The almond tree: cultivation requirements, plant material and nutrition

INTRODUCTION

The almond tree is one of the most widely cultivated fruit crops , after olive groves and vineyards, both in Andalusia and nationally. Traditionally, it has been grown under challenging environmental conditions (topography, climate, and soil), and is cultivated under dryland farming conditions.

In much of the cultivated area, these adverse conditions are extreme, resulting in a marginal crop with yields that do not exceed 200 kg/ha of shelled almonds. The low productivity of these plantations has a significant component in the insurmountable limitations imposed by the poor environmental conditions (poor soils, dryland farming, etc.), but there is also another negative component due to the management practices that are relatively common (insufficient application of fertilizers and pesticides, poor pollination, early varieties susceptible to frost and disease, etc.). As in other economic sectors of our society, in recent years agriculture has needed to evolve to guarantee the profitability of farms. The search for more productive and efficient systems has therefore been the common goal of many fruit crops.

In countries where almond cultivation has been more recently established, such as the United States, the approach has been "intensive" cultivation , under optimal soil and climate conditions, with high irrigation levels and advanced technology. Under these circumstances, they are achieving production levels of around 2.500 kg/ha of shelled almonds.

In recent years, the almond tree has gone from being a marginal crop grown in the poor, arid soils of the Spanish countryside to an alternative to other fruit trees , olive groves, citrus fruits, and even extensive herbaceous crops. This development is further fueled by the increased demand for almonds, especially in the United States, where California, the world's largest producer, is unable to meet its own domestic demand.

 

This situation, combined with other factors such as the promotion of new late-blooming varieties (March and April) that are not affected by frost, the implementation of irrigation (generally deficit irrigation), and mechanization , has led to a significant change in almond cultivation in Spain. These new plantations have very different characteristics from traditional ones, requiring new cultivation practices adapted to this type of planting.

According to the FAO (FAOSTAT), the total area dedicated to almond cultivation in 2018 was 2.071.884 hectares, with Spain having the largest area dedicated to almond cultivation at 657.771 hectares, followed by the United States with 441.107 hectares. In terms of production, the United States leads with 1.871.500 metric tons, followed by Spain with 337.915 metric tons. It should be noted that average yields in Spain range between 350 and 400 kg/ha, while in the United States they can reach up to 4.000 kg/ha, due to the more intensive nature of its plantations, water availability, the use of high-yield varieties, and their location in areas that are practically frost-free.

In Spain, almond production is concentrated mainly in the Mediterranean coastal regions: Andalusia, Castile-La Mancha, Valencian Community, Murcia, Aragon, and Catalonia (Figure 1). This production is highly variable due to drought and the incidence of frost , which affect flowering and subsequent fruit set.

DEMANDS

The almond tree is a typically Mediterranean crop , adapting to very diverse soil and climate conditions. It can withstand high summer temperatures and intense winter cold . It can survive long periods of drought and be cultivated in very poor soils . However, all these circumstances will negatively affect its yields , which are higher the better the environmental conditions.

Climate

• Temperature

For temperate zone fruit trees, such as the almond tree, the optimal temperature range for photosynthetic activity is between 25-30 ºC, with a sharp reduction occurring at temperatures below 15 ºC or above 35 ºC.

For the vegetative period to begin and for proper flowering and fruit set to occur, certain winter chilling requirements must be met , followed by certain heat requirements. To quantify winter chilling requirements, the concept of chilling hours (CH) is used , which is the number of hours the tree must spend at temperatures below 7,2°C . The almond tree has low CH requirements (between 100 and 400, depending on the variety), similar to those of other typically Mediterranean fruit trees, such as the fig or olive.

The almond tree is one of the most cold-resistant fruit species during its winter dormancy period, able to withstand temperatures below -15°C. Newly set fruit are the most sensitive parts to frost, followed by the flowers and swollen buds . These parts can be damaged at temperatures slightly below 0°C. The almond tree is one of the earliest flowering fruit trees, although, as will be discussed later, there are significant varietal differences in flowering time . Therefore, in areas with a risk of frost, special care must be taken to select late-flowering varieties (Arquero, 2013).

