Fertilizer International 511 Nov-Dec 2022
30 November 2022
Fertilizers for small fruits
CROP NUTRITION
Fertilizers for small fruits
Small, soft fruits such as blueberries, raspberries and strawberries thrive on fertile, well-drained soils rich in organic matter. Their nutrient needs can vary widely according to yield expectations and soil characteristics.
Blueberries, raspberries and strawberries are widely consumed as fresh fruit or in preserves (jams), juices and cordials. In response to their growing popularity, world production of small fruits is on the rise – thanks to a larger global cropping area, new varieties and better use of fertilizers and other crop inputs.
For small fruits, the length of the growing season depends on factors such as berry type, the particular cultivar and the individual climate of each growing region. Nutrient needs can vary widely, being influenced by both the cropping system and yield expectations.
In general, small fruits typically prefer fertile, well-drained sandy loam or silt loam soils which are rich in organic matter and of neutral-to-acid soil pH. Surface water drainage and air drainage away from plantings are also important.
Fertilization practices for strawberry – the most widely cultivated small fruit – are correspondingly the best documented. Importantly, small fruit are typically chloride-sensitive (particularly gooseberries, currant varieties and strawberries) and applying chloride-containing fertilizers to many types of berry is therefore best avoided.
This article provides an overview of the fertilization of three small fruits, blueberries, raspberries and strawberries, with a focus on outdoor growing in soil.
Blueberry – the superfood
The popularity of blueberries (Vaccinium sp.) is linked to its reputation as a ‘superfood’ and its widely reported nutritional qualities. These include exceptional richness in vitamins, mineral salts, fibre and polyphenols (oxoflavoids and anthocyanins in particular). In addition, blueberries have the highest antioxidant content of any fruit apart from prunes and raisins.
Blueberries are a deciduous fruiting plant native to North America. They can live beyond 15 years and still be productive before re-planting is necessary. Cultivated blueberries are generally of the ‘highbush’ variety. These have larger berries growing on taller bushes. Blueberries are pale greenish at first, then, during the maturation process, they become reddish-purple, before finally turning dark purple on ripening. Mature berries have a sweet taste with variable acidity. Fruiting times are dependent on local conditions including the climate.
Bushes prefer an acidic soil ideally below pH 5.5. Soil pH can be corrected, if necessary, prior to planting or during establishment. Fertilizers are usually applied at the start of spring growth and immediately after harvest.
Fertigation with water soluble fertilizers needs to be carried out with care as blueberry plants can be sensitive to excessive nutrient levels. Because of this, sufficient water needs to be applied during the fertigation process to limit the concentration of nutrients in solution. Ammonium is often the preferred nitrogen source due to its soil acidification effect. The optimal fertilization balance for blueberry is around 1 N – 0.6 P2 O5 – 0.75 K2 O.
Low calcium content contributes to poor storage quality in soft fruit. Calcium fertilization is therefore necessary when Ca availability and uptake need to be improved (see box). Blueberries can be prone to calcium deficiency due to their cultivation in acidic soils and soils with a low cation exchange capacity (CEC). Although rare, deficiency symptoms include interveinal chlorosis of younger leaves and scorching of the margins of older leaves. For these reasons, calcium supply and deficiency need to be monitored and corrected for, if necessary.
Ever-popular strawberries
Strawberry (Fragaria x ananassa) is a small and fleshy fruit popular in many countries globally. Strawberries are mostly eaten fresh but are also enjoyed preserved with sugar in the form of jam. They are also consumed in cakes and pastries and as a flavouring in drinks and deserts such as milkshake and ice cream.
Consumer interest in berries and their health benefits has grown dramatically in recent years. Strawberries are rich in antioxidants and are also an excellent source of fibre, vitamins, folic acid, fatty acids, polyphenols and minerals.
Strawberries take an average of 30 days to mature from flower to fruit. Berries are generally picked every three days and fields must be re-planted every year. Strawberries thrive best in nearly neutral soils (pH 6-6.5) and need moderate levels of fertilization and irrigation.
