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Definitions Below we present some examples to show how fertilizer dosage is calculated whenever recommendations are available. Recommendations are published in the following form:
All recommendations should be carefully studied and evaluated. Since some recommendations are in terms of pure P and K instead of the oxides. |
Calculation of fertilizer dosage
Example for field crops fertilization
The following fertilization recommendation has been given a farmer growing a drip- irrigated squash crop:
Basic dressing (as pre-transplant application) of 60 Kg N, 120 Kg P2O5 and 70 Kg K2O per hectare.
Top dressing during the 10 mid-season weeks 210 Kg of N, 70 Kg of P2O5 and 210 Kg of K2O should be fertigated per hectare.
Calculating the pre-plant application
The recommended rates are 60 – N; 120 – P2O5 and 70 – K2O, given ratio of 1:2:1.2.
For this recommendation “Gatit 14-28-18+TE” fertilizer is the best approximation to the above recommendation. The following amount of “Gatit 14-28-18+TE” is required:
| 60Kg N / ha × 100 | = 430 Kg “Gatit 14-28-18+TE” / ha |
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This quantity supplies the required amounts of N, P2O5 and K2O.
Calculating the mid-season application
The recommended rates are 210 – N, 70 – P2O5 , 210 – K2O , given ratio of 3:1:3 “Gatit 21-7-21+TE” fertilizer fits the recommendation.
The required amount of fertilizer is:
| 210Kg N / ha × 100 | = 1000 Kg “Gatit 21-7-21+TE” / ha |
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A total amount of 1000 kg of fertilizer should be applied during a period of ten weeks. Therefore, every week:
| 1000Kg “Gatit 21-7-21+TE” / ha × 100 | = 100 Kg “Gatit 21-7-21+TE” / ha / week |
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should be applied
A second alternative is to inject the fertilizer solution at constant concentration into the irrigation water (proportional fertigation)
If during that 10 week period, a total of 3000 m³ water/ha are to be applied, then the following “proportional” dosage should be maintained:
| 1000Kg “Gatit 21-7-21+TE” / ha × 100 | = 0.330 Kg “Gatit 21-7-21+TE” / m³ |
| 3000m3water / ha |
Example for greenhouse fertigation
The following recommendation has been given to a grower of greenhouse carnations: Nitrogen (N): 120 ppm; Phosphorus (P2O5): 60 ppm; Potassium (K2O): 240 ppm. The relationship between the three nutrients is: 2:1:4.
Gat fertilizers “Shafir4-2-8” has the adequate ratio of nutrients.
The following calculations are performed:
To reach a concentration of 1 ppm N, 1 gram N should be added to each cubic meter of irrigation water, (1 gr N per 1,000,000 gr water = 1 gr N /m³). Therefore, in order to reach 120 ppm, 120 gr N (0.12 Kg N) should be added to each m³ of irrigation water.
The required weight of “Shafir4-2-8” is calculated as follows:
| 0.2Kg N / m³ × 100 | = 3 Kg “Shafir 4-2-8” / m³ |
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The volume weight of “Shafir4-2-8” is 1.17 gr/cm³ (=1.17 Kg per Lt)
Therefore, the volume of fertilizer solution to be injected into the irrigation water is:
| 3Kg “Shafir 4-2-8” / m³ | = 2.56 Lt “Shafir 4-2-8” / m³ |
| 1.17Kg / Lt |