High temperatures can cause serious damage to plants. As mentioned previously, above 35°C, photosynthetic activity is significantly reduced , causing trees (especially those in dryland conditions) to enter what is known as summer dormancy . Higher temperatures, above 40°C , can cause dehydration, necrosis and leaf drop, fruit damage, and wood burn.

• Pluviometria

This is one of the main limitations to production . Rainfall in Mediterranean climates is typically scarce and unevenly distributed throughout the year, resulting in a period of water deficit that extends through the summer and into spring and autumn. This is compounded by the frequent occurrence of dry years. The almond tree is a crop well-adapted to dryland conditions, although its production increases significantly with irrigation.

• RH

This is a crucial factor, as very high or excessively low relative humidity (RH) levels induce stomatal closure in plants, negatively impacting vegetative and reproductive activity. Similarly, the incidence of diseases is greatly favored under high RH conditions, which can become a limiting factor for crop establishment.

We can partially control RH within the plantation through cultivation techniques. For example, plantation design and training and pruning systems can reduce tree and canopy density, thereby improving ventilation within the plantation.

• Wind

Strong winds significantly increase the transpiration rate , causing water stress in the plant. They also amplify the negative effects of extreme temperatures, both low in winter and high in summer. Tree formation is greatly hampered in areas with constant winds. As with rain or fog, bee activity is reduced by strong winds, negatively impacting pollination. Finally, very strong winds can cause physical damage to the trees : falling flowers and fruit, broken branches, and even the uprooting of entire trees (Arquero, 2013).

In areas with strong and frequent winds, stakes are necessary to ensure the tree's anchorage and upright position in the early years. It's also important to design the planting properly to facilitate air circulation, as well as pruning with adequate intensity to thin the canopy sufficiently so that it doesn't create a screen effect.

Land

• Physical properties

  • • Texture

Heavy or clayey soils can present problems due to poor aeration and limited water mobility. In contrast, they have a greater capacity to retain water and nutrients. Silty soils suffer from similar problems, in addition to a poor structure that favors crust formation and erosion. Sandy soils are highly permeable, barely retaining water or losing it very quickly through percolation. Although they facilitate root growth and the movement of air and water, they are also loose and easy to till, but result in infertile soils with a poor water retention capacity.

 

Heavy soils can lead to waterlogging and poor aeration , causing root asphyxiation , to which almond trees are very susceptible. To prevent this, it is recommended to use resistant rootstocks or those with a shallow root system, to ensure proper drainage before planting, and/or to plant on raised beds.

• • • Effective depth

The effective depth of soil is usually determined by the presence of a water table or a hardened horizon. The latter can be petrocalcic (accumulation of calcium carbonate), argillic (accumulation of clay), or composed of bedrock. All of these physically impede root development , affecting the roots' ability to extract water and nutrients from the soil. This, in turn, affects the vegetative and productive stages of the crop, especially under rainfed conditions.

Deep subsoiling work prior to planting can significantly improve the usable depth by breaking up petrocalcic horizons, as well as reducing soil compaction or soil erosion.

The almond tree can be grown in shallow soils, although it must always be kept in mind that, with greater effective soil depth, we will have greater root development and a higher availability of water and nutrients , considerably improving the vegetative and productive states of the tree.