Strawberry’s fertilizer needs are primarily determined by the export of nutrients in the fruit. One tonne of strawberries typically removes:
- 2 kg of potash (K2 O)
- 1 kg of nitrogen (N)
- 0.5 kg of phosphate (P2 O5 )
- 0.3 kg of calcium (CaO)
- 0.2 kg of magnesium (MgO).
In general, the preferred fertilization formula for strawberries is: 1 N – 0.5 P2 O5 – 1.5 K2 O – 0.2 MgO – 0.6 CaO.
Raspberry – two cultivars
Raspberries are an important commercial fruit crop and are widely grown in many of the world’s temperate regions. As with other small fruits, interest in the claimed health benefits of raspberries has grown in recent years. Red raspberries contain a range of strong antioxidants such as vitamin C, quercetin and gallic acid. These have anti-inflammatory properties and are said to have a role in the prevention of cancer and cardiovascular disease.
Raspberries prefer well-drained, sandy loam soils which are rich in organic matter. They do not grow well in waterlogged soils or shallow chalky soils. Two types are grown:
- Summer-bearing cultivars (non-rising varieties). These produce fruit only on second-year canes (floricanes) during a relatively short period during the summer. Such varieties are generally preferred by commercial grwers as they can be efficiently harvested at a lower labour cost.
- Double- or ever-bearing cultivars (rising varieties). These cultivars, as well as bearing summer fruit on second-year floricanes, also bear some fruit on first-year canes (primocanes) in late summer and autumn.
Raspberries prefer a soil pH of between 5.6-6.2. The fruit is ready to harvest when the fruit has turned a deep colour and easily comes off its receptacle. Splitting fertilizer applications (NPK) into three equal parts is recommended in new plantings – with the first part applied two weeks after planting, the second one month later, and the third part one month after the second. Fertigation is also a useful way of ensuring that the total fertilizer requirement is applied incrementally in phases. The optimal balance for raspberry fertilization is around: 1 N – 1.3 P2 O5 – 2.4 K2 O.
General fertilization guidance
University agricultural extension services in North America provide good general guidance on small fruit fertilization. These provide the following growing advice1 :
- Good drainage and correct soil pH are important for all berry crops. Blueberries grow best on at pH 4.5-5.5, raspberries at pH 5.6-6.5 and strawberries at pH 5.0-6.5.
- Most fruit plants are fertilized in spring at flowering. Strawberries are the exception – being fertilized in August.
- Annual applications of nitrogen, phosphorus, and sulphur will be required to produce maximum yields and promote stand longevity. Soil levels of magnesium, potassium, and boron should be monitored.
- Granular fertilizers should be broadcast over the root zone when foliage is dry and then watered in if rainfall is not expected. Any residual fertilizer should be brushed from leaves.
- Move mulch away from plants when applying fertilizers to ensure direct contact with soil – and then reapply mulch after fertilizing.
- Keep fertilizers from contacting the crowns (base) and lower stems of small fruit plants.
- Once plants are in the ground, do not disturb shallow roots by working fertilizer into the soil.
- Fertilization regimes for establishing plants are different from those for fruit-producing plants.
- Late summer and autumn fertilization – except for strawberries – interferes with the hardening-off process and can lead to winter damage of tender growth.
- Good insect, disease, and weed control all help to ensure maximum yields.
Typical nutrient requirements of small fruit are shown in Table 1. The contrasting nutrient uptake curves for blueberry, strawberry and raspberries during vegetative growth, fruit set, fruit development and ripening are shown in Figures 1-3, respectively.
The following fertilization recommendations and nutrient needs are based on North American growing practice for small fruits1 .
Nitrogen for growth
Nitrogen is necessary for:
- Vigorous vegetative growth
- Fruit bud initiation
- Fruit set.
Small fruit plants respond readily to nitrogen and annual applications are necessary. Too little nitrogen results in poor growth, spindly plants and poor yields. While too much nitrogen results in excessive shoot and foliage growth, and soft, poorly coloured fruit which can be susceptible to rot.
Small fruits require continuous nitrogen supply. Around 50 percent of nitrogen uptake eventually accumulates in the fruit. Excessive nitrogen availability during later growth stages should be avoided as it can soften the fruit and delay ripening. Regular nitrogen applications throughout the growing season are therefore recommended as best practice.
