Avocado fertilization

CROP NUTRITION

Avocado fertilization

Mexico supplies more than half of the international avocado market. This oil-rich, nutritious fruit is also widely grown in Colombia, the Dominican Republic and Peru. We look at the nutrient needs of this regionally-important cash crop.

Central American native

The avocado (Persea americana) is a native of Central America and the West Indies. Written accounts date back to the early 1500s when the Spanish conquistadors encountered extensive avocado growing by the Aztec and Incas. The fruit was later introduced into Florida, California and Hawaii in the early 1800s and is now cultivated widely globally where growing conditions are suitable1 .

World production of avocados exceeded eight million tonnes in 2020, having more than doubled in a decade (Figure 1). Four Latin American and Caribbean countries – Mexico, Colombia, the Dominican Republic and Peru – dominate production being collectively responsible for almost 60 percent of global output (Figure 2). Mexico is both the world’s lead avocado producer (2.4 million tonnes) and exporter (1.3 million tonnes).

Global exports of avocado reached 2.3 million tonnes in 2020. Peru, Colombia and Kenya are the three emerging exporters with a combined global trade share of 25 percent The US is the largest import market. The EU also accounts for a substantial share of world consumption.

Looking ahead, world avocado production is likely to expand to 12 million tonnes by 2030, according to OECD-FAO projections, triple its 2010 level.

Heavy fruiting evergreen

Avocado is evergreen tree whose growth habit varies from tall and upright to well-shaped and spreading. The fruit has one large central seed and can vary greatly in size, shape, colour, texture and flavour. The edible part – the flesh between the seed and the skin – is cream to yellowish-green in colour with the consistency of soft butter when ripe. The fruit is unusual in that it does not ripen until harvested and can be left on the tree for some time on reaching maturity1

There are three ‘races’ of avocados – Guatemalan, Mexican and West Indian – as follows1 :

• The Mexican avocado is the hardiest and most tolerant of the cold. Mature trees can withstand temperatures down to minus five centigrade without damage, although its flowers are frost-prone. Zutano, Bacon, Shephard and the rootstock Duke are all Mexican types.
• The Guatemalan avocado originated in the tropical highlands and requires a cool tropical climate without extremes of humidity and temperature. Trees can withstand light frosts down to minus 2°C. Gwen and Reed are two common varieties.
• The West Indian avocado originally came from the humid lowlands of tropical Central America. This race is the most tolerant to saline soil/water conditions while being the least tolerant of cold weather.

Despite their distinctive features, all three avocado races can be cross-pollinated to develop new varieties. Hass – the most popular variety grown today – is one such hybrid.

Avocados can contain from between 5-40 percent oil, the percentage depending on the variety and the growing and seasonal conditions. Only ripe olives have a higher oil content. This oil is rich in mono and poly unsaturated fatty acids with Hass fruit containing up to 83 percent, for example. Avocados are also vitamin-rich with high levels of B complex, vitamins A and E, folic acid and iron. The fruit is also cholesterol free.

Fig. 1: World avocado production, 2010-2020

Fig. 2: Top 10 avocado producers, 2020

Mexican growing experience

In 2014, Tessenderlo Group published a handbook on avocado crop nutrition2 in collaboration with its partners Mexico’s National Institute of Forestry, Agriculture and Livestock Research (INIFAP) and the Sulphate of Potash Information Board (SOPIB). This Spanish language publication provides useful evidence and advice on avocado nutrient management using soil-applied and foliar fertilizers to increase crop productivity and quality.

The handbook is based on agronomic experience gained in Michoacan, Mexico’s foremost avocado growing region. This article partly draws on this handbook and its main findings, as kindly translated into English by Nicolas White, Portfolio & Knowledge Director at Tessenderlo Kerley International.

Maintaining soil fertility in the root zone is particularly important in Michoacan. Many of the region’s avocado orchards are grown on ‘tupure’, a deep and well drained andosol. This soil type is low in organic matter and nutrient deficient – mainly for nitrogen, phosphorus, calcium and micronutrients such as zinc, boron and manganese. Low soil fertility is compounded by high rainfall conditions2 .

Nutrient requirements

In general3 :

• Adequate nutrition of avocados is very important for sustained performance.
• Inputs of nutrients should be increased when trees have set a heavy crop.
• Avocado trees may not show deficiency symptoms in the orchard. Yet these may reduce crop yield and tree performance. l Nitrogen, boron and zinc are particularly important in avocado nutrition and target levels in leaves must be maintained.

Example target nutrient levels for Hass avocado3 are shown in Figure 3.

