Irrigation and fertiliser guidelines for strawberries

Page last updated: Thursday, 26 May 2016 - 9:37am

Please note: This content may be out of date and is currently under review.

It is difficult to provide prescriptive guidelines for strawberry irrigation given the wide range of planting configurations and irrigation layouts. This webpage helps growers in Western Australia understand the factors affecting their crop requirements so they can adapt their practices accordingly.

The fertiliser requirements for strawberries differ between varieties although many growers use only one recipe for all varieties.

Water and soil

Irrigation water for strawberries needs to be excellent quality, as strawberries are very sensitive to salts, in particular chloride (but not sodium).

Chloride will start to depress yield at very low levels. Ideally the electrical conductivity (EC) of irrigation water needs to be below 0.75dS/m (750uS/cm) or total dissolved solids (TDS) of 400mg/L.

Yield will drop by about 25% if water contains 650mg/L TDS (1.20dS/m), and even more if the water is saltier.

If your water contains more than 0.5 parts per million (ppm) iron, then some treatment to remove the iron may be needed to avoid dripper blockages.

Soil pH should be in the range 5.5 to 7.5 (slightly acid to neutral). If pH correction is required, this should be done prior to planting using lime and/or dolomite.

It is not necessary or desirable to apply any nitrogen (N), potassium (K) or phosphorus (P) fertiliser prior to planting if fertigation is used.  In the Western Australian coarse sands they will leach and be wasted before the plants develop enough roots to be able to access the nutrients. A base dressing of mixed trace elements may be applied pre-plant if desired since they are less easily leached. Trace elements can also be applied through the irrigation with the rest of the fertiliser. If adding compost to the soil, apply it close to planting to avoid leaching of nutrients.

Very high levels of salts have been observed early in the season when pre-plant compost is applied. This is something to be aware of and monitor, especially with the more sensitive varieties such as Fortuna.

High levels of fertiliser salts may damage young roots and allow access of pathogens that lead to disease. High levels of nitrogen early will promote vegetative growth at the expense of flowering and fruiting.

Poultry manure

Application of raw poultry manure is banned in several shires and city councils on the Swan Coastal Plain, from Gingin to Harvey, as it provides a breeding ground for stable flies, which are a serious pest to animals and humans. Poultry manure also contains easily leachable nutrients and similar to applying a base dressing of NPK fertiliser, much is wasted before plants can access it.  Other products such as composted chicken manure or other types of compost do not breed stable fly but as mentioned above, are still able to leach nutrients.

When forming beds, ensure the soil is rolled well. Fluffy soil will decrease the lateral spread of water and plant establishment will be adversely affected.

Irrigation and fertiliser practice

The following guidelines are based on research conducted in Wanneroo over many years. With good fertiliser and irrigation practices, and depending on variety, strawberries can consistently produce up to 1.5 kilograms of fruit per plant each season.

The industry average is closer to 0.50–0.75kg but one kilogram should easily be achievable by following recommended guidelines.

Irrigation systems

Overhead sprinklers are used for establishment and for cooling plants in hot weather. Drip irrigation is recommended throughout the season.

Drip irrigation is more efficient, as it uses less water and applies it more evenly. Fruit is kept dry, lowering disease incidence.

Overhead irrigation for establishment

Overhead irrigation should be applied to prevent new runners from wilting. When plants arrive they have no active roots. Reducing plant stress (wilting) will promote quicker root establishment and the take up of nutrients, enabling good plant growth.

The frequency and duration of overhead irrigation will depend on weather and the condition of the runners at planting. Compact runners with smaller leaves require less irrigation as they have less leaf area than long large-leafed runners that lose water more readily.

Many growers use ‘leaf-off’ plants. These generally arrive about two weeks later than leaf-on plants and have experienced a larger degree of chilling. They are less fragile than leaf-on plants but still require some overhead cooling.

When planting in February or early March, cooling may be required between once and four times an hour during the heat of the day until roots are established and drip irrigation is effective.

Growers can use a significant proportion of their water allocation during this period. As the goal is cooling rather than irrigation, using a low output nozzle with a finer droplet size may reduce water use.

Good irrigation uniformity and cooling can be achieved by using smaller nozzles at the right pressure. Twin-nozzle sprinklers may use up to 1000 litres per hour whereas some newer low-flow sprinklers may only need 500L to achieve the desired result.

Drip irrigation should be used in addition to overhead irrigation during establishment to promote root establishment and prevent soils from drying out between planting holes.

Sands in Western Australia can become water repellent or non-wetting if soil moisture is not maintained and are almost impossible to re-wet using drip irrigation. Even in uniformly wet sandy soils, the lateral spread of water is no better than about 15–18cm. When sandy soils dry out, water follows preferred pathways and even that limited spread may not be achieved.

Other factors can have major effects on the penetration of overhead irrigation and rain, such as:

  • the size and shape of the planting hole punched in the plastic
  • the evenness of the bed surface, and
  • how tightly the plastic has been laid.