Blueberries have a higher nitrogen need than other berry crops, being essential for optimum plant growth, production of fruiting wood, and desirable berry size. Ammonium sulphate (21-0-0-24) or urea (45-0-0) are generally applied. The total amount of applied N is split between three separate applications during the growing season1 :
- 50 percent should be applied at bud break alongside all of the P and K requirement
- 30 percent in late spring. e.g., late May
- And the remaining 20 percent in midsummer, e.g., early July.
Winterkill can result when fertilizer is applied too late in the season.
Around 85-110 Kg/ha (75-100 lb/ ac) of nitrogen is required for good blueberry yields, with amounts adjusted up or down according to yearly plant growth. In general, blueberry plants need to produce strong, new unbranched shoots each year to replace old canes and the weak ‘twiggy’ growth removed during annual pruning. This usually translates to at least threeto-five strong canes arising from the base of the plant or halfway up the old canes1 .
Nitrogen should be applied to established strawberries at around 40-45 kg/ ha (35-50 lb/ac) in late summer to early autumn, e.g., between mid-August and early September. During a fruiting year, nitrogen should not be applied in the spring as it will often cause excessive foliage, soft berries, and increased fruit rot. However, if plants do show a need for spring N, then no more than 20 kg/ha (around 15 lb/ac) should be applied1 .
An annual nitrogen application of around 55-75 kg/ ha (50-65 lb/ac) is recommended for raspberries.
“Annual applications of nitrogen, phosphorus, and sulphur will be required to produce maximum yields and promote longevity?”
This is applied to the soil surface in the spring along the row or banded with the required P application. Nitrogen is the most important factor controlling internodal length and more N can therefore be applied (around 75-85 kg/ha, 65-75 lb/ac) if cane growth is inadequate and internodal length is less than the ideal distance of 10 centimetres (4 inches)1 .
Phosphorus for crop development
Phosphorus is important for ensuring good rooting and leaf growth during early crop development stages. In productive plants, internal redistribution of P takes place during fruit development, with around 40 percent of total uptake ending up in the berry.
All berries will respond to a band applications of phosphate fertilizers if soil tests reveal low P levels in the upper soil profile. Phosphorus is spring-applied for blueberries and strawberries and autumn-applied for raspberries. Application rates (P2 O5 ) as high as 100-150 kg/ha (90-135 lb/ac) can be necessary1 .
Potassium for fruiting
Berries require potassium in large quantities – showing a potassium need above that of nitrogen – with almost 60 percent of the K requirement found in the fruit at harvest. Potassium demand peaks, with uptake exceeding that of any other nutrient, from the early fruit formation stage through to maturity.
For maximum yields, blueberries, raspberries and strawberries all require adequate potassium availability, as determined by soil testing. Application rates (K2 O) of 90-100 kg/ ha (80-90 lb/a) can be necessary1 .
SOP (sulphate of potash, K2 SO4 ) or other low chloride K sources are generally recommended for berries, and blueberry in particular, due to chloride toxicity. Potash is spring applied in raspberry and blueberry, and autumn applied in strawberry. Potassium fertilizers can be broadcast between the rows or banded with P and N fertilizers1 .
Secondary and micronutrients
Sulphur can be applied as elemental sulphur, ammonium sulphate, gypsum, SOP or thiosulphate with rates of around 30-35 kg/ha (25-30 lb/ac) being required annually. The application of magnesium (e.g., as magnesium sulphate or potassium magnesium sulphate) at rates of around 550 kg/ha (500 lb/ac) is recommended for Mg-deficient soils. Boron fertilization (up to 1 kg/ha) is also advised for both raspberries and strawberries if testing reveals low B levels (less than 0.5 ppm) in the soil profile1 .
Products and producers
Specific products, crop guides and fertilization recommendations for berry crops are available from leading speciality fertilizer producers, including:
- Haifa
- ICL
- Omex
- SQM
- Tessenderlo Kerley International
- Yara International.
