In late May, the olive tree moves into flowering and fruit set, and the grapevine passes from flowering into berry development. These are precisely the moments when both crops take up the most nutrients, and a late or wrong surface application now shows up at harvest. The right move starts with correctly reading three numbers on the bag.
What the NPK figure shows
The three consecutive numbers on the label, for example 20-20-20 or 11-15-15, show the percentage by weight of the three main nutrients, nitrogen (N), phosphorus (as P₂O₅) and potassium (as K₂O). A 20-20-20 holds 20% of each and counts as balanced. Urea 46-0-0 is a purely nitrogen fertilizer, while an 11-15-15 leans on phosphorus and potassium, which is why it often goes into the base fertilization of fruit trees. The larger the numbers, the more concentrated the product, so the smaller the amount needed per unit area.
It is worth separating fertilizer units from kilos of product. When a soil analysis recommends, say, ten units of potassium per stremma, it means ten kilos of pure K₂O, not ten kilos of the bag. The amount of the bag is determined by dividing the units by the percentage of the element.
What nitrogen, phosphorus and potassium each do
Nitrogen feeds vegetative growth, phosphorus supports the root system and flowering, and potassium improves the size, quality and durability of the fruit. Then come the secondary elements, calcium, magnesium, and sulfur, and finally the trace elements, iron, manganese, zinc, boron, copper, and molybdenum. Although they are needed in small amounts, their absence costs. A boron deficiency at fruit set, or iron in calcareous soils showing up as chlorosis, noticeably cuts the yield.
How much the olive and the vine need now
For the olive, nitrogen is the most decisive element. The tree responds almost always to nitrogen fertilization with abundant growth, more perfect flowers and better fruit set, while its absence leads to alternate bearing. According to the Greek national fertilization guide, the annual needs of a productive olive tree run around one to one and a half kilos of nitrogen, two hundred to four hundred grams of phosphorus, one to one and a half kilos of potassium and forty to forty-five grams of boron. In non-irrigated groves, the level of nitrogen is tied to rainfall, and the spring nitrogen application is the one that supports the current crop.
The grapevine is a potassium-loving plant, and this period is the most critical for that element. Most of the potassium is taken up in the first weeks after flowering, and it is potassium that raises the sugars, strengthens the colour and aroma of the berries and improves overall quality. As for nitrogen, demand is low until flowering and peaks in the early stages of berry development, so the present period is pivotal. The vine is also sensitive to magnesium and boron deficiency, worth watching at this time.
Categories of fertilizer
By form, granular fertilizers are applied to the soil and release gradually, the usual choice for base fertilization. Crystalline, water-soluble fertilizers dissolve completely and are intended for fertigation and foliar application, while liquid fertilizers provide fast, even distribution through the irrigation network.
By method of application, soil fertilizers meet the crop's basic needs, foliar fertilizers are absorbed through the leaves and quickly correct deficiencies at critical stages without replacing soil fertilization, and fertigation delivers nutrients through irrigation water with high efficiency and lower losses.
By origin, inorganic or chemical fertilizers have a known, stable composition, organic and organo-mineral ones also improve the structure and biological activity of the soil, and certified organic fertilizers are permitted in organic farming. Finally, there are stabilized, slow-release fertilizers, which, through nitrification or urease inhibitors, slow the conversion of nitrogen and reduce losses from leaching and volatilization, performing better in sandy soils and in areas of heavy rainfall.
How to choose a fertilizer
Before any decision, a soil analysis reveals what the field already contains and what it lacks, so that no surplus fertilizer is added. The pH matters too, since in alkaline soils phosphorus and several trace elements can become locked up, reducing their availability. Then come the crop's needs at each stage, with more nitrogen during growth and greater weight on potassium toward ripening, and finally the method of application the farm supports, which determines whether you need a granular, water-soluble or liquid product.
In practice, base fertilization is applied before or at planting, while surface and foliar applications are spread throughout the growing season. That is why late spring is such an active window, since most fruit trees are now passing from flowering into fruit set.
