# How to Calculate Plant Population

This article is a quick guide to how to calculate plant population in the farm. Farmers are usually faced with what number of seeds to plant per hectare. Understanding this vital step in crop production is essential to nutrient uptake and utilization per crop.

Plant population refers to the NUMBER of plants per unit area of land.
Example: 40 000 plants per hectare (e.g. cabbage) or
100 plants per square metre (e.g. carrot)
Plant spacing, on the other hand, refers to the ARRANGEMENT of plants on the area planted.
Example: Widely varying plant spacings such as 1 000 mm x 10 mm, 500 mm x 20
mm and 100 mm x 100 mm, all give a plant population of 100 plants per
square metre.

## Plant Population: How to Calculate

1. The Plant population of any field is given by multiplying the between plants spacing with the spacing between the rows.
2. The total area of a hectare is 10000 square metres.
3. Divide 10000 by the result of multiplying the between plant spacing and the between rows spacing as given in as shown below
Plant population = 10 000
_____________________
between plants spacing (m)x between rows spacing row (m)
An example
If the between plant spacing of tomatoes is 30cm and between row spacing is 90cm what is the plant population per hectare?
1. First convert cm to m
30cm =0.3m, 90cm =0.9
2. Multiply between plants spacing and the between rows spacing
0.3m x 0.9m = 0.27 sqm
3. Divide area of 1 hectare by 0.27sq m
10000 sq m / 0.27 sq m = 37037
4. Therefore Plant population of potatoes per ha is 37037

According to the Traditional Field Crops publication, another good example would be;

### Plant Population: Calculation of final stand

The calculation of the final stand is accomplished by the following formula:

Plant population/ha = [100,000,000 cm² /ha ] / [seed spacing in the row in cm × row width in cm]

For example, if the row width is 40 cm and seeds are spaced 10 cm apart. the final stand, assuming 100 percent germination and no plant mortality, would have:

100,000,000 / 40 x 10 = 50,000 plants

Likewise if the crop is planted in hills the calculation made is:

Plant population/ha = [100,000,000 (cm²/ha) x number of seeds/hill] / row width (cm) x hill spacing (cm)

Thus planting in 50 cm width with 50 cm between hills and two seeds planted per hill yields:

100,000,000 x 2 / 50 x 50 = 80,000 plants/ha

The same formula can be used to calculate the in-row seed spacing needed to provide a given population at various row widths. For example, if an optimal population of 100,000 plants/ha is desired, then:

100,000 plants/ha = 100,000,000 / [row seed width × seed spacing (cm)

or:

row width x seed spacing = 1000 cm²

This spacing can be achieved using:

10 cm seed-spacing in 100 cm row width,
20 cm seed-spacing in 50 cm row width,
15 cm seed-spacing in approx. 70 cm rows, etc.

Note again that the calculation does not account for losses due to poor germination or plant mortality. You may want to plant 15 or 20 percent more than the amount you wish to harvest in order to account for these probable losses.

### Plant Population: How to determine amount of seed needed to plant a given field size

You first need to know how many seeds of each crop are contained in a kilogram. The most accurate way of calculating this is to weigh out a 60 g sample of the seed and count it if you can find a reliable scale (i.e. at the post office or at a pharmacy). Multiplying the number by 10 will give the number of seeds per kilogram. Otherwise, you can use the table below as a rough guide:

The table below shows Number of Seeds per Kilogram

 Maize 1760-2860 Sorghum 26,400-44,000 Peanuts 1100-1540 Beans 3000-3960 Cowpeas 3960-4040

To find the kilograms of seed needed per hectare, simply divide the number of seeds needed by the number of seeds/kg. Multiplying this times the size of the field in hectares will give the total amount of seed required.

## Plant Population: How to determine a farmer’s actual plant population

When troubleshooting a farmer’s field, it is usually valuable to check out his plant population, since this has an important influence on yield potential and response to fertilizer. This can be easily done by counting the plants in 510 randomly selected strips of row each equal to 1/1000th of a hectare.

Step 1: First determine the field’s average row width by measuring the distance across 10 complete rows and then dividing by 10. Do this at several random locations to get a representative average.

Step 2: Refer to the 1/1000th hectare row length chart for the proper random selection procedure.

Step 3: Select at random five to ten row strips of the appropriate length and count the number of plants in each and record it.

Step 4: Multiply the average number of plants in the row strips by 1000 to yield the plant population per hectare.

## Plant Population: How to Make A Pre-Harvest Yield Estimate

A pre-harvest yield estimate can be accurate to within 5 percent of the actual harvested yield if the correct procedure is used. When working with trial and demonstration plots, you should always take such a pre-harvest yield sample of both the test plot and the control plot. There is always the chance that the plots might be inadvertently harvested before the agreed-upon time without the yields being measured. Pre-harvest yield sampling is also a quick way of estimating crop yields in farmers’ fields.

## Plant Population: General Principles Of Yield Sampling

1. Samples should be collected at random for various portions of the field or plot. Do not purposely select samples from higher- or lower-producing areas within the plot or your estimate may be very inaccurate A random sampling pattern should be determined before you enter the field so you will not be tempted to choose them by visual appearance.
2. Don’t collect yield samples more than one week before the actual harvest.
3. When taking each sample, the area (or row length) to be harvested must be precisely measured. Do not estimate’ Remember that any error in the sample area size will be magnified hundreds of times when converting the yield to a larger land unit basis.
4. You must adjust the sample weights to account for factors like excess moisture, damage, and foreign matter.

