CULTIVATION PRACTICES

1.         SOIL PREPARATION

Soybeans require a well-prepared, fine, weed-free seedbed for good germination. Large clods, furrows or ridges must be avoided in order to ease the planting operation and ensure a good stand.

Herbicides are also more effective in a fine well-prepared seedbed.

2.         PLANTING TIME

Early planting does not have the same beneficial effect on yield as it does in crops such as maize. In fact, in very hot areas with a high number of daily heat units it is important not to plant too early. It will merely stimulate excessive vegetative growth which will later lead to lodging problems without any yield advantage.

On the other hand, very late plantings result in insufficient vegetative growth, a low pod height and lower yields.

3.         CONDITIONS AT PLANTING

Soybeans should emerge within 5-7 days after planting. Slow germination leads to weak seedlings and low yields.

The following factors should be taken into consideration in this regard:

a)         Soybean seed must imbibe moisture equal to at least 50 percent of its own weight before it will germinate – compared with 30 percent in the case of maize.

Soil moisture at planting must therefore be adequate.

b)         Soil temperatures of at least 18ºC favor rapid germination.

c)         Planting depth will depend on soil type.  A depth of 3-5 cm is recommended.

d)         In the event of a heavy shower of rain after planting which may possibly result in a hard crust, the soil must be loosened somewhat before emergence to prevent breaking of the “neck” (a rotary cultivator may be used 3-4 days after planting to break up a soil crust and has the added benefit of controlling germinating weeds). However, extreme care should be exercised during this operation so as not to damage the germinating seedlings while loosening the soil.

4.         SPACING

Soybeans must be planted close together in order to achieve A good plant population, Maximum pod height and a good yield

A plant population of 100,000-200,000 plants per acre should be aimed at with a yield optimum at around 160,000 for high potential rain fed plantings. Low potential dry land plantings may require a population of 100,000, while high potential irrigation plantings may be more suited to populations of 180,000. Optimum yields with late planting may be obtained with plant populations as high as 200,000. 

Seed requirements for the projected plant population will vary from 20 -40 kg/acre. A spacing of 45cm x 25cm for single seeds and 60cm x 10cm for two seeds.

The later planting takes place, the higher the planting rate should be as late planting results in shorter plants and lower pod height. A higher plant population tends to increase pod height.

 

5.         FERTILIZATION

The soybean plant has a strong tap-root system and is able to utilize nutrients in the sub- soil very effectively.  It does not respond as dramatically to band-placed fertilizer as maize, but responds well to soil reserves that have built up over a long period through fertilization of previous crops in rotation.

a)         Nitrogen (N)

Although soybeans are heavy users of nitrogen, nitrogen applications are not recommended.  No increase in yield is achieved and it only leads to excessive vegetative growth. The nitrogen supply to soybeans must be ensured by inoculating the seed with the correct nitrogen-fixing bacterium, Bradyrhizobium japonicum, before planting. This enables the plant not only to fix sufficient nitrogen to meet its own requirements, but also to carry over 12-20 kg of available nitrogen/acre to the following crop.

b)        Phosphorus (P) and Potassium (K)

Soybeans remove large quantities of phosphorus and potassium per ton of yield compared with maize.   These nutrients must, therefore be replenished by means of fertilizer application if the soil levels are low.

Band placement of fertilizer is not recommended as soybeans do not readily respond to it and also because soybean seed is very sensitive to fertilizer burn. Soil reserves of these nutrients must be gradually built up by fertilization of other crops planted in rotation. However, where levels are very low fertilizer must be applied before planting. An application of approximately 80 kg super phosphate and 40 kg potassium chloride per acre, broadcast and incorporated, is normally sufficient.

c)         Lime (Ca and Mg)

Lime application is recommended where the soil pH is lower than 4.5-5.0. Although soybeans can be grown in soils with acid sub-soils, the nitrogen fixing bacteria are more effective above a pH of 5.0.

d)        Trace elements

Boron, zinc and iron deficiencies may occur in soils with a pH of 6.0 and higher, while molybdenum deficiency usually occurs in soils with a low pH.