• Chemical properties

Among the most important chemical properties of the soil are (FAO, 1984):

• • • Organic material: The optimum should be around 2%, although in many soils where almonds are grown it is common not to exceed 1-1,2%. Therefore, organic fertilization is one of the aspects that needs to be improved in the crop.
    • Carbonates, have a positive effect on soil structure and on the activity of microorganisms, but can reduce crop yields by limiting the response to fertilization when found in concentrations above 20%.
    • active limestone, are fine carbonate particles, very chemically active and which can interfere with the normal development of the almond tree when they exceed concentration values ​​of 9%.
    • pH, greatly influences crop development by determining the solubility of nutrients, the growth of microorganisms, the speed of humification and mineralization processes, and the cation adsorption capacity of the exchange complex. Almond trees tolerate soils with pH values ​​between 5,5 and 8,4.
    • Salinity, measured through Electrical Conductivity (EC), which determines the main cations (sodium, calcium, magnesium and potassium) and anions (sulfates, chlorides, carbonates and bicarbonates) in the soil. The almond tree is a crop relatively tolerant to soil salinity, being able to grow up to EC values ​​of 4 dS/m; however, above 1,6 dS/m, the crop's productive capacity would begin to decline.
    • Fertility levelsA C/N ratio of 10-12 with a total nitrogen percentage (Kjeldahl) of 0,11-0,2 would be optimal for the almond tree.

Irrigation water

The main negative consequences of using poor quality irrigation water are soil salinization and sodification , as well as phytotoxicity in the plant, which can cause severe damage to the crop and even seriously alter the physicochemical characteristics of the soil.

A sodium absorption ratio (SAR) greater than 9, chlorine levels above 355 mg/l, and boron levels above 3 mg/l represent a high level of restriction for almond cultivation. The EC of irrigation water will have a greater or lesser impact depending on the SAR, although normal values ​​for this parameter can be estimated to range between 1,1 and 3,2 dS/m (Micke, 1996).

PLANT MATERIAL

As with other fruit species, the almond tree is composed of two individuals: the variety , which constitutes the aerial part of the tree, and the rootstock , which constitutes the root system.

Current genetic improvement programs were created with the aim of contributing to increasing the competitiveness of plantations, through the improvement of the quality of plant material.

Patterns

When choosing a rootstock, the following aspects, among others, must be taken into account: compatibility with the variety, plant uniformity, vigor, plantation longevity, behavior under adverse soil conditions, adaptation to extreme climatic conditions, suitability for the cultivation system (irrigation/dry land, replanting, etc.), resistance to soil-borne pests and diseases, influence on productivity and fruit characteristics.

The most commonly used patterns in almond trees and their main characteristics are (Arquero et al., 2013):

Considering the specific requirements of the soil and the sensitivity of almond rootstocks to waterlogging , Cultifort recommends using OXIFORT to prevent and/or recover almond trees that have suffered root asphyxiation . This long-lasting and effective soil oxygenator not only improves aeration and water circulation in the root zone , increasing soil porosity and sponginess, but also inhibits the growth of anaerobic microorganisms, promoting beneficial aerobic microbiota and thus improving soil fertility.

Variety

The availability of quality plant material is a fundamental factor in establishing new plantations. Traditional varieties have remarkable characteristics, but also significant limitations. For example, Marcona and Desmayo Largueta, among the most widespread, produce fruit highly valued by Spanish consumers; however, they flower early (making them susceptible to frost damage), require pruning, are prone to disease, and so on.

This highlights that the variety with the highest yield potential does not always achieve the highest profitability . Frost, pest and disease infestations, pruning costs, etc., negatively affect almond tree yields. Therefore, the specific objectives of genetic improvement programs have focused on obtaining new varieties, taking into account characteristics such as:

• Phenology

The new Spanish varieties are late or very late flowering , which helps to avoid or minimize frost damage . The delay in the flowering date also helps to improve the effectiveness of pollinators , potentially increasing fruit set, as they work under better weather conditions (Vargas et al., 2009).

