The role of potassium is directly related to the quality and production. The increment of the potassium levels improves the plant performance.

The essential roles of potassium are found in the protein synthesis, the photosynthetic processes and the sugars transport from the leaves to the fruits. A good potassium supply will sustain, therefore, from the beginning the leaf function in the fruit growing. This will contribute to the positive potassium effect in the plant yield and in the higher soluble solids´ content (more sugar) in the fruit at harvest time. Approximately 60 to 66% of the potassium absorbed by the plant is found in the fruit (Winsor et al, 1958). The potassium action in the protein synthesis reinforces the conversion of the nitrate absorbed in proteins, contributing to better efficiency of the supplied nitrogen fertilizer. Potassium is a cation involved in the maintenance of the plant osmotic potential (cell turgidity). An implication of this is the stoma movement; when stomata are open allow the plants to exchange gas and water with the atmosphere. This permits the plants to maintain an adequate hydration under stress conditions such as salinity or water shortage. In fact, the tomato crop with a high content of potassium generally shows a greater efficiency in the water use, this means, that this consumes relatively less water than potassium deficit crops for producing the same biomass quantity. In addition, potassium is involved in the fruit maturity processes such as the synthesis of the licopene pigment, which is responsible of the tomato red color. Potassium promotes a high acid content, which is essential for the good fruit flavor.

Deficiency is commonly noted by curling-up of the leaf edges (margins). This response occurs in most fruit tree types. Leaf margin chlorosis followed by necrosis (death) may subsequently occur. Symptoms generally manifest themselves first in mature leaves. New shoot extension may be reduced in affected trees. Fully-grown fruits may be smaller. Such fruits may also exhibit reduced colouration at maturation. Shelf-life may additionally be reduced.

Images from the book Symptons of Nutrient Imbalances in fruit trees, Bruno Razeto.

Early leaf symptoms of K-deficiency in 'Red Delicious' apple.

'Granny Smith' apple leaves showing advanced symptoms of K-deficiency.

Leaf symptoms of K-deficiency in 'Gala' apple (photo courtesy Gabino Reginato).

Leaf symptoms of K-deficiency in peach.

Leaf symptoms of K-deficiency in nectarine (photo courtesy Rafael Ruiz).

Leaf K-deficiency symptoms in cherry.

Leaf K-deficiency symptoms in 'D'Agen' prune.

Leaf K-deficiency symptoms in 'Thompson Seedless' ('Sultana') grape.

Leaf symptoms of K-deficiency in 'Chardonnay' grape.

Leaf K-deficiency symptoms in 'Thompson Seedless' grape.

Kiwifruit leaves showing initial signs of K-deficiency.

Leaf symptoms of K-deficiency in kiwifruit at the time of fruit set.

Fruits and leaves of 'Hass' avocado. Left - From a non-nutrient deficiency tree. Right - From a tree deficient in both K and Zn.

Leaf symptoms of K-deficiency in olive.

K-deficient olive leaves (increasing severity - left to right).

K-deficiency in a banana leaf (photo courtesy CORBANA and INPOFOS).

Leaf symptoms of K-deficiency in banana (photo courtesy CORBANA and INPOFOS).

Leaf symptoms of severe K-deficiency in banana.

To assess the response of table grape to potassium fertilisation, a field test was performed to evaluate the effect of 3 doses of potassium, applied with Ultrasol® NKS and Ultrasol® SOP,on fruit yield. The experiment took place at the Agrícola Viñedos Costa in the locality of Hermosillo, Sonora State, Mexico. The tested crop was a 13 years old Flame Seedless variety, planted on ungrafted rootstock with a density of 1.730 plants/ha. The soil was designated as clay with a pH of 7,67, an EC of 2,35 mS/cm and 1,74 meq K/100 g (sampled at a depth between 0 and 30 cm). The 3 different potassium fertiliser treatments under study (Table 1) were arranged in a randomized complete block design with 10 repetitions; the plant being the experimental unit.

Figure 1. The applied irrigation system.

The application moment of the treatments was started at mid bud break with an interval of approximately 3 days and ended 2 weeks before harvest.

There were 3 fruit harvests, starting on 20/05/2011 and ending on 03/06/2011. The harvest was hand-picked and each time and inventory was made of grapes fit for commerce and wasted grapes. The calibres and the degrees Brix° were measured also.

Grape yield, calibre and degrees Brix were analyzed according to the ANOVA test and the mean of one group was compared with the mean of another using the LSD test (0,05).

To evaluate the effect of potassium on the degrees Brix of the grapes, the average value of the measures degrees Brix of the 3 grape harvests was used.

Figure 2. Total and commercial grape yield (kg/plant).

Agronomic analysis and economic results.

• Fertilisation T2 (100 kg K2O/ha) applied on table grape of the Flame Seedless variety had a positive effect on both total and commercial grape yield (Figure 2).

• Treatment T2 resulted in far more total and commercial grape yield than the treatments T1 and T3.

• Treatment T2 also resulted in the highest earliness in grape harvesting.

• The treatments T2 and T3 resulted in bigger commercial fruit calibres compared with treatment T1 (Figure 3).

• There were no statistically significant differences between the obtained degrees Brix° (T1=15,9, T2=16,4 and T3=16,1).

• Treatment T2 resulted in 9.610 US$ per ha more net income compared with the control (+80%).