Nutrient deficiencies can negatively affect avocado yield and fruit quality. Deficiencies can reduce fruit yield by up to 80 percent, for example. Sufficient nitrogen supply, in contrast, can increase yield and fruit size by at least 30 percent. Nutrient shortfalls can also make trees more prone to physical damage. Magnesium and manganese deficiencies, for example, are linked to a higher risk of frost damage2 .

Nutrients such as K, Mg and Ca, even when supplied in adequate quantities, may still not be fully plant-available. This can be due to the soil’s low cation exchange capacity (CEC) and poor soil moisture levels at the driest time of the year. These three cations also need to be present in the soil at an ‘ideal’ ratio for good avocado cultivation2 :

• Ca/Mg = 6.0
• Ca/K = 12.0
• K/Mg = 3.4

For avocado, it is important to provide nutrients in the root zone while maintaining soil moisture levels (60-85%) yet without over saturation, as this promotes leaching and nutrient percolation. Nutrient application timings and application rates are also important factors for fruit quality and yield2 .

Fig. 3: Target nutrient levels for Hass avocado

Nutrient removal

Nutrient removal by avocado depends on factors such as variety, planting density and productivity. Examples for Hass avocados are provided in Table 1 based on a fruit yield of 10 t/ha. Potassium is the most in demand nutrient (45-153 kg) with the quantity removed exceeding that of nitrogen by a factor of 1.5-2. Hass avocados, nevertheless, do still take up nitrogen in large amounts (40-70 kg). Phosphorus and calcium removal are lower but broadly similar (6-11 kg) and should also be included in the fertilization programme2 .

Additionally, avocado requires quite a lot of sulphur. Sulphur removal can even exceed that of phosphorus by as much as four times. Sulphur can be supplied alongside potassium in low-chloride products such as sulphate of potash (SOP) and polyhalite.

Table 1: Nutrient extraction by avocado (kg) for fruit yield of 10 t/ha

Table 2: Suggested annual fertilization for Hass avocado (Kg/ha). These amounts are needed to replace exported nutrients and maintain soil fertility

Nutrient application

Examples of yearly macro nutrient and micronutrient application rates for Hass avocado are shown in Table 2. These are required to deliver a fruit yield of at least 10 t/ha and large size avocados (at least 25 percent of fruit greater than 200 grams). Any annual fertilization programme does however require the monitoring of soil nutrient availability and measurement of nutrient concentration in mature leaves.

Nutritional status

The nutritional status of avocado can be monitored by measuring leaf nutrient concentration (N, P & K etc.). Within leaves, individual nutrients peak at different times during the growing season. In Mexico, nitrogen’s maximum concentration occurs in March, for example, while that of phosphorus and potassium takes place in May. Leaf nutrient concentrations for K and P are also more variable than that of N.

Typically, leaf nutrient levels will reflect soil nutrient availability over the preceding 30 days. Plant take-up of N, P and K, and other important nutrients such as calcium, magnesium and zinc, therefore requires frequent fertilizer applications at low doses to maintain nutrient supply2 .

The importance of potassium

While nitrogen is essential for avocado tree growth and development, potassium is the key activator of internal processes within the tree. Although soil nutrient levels of more than 200 ppm are typically maintained in orchards, not all of this potassium will be plant-available. Indeed, uptake of potassium by avocado generally requires soil moisture to be kept above 50 percent2 .

The important functions of potassium include:

• The synthesis of carbohydrates and their transport to reserve bodies during photosynthesis
• The synthesis of amino acids and proteins from ammonium, which requires K availability in the soil
• Increases plant resistance to drought, as it influences the opening and closing of the stomata which regulate water transpiration
• Increases the consistency and hardness of plant tissues
• Strengthens the root system
• Increases the resistance of plants to frost.

Potassium also plays a vital role in quality by intensifying and conserving the colour and flavour of fruits. Excessive K supply should, however, be avoided as it can decrease the availability of Cu, Ca, Mg, P and Zn2 .

Typically, each tonne of avocado fruit extracts 4.5 kg of potassium versus 2.5 kg for nitrogen and 0.6 kg for phosphorus. Potassium generally accumulates in areas of vegetative activity and when there is a shortage will move to areas of greatest activity such as young leaves. Deficiency is therefore usually shown in older leaves first2 .

While potassium influences plant processes as diverse as breathing, photosynthesis, chlorophyll and water content in leaves, its key role is arguably as an enzyme activator. This explains its presence in high concentration in the newly formed parts of branches.

