Fertiliser
Fertiliser should be applied daily at least. If the system is able to maintain pressure between irrigations then fertiliser can be applied with each watering.
An efficient system requires a fertiliser injector and two or even three different tanks.
With the two tank system, calcium nitrate and half of the potassium nitrate are dissolved in Tank A and the other fertilisers are dissolved in Tank B. Tank B may be divided further so that sulphates and phosphates are kept separate, making three tanks.
The most basic system uses a venturi, where a given amount of fertiliser is simply sucked into the line. In this case, the grower may only have one tank. Since some nutrients cannot be mixed together, then two different feeds may be used on different days.
Growers who feed manually often fertilise less often because the process is labour intensive. Every three to four days is common. Soil nutrient monitoring shows fertiliser levels decline rapidly between feeds. This will reduce plant growth.
Table 1 is an example fertiliser program. Other fertilisers may be used to make up the suggested quantities of each element required. Note that the amounts are given per hectare of bed area, that is, pathways are excluded.
| Product | Rate (kg/ha/day) | Applied nutrients (kg/ha of bed area) | ||||
|---|---|---|---|---|---|---|
| N | P | K | Ca | Mg | ||
| Calcium nitrate (15.5% N, 19% Ca) | 6.5 | 1 | 1.2 | |||
| Potassium nitrate (13% N, 38% K) | 6.4 | 0.8 | 2.4 | |||
| Magnesium sulphate (9.9% Mg) | 3 | 0.3 | ||||
| Mono-ammonium phosphate (11% N, 22.8% P) | 1.9 | 0.2 | 0.4 | |||
| Total nutrients applied per day | 2 | 0.4 | 2.4 | 1.2 | 0.3 | |
This fertiliser program applies approximately 450kg of nitrogen, 100kg of phosphorus, 580kg of potassium, 288kg of calcium and 76kg of magnesium per hectare per season (April to November). Rates of nitrogen in excess of 450kg/ha risk compromising fruit quality and taste.
Despite a common belief that strawberries cannot be given nitrogen (N) in the ammonium form, trials around the world have shown better N uptake when some ammonium is present. One quarter of the N requirement can be given as ammonium when growing in soil as opposed to hydroponics. More than that will affect fruit quality making it soft (more prone to bruising and fungal rots) and flavourless.
Trace elements
You can add all trace elements at the start of the season as a broadcast application which is then rotary hoed in (shown in Table 1) or you can add them to your nutrient solution mixture above.
If they are added as sulphates, rather than chelates, then you need to keep them in the ‘B’ tank.
| Fertiliser | Formula | Quantity |
|---|---|---|
| (g/1000L) | ||
| Iron chelate | Fe-EDTA (13% Fe) | 860 |
| Manganese sulphate | MnSO4.H20 | 169 |
| Borax | Na2B4O7.10H2O | 953 |
| Zinc sulphate | ZnSO4.7H2O | 201 |
| Copper sulphate | CuSO4.5H2O | 19 |
| Sodium molybdate | Na2MoO4.2H2O | 12 |
Alternatively, use a proprietary trace element mix and add to the appropriate tank at recommended rates.
Note:
- The electrical conductivity (EC) of the nutrient solution emitted from your drippers should not exceed 2μS/cm. High salt concentrations can damage the crop.
- Calcium nitrate should not be mixed in the same tank as fertilisers containing phosphates or sulphates.
- When fertigating, keep the soil at or near field capacity to avoid concentrating salts in the root zone. As the soil dries, the concentration of salts increases.