At Cultifort, we've already addressed the problem of frost damage in woody crops in previous webinars, where we covered all the principles used to control low temperatures, both before and after planting, including the application of nutritional products and/or biostimulants. In this case, our recommendation was BVC EVOLUTION (formerly BVC 2021) , a plant metabolic activator that works even under adverse conditions (temperature, light, wind, etc.), based on amino acids and algae in emulsion. Besides increasing resistance to low temperatures and adverse weather conditions , it generally helps plants overcome stress. It increases the rate of photosynthesis, improves nutrient assimilation and translocation, increases the synthesis of amino acids, peptides, and proteins, and delays leaf senescence, among many other effects.

 

The flowering date has both a genetic and an environmental component . Because it depends on the prevailing temperature regime, the flowering time for the same variety and year will differ in areas with varying climatic conditions. For the same reason, for the same variety and location, very different flowering dates can be recorded in years with very different weather patterns (Arquero et al., 2013).

It's relatively common to choose varieties that don't fully coincide in flowering dates in self-incompatible multivarietal plantations, resulting in poor pollination and a reduced yield. For all these reasons, flowering time must be an important factor to consider when choosing a variety.

Another important phenological aspect to consider is the ripening time , especially in multi-varietal plantations, to establish the desired degree of staggered harvesting.

• Compatibility

The almond tree is a species with complete flowers (possessing both male reproductive organs, stamens, and female reproductive organs, pistils) and entomophilous pollination by insects (mainly bees). There are self-incompatible and self-compatible varieties . In self-incompatible varieties, fertilization of the flowers with pollen from the same variety is not possible, while in self-compatible varieties, self-pollination (fertilization of the ovule with pollen from the same variety) is possible. If the self-compatibility trait is complemented by a floral morphology that has a suitable arrangement of the anthers relative to the stigma, such that pollen grains can be positioned on the stigma spontaneously without needing to be transported by insect vectors, the trait of self-fertilization or autogamy is achieved for a variety (Figure 4).

The self-incompatibility characteristic requires the planting of at least two intercompatible varieties that also bloom at the same time . It is advisable to plant at least 30% of the area with the pollinating variety(s) and the remainder with the main variety. Likewise, the presence of bees is strictly necessary , as are favorable climatic conditions (absence of rain, wind, and fog, and moderate temperatures) during the flowering period to allow for good bee activity (Arquero et al., 2013).

Self-compatible cultivars allow for single-variety plantings . Furthermore, if they are self-fertile, pollination becomes less dependent on insect vectors. Therefore, self-fertility is essential for single-variety plantings and/or those with a low bee population.

Cultifort recommends applying FOLIQUINO to improve the activation of the almond tree's vegetative cycle, achieve higher quality flowering, and more effective fruit set . This energy-rich biostimulant, formulated with organic acids , is easily absorbed by leaves, branches, woody parts, and the root system . Its effectiveness is based on the potent systemic action of aluminum lignosulfonate, whose upward and downward mobility within the plant helps balance the aerial parts and the root system , improving the sprouting of dormant buds and thus resulting in a greater quantity and quality of flowering . Among its physiological and metabolic functions, it is notable for increasing the synthesis of various metabolites, primarily polyphenols (activation of the shikimic acid cycle), but also phytoalexins . Both polyphenols and phytoalexins are part of the plant's natural defense mechanisms : polyphenols as natural antioxidants, and phytoalexins as antimicrobial compounds with fungicidal and bactericidal action, which help limit the development and proliferation of pathogens. FOLIQUINO also plays a role in the construction of new cell structures and the regeneration of vascular tissues , making it an important ally in the prevention of wood diseases . Furthermore, it promotes nutrient penetration and improves the systemic action of phytosanitary treatments.

 

 

• Ease of pruning

In well-managed irrigated orchards, the trees can reach a considerable size. The limiting factor is sunlight , not water scarcity as in traditional dryland farming. Where sunlight doesn't reach, fruit doesn't develop and the branches age more rapidly. Varietal vigor, along with the almond harvesting machinery used, will determine the planting density.