Drip irrigation

The most common configuration for commercial strawberries on sand is two lines of drip irrigation per four-row bed with each line laid between the two outer rows of plants.

However, growers use a range of planting and irrigation configurations including two, three and four-row beds using one, two or even three rows of drip tape. Some growers plant two ‘pairs’ of rows with plants offset rather than four rows equally spaced across the bed.

Bury the lines slightly so they stay in place and don’t move around. This gives the best chance of all plants receiving the same amount of water and fertiliser.

A 25cm dripper spacing is common but a closer spacing, if scheduled effectively, will lead to more even irrigation and may give better yields and quality.

Dye tests and irrigation monitoring in the Wanneroo area have shown that increased volumes (longer duration) will not increase the spread of water beyond about 15–18cm total or 7.5–9cm on either side of the dripper. This indicates that 15 to 20cm distance between drippers would deliver irrigation more evenly to the crop than 25cm.

Experience in Wanneroo including dye testing showed that a yield advantage was achieved with a 10cm dripper spacing but that may not be practical for many growers.

Longer run times only waste water (and fertiliser) by sending it deeper into the soil profile, past the root zone.

Questions exist as to what degree plant roots will grow towards soil water and what detriment, if any, this may have on yields. In Florida, soils are also sandy but have a higher proportion of finer silt particles, so lateral water movement is better. Beds are constructed 30cm higher than the pathways and growers aim to promote extensive root growth to fill the bed as a matter of priority after planting, to maximise nutrient uptake and efficiency.

Cultivar differences

Strawberry cultivars vary in their vigour, leaf area and fruit production which suggests that different amounts of irrigation and fertiliser are required. For example, Fortuna is recognised as being a smaller, less vigorous plant. Its root system may not explore the soil to the same degree as Camarosa and Festival.

Most growers in Western Australia treat all varieties the same but this may not be the best option, especially for water and fertiliser.

Research at Wanneroo has shown that stand-alone evaporation-based irrigation scheduling does not work well with strawberry crops grown under black plastic mulch, high tunnels or plastic cloches.

Irrigation requirements

Evapotranspiration and the impact of rainfall are altered by plastic mulch and cloches or tunnels.

When rain falls throughout the cooler months, growers often reduce or stop watering in the belief that water falling in the pathways can be accessed by the plants. However, on coarse sandy soils this is not so. Some water does enter the beds during heavy falls (25mm or more) but it is largely below the root zone and serves mostly to leach fertiliser.

Soil moisture monitoring is essential to determine the effectiveness of water applied and the water potential of the soil. Details on using soil moisture sensors to fine-tune irrigation can be found here.

Previous research in Western Australia suggested that replacing 70% of evaporation was adequate to supply plant needs over most of the season. More recent experience indicates that water requirements could be as high as 100% of evaporation for maximum production.

Using soil moisture monitoring equipment to fine-tune irrigation for each particular variety/plant spacing/irrigation configuration and soil type is the best practice.

In Western Australia's sandy soils irrigation should be split into two or three times a day, even four times when daily evaporation gets to around 10mm per day. The aim is to apply small amounts more often (<3mm at a time) to keep fertiliser in the root zone rather than flushing it below the roots.

Many modern soil moisture probes also monitor EC (the concentration of salts in the root zone). That can be useful when levels start to climb, given strawberries are a salt-sensitive crop. If levels rise apply water only, for a day, to leach out the build-up of salts.

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.

Table 1 Example of nutrient solution applied through irrigation
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.

Calibrating nutrient solution injection pumps

  1. Start irrigation and injection pumps.
  2. Adjust each injection pump to the same output.
  3. Measure the EC at the dripper and adjust each injection pump up or down to get an EC of about 2.2µS/m in total, for example 1.7µS/m for the fertiliser and 0.5µS/m for the bore water.
  4. Measure the output of the injection pump for one minute to work out the flow rate.

Calculation of injection time (worked example)

Assume you are using two 1000L tanks for your fertiliser stock solution.  The following amounts of fertiliser are contained in your two tanks.

75kg calcium nitrate

75kg potassium nitrate

36kg MAP

18kg magnesium sulphate

 

There is 23.4 kg of nitrogen containing fertiliser in the above recipe. If you have two pumps, each with a flow rate of 120L/hour it will take 2000/120 = 16.6 hours before the tanks empty.

Injection rate – 23.4kg N/16.6 hours = 1.40kg N/hour = 23.5g N per minute

If the area for one station is:

Nine 4-row beds each 125m long x 1.2m wide = 1350m2

and the amount of nitrogen needed is 2kg/ha per day

= 0.2g/m2/day

= 0.2 x 1350m2

= 270g

Injection time = 270/23.5 = 11.5 minutes

To increase or decrease the amount of N, you can either change the injection time or increase the concentration of the recipe provided the final EC does not exceed 2.2µS/m.

Further reading

The external websites listed provide useful information from other growing areas.