## Plant Population: How to Take Samples and Estimate Yields

1. The Sampling Procedure

a. Number of samples: For plots less than 0.5 ha, take a minimum of five samples. For plots of over 0.5 ha, take between five and ten. If crop growth is not very uniform, take ten samples.

b. Size of each sample: Take each sample from the same sized area or same amount of row length. Individual sample size should be between 2.5 and 5.0 square meters. For row crops, the area of a sample is determined by multiplying row length by row width. (Harvesting three meters of corn row planted in rows one meter wide will give you a sampling area of three square meters.) Alternatively, use a section of row length equal to 1/1000th of a hectare. This will make later math calculations simpler, and the 1/1000 ha row length can be taken from the following table.

 Row Width 1/1000th hectare Row Length 50 cm 20.00 m 60 cm 16.67 m 70 cm 14.28 m 75 cm 13.33 m 80 cm 12.50 m 90 cm 11.11 m 100 cm 10.00 m 110 cm 9.10 m

c. Taking a random sample: Decide on the sampling pattern before entering the field, and do not deviate from it. To randomize, the field can be divided up into sections and each section given a number drawn from a hat. Or you can pick randomized starting points at the side of the field and then enter random distances from the starting point. A good system for row crops is to number the rows and select them at random, then select the distance into the row (field) at random. NOTE: Exclude three meters or four rows of perimeter from your sampling area along all four sides of the plot to ensure sampling from the heart of the plot.

2. Accuracy: Use a tape to measure each sampling area or row length. Use an accurate scale to record the total weight of the samples within one plot.

3. Handling the Samples: The samples should be harvested and processed according to local prevailing methods. If drying is required before shelling or threshing, be sure the location is secure and free from rodents or birds.

4. Weighing the Sample: Use an accurate portable scale. You do not need to weigh individual samples separately, but only the fatal collective sample from the plot. If you cannot find a good portable scale, have the grain weighed in town.

5. Checking Grade: Take a random sample of the collective sample and have it checked for moisture content and any other graded qualities if necessary. (Refer to the storage section in Chapter 7 for how to determine grain moisture content.)

6. Yield Calculations:

Size of total sample area = No of samples × size of individual sample areas

7. Correcting for Moisture: Yields are usually based on grain that is dry enough to store in shelled form (usually 13-14 percent moisture content). If you base your estimates on the weight of a high moisture sample, you should revise the yield downward using this simple formula (otherwise, dry the grain first).

Grain weight after drying = [% dry matter before drying / % dry matter after drying] x original grain weight

Example: Suppose you weigh a collective sample of “wet” grain and then estimate the plot yield to be equal to 3500 kg/ha. A moisture test shows the sample has 22 percent moisture; what is the actual yield based on 13 percent moisture?

22% moisture = 78% dry matter,
13% moisture = 87% dry matter

78% / 87% x 3500 kg/ha = 3138 kg/ha yield based on 13% moisture

A Yield Estimate Example

Suppose you are taking a yield estimate on a farmer’s maize plot which is slightly less than 0.5 hectare. The rows are planted 90cm apart, and you decide to take six samples, each consisting of 1/1000th hectare of row length. The collective weight of the shelled, dried maize is 18 kg. What is the estimated yield on a per hectare basis?

Solution:

area of collected sample = 6/1000ths of a hectare = 60 sq. meters

18 kg x [10000 sq.m (1 hectare) / 60 sq. meters] = 3000 kg/ha estimated yield

## Plant Population: Vegetables

To determine plant population, there is no precise answer to the question of the specific plant spacing requirement for all crops. Factors such as climate, soil, cultivar, market requirements, managerial ability of the grower, and many others, all play a role. For this reason, one will often find that a range of spacings or populations is recommended.

For example, a recommendation for cabbage may be a plant spacing of 350 mm to 500 mm in rows drawn 500 mm to 700 mm apart, and a plant population of 35 000 to 45 000 plants per hectare.

At any specific plant population, individual plants are likely to perform best where a uniform spacing of plants, equidistant from one another in all directions, is adopted.

However, it is usually more practical to plant fairly closely in rows, with the rows being spaced wider apart. This allows for easier access into the planting for inspections, weeding, pest and disease control and harvesting.

The size and shape of the root system of most plants are generally in proportion to the size and shape of their top growth. Thus we find that plants like lettuce, cabbage and cauliflower, with a fairly compact “rounded” top growth, generally have compact, rounded root systems.

Lettuce, being smaller, should be planted closer together than cabbage for
optimum yields. Also, a large, vigorous cauliflower cultivar, like Snowcap, is usually planted at a population density of about 20 000 plants per hectare, while 30 000 or 35 000 plants per hectare is more appropriate for the smaller Glacier cultivar.

Small-growing (short), more upright growing crops, like onion or carrot, have relatively shallow roots, with limited lateral (sideways) spread.
Rambling crops, such as pumpkin, Hubbard squash or butternut, on the other hand, tend to have rather sparse.

Spreading, root-systems, similar in size and spread to that of the top growth. Butternut, being less vigorous than the others mentioned, requires a closer spacing (a higher plant population) for optimum yields.

In cases where climate, soil and nutrient status are all favourable for growth, plants will grow larger and have better-developed root-systems and this could require a wider than normal spacing.

A lower plant population is also justified when conditions such as limited soil moisture are a likely limitation to the crop. With an understanding of a plant’s growth behaviour, and the conditions under which it is to be grown, it is possible to make a good estimate of a suitable plant spacing for most
vegetable crops.

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### One Response

1. Mereba Mesu says:

Good idea, but what’s the different between plant population and plant number per unit area?