Molybdenum in particular is very important for soybeans – it plays a role in the nitrogen-fixing process as well as in the processing of nitrates within the plant – and seed treatment or foliar spraying with this element is recommended where soil deficiencies occur. For foliar spray at the seedling stage, the recommended rate is 60g sodium molybdate in 80 liter water/acre. Seed treatment should not be necessary as high levels of molybdenum are fixed in certified seed during the production process. However, if a molybdenum seed treatment is used, the amount of soybean inoculant used should be doubled since direct contact with the molybdenum will be toxic to the nitrogen-fixing bacteria.

 

6.         SEED AND CULTIVARS

a)          General

The use of pure, high quality seed from a reliable source is the first prerequisite for successful soybean production.  Seed and seedling vigour, which is so vital for a good plant population, may be affected by many factors and the use of seed from an unreliable or unknown source may therefore be cheaper at first, but eventually lead to a great financial loss.

The increasing trend to keep back farm-saved seed or buy “brown bag” seed is not only agronomically unsound, for reasons mentioned above, but also threatens the breeding of new varieties for commercial release. The long term repercussion of this is that new varieties will cease to be released due to seed companies being unable to recover their costs. Farmers will become dependent on existing varieties and with no future varieties being released eventually yield potential and agronomic qualities will decline.

Important factors which may affect seed and seedling vigour in soybeans are the following:

•  High humidity before or after harvesting favours the incidence of diseases, which reduces vigour.

•  Warm weather before harvesting results in a crinkled seed-coat which favours the development of seed diseases, which reduces vigour.

•  If the seed is harvested too “wet”, the seed-coat is easily damaged which may also lead to mould and reduced vigour.

•  Variations in humidity and temperature during the storage of soybean seed have a detrimental effect on seed vigour.

•  Vigour deteriorates the longer soybean seed is stored.

Various fungal, bacterial and viral diseases may be transmitted on and within soybean seed.  The use of fresh seed, from disease-free lands, which has been properly cleaned and treated, is therefore essential.

b)         Seed Treatment

All soybean seed must be inoculated with the nitrogen-fixing bacterium Bradyrhizobium japonicum before planting, as these bacteria are absent in virtually all of our soils.  The effectiveness of the inoculation depends on the freshness and viability of the inoculant and the method of application. There are two methods of introducing the required bacteria into the soil. The seed may be inoculated with the bacteria or sprayed as a water solution into the seed furrow with the seed at planting (60 liters water/acre).  Full particulars regarding the method of application are given by the suppliers but special attention should be given to the following:

•  Inoculation must be done in a cool place and in the shade.  The bacteria are very sensitive to sunlight.

•  Methyl cellulose is the recommended sticker.   Skimmed milk and sugar solution may also be used.

•  Do not dilute the inoculant with too much liquid as it may cause the seed coat to shrink and burst.  Just enough liquid must be added to moisten the seed slightly – approximately 500 ml/50 kg seed.

•  Seed must be planted as soon as it is dry enough and within a day of treatment.

Do not, therefore, treat more seed than can be planted within a day.

• On soils where soybeans are being planted for the first time, it is recommended that the normal dose of bacterial inoculant be doubled.

•  Where molybdenum seed treatment is required, it must be applied in the form of sodium molybdate, which is less toxic to the bacterial inoculant.

The effectiveness of the inoculation may be easily determined by inspecting the roots of the soybean plant during the growing season.  N-fixing nodules easily come away from the roots and are pink to red in colour when dissected. Nodules that are white in colour are inactive, while green nodules once fixed nitrogen but are no longer active.

Active nodules are pink to red in colour when dissected

7.         IRRIGATION

Heavy irrigation during the vegetative growth stage is not recommended as it stimulates unnecessary stem and leaf development which may lead to lodging and disease problems.

