Farm Machinery and Equipment-I 3(2+1)

Lesson 23. Seed metering devices for solid drilling and single farming, Furrow openers used in drills and planters

Factors affecting Germination and Emergence:

• Factors affecting seed emergence rate are influenced by:

• Viability of seed (% germination under controlled laboratory conditions

• Soil temperature

• Availability of soil moisture to seeds

• Soil aeration

• Mechanical impedance of seedling emergence (resistance of soil to penetration by seedling)

These are influenced by:

• Soil type

• Physical condition of the soil

• Depth of planting

• Intimacy of contact between seeds and soil

• Degree of compacting of soil above the seeds

• Formation of surface crusts after planting

• Final field stand is also due to disease, insects, and adverse environmental conditions.

Field emergence rate of 80% to 90% are typical for corn and other crops that tolerate a fairly wide range of planting conditions.

Field emergence with sugar-beets and many small seed vegetable crops is so low and unpredictable i.e. 35% to 50%.

Functions of Seed Planter:

            Seed planter is required to perform following mechanical functions:

1) Open seed furrow to proper depth

2) Meter the seed

3) Deposit the seed in the furrow in an acceptable pattern

4) Cover the seed and compact soil around seed to the proper degree for type of crop involved

5) Planter should not damage the seed enough to appreciably affect the germination

6) Seed should be placed in the soil in such a manner that all the factors affecting germination and emergence will be as favorable as possible.

Effect of Planter or Planting System upon Emergence Factors:

1) Good planter performance is essential for obtaining an adequate stand with crops whose emergence is critical.

2) Precise depth control, placement of seeds into moist soil and non-crusting conditions above the seeds are important for small seed vegetables and some other crops.

3) Packing of soil by planter can affect the availability of moisture, availability of oxygen and mechanical impedance.

Devices for Metering Single Seeds:

a) Devices having cells on a moving member (cells sized to accommodate one seed or group of seeds) e.g. Horizontal plate planter.

• Horizontal plate planter has a spring loaded cut-off device that rides on top of the plate and wipes off excess seeds as the cell moves beneath it.  A spring loaded knock-out device pushes the seeds from the cells when they are over the seed tube.

• Plates with round or oval holes are used for drilling or hill dropping of various row crops.

• Edge-cell, edge-drop plates are used for planting relatively large, flat seeds like corn.

• Inclined plate metering devices have cups or cells around the periphery that pass through a seed reservoir fed under a baffle from the hopper, lift the seeds to top of plate travel and drop them into delivery tube. A stationary brush is provided for more positive unloading. Seeds are handled more gently than with horizontal plate unit because there is no cut-off device.  The metering unit has an edge-cell plate with sizes available to fit various kinds of small vegetable seeds.  Plate and surrounding ring are accurately machined to provide uniform cell sizes for precision metering.

• Vertical-rotor metering devices are used for precision planting of vegetables and sugar-beets.  In some units seed tube is omitted and rotor placed as low as possible and discharge seeds directly into the furrow.  These units also have seed cups which move up through a shallow seed reservoir, pick-up single seed, carry them over the top of the circle and discharge them during the downward travel.

• Cells in a Belt is also another type of precision metering device in which seeds are fit to size.  Seeds from hopper enter the chamber above the belt through opening and are maintained at a controlled level.  As belt moves clockwise, counter-rotating seed repeller pushes back excess seeds so there is only one in each cell.  Seeds in cells are conveyed over the base and discharged from belt beneath the seed repeller wheel.  Lack of positive unloading device causes some variability in seed spacing.

• Single seed metering devices that do not have cells are used for all type of corn seeds.  In this twelve spring-loaded cam operated fingers on radial arms rotate, gripping one or more seeds as they pass through the seed reservoir.  One seed is released as each finger passes over to small indents near top of stationary disk.  As finger continues to rotate it throws the remaining kernels into one of 12 cells in the adjacent, rotating seed wheel and seed wheel discharge the individual kernels into furrows.

• Pneumatic (air-pressure) Metering System has a centralized hopper and metering unit that serves 4, 6 or 8 rows.  The ground driven seed drum has one circumferential row of perforated seed pockets for each planter row.  A shallow reservoir of seed is automatically maintained in the drum by gravity flow from the hopper.  A PTO driven fan supplies air to the drum, maintaining a pressure of about 4 kPa (0.6 psi) in the drum and in the hopper. Air escapes through holes in the seed pockets until a seed enters the pocket. Differential pressure holds the seeds, the revolving drum carries a stationary brush near the top that knocks off any excess seeds. Air-cutoff wheels on top of drum momentarily block the holes, causing seed to drop into seed-tube manifold.  Air flow through the tube carries the seeds to the planting units and deposits them in the furrows. This is generally used for crops such as corn, beans, grain sorghum.

  • Pneumatic principle is also used for single row metering devices for unit planters. Small blowers driven by electric motors connected to the tractor’s electrical system provide air-pressure in metering chamber.  Seeds are held against the holes in the rotor. Seed pockets are carried upward and around in a counter-clockwise direction in the unit.  The seeds are released into seed tube when pockets pass a baffle that cutoff the inner air pressure to the front (left hand) portion of the rotor.  Different disc rotors are used for different kinds of seeds.