Tessenderlo Kerley International offers a wide range of thiosulphate and SOP products for berry crops. These can be supply nutrients such as nitrogen, potassium, calcium and sulphur at different growth stages using a variety of methods, including fertigation, foliar and soil application (Table 2).
For blueberry, liquid fertilization with potassium thiosulphate (KTS® ) as a potassium source can deliver substantial benefits compared to traditional fertilization. In a 2018 trial on drip irrigated Biloxi blueberry plants in Isidro Mazatepec, Jalicso, Mexico, the grower observed that the number of harvested berries increased from 300 to 400 boxes per hectare after the first week of KTS® application. This translated to a yield increase of 1.4 t/ha with thiosulphate applications, providing the grower with substantial extra income.
Supercharged calcium for blueberry
Applying Omex’s Calmax Gold foliar fertilizer to blueberry plants improves overall uniformity of fruit ripening. Beneficially, this shortens the harvesting period by reducing the number of fruit pickings required. Dr Terry Mabbett outlines how foliar-applied calcium can increase yields and improve the quality of this ‘superfruit’ – as well as helping to prevent plant disease.
Recent years have seen blueberry take on the mantle of the world’s ‘superfruit’. Primarily due to the exceptionally high antioxidant content of this small fruit, but more generally because berries are rich in a range of mineral nutrients and vitamins, as well as being a good source of fibre.
Maintenance of these qualities in commercial fruit growing requires strict adherence to crop nutrition requirements. This means providing blueberry plants with all the essential nutrients, including: the macronutrients (nitrogen, phosphorous and potassium); micronutrients such as manganese, copper, and zinc; and so-called secondary nutrients like calcium and magnesium.
Raw blueberries are exceptionally rich in manganese while containing generous amounts of phosphorous, magnesium and calcium. These nutrients, especially calcium and phosphorous, have important roles in fruit quality and boosting the resilience of crop plants to disease. Calcium is arguably the most important and intriguing nutrient in this respect.
Fight the fungus
Blueberries have only been under intense cultivation for a relatively short period of time. But this has not stopped a range of plant diseases evolving alongside the crop to cause potentially serious economic levels of crop damage and loss.
The most widespread and serious of these is is root rot. This is caused by a fungus-like pathogen called Phytophthora cinnamomi. Like other root infecting pathogens, this resides on debris in the soil and is therefore a pernicious problem for perennial bush crops like blueberry which – by their very nature – will spend many years growing and yielding in the same soil.
Root rot of blueberries has traditionally been managed by using systemic, site-specific chemical fungicides, but such options are decreasing as established products lose their approval and are removed from the market. Furthermore, chemical fungicides have been shown to degrade fruit quality.
Blueberries have a pale waxy coating which acts as a natural barrier against fungus and helps preserve the fruit’s moisture. Many agrochemicals wash off this protective coverage, reducing the quality of the fruit. Foliar fertilisers such as Cal-max Gold, in contrast, do not affect this natural barrier and so help keep blueberries fresher for longer.
Furthermore, consumers are increasingly demanding organically produced fruit and vegetables requiring zero use of chemical fungicide. For organic growers and consumers, there’s little point in producing and eating blueberries for health reasons if this requires treatment with chemical fungicide.
Strengthening the blueberry
Fortunately, alternative disease avoidance and management options are now available to growers – making blueberry cultivation and production without fungicide a reality. Research shows that the manipulation of blueberry crop nutrition, for example, can be highly effective at managing diseases like root rot. All essential nutrients are required for proper plant growth and development. But one stands out from the rest in blueberry cultivation and that nutrient is calcium.
Soil calcium has been shown to disrupt the root infection process of northern highbush blueberries in the United States. However, relying on soil calcium alone is not a fail-safe option. While the soil may well record high levels of calcium, a lower proportion is likely to be present in a soluble form available to the roots.
Typically, a high proportion of soil calcium is ‘locked up’ as insoluble salts such as calcium phosphate, especially when soils are alkaline with a correspondingly high pH. Neither is supplementing the soil with an applied calcium fertilizer the best option because most granular calcium compounds, like agricultural lime (ground limestone) for example, will raise soil pH – to the dislike and detriment of ‘acid loving’ blueberries.