The most critical moisture requirements of the soybean plant are as follows:

a)        Planting

The soybean seedling is very sensitive to breaking its “neck”, and needs adequate moisture to germinate.  Irrigation at planting, or 3-4 days later, will encourage rapid germination and prevent possible crust formation.

b)        Flowering

Although this stage is not as critical as in the case of maize as soybeans flower over a longer period, adequate moisture at flowering will ensure that the maximum number of flowers will be fertilized and produce pods.

c)         Pod-filling

The most critical stage is the pod-filling stage. Adequate moisture is necessary to avoid pods being aborted and to maximize the number of seeds per plant and the size of the seeds – all factors affecting the ultimate yield.  The soybean plant removes ± 30 percent of its potassium and 40 percent of its phosphorus requirements from the soil during the pod-filling stage. Maize, on the other hand, has removed all its potassium and 70 percent of its phosphorus and nitrogen requirements at this stage.

d)        Seed-filling

Stress during late grain-fill can reduce yields by as much as 30%.

8.         WEED CONTROL

At the correct plant spacing an effective canopy may be obtained 5-6 weeks after planting.  Weed problems after this period are unlikely if a good plant population is maintained.

Effective weed control is necessary at and shortly after planting to protect the seedlings:

·         Good seedbed preparation and the use of a rotary cultivator 3-4 days after planting will control young germinating weeds and at the same time prevent a soil crust.

·         Pendimethalin and trifluralin can be incorporated prior to planting.

·         A wide range of pre- and post-emergence herbicides are registered for use with soybeans.

·         The rotary cultivator may be used until the seedlings are approximately 15 cm high.  In order to minimize plant damage, this operation should be carried out during the warmest period of the day.

·         Should mechanical cultivation be necessary between the rows, ridging must be strongly avoided as it may complicate mechanical harvesting.

9.         INSECT CONTROL

·         Soil insects such as cutworm, snout beetle and wireworm may affect the plant population and it is important that these pests are controlled prior to or shortly after planting by means of a bait or spray.

·         African (formerly American) Bollworm and Loopers often occur and damage the leaves and pods. Spraying with registered contact insecticides when the pests are noticed usually gives good control.

These control measures should be considered as standard practice.

·         Aphids and Red Spider occur sporadically, particularly during warm, dry periods and can be effectively controlled with the correct registered chemicals.

·         Damaged plants and/or pods are more easily infected with disease and it is important, therefore, that control measures are implemented timorously.

10.       DISEASE CONTROL

i)       Soybean Rust occurs in the whole of Uganda. It is a fungal disease that is mainly spread by wind. New infections do not start from seed but rather from windborne spores that have over-wintered on other leguminous plant species.

Infected plants have lesions containing rust pustules on the leaves. Infected leaves usually turn yellow and drop off prematurely. Infection results in fewer filled pods per plant, fewer seeds per pod and smaller seeds. Losses of 35% are common if this disease is left untreated. Symptoms are usually only seen post-flowering.

Fungicides from the triazole chemical group are very effective in controlling soybean rust.

Soybean rust symptoms on a leaf are easily distinguished by the sporulating pustules on the underside of the leaf

ii)      Soybean Mosaic Virus is transmitted on infected seed, by aphids and also mechanically by implements which have come into contact with infected material.

Infected seed is usually discoloured, with the discolouration the same colour as the hilum. The discolouration originates from the hilum and spreads over the rest of the seed coat.   Infected seed coats do not necessarily mean infected seedlings. Infection is dependent on the environment and the severity of the seed infection.

Infected plants may have discoloured leaves, which typically are thickened, deformed or dwarfed and also stay green long after the normal plants have matured. The leaves appear crinkled with a green mosaic and can have a yellow margin. Pod set is usually poor.

No effective control measures against this disease exist except to ensure that disease-free seed is planted.

Symptoms of soybean mosaic virus on soybean seed

iii)     Bacterial Blight is particularly prevalent during wet seasons and when diseased plant residues have not been incorporated properly.  Symptoms vary from small yellow spots on the leaves to large yellow spots or angular, dead lesions with a yellow margin. Leaves of infected plants drop early. This disease seldom causes serious economic losses.

 Leaf symptoms of bacterial blight

iv)     Sclerotinia Stem and Root Rot is a fungal disease characterised by the sudden wilting and death of the plants, usually after flowering.  A coarse white fungal growth can be seen on the roots or lower parts of the stem when the infected plant is pulled out of the soil.