Potassium deficiency can be prevented by:

• Supplying 100-450 g/tree/year to young trees (1 to 4 years)
• Supplying 900-1,100 g/tree/year to adult trees (over 6 years).
• Trees also respond well to foliar application of K.

High-chloride fertilizers such as potassium chloride (MOP) cannot be used for avocado. Trees will not tolerate high levels of chloride, either in the soil or in the irrigation water. The maximum chloride tolerance in leaf dry matter is 0.25 percent with plants showing toxicity symptoms above this level2 .

Potassium sulphate (SOP)

The application of Tessenderlo’s foliar booster product K-Leaf® (K2 SO4 , 00-0052+18.5S) has been trialled on avocado in Michoacan, Mexico, with results. Foliar K was applied as a three percent spray solution with a leaf penetrant (HCA). This foliar application was to supplement not replace soil fertilization with potassium. Recommendations (K2 O) are to maintain this at 150-300 kg/ha using Tessenderlo’s soil-applied SOP product Granupotasse®2 .

OMEX foliar programme finds success in Mexico

Avocados are mainly produced in the central areas of Mexico – with two regions, Michoacán and Jalisco, accounting for 90 percent of the country’s avocado cultivation. Avocado fruit are produced all year round from 2-3 flowering seasons.

Drawing on its experience in Mexico, OMEX Agrifluids offers a complete programme of foliar fertilizers and biostimulants for avocado, including:

• The seaweed-based biostimulant Bio 20 containing macro and chelated micronutrients
• CalMax Super (Gold) containing foliar calcium together with nitrogen, magnesium and trace elements plus amino acids
• The high concentration liquid suspension product Kingfol Zn for low rate applications
• Highly concentrated suspension fertilizer K41 containing foliar potassium with nitrogen, magnesium and sulphur
• The potassium phosphite product DP98 which promotes root growth in young plants.
• Highly concentrated Sulphomex fertilizer containing water-soluble sulphur and nitrogen
• All the following foliar fertilizer recommendations for avocado are at an application rate of two litres per hectare unless otherwise stated.

Young avocado trees (1-3 years)

Plant growth and structure benefits from the foliar application of Bio 20 and Kingfol Zn (0.5 litres per hectare) to promote a physiological response in the plant tissues. Treatment with DP98 also improves plant health and disease resistance.

Mexican avocados grown with OMEX foliar products.

PHOTO: OMEX

Mature avocado trees (more than 3 years old)

When flowering begins, CalMax Super and Bio 20 are applied to leaves to promote flowering and fruit set. Avocado fruit size and quality can also be improved by the application of K41. Foliar treatment with CalMax Super (Gold) (1 litre per hectare), meanwhile, has multiple benefits, including better fruit set, increased yield and improved firmness, storability, colour and finish.

Additionally, OMEX recommends foliar applications of Kingfol Zn (0.5 litre per hectare) and DP98 throughout the crop cycle to promote plant health and stimulate roots.

Finally, Sulphomex mixed with fungicide and insecticide can be applied as part of the foliar fertilization program to control (2-3 litres per hectare) or prevent (3 litres per hectare) disease and mites.

Supplementary treatment with K-Leaf® increased total foliar potassium levels. This improved supply was reflected by a higher fruit yield of 14 kg of fruit per tree, relative to soil applied SOP alone, or a 40 kg/tree increase versus no potassium application at all. This translates to a marketable yield improvement of about 1.4 t/ha or 4.0 t/ha, respectively. The treatment also had a direct effect on fruit quality by increasing oil content and pulp dry matter. This improves shelf life as well as fruit flavour, consistency and colour, all qualities valued by the consumer2 .

Avocado trial results in Michoacan, Mexico, during 2004-2006 have demonstrated the benefits of potassium sulphate (SOP) fertilization as part of a nutrient management programme alongside adequate nitrogen, phosphorus and micronutrients. Supplying 450 kg of Granupotasse® in three soil applied applications (150 kg/ha each) combined with 3 percent foliar applied K-Leaf® was shown to increase fruit yield by 40 percent versus the control2 (no potassium application).

References

1. Dirou, J., 2003. Avocado growing. Agfact H6.1.1, Third edition. New South Wales (NSW) Centre for Tropical Horticulture, Alstonville.

2. Tapia Vargas, L. M., 2014. Avocado nutrition handbook. Experience in the region Michoacán region, Mexico. [In Spanish.] National Institute of Forestry, Agriculture and Livestock Research (INIFAP), SOPIB, Tessenderlo Group.

3. Sale, P., 1996. New Zealand Avocado Growers Manual. NZ Avocado Growers Association.