• Vacuum Pick-Up Devices are also available for seed metering.  In this there is a central vacuum pump with valves to each pick-up orifice, seal between stationary piping and rotating pick-up assembly.  A stationary cam extends the piston to produce pressure for unloading the seeds and a spring retracts it to develop the vacuum for seed pick-up. Vacuum pick-up devices can perform effectively with small, irregular shaped seeds like lettuce. These are sensitive to dust and dirt.

  • The average spacing of seed and hill is determined by ratio of linear or peripheral speed of seed pick-up units (cells, fingers etc) to the forward speed of planter and by the distance between seed pick-up units on the metering unit. Changing the speed ratio is the most common method of changing seed spacing.

• Seed Tape Planting System is the precision planting system.  Seeds are deposited either singly or in groups (hills) on a water soluble table in a laboratory under controlled conditions.  Equipment is available for single out and spacing small, irregular shaped seeds on tape with high degree of accuracy. The seeds are placed on the tape at the desired field spacing and a continuous strip of tape is un- released and placed beneath the soil by a simple planting unit.  The tape is a polyethylene oxide that is stable under normal atmosphere conditions but dissolves in one or two minutes when placed in most soil.  It is used for planting lettuce, tomatoes, cucumbers and some other vegetable crops.  The tape is expensive and good soil preparation is imperative.  A large amount of tape per hectare is needed especially for close spaced rows i.e.20 km/ha for 51 cm row spacing Precise depth control is difficult to maintain but planting can be done at relatively high forward speed.  Seed spacing in the row is pre-determined when seed tape is made and is precise in the field. Increased yields are reported with this for lettuce and cucumbers. A special planter that cuts the tape into single seed sections and deposits these sections in cone-shaped pockets are pressed into the soil.  An arrangement is provided to meter a charge of non-crusting soil amendment (vermiculite) into each pocket to cover the tape. Perforations along one side of seed tape synchronized the depositing of seeds on tape and cutting the tape into sections by planter.

Precision Planting:

            It means accurate spacing of single seeds in row, precise control of planting depth, especially for shallow planting of vegetable crops, and creates a uniform germination environment for each seed.

            The primary objective of precision planting is to obtain single plant spaced far enough apart so that thinning can be done mechanically or with a minimum of hand labour. So, with precision planting:

a) Reduce thinning cost by doing thinning in less time

b) Reduce shock to the remaining plants during thinning

c) Maturing of crop is more uniform and thus increases the feasibility of non-selective   

harvesting of crops such as lettuce

Hence, principal requirement for precision planting with a cell type metering devices are:

a) Seeds must be uniform in size and shape, preferably about spherical

b) The planter cells must be of proper size for seeds.  Plates and other critical parts of metering device must be accurately made.

c) The seeds must have adequate opportunity to enter the cells.  Plate speed and exposure distance of cells in hopper are basic parameters, with low speed.

d) A good cut-off device is needed to prevent multiple cell fill without causing excessive seed breakage.

e) Unloading of seeds from cells must be positive.

f) The seeds must not be damaged enough to appreciably affect germination.

g) The seeds must be conveyed from the metering unit to the bottom of furrow in such a manner that spacing pattern produced by the metering device is maintained.

h) The seeds should be placed at proper depth in a furrow with minimum of bouncing or rolling in the furrow.

Factors Affecting Cell Fill and Seed Damage:

Percent cell fill is defined as the total number of seeds discharged divided by the total number of cells passing the discharge point.  According to this definition 100% cell fill does not mean that every cell contains a single seed but implies that any empty cells are offset by extra seeds in multiple fills. The most uniform seed distribution is obtained with combinations of seed size, cell size, and cell speed that give about 100% cell fill.  Percent cell fill is influenced by:

1) Maximum seed size in relation to cell size

2) Range of seed sizes

3) Shape of seeds

4) Shape of cells

5) Exposure time of a cell to seed in the hopper

6) Linear speed of cell

• Effect of cell speed appears great for rough surfaced seeds than for large, smooth seeds like corn.

• Double seeds mostly occur at low speed with small seeds than with large seeds.

• In general cell diameter or length should be 10% more than maximum seed dimensions and cell depth should be equal to average seed diameter of thickness.

• Performance is improved by grading seed within close size tolerances.

• Cell size less critical with pneumatic metering devices and not a factor in vacuum pick-up devices.

Controlling Seed between Metering Devices and Furrows:

            Precise metering is when seeds are controlled so that each requires same time from meter to furrow.  Variation in drop time can be minimized by one of the following:

a) Having short, smooth, small-diameter drop tube with discharge and close to bottom of furrow.

b) Discharging the seed directly from the metering device within a few centimeters of furrow bottom.

c) Mechanically transferring the seed from the metering unit to the furrow as is done with transfer wheels on some hill-drop planters.

d) Slow cell speed or trajectory shaped seed tubes for high plate speeds minimize bouncing.

e) Seed movement in furrow can be minimized by having narrow and imparting rearward velocity component to the discharged seed to partially offset forward velocity of planter.

f) Improved uniformity by angling seed tube 15⁰ to 30⁰ from vertical.

g) High downward velocity increase seed bouncing and displacement in furrow.