How to convert % of NPK to kg/ha. I have a 2 kg of soil in a polybag size of 12" x 12" inch (Diameter=16.6 cm, width=14cm and height=22.4cm). The NPK ratio I bought from nursery is 8%N:8%P:8%K. I am going to cultivate mustard plant. Optimal NPK ratio recommended for this plant is 180-100-100 kg/ha. How many quantity of N8:P8:K8 should be added to meet the N requirement of the plant?
To determine the quantity of N8:P8:K8 fertilizer required to satisfy the nitrogen needs of your mustard plants, we must delineate the procedures involved:
The volume of your polybag in cubic centimeters:
Volume = Diameter * Width * Height
Volume = 16.6 cm * 14 cm * 22.4 cm
Volume = 5,215.36 cubic centimeters
Convert the volume to cubic meters:
1 cubic meter = 1,000,000 cubic centimeters
Volume = 5,215.36 cubic centimeters / 1,000,000 cubic centimeters/cubic meter
Volume = 0.00521536 cubic meters
The weight of your 2 kg soil in grams:
1 kg = 1000 grams
Weight = 2 kg * 1000 grams/kg
Weight = 2000 grams
The density of your soil:
Density = Weight / Volume
Density = 2000 grams / 0.00521536 cubic meters
Density = 383,561.64 grams/cubic meter
The total soil volume required for one hectare:
1 hectare = 10,000 square meters
Assuming a soil depth of 20 cm (0.2 meters), the total volume of soil needed would be:
Volume = 10,000 square meters * 0.2 meters
Volume = 2,000 cubic meters
The total amount of N needed for 1 hectare
Total N = 180 kg/ha
The amount of N8:P8:K8 fertilizer needed to provide the total N:
Given that the N8:P8:K8 fertilizer contains 8% nitrogen, we must multiply the total nitrogen by 100/8 to determine the total quantity of fertilizer required:
Total fertilizer = Total N * (100/8)
Total fertilizer = 180 kg/ha * (100/8) *
Total fertilizer = 2250 kg/ha
The amount of N8:P8:K8 fertilizer needed for your polybag:
Given that your polybag has a volume of 0.00521536 cubic meters, we can determine the requisite amount of fertilizer by multiplying the total fertilizer per hectare by the volume of your polybag and subsequently dividing by the volume of soil per hectare:
Fertilizer for polybag = Total fertilizer * (Volume of polybag / Volume of soil/ha)
Fertilizer for polybag = 2250 kg/ha * (0.00521536 cubic meters / 2000 cubic meters/ha)
Fertilizer for polybag = 0.005844 grams
Consequently, around 0.005844 grams of N8:P8:K8 fertilizer is required to fulfill the nitrogen needs of your mustard plants in the polybag.
Note : Please keep in mind that the soil density used in this estimate is 383,561.64 grams/cubic meter. The real density of your soil may be different, so you may need to make changes to the math.
The calculated amount is quite small. Considering the critical limits of the elements in the soil, I suggest you add more fertilizer to your soil. It is worth mentioning that plant growth depends on other parameters, such as soil texture, organic matter, and other factors.
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