• In Table 2 only the control (T1, 0 kg K2O/ha) was compared with T2 (100 kg K2O/ha)

Figure 3. The average fruit calibre of the 3 harvests.

The two nutrient salts most utilized by crop plants are nitrate (NO₃-) and potassium (K+). Potassium nitrate (KNO₃) can thus be considered an ideal fertilizer. Its application in fertigation, direct soil application and hydroponics is well documented. Potassium also has great benefits when used as a foliar in fruit trees and other crops. It has additionally been used as a rest breaking agent in deciduous fruit trees. KNO₃ has also been found to suppress scales and aphids after foliar application, and to reduce fruit splitting.

Potassium per se is known for its benefit in enhancing produce quality. Potassium's role in fruit trees and other crops is entirely regulatory, it having a major role in facilitating the movement of sugars and organic acids in the phloem, the living conducting tissue of plants. It facilitates movement from leaf to fruit, or from leaf to root, for example. Increased efficiency of assimilate movement translates to benefits in yield and quality.

It is a general phenomenon that KNO₃ sprayed on fruit trees during the period of fruit growth and development increases fruit size, yield and quality. Increases in this regard are commensurate with the degree of relative deficiency of potassium or nitrogen. Relative deficiency refers to less than optimal tree levels which are not such that deficiency symptoms are observed. In general, the fruits set on a fruit tree require relatively high amounts of potassium, more than any other nutrient, and often more than the roots can provide while the fruits develop and grow. There is therefore the need for extraction from tree reserves. Potassium may even be extracted from the leaves to support fruit growth. A challenge growers generally face is to ensure sufficiency of potassium prior to flowering and the commencement of fruit development. Soils rich in calcium or magnesium are often difficult in the sense of being conducive to reduced potassium uptake by roots. Spray application represents a convenient alternative in managing tree-potassium, particularly where soil conditions do not favour potassium uptake.

Early morning spraying of trees in flower or when much of the trees have young developing leaves on it is conducive to elevated nutrient uptake.

Zimmer et al., (1996), after testing the effects of various nitrogen-sprays on apple trees of "Gala" and "Jonagold", noted a remarkable increase in fruit size as a result of KNO₃ foliar application. Fruit colour was also improved in Jonagold. The sprays were applied in solution to the leaves and flowers.

Southwick et al. (1996) sprayed French prune trees with KNO₃. Spray applications were compared with single annual soil applications of potassium chloride (1.4-2.3 kg/tree) or sprays of urea + KNO₃ with respect to leaf potassium and nitrogen concentrations, fruit size, drying ratio and dry yield. KNO₃ sprays were as effective or better than soil-applied potassium chloride at maintaining adequate levels of potassium throughout the season. Lowest leaf potassium values, below the adequate level of 1.3% potassium, were found in the trees where no potassium was applied. These trees developed potassium deficiency symptoms. Trees showing below optimum leaf-potassium levels showed a clear yield benefit following spraying. Trees deprived of potassium were the lowest yielding. It was concluded that foliar KNO₃ sprays applied four times throughout the growing season can correct relative potassium deficiency in French prune.

Ebrahiem et al. (1993) sprayed Mandarin Balady trees growing at Mallawy in Egypt in a sandy soil with 0.5 or 1.0 % (K₂O) with KNO₃, potassium sulfate or potassium chloride. Best results were obtained following KNO₃ spraying, this giving rise to greatest growth, leaf potassium and nitrogen content, number of fruits set and fruit weight, yield, TSS, total sugar content and vitamin C content. Correspondingly, fruit peel weight, thickness and total acidity were reduced to the greatest extent following KNO₃ spraying. Alternate bearing was minimized when three sprays of KNO₃ at the rate of 0.5% (K₂O) were applied.

Diffusion is the process which gives rise to nutrient uptake by the leaf. Tissues which are soft and young take up greater quantities of nutrient than those that are hardened and mature. Mature leaves have a well developed cuticle and wax layers which are less conducive to diffusion, whereas blossoms and inflorescences are least resistant to diffusive movement of nutrient salts. Degree of uptake also relates to surface area, the bigger the better. Surface area of soft tissues is greatest when the trees are in flower. Spraying then generally gives rise to greatest uptake. Extent of uptake also depends on the duration of wetting. When conditions are dry and hot, wet-time is far less than when conditions are humid and cool. Nutrient uptake when spraying early in the morning or late in the afternoon is thus preferable. Early morning spaying of trees in flower or when much of the tree has young developing leaves on it is conducive to elevated nutrient uptake. The concentration of the solution sprayed must however be such (low enough) not to cause burn (pytotoxicity).

Literature Cited
Ebrahiem, T.A., Ahmed, F.F., and Assy, K.G. 1993. Behaviour of Balady mandarin trees (Citrus reticulata L.) grown in sandy soil to different forms and concentrations of potassium foliar sprays. Assiut Journal of Agricultural Sciences 24:215-227.
Southwick, S.M., Olson, W., Yeager, J., and Weis, K.G. 1996. Optimum timing of potassium nitrate spray applications to 'French' prune trees. Journal of the American Society for Horticultural Science 121:326-333.
Zimmer, J., Handschack, M., and Ludders, P. 1996. Influence of flower thinning with nitrogen-containing fertilizers on growth and fruit quality of apple. Erwerbsobstbau 38:81-85.