Procymidone can be applied from flowering in fields with a history of Sclerotinia. Crop rotations with non-host crops, e.g. maize and sorghum, can also help to decrease disease incidence.  There are a large number of susceptible crops which include sunflowers, dry beans, groundnuts and many vegetables.  Fresh organic matter coming into contact with the stem or roots aggravates the disease as this acts as a food source for the fungus.

 Soybean infected with Sclerotinia showing sclerotia on the stems

v)      Damping-off is caused by a number of different fungi and can cause large reductions in plant stand and subsequently yield loss. Seed stored for a long period has reduced vigor and is more prone to fungal infection. Damping-off as a result of cool, wet soils is usually caused by Pythium spp. Another fungus that can cause severe seedling loss is Fusarium. Any other seed borne disease can reduce the stand but is usually not as severe as the above two fungi.

 

vi)     Purple Seed Stain is characterised by a pink to purple discolouration of the seed.  It is caused by a fungus and occurs under wet conditions.  Usually the discolouration only appears in the form of small spots on the seed, but under severe conditions the whole seed surface may be discoloured which may affect grading. Planting infected seed can result in seedling blight. The incidence of this disease increases where soybeans mature in wet weather conditions. Certain cultivars are fairly resistant to this disease.

Soybean seed exhibiting symptoms of purple seed stain

C)      HARVESTING

·         Harvesting must commence when most of the leaves have been shed, but while the stems are still pliable.  At this stage only a few of the pods will shatter and the kernels will not be dry enough to break yet.

·         The combine must cut the plants as close to the soil surface as possible in order to prevent too many pods remaining behind.

·         The maturing period is fairly short and the availability of adequate harvesting facilities is thus imperative, especially where unfavourable weather conditions may be expected during this period.

D)      UTILISATION

a)          Grain

The following factors are taken into account during the grading process:

•      Presence of noxious weed seeds e.g. Datura. No such weeds are allowed.

•      Moisture content.

•      Presence of foreign material.

•      Presence of mechanically damaged kernels, including small fragments, broken or split kernels and diseased, immature and discoloured kernels. Broken or split kernels, although initially sound, are subject to chemical change and fungal infection and must therefore be avoided.

b)          Livestock Feed

Soybean grain is an excellent source of protein and may be used for all types of livestock.  The average analysis of soybean grain is as follows:

Protein                                 38%

Carbohydrates                  34%

Oil                                          19%

Fibre                                     5%

 Minerals                             4%

It is therefore a highly concentrated feed.

In the case of cattle and sheep, soybean grain may be fed in its natural form, but for pigs and poultry the grain must be boiled or heat-treated before use. Soybeans contain a trypsin inhibitor which is detrimental to mono-gastric animals.   During the boiling process this constituent is broken down and neutralised.

The whole plant may be cut after pod formation and used as hay.   For this purpose planting must take place earlier to promote maximum leaf and stem growth.

Milled soybean must not be stored for long periods.  As a result of the high oil content the feed becomes rancid and unpalatable. Even grain stored under warm conditions for a long time undergoes the same undesirable changes.

c)          Rotation

The soybean is an excellent rotation crop for maize, rice and grain sorghum. Apart from the beneficial effects in reducing disease incidence, it carries over 12-20 kg/acre available nitrogen to the following crop, which represents a significant saving in nitrogen fertilization. It must be remembered though, that soybeans are very sensitive to atrazine damage and cannot be planted after maize if this herbicide was used during the previous season without testing for carry-over. The waiting period before planting soybeans after atrazine use is 18 months.

Soybeans must not be rotated with crops such as dry beans, sunflower, groundnuts or a large number of certain vegetables as all these crops are infected by Sclerotinia sclerotiorum and may result in a build-up of this pathogen in the soil.

d)          Silage

Soybeans and especially the forage types, make excellent silage when combined with maize.  (Ensile maize and soybeans at a ratio of 2:1).  When planting for silage, plant at the same time or before maize. Plant alternative strips of the same area of maize and soybeans.   This will ensure the correct ratio of 2:1 maize/soybean in the silage. When ensiled at this ratio no additional sugar in the form of molasses is required.