Cashew - Climate, Soils And Nutrients

Temperature

The most suitable average annual temperature for cashew trees is 24 - 28 degrees Celsius, the average maximum temperature is 38 degrees Celsius and the average minimum temperature is 18 degrees Celsius. Cashew trees prefer high temperatures, which does not mean that they require constant temperatures. For instance, in East Africa, the temperature varies according to the seasons as below:

- Dry winter: average temperature is 18 - 24 degrees Celsius, average maximum temperature is 25 - 30 degrees Celsius, average minimum temperature is 12 - 19 degrees Celsius.

- Humid summer: average temperature is 26 - 28 degrees Celsius, average maximum temperature is 30 - 32 degrees Celsius, average minimum temperature is 20 - 24 degrees Celsius.

At the production stage of the cashew plant, low temperatures will affect flowering and fruit production, while in the development stage of young fruits, temperatures of 40 degrees Celsius sometimes cause flowers and fruits to drop.

Young cashew trees are sensitive to low temperatures, while mature cashew trees can withstand temperatures at 0 degrees Celsius for a short time. In southern Mozambique (latitude 26 degrees south) there are cashew varieties that can withstand low temperatures of 7 - 8 degrees Celsius during the nights of July and August. No damage to the leaves or soft tissue rot can be seen, the growth and reproduction phases become active again as soon as the temperature increases. From this fact, perhaps not far away, cashew trees will also be grown at higher altitudes (lower temperatures) thanks to genetic modification techniques as done with other crops.

Sunshine

As known, cashew is a sun-loving tree and produces fruit at the tips of the branches. Sunshine is one of the key factors for the cashew tree. To complete its reproduction phase (occurs during the rainy season) and production phase (occurs during the dry season) and to give a full yield, it must have required sunshine uniformly distributed over its entire foliage. On average, the optimal amount of sunshine is 2,000 h/year. For example, in Togo, the optimal amount of sunshine is held to be 2,464 hours/year with 1,285 hours in the flowering/fruiting time. In Brazil, the optimal sunshine lies between 1,500 and 2,000 h/year, while the amount of sunshine for India is considered to be 9 h/day from December to May and down it goes from June to November. In general, cashews are best grown in sunny, outdoor areas. Cloudy conditions reduce pollination and are favorable for outbreak of insect pests and diseases. Cloud cover, therefore, is also an important indicator to be considered. The suitable value of cloud cover for the cashew growing area is 3 - 5 tenths, the average is 4 tenths, 5 tenths is the final limit that is no longer suitable for cashews.

Relative humidity

Cashew trees are suitable for relative air humidity of 65 to 80%. In the flowering and fruiting time, low relative humidity will be fine for better output, whereas high relative humidity will affect the quantity and quality of production. However, in areas with minimum rainfall, high relative humidity during flowering and fruit-set is beneficial because cashew trees can balance their water requirements and can overcome periods of severe water shortage at the end of the dry season.

Wind velocity

Cashew is mostly grown along the sea-coast in almost all the cashew growing countries, where it is exposed to windy conditions throughout. Wind plays an important role in the dispersal of pollen and helps cross-pollination. The optimal wind speed for cashew growing areas may be estimated around 2 to 25 km/h. However, high velocity wind is harmful and causes flower and young fruit-drop, leading to failure in cashew planting. As a telling example, in the Fiji Islands, Antilles or salty wind (the wind contains salt) causes sunburn in young leaves and sprouts, or dry winds in West Africa increase water evaporation, leading to physiological imbalance during the flowering and fruiting period.

In summary, the climatic factors affecting cashew trees are:

- Dryness during the flowering and fruiting period will ensure better output.

- Cloudy conditions during flowering will increase flower wilting due to tea mosquito infection.

- Heavy rain during the flowering period will harm production.

- High temperatures (39 - 42 degrees Celsius) at the stage of young fruit growing (the size of a marble) will cause fruit-drop.

