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Test Oranges in Sofruco Agricultural Society
Research details

For the present research study an orchard of the “La Rosa Sofruco S.A.” farm was selected. This orchard had the necessary characteristics for the study: The proper variety (Valencia: the most planted in Chile); being in full production, considering the great potassium demand by the fruit; homogenous threes, with excellent management and sanitary control; besides, presenting the lowest possible potassium foliar level.

Trees were orange Valencia variety (Citrus sinensis (L.) Osbeck), drafted over Troyer citrange. The orchard had 18 years at the trail’s initiation date and the planting was 7 x 7 m distance. The orchard was irrigated by furrow system and weeds between the rows were controlled with a harrow approximately with three annual operations, and in the rows with herbicides. The orchard’s fertilizer program in previous years to the trial was only an urea application dose of 980 g per tree at the end of the winter season, which was maintained during the research period.

The soil corresponded to the “La Rosa Series”, classified as Coarse loam, mixture, thermal family; Calcic Halloxeroll sub group; Mollisol order (CIREN, 1996). This soil is of alluvial origin found in a terrace in the Cachapoal River, deep, of silty loam texture, and from clay to sandy clay loam in the surface. The drainage is good, the permeability is moderately rapid, and the humidity regime is xeric. The average annual precipitation is 505 mm and the average annual temperature is 19.1° C with a frost free period of 11 months.

The treatments were: 1. Control (without potassium application). 2. Potassium chloride (KCl). 3. Potassium nitrate (KNO3). 4. Potassium sulfate (K2SO4) 5. Potassium magnesium sulfate (K-MgSO4). Fertilizers were manually applied at the beginning of the Spring Season each year (1990, 1991, 1992) in two furrows, one each side of the tree’s projection canopy, at 25 cm depth. Each year an equivalent dose of 3 kg of K2O per tree was used.

The experimental design was a complete randomized block with five treatments and six replications. The experimental unit (a tree) remained completely isolated by eight trees as borders without treatment.

Foliar analyses were performed before establishing the trail and afterward these were done annually. Leaves with petioles were taken for these analyses (following recommendation of Embleton et al. 1978) for each experimental unit during the first week of March, corresponding to the spring sprout.

Potassium concentrations in the leaves

As follows it can be proved that the first year, K applications did not significant increased the concentration of this element in the leaves. In the second year the larger values corresponded to the KNO3 and K2SO4. In the third year the concentrations of all K application treatments were significant higher that the control, which would explain the slow response that fruit threes to the potassium application to the soil (Razete, 1999). This slow response could be to potassium fixation by the soil clay (25% clay). Wolf (1999) indicated that exchangeable potassium in a soil with 25% clay should be ten time higher in comparison to a sandy soil (2.5% clay), for obtaining an adequate potassium concentration in the soil.

K concentration in the leaves according to different potassium sources
K concentration in the leaves according to different potassium sources.
Source: Razeto, B. and Opazo, J. 2001. Effect of different potassium fertilizer on the nutrient foliar content, production and quality of orange fruit, Valencia variety. Agricultura Técnica (Chile) 61 (4): 470 – 478.

The concentration in the control treatment was decreasing up to near reaching a critical value which was able to affect the yield. This would indicate that the natural potassium supply to the soil, together with an annual supply of irrigation water of approximately 23 kg/ha of K, was insufficient for compensating the extraction made by the tree, especially through the fruit. In fact, the lowest yield value was obtained by the control. There is a difference of almost 12 ton/ha between the highest value and of the control.

For Valencia variety, Carranca et al. (1993) showed a optimum K range in the sprouts with leaves without fruit between 4.0 and 5.3 g/kg. In this experiment, in the third year only the control presented a value near to the lowest value from the optimum foliar potassium. Even though, according to the standards used in California (Embleton et al., 1978), the control would have found in a low level, trees treated with KCl, K2SO4 and K- MgSO4 were in a medium to low level, and only those trees treated with KNO3 reached an optimum level. Besides, according to the Alva and Tucher (1999) standards, none of the treatments reached an optimum K level of 12 to 17 g/ha.

Yield and fruit number per tree

The following table shows yields per three and fruits number. There were no significant differences between treatments. Assuming an average yield of 220 kg per tree, the estimated yield for the orchard is 45 ton/ha, which is considered high and very similar to the one reported by Koo (1985) for the USA conditions.

Effect of Different Potassium Sources over Yield and Weight
Effect of Different Potassium Sources over Yield and Weight.
Source: Razeto, B. and Opazo, J. 2001. Effect of different potassium fertilizers on the nutrient foliar content, production and orange fruit quality of Valencia variety. Agricultura Técnica (Chile) 61 (4): 470 – 478.

The average fruit weight significant increased at the third year of application, which indicates that potassium is a determinant factor in the caliber problem (fruit size). This coincides with Embleton et. 1978), who indicated that a K concentration below 13.0 g/kg in the leaves, the application of K increased the fruit size. Koller and Schawarz (1995) in Citrus sinensis x Citrus reticula, also found that high K levels increased the average fruits weight.

Potassium chloride (KCl) and potassium nitrate (KNO3) significant increased the average fruit weight. Values in the potassium sulfate (K2SO4) and potassium magnesium sulfate (K-Mg SO4), even though larger than the control, the were no significant different, which would indicate that the effect of these last fertilizers over the fruit weight would be slower.

Concentration of chloride, magnesium and nitrogen in the leaves in the third application year

Mg was significantly increased in the treatments with K-MgSO4 reaching the optimum range shown by Embleton et al. (1978). In the other treatments the Mg was found in a low level according to the authors. It is interesting to highlight the foliar increment levels of K and Mg obtained with this fertilizer, if considered that both elements are antagonist in fruit threes, which it would explain by the simultaneous supply of both elements.

Concentration of N, Mg and Cl in the Leaves according to Different Potassium Sources
Concentration of N, Mg and Cl in the Leaves according to Different Potassium Sources.

The treatment with KNO3, though had the highest N concentration value in the leaves, this was not significant different from other treatments. Probably this could be explained by the high N level existing in all trees of the orchard, as a result the annual fertilizer application with urea.


From the obtained results under the conditions that the present research was conducted the following conclusions were formulated:

In orchards under conventional irrigation, in a soil with 0.5 cmol/kg exchangeable K in the first 20 cm and 0.25 cmol/kg between 20 and 40 cm, and with K concentrations medium to low in the leaves, the yield is not incremented neither the fruit number per tree, after three years of fertilizers application to the soil. But, the fruit size was incremented (weight and caliber) in the third year.

Out of the four compared fertilizers (potassium chloride, potassium nitrate, potassium sulfate and potassium and magnesium sulfate), the first two ones appear more rapid and efficient in their effect on the fruit size.

Potassium fertilizers applied to the soil with conventional irrigation are slower in thier action, since the potassium level in the leaves significantly increased only at the third year application, except potassium nitrate and potassium sulfate which effect was presented at the second year.

The foliar potassium level was diminishing year by year in the trees which were not fertilized with this element.

Double potassium and magnesium sulfate significantly increased the magnesium concentration in the leaves.

Potassium chloride applications significantly increased the chloride concentration in the leaves, but within normal levels.

Potassium nitrate applications, as potassium supply, did not increase foliar nitrate to an excessive level.