The foliar solution
With blueberry, the foliar application of soluble calcium, which results in rapid nutrient uptake by plant leaves, is the secret to success. This provides sufficient calcium to blueberry bush crops, and is an efficient way of suppling what is otherwise a poorly mobile nutrient.
Omex’s CalMax foliar range will correct any calcium deficiency, improve fruit set, enhance yield, and assist in maximising fruit firmness, storability, fruit colour and finish.
The latest field trial results from Chile, one of the fastest growing producers of this berry fruit and a world-leading exporter of blueberries, clearly show the benefits of foliar feeding with CalMax Gold, a high-calcium liquid formulation. Three applications of CalMax Gold (300 ml product in 100 litres water) were applied to the blueberry variety ‘Brigitta’ (Vaccinium corymbosum – highbush blueberry), first at the beginning of blossoming, second at 50 percent bloom and the third at full bloom.
Compared with the ‘no treatment’ control, CalMax Gold showed the following benefits:
- Increased number of fruits borne on each spur
- Higher yield by achieving more uniform fruit ripening
- Shortening of the harvest period by 2-3 weeks
- Reduction in the number of fruit pickings required
- Significant improvements in fruit quality
- Better fruit firmness even after a six-month storage period.
Quality parameters such as brix, firmness and size are of key importance for berry growers, suggests Tessenderlo, due to their ability to add value to small fruits. These valued characteristics are all influenced by crop fertilization.
Foliar applications of Tessenderlo’s SOP product K-Leaf® , for example, have been shown to increase blueberry, raspberry and strawberry fruit size. The company’s calcium thiosulphate product CaTs® should also increase berry firmness, as has been shown with other fruit types. Excessively high nitrogen levels should be avoided, suggests Tessenderlo, as these are likely to decrease brix. Using MOP as a potassium source for small fruits is also not advised due to their chloride sensitivity. There is also evidence that MOP has a negative impact on strawberry firmness.
Fertilization practices for small fruits are influenced by growing practice, particularly whether plants are grown in open field or under plastic and in glasshouses. Growing in tunnels typically favours fertigation, reports Tessenderlo, due to the need for frequent watering. This makes it practical to provide plants with small amounts of nutrients with each watering. Combining watering with fertigation is also favoured when growing in a substrate, especially in tunnels, since plants have a limited root zone.
Chile’s SQM is a world-leading primary producer of potassium nitrate (KNO3 ), The company reports that using potassium nitrate – commonly known as NOP (nitrate of potash) – in strawberry nutrient management results in higher yields and stronger plants2,3 . Other research shows how the use of potassium nitrate and calcium nitrate can ameliorate the negative effects of salinity on strawberry plants4 .
In trials, NOP was found to promote earlier bud break and more rapid flower development, leading to quicker fruit setting and larger size fruit. Feeding strawberries with nitrate via fertigation also promotes greater yields by increasing biomass and raising carboxylate and calcium content. Foliar NOP applications, meanwhile, are linked to greater plant leaf area, longer root and petiole length, and higher chlorophyll content. The use of chloride-free NOP also prevents injury to chloride sensitive strawberry plants.
Crop trial evidence suggests that foliar applied NOP, because it is effective at inducing bud break, has a beneficial effect on strawberry plant growth and development. Applying doses of NOP via a foliar spray, by outperforming other dormancy breaking agents, increased strawberry plant flowering and fruit weight.
Similarly, SQM reports that foliar NOP treatments are highly effective at increasing firmness, fruit diameter and fruit weight in blueberries.
SQM’s provides a range of water soluble and micronutrient-enriched NPK fertilizers for fertigation use with berries. These include:
- Ultrasol® Berries Soil – a formulation specifically designed to enhance nutrient uptake for raspberry, blackberry and cranberry
- Ultrasol® Strawberry Soil – a formulation specifically designed for strawberry crop nutrition
- Ultrasol® Blueberry Soil – a formulation tailored for blueberry growing.
The Ultrasol® strawberry formula provides this berry type with potassium and nitrogen at an ideal N:K ratio of 1:1.5. The blueberry formula, meanwhile, is acidified to enhance the nutrient uptake by this acid-loving berry.
References