- Shorter dry periods, better cashew trees grow.

Soil

Cashew is often grown on idle lands, grown on many types of soil: loose sandy soil, volcanic soil, alluvial soil, iron-containing soil, Feralit soil, but they prefer friable, deep, well-drained soil (cashew trees do not like any form of water stagnation) with a pH of 4.5 to 6.5. With hard soils with a solid surface or shallow layers of gravel, despite natural fertility in soil, the cashew tree still grows slowly because this type of soil prevents the root system from finding water and nutrients. in a large volume of land to grow plants, especially during the dry season. It is suitable for fairly steep slopes with shallow top soils because its large canopy and surface root system protect the soil very well from runoff and erosion during the heavy monsoon rains. Thus, the cashew tree is more sensitive to physical properties than to the chemical properties of the soil, and the physical quality of the soil has a great influence on the results of cashew growing. The lack of certain nutrients in the soil that plants need can be easily overcome with appropriate and timely fertilization.

Nutritional requirements

Although cashew is an easy tree to grow, cashew trees also need to be provided with adequate nutrients for growth to obtain their high yield, especially in poor soil conditions.

According to a study written by Ohler and Coestere (1973), the manifestations of nutritional deficiencies affecting the growth of small cashew trees can be divided into the following three groups:

- Dangerous deficiencies that are lethal; in order of seriousness: Fe, Mg, K, N and Mo.

- Deficiencies that developed early but were not lethal. In order of severity: S, Ca and Zn. 

- Deficiencies that developed slowly and did not cause severe effects; in order of importance: P, B and Cu.

Only through soil analysis can specifically determine nutrients that are lacking to supply  cashew trees with these  substances.

Results of studies conducted in Brazil and India showed that cashew plants respond well to Nitrogen and Phosphorus treatment but no response and less response with Potassium. On the other hand, calcium has a negative impact on the plant because this plant prefers soil with weak acidity.

According to Mohapatra and Vijaya Kumare Bhat (1973), a mature tree with high yield annually consumes 2.84 kg N, 0.752 kg P2O5 and 1.265 kg K2O. On this basis, it is possible to calculate the amount of fertilizer needed for cashew plants and plans for soil improvement.

Table - Optimal ecological characteristics for developing cashew production

Vietnamese conditions suitable for the ecological characteristics of cashews:

1. Temperature

The South Central Coast, Central Highlands, Southeast and Mekong Delta all have temperatures very suitable for cashew trees. However, in the Central Highlands, cashews should not be grown at altitudes above 600 m because the temperature is low there.

2. Rainfall

In the entire territory of Vietnam, the mean annual rainfall is suitable for cashew trees. However, the north is more favorable for cashew growth than the south because it has two distinct rainy and dry seasons and a dry season is long enough and coincides with the flowering and fruiting season of the cashew tree. The relative humidity of the air on average during the year and during the dry season in the south is also lower than in the north, which will be favorable for cashew trees to bear flowers and fruits.

3. Sunshine

Only southern provinces have enough sunshine hours (2000 hours/year) for cashew growth.

4. Soil

In southern Vietnam, there are many types of soils that can be planned for growing cashews without worrying about competing with other economic crops. These types of soils are all located in the cashew ecological zone (especially the Southeast region and South Central Coast) such as red sandy soil on the Binh Thuan coast, white sandy soil on the South Central coast, ancient gray alluvial soil (the main soil type occupying a large area in the Southeast region), basalt (including 3 main types: red basalt soil, degraded basalt soil and basalt soil mixed with pumice-distributed mainly in the Central Highlands provinces). These types of soil are mostly bare hills and mountains that need to be greened, so they are very convenient for plans to expand cashew growing areas.

Table - The guidelines for selection of soil for cashew plantation (Mahopatra & Bhujan, 1974-Land selection for Cashew plantation - A survey report)

REFERENCE:

Thanh P. D. Cashew Nuts - Production and Processing (2003).