In broadcasting, the only basis for planting is the total seeding rate usually expressed in weight of seeds per unit area. Example: 50-100 kg per hectare for lowland rice. There are no rows and uniform plant-to-plant distances are not considered.
It is possible to calculate the total plant population and the average number of plants per unit area upon germination if the average weight and the germination percentage of seeds are known. Replicated sampling of stand count per square meter or any sample area can also give a good estimate of total population.
In both the hill and drill methods of planting by direct seeding, there is a desired row-to-row spacing. Take mungbean, for example. The distance between adjacent rows may be 60 cm.
If planted by hill, 12 seeds may be sown in 4 hills 25cm apart in each linear meter in a row at 3 seeds per hill. Otherwise, if planting is by drill, the 12 seeds will just be dispersed in one linear meter without considering uniformity in plant-to-plant spacing within the row. With 12 seeds per linear meter (1m = 100cm), the mathematical, but not actual, average for the plant-to-plant distance will be 8.3cm (i.e., 100cm ÷ 12 = 8.3cm).
But if broadcasted, no reference to any distance nor spatial arrangement is necessary. For uniformity, however, average seeding rate should be 20 seeds per square meter or 12 seeds in every 0.6-sq m ground area. In all these planting methods, total seeding rate is constant at 200,000 seeds per hectare.
In the hill method, the hills are either occupied by a single plant or any multiple number of plants and spaced uniformly within each row. Therefore the row distance, hill distance, and number of plants (or seeds) per hill are always given or otherwise computed from a given plant population density.
In contrast, there is no uniform spacing between plants within the row in the drill method. Instead, seeds are released over the intended row in uniform seed weight or number of plants per linear meter.
In Broadcasting. If the required seeding rate is 100 kg per hectare, the seeds have to be broadcated at an average rate of 0.01 kg or 10 g per sq meter. Assume further that the crop to be grown is rice and the seeds to be used for planting have an average weight at 14% moisture content of 30 grams per 1000 grains (equivalent to about 33,333 grains per kilogram). This will be equivalent to an average seeding rate of about 333 grains per sq. meter or a total seeding rate of 3,333,333 grains per hectare. If seed germination is 100%, the same figures will apply to seedling emergence.
In Hill Planting. Assuming that the recommended planting distances for corn is 75cm x 25cm, this means a spacing of 75 cm between rows and 25 cm between hills. If the planting rate is 1 seed per hill, the computed total seeding rate per hectare will be 53,333.3 seeds. The decimal point indicates that this value is just an estimated average. Formula:
PD (or TSR) = [A ÷ (Dr x Dh)] x NPh where: PD = total plant population density (no. of plants) TSR = total seeding rate (no. of seeds) A = area to be planted (m²) Dr = distance between rows (m) Dh = distance between hills (m) NPh = number of plants (or seeds) per hill
In Drill Planting. Given the same assumptions under broadcasting, i.e., seeding rate of 100 kg (=100,000 g) per hectare and 1,000-grain weight= 30 grams but planting the seeds in drill: If the rows are 20 cm apart, the calculated average seeding rate per linear meter in the row will be 2 grams. With 1000 grain weight of 30 grams for rice, this is equivalent to an average seeding rate of about 66.7 seeds per linear meter. But if the row distance is widened to 25 cm, the average seeding rate will increase to 2.5 grams or 83.3 seeds per linear meter. Formula:
PD (or TSR) = (A ÷ Dr) x Nplm where: PD = total plant population density (no. of plants) TSR = total seeding rate (no. of seeds) A = area to be planted (m²) Dr = distance between rows (m) Nplm = no. of plants (or seeds) per linear meter
1. To calculate the average seeding rate (ASR) in weight of seeds (grains) per square meter, divide the recommended seeding rate by the area to be seeded. This is the desired amount of seeds that should be broadcasted per square meter for uniformity throughout the crop area.
ASR (i.e., weight of seeds/m²) = recommended seeding rate (kg) ÷ area (m²) = seeding rate per hectare (kg) ÷ area of 1 hectare (m²) = 100 kg/ha ÷ 10,000 m²/ha = 0.01 kg/m² or 10 g/m²
2. To calculate the average weight of a single seed (grain), divide the weight by the given number of grains. Here we have presumed that we have a data providing that 1000-grain weight is 30 g.
Ave. Seed Weight (g/grain) = Weight of seeds ÷ no. of seeds = Weight of 1000 grains (g) ÷ 1000 grains = 30 g ÷ 1000 grains = 0.03 g per grain
3. To calculate the average seeding rate (ASR) in number of seeds (grains) per square meter, divide the ASR (weight of seeds/m²) by the average seed weight.
ASR (i.e., no. of seeds/m²) = ASR (g/m²) ÷ Ave. seed weight (g/grain) = 10 g/m² ÷ 0.03 g/grain = 333.3 grains per sq meter
4. To calculate the total seeding rate (TSR) in number of seeds (grains) per hectare, divide the required (or recommended) seeding rate per hectare by the average seed weight or, given the ASR in number of seeds per sq. meter (no. 3), multiply it by 10,000, the area of a hectare. Make the necessary conversion to the same units of measurement. The result will also be the total perfect plant population if germination of the broadcasted seeds and plant establishment are 100 percent.
TSR (no. of seeds/ha) = Seeding rate per hectare (g/ha) ÷ Ave. seed weight = 100 kg/ha ÷ 0.03 g/grain = 100,000 g/ha ÷ 0.03 g/grain = 3,333,333.3 grains per hectare