Both vigor and growth habit will determine the type and severity of pruning to be applied to each variety. When working with varieties that are not very vigorous and begin producing very early, the development of a good tree structure should take precedence over production to avoid potential tree exhaustion. Conversely, in vigorous varieties, formative pruning can be less intensive to encourage a rapid start to production. Furthermore, in varieties with a compact growth habit, the opening of the main branches should be encouraged. Conversely, in varieties with a very open growth habit, the most compact main branches should be prioritized (Miarnau et al., 2015).

• disease resistance

There are clear varietal differences in susceptibility to disease . This must be kept in mind in areas with environmental conditions conducive to disease, as its presence necessitates a greater number of phytosanitary treatments, and if effective control is not achieved, the resulting damage can be substantial.

Figures 5 and 6 show the varietal differences regarding the incidence of Fusicoccum ( Phomopsis amygdali ), and Ocher Spot ( Polystigma amygdalinum ), (Miarnau et al., 2015).

 

• Productivity

There are clear varietal differences in yield . Early-flowering cultivars generally have lower productivity , and very late-flowering cultivars are significantly less productive . Table 4 shows the yield performance of the main late-flowering varieties.

The almond tree is one of the species with the earliest entry into production , being able to achieve considerable harvests in the 3rd - 4th year.

However, it is important to distinguish between varietal behavior regarding early entry into production and its productive level or potential at maturity , as these are not always related. Thus, some varieties are very early maturing but have low productive potential at maturity, and vice versa.

Another important production aspect is the degree of alternate bearing or biennial bearing (Figure 7). It is always advisable for a variety to maintain regularity in its harvests (Arquero et al, 2013).

• Characteristics of the fruit

Commercially, it is desirable for the almond to have a high kernel weight and a low percentage of double kernels , although these values ​​do not usually influence the purchase price for farmers.

The commercial value of almond cultivation lies in the fruit's kernel (almond kernel, seed, or kernel). The farmer delivers the almonds in their shells, after removing the outer husk. A sample is taken from the delivered batch, and the weight of the kernel or seed is determined relative to the weight of the whole fruit (almond in shell). This ratio, expressed as a percentage, is called the shell-to-kernel yield and is used to determine the total quantity of kernels, upon which payment is calculated.

It is common to associate a higher hull-to-kernel yield with a better productivity level. This statement is incorrect , since productivity is determined by the weight of kernels produced per unit area, and is independent of the hull-to-kernel yield. Thus, there are varieties like 'Cristomorto' that have high productivity with a

low yield; while others, such as 'Supernova', have high yield but medium/low production (Arquero et al, 2013).

NUTRITION AND FATTENING TREATMENTS

Traditionally, it has been considered that fertilization should replenish the soil with the nutrients extracted by the crop. This method of replenishment fertilization has serious shortcomings that make it inadvisable , the two most important being the lack of response to fertilization when an element is available in sufficient quantity in the soil solution, and the failure to quantify luxury nutrient consumption.

Currently, the annual fertilization plan for woody crops is considered to be based on the diagnosis of the plant's nutritional status , determined by foliar analysis , and should be complemented with a series of information or references, such as: soil characteristics, symptoms presented by the plant, vegetative and productive states, and crop management.

Foliar analysis is the best method for diagnosing a tree's nutritional status . It allows for the detection of low nutrient levels before deficiencies occur, establishes the response to applied fertilizers, and detects toxicities from chlorine, boron, and sodium.

The results of the foliar analysis must be compared with the critical levels for each nutrient. Table 7 shows the established values ​​for almond trees. Concentrations below the appropriate level lead to decreased growth and yield.

Almond fertilization is primarily carried out through soil application or foliar application.

