulosic wall. These symptoms usually occur in acidic soils and are associated with Al and Mn toxicities. | |
| Magnesium | The older leaves show internerval chlorosis (light yellow) with pale green veins. | |
| Sulphur | Uniform chlorosis which is similar to N deficiency but occurs in young leaves, whereas chlorosis due to N deficiency starts in older leaves. |
Sources: Embrapa, Brazil; IPNI; IPI
Key aspects in soybean nutrition and fertilisation
Soybean requires good nutrition for optimum growth and development. Lack of availability of nutrients results in abnormal or sub-normal plant growth, with plants showing typical nutrient deficiency symptoms as described above.
Soybean is a high-protein crop and requires a large quantity of nitrogen (N) to synthesise amino acids and protein, in the region of 80 kg N/t grains. The main source of nitrogen for soybean crops is biological nitrogen fixation (BNF) where atmospheric nitrogen (N2) is converted in the root nodules into ammonium (NH4+).
To improve the biological nitrogen fixation (BNF) and remove the need for nitrogen fertiliser, farmers must inoculate the soybean seeds with Bradyrhizobium (minimum 1.2 million viable cells per seed).
The ideal soil pH for soybean is from 5.5 to 7.0. Acidic soils will require liming to raise the pH of the soils above 5.5. Liming also provides calcium and magnesium and raises the pH neutralising the toxic effects of aluminium, while increasing the efficiency of fertilisation and BNF.
Soybean root systems need good levels of calcium in the soil profile. Soybean has a low tolerance to high concentrations of aluminium in the soil. To reduce or eliminate aluminium toxicity, and increase calcium in deeper soil layers (>20 cm), gypsum must be used (CaSO4), or fertilisers that contain gypsum in their composition.
Potassium and phosphorus must be available in the soil. In general, soil P levels must be 20 mg/dm3 or more. In slightly acidic to alkaline soils, where the more commonly used P extractors are Bray-1 and Olsen, the P2O5 rates vary from 20 to 90 kg/ha as a function of the amount of P in the soil. On the other hand, in more acidic soils with a greater capacity for fixing P by iron and aluminium oxides, the main extractant is Mehlich-1, and the nutrient doses are higher, at least 60 kg/ha of P2O5, reaching values more than 150 kg/ha. In these cases, it is recommended to part apply P prior to planting (as a corrective) and apply the remainder during sowing. Water-soluble fertilisers must be used to maintain the yield potential of the crop. The main sources of P are single superphosphate (SSP), triple superphosphate (TSP) and MAP.
Potassium levels must be above 120 mg/dm3. The main source of K is KCl, but there are some alternative sources that supply K and other macronutrients (for example ICL’s Polysulphate which supplies K, Ca, Mg and S). Application rates vary according to the amount of K in the soil, but they typically range from 50 kg/ha to 120 kg/ha of K2O.
In areas where gypsum is not applied to reduce aluminium toxicity, sulphur should be applied together with PK fertilisation. In Oxisols and Ultisols, the suggested dose is from 3 to 5 kg/ton of grain produced. In Mollisols, Alfisols and soils with high organic matter content, there is usually no economic response from sulphur application. However, it is suggested to apply sulphur for crops such as alfalfa and maize in fields with a history of low productivity at doses from 10-15 kg/ha of S.
Calcium and magnesium are normally supplied by liming. However, in Oxisols with a lower CTC, a positive response is achieved from the use of water-soluble fertilisers containing these nutrients. The suggestion would be to provide 15-20 kg/ha of Ca and 5-10 kg/ha of Mg.
Including micronutrients in the fertiliser plan, including molybdenum (Mo) and cobalt (Co), supports biological nitrogen fixation. In acidic Oxisols and Ultisols, the main deficiencies are boron (B), copper (Cu), molybdenum (Mo), manganese (Mn), and zinc (Zn). These can be applied with seeds, to the soil, or as a foliar fertiliser. In Alkaline soils, the main deficiencies are iron (Fe), manganese (Mn), and zinc (Zn).
Soybean Trials
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