The traditional and most widely used method is direct application to the soil . The fertilizer applied in this way passes into the soil solution, from which it is absorbed by the roots. This system has the lowest efficiency , expressed as the percentage of the element absorbed by the plant compared to the total applied. The main losses are due to leaching and volatilization into the atmosphere. To improve the efficiency of soil fertilization, certain application criteria or recommendations must be followed. Fertilization should be carried out when the tree is actively growing and, therefore, has the greatest capacity for nutrient absorption. It is recommended to split the application into two or three applications throughout the growing season. The fertilizer must be distributed uniformly across the entire surface, since the root system, in mature trees, explores the entire area. To avoid losses due to volatilization, localized applications should be made, incorporating the fertilizer into the soil through surface tillage, or it should be applied to the surface immediately before rainfall, so that the water incorporates it into the soil.

Foliar fertilization involves applying a nutrient solution to the tree canopy via spraying, where it is absorbed by the leaves. This system allows for rapid and efficient nutrient utilization . It is especially recommended for micronutrient application and for "shock" applications to achieve a quick response. For greater efficiency, applications should be made when the tree is actively growing and the leaves are fully expanded; it should not be carried out in strong winds or high temperatures.

In plantations with localized irrigation, it is recommended to apply fertilizers dissolved in the irrigation water, a technique known as fertigation . This system is the least expensive to apply and the most efficient , since the fertilizer is applied in a humid area with a high concentration of roots, maximizing nutrient absorption by the plant. Furthermore, by incorporating it with the irrigation water, fertilizer loss through volatilization is minimal . It is advisable to apply fertilizers as frequently as possible, which can be weekly or daily, throughout the entire irrigation period. This way, fertilizer concentrations in the water remain low, and constant levels are maintained in the soil (Arquero and Serrano, 2013).

It is very important to split fertilizer applications , especially nitrogen, which is specifically recommended in spring, summer, and autumn. Post-harvest fertilization should be used to supply or replenish nutrients that have shown deficiencies in foliar analysis and to ensure adequate levels of elements that will be most needed by the almond tree during the next flowering and the beginning of vegetative growth. Potassium applications are essential for obtaining good, uniform yields with well-sized fruit and for guaranteeing production levels the following year.

In any case, the products, concentrations, methods, and application times permitted or recommended for fertilizer applications must always be respected.

One of the cornerstones of almond tree nutrition is organic fertilization , a key factor in improving communication between the inert or mineral components of the soil and the plant. At Cultifort, we offer the best product on the market in its category to meet the requirements of organic fertilization. This product is MICROVITAL – L , formulated over 35 years ago, and its proven track record speaks for itself. It is an organic soil bioactivator of plant origin, rich in magnesium, micronutrients, organic complexes, and flavonoid molecules . In addition to its beneficial influence on the soil's physical, chemical, and biological aspects (activating the microbiota), it prevents deficiencies in magnesium and other key micronutrients, induces root formation and plant metabolic activity , and protects against the harmful effects of ultraviolet radiation thanks to flavonoid molecules (polyphenols). These molecules also exert antioxidant activity, delaying leaf senescence, protecting the plant against adverse conditions, and conferring greater resistance to disease. The organic compounds provided by MICROVITAL-L not only complex the nutrients contained in its formulation but also those present in the soil that are immobile or bound, substantially improving soil fertility and plant nutrition.

 

Treatments focused on almond ripening and fattening should always be considered if high yields are desired . It's important to remember that from spring onwards, after petal fall, the almond grows very rapidly, and any disturbance can affect it. If there has been a good fruit set, the tree will typically begin to purge the almonds, meaning that small almonds will start to fall. This is usually due to several factors, such as:

• Variety:There are varieties that produce more flowers than fruit, and others that produce small almonds.
• NutritionAlmond trees have nutritional reserves of macro and micronutrients from the previous year, so if these reserves are low, it's logical that they will drop more almonds. Hence the importance of fertilization in the final phase of the cycle, post-harvest. Applying nitrogen alone is not enough.
• Irrigation: If there is excess or deficiency in irrigation, it is also a cause of excessive almond fall.
• Wind/Hail: Logically, we will not be able to influence anything here, just have the tree with good reserves so that it costs more work to throw it away.

Regardless of the tree's fruit set, we will always need to use specific almond fattening products at this stage if we want to achieve high yields . The reasons are as follows:

1. If we have had a poor curdling, at least the almond we have has a big caliber, and consequently we have kilos of seeds that we can defend throughout the year.
2. On the contrary, if we have had a great set and if we do not take into account the nutritional value, the almond will remain small and the weight will be less than expected at first.
3. With small increases in production justify fully applications.
4. In varieties that carry a lot of weight, and the almond is not very large (Penta) it is highly recommended.

The timing and duration of the treatments will depend on the variety, climate, region, etc. The general rule is to begin applying them when the almond is at least the size of a 20-cent coin, and you would have until the almond shell hardens completely (while the kernel is still milky). At this point, the skin will no longer grow, and consequently, the almond kernel will no longer be able to grow.

In the Cultifort catalog we have different formulations specially developed for the ripening and fattening stages of the fruit.

MACROFOL RED PLUS is a soluble NPK fertilizer with a 15-5-30 ratio, formulated with magnesium and micronutrients. Its composition is designed to promote fruit development, growth, ripening, and firmness, increasing carbohydrate and protein storage . It also provides an ideal nitrogen supply at a concentration that does not negatively affect fruit ripening during the phenological stages when its application is most recommended. MACROFOL RED PLUS is a highly soluble and stable product that mixes well with other products on the market. It does not clump during dissolution, has a slightly acidic pH, and best of all, it is chlorine-free.

CULTIFORT K and CULTINEUTRAL K are two high -potassium , chlorine-free liquid formulations . They are designed to promote fruit growth and ripening, increasing fruit size and uniformity, boosting sugar synthesis and accumulation, and improving color and firmness . Thanks to their formulation technology, they are rapidly assimilable products with high absorption, mobility, and translocation within the plant. They are similar in potency, and their main differences lie in their pH (9 in CULTIFORT K and 6 in CULTINEUTRAL K ) and density (1,5 and 1,24 kg/L, respectively).

REFERENCES

Arquero, O., Casado, B., Fernández, JL, García, A., Lovera, M., Ramírez, A., Romacho, FJ, Romero, J., Salguero, A., Serrano, N. and Viñas, M., 2013. Plant Material . In Almond Tree Manual (pp. 22-31). Seville (Spain). Ministry of Agriculture, Fisheries and Rural Development. Regional Government of Andalusia.

Arquero, O. and Parra, MA, 2013. Environmental Requirements . In Almond Tree Manual (pp. 16-21). Seville (Spain). Ministry of Agriculture, Fisheries and Rural Development. Regional Government of Andalusia.

Arquero, O. and Serrano, N., 2013. Fertilization . In Almond Tree Manual (pp. 52-54). Seville (Spain). Ministry of Agriculture, Fisheries and Rural Development. Regional Government of Andalusia.

Brown, PH and Uriu, K., 1996. Nutrition deficiencies and toxicities: Diagnosing and correcting imbalances . In Micke, WC Almond Production Manual (pp.179-188). University of California (USA), Division of Agriculture and Natural Resources.

Grasselly, C., and Duval, H., 1997. L'amandier. Paris: Ctifl. Ed.

MAGRAMA, 2018. Ministry of Agriculture, Food and Environment. Statistical Yearbook 2018.

Miarnau, X., Torguet, L., Batlle, I., Romero, A., Rovira, M. and Alegre, S., 2015. Agronomic and productive performance of new almond varieties. National Symposium on Almonds and other Nuts. September 2015. Lleida, Spain.

Micke, W. C. 1996. Almond Production Manual. University of California. Division of Agriculture and Natural Resources.

Vargas, FJ, Romero, M., Clave, J., Batlle, I., Alegre, S. and Miarnau, X., 2009. Important traits in IRTA's new almond cultivars. 5th International Symposium on Pistachios ^and Almonds, October, 2009. Sanliurfa, Turkey. Acta Horticulturee , 912: 359-365.