Print version ISSN 0103-9016
Sci. agric. vol.56 n.4 Piracicaba Oct./Dec. 1999
Roberto Anjos Reis Jr.2,6*; Paulo Cezar Rezende Fontes3; Júlio Cesar Lima Neves4; Nerilson Terra Santos5
2CCTA/UENF, CEP: 28015-620 - Campos dos Goytacazes, RJ.
3Depto. de Fitotecnia/UFV - C.P. 308 - CEP: 36570-000 - Viçosa, MG.
4Depto. de Solos/UFV.
5Depto. de Matemática/UFV.
ABSTRACT: Soil K+ to Ca2+ and Mg2+ ratio as well as the total salinity were evaluated in response to potassium fertilizer application onto potato. Potassium was applied at six different rates (0, 60, 120, 240, 480 and 960 kg ha-1 of K2O), as K2SO4, and was placed during planting time in the furrow. Soil from the 0-200 mm layer was collected in the furrow, 20 and 48 days after plant emergence (DAE) to evaluate soil pH, K+, Ca2+ and Mg2+ contents and the total electrical conductivity (EC). A factorial design (6x2), with six K rates and two sampling times was set up in a randomized block design with four replications. The application of K fertilizer increased exchangeable K, did not affect pH and exchangeable Ca and Mg contents, but caused a linear increase of the soil K+/(Ca2++Mg2+)1/2 ratio as well as EC. At 20 DAE, the critical soil K+/(Ca2++Mg2+)1/2ratio and the EC associated with maximum tuber yield (30.5 Mg.ha-1, with 353.4 kg ha-1 of K2O) were 1.79 and 1.6 dS m-1, respectively. The highest soil K+/(Ca2++Mg2+)1/2 ratio and EC were obtained with the highest application of K fertilizer, which led to a reduction in the potato tuber yield.
Key words: Solanum tuberosum, potassium, calcium, magnesium, salinity
Condutividade elétrica e níveis críticos da relação entre K+ e Ca+ + Mg+ no solo para cultura da batata
RESUMO:Com o objetivo de avaliar a relação entre K e Ca + Mg e a salinidade no solo em resposta à adubação potássica no cultivo da batateira (cultivar Baraka), foi instalado experimento fatorial a nível de campo com seis doses de potássio (0, 60, 120, 240, 480 e 960 kg ha-1 de K2O) e duas épocas de amostragem, 20 e 48 dias após a emergência das plantas, (DAE) delineado em blocos casualizados com quatro repetições. O potássio foi aplicado como K2SO4 no sulco de plantio. O solo foi amostrado (0-200 mm de profundidade) para avaliar o pH, a condutividade elétrica e os teores de K, Ca e Mg. A adubação potássica aumentou o K trocável, não afetou o pH e os teores de Ca e Mg trocáveis no solo, e elevou linearmente a condutividade elétrica e a relação K+/(Ca2++ Mg2+)1/2. Aos 20 DAE, a máxima produção de tubérculos foi associada com uma relação K+/(Ca2++Mg2+)1/2 de 1,79 e uma condutividade elétrica de 1,6 dS m-1. Com as maiores doses de potássio obtiveram-se as maiores relações K+/(Ca2++Mg2+)1/2, e condutividade elétrica do solo, que podem ter contribuído para a redução da produção de tubérculos.
Palavras-chave: Solanum tuberosum, potássio, cálcio, magnésio, salinidade
Potato (Solanum tuberosum L.), a staple food in many countries (Mclaughlin et al., 1994), is an important crop in Brazil, cultivated in 170,829 ha (FNP Consultoria & Comércio, 1998) with increasing commercial importance (Campora, 1994). The potato crop receives high level of fertilizers. Among the nutrients usually used for potato fertilization, K is of great importance since it is the nutrient taken up in the greatest quantity by the potato plant (Perrenoud, 1993). Potassium is needed for sugar translocation, starch synthesis (Reis Jr & Fontes, 1996) and to promote high potato tuber yield (Westermann et al., 1994b) of good quality (Westermann et al., 1994a). Although K can promote high tuber yield, its excessive use in agriculture, can reduce tuber yield. High mineral fertilizer application rates, such as those often used in agriculture, are usually criticized, partly due to fears of environmental impact (Eppendorfer & Eggum, 1994). Therefore, research should be carried out to evaluate high potassium fertilizer inputs onto potato crop, in order to avoid tuber yield reduction, fertilizers losses and environmental contamination. High inputs of potassium fertilizer may alter soil K, Ca and Mg ratios and salinity. Little systematic research has been carried out under field conditions to evaluate how these factors affect the potato tuber yield, under high potassium fertilizer input.
Interactions among K, Ca and Mg occur in plants as well as in the soil, which is object of several studies (Reis Jr, 1995). Interactions among K+, Ca2+ and Mg2+ are common, because these ions have specific chemical properties that are sufficiently similar to compete for adsorption, absorption and transport sites on the plant root surfaces (Fageria et al., 1991). The ratio of K, Ca and Mg affects the K content in the soil solution (Raij, 1982), and its absorption by the plant. When the K+ supply is abundant, "luxury consumption" often occurs, which affects plant composition and interferes with the uptake and physiological availability of Ca2+ and Mg2+ (Marschner, 1995). This interaction usually occurs when plants are grown in soils with low Ca2+ or low Mg2+ contents, or alternatively, when the plants require high levels of K to produce high yields (Usherwood, 1982). Several authors have tried to improve the recommendations for K fertilizer using as a parameter the ratio of K to Ca and Mg (Prezotti & Defelipo, 1987). Although soil exchangeable K content is traditionally used to make recommendations of K fertilizer application in several crops, Castro & Meneghelli (1989) verified that crops growing in some soils with low exchangeable K+ content still do not respond to potassium fertilizer application, while others, with satisfactory K+ content, will respond to potassium fertilization. It is possible that the soil K to Ca and Mg ratios could help to explain this observation. Soil K to Ca and Mg ratio will change as a function of K fertilizer application, and the influence of this ratio on the potato tuber yield are research topics that have not been intensively examined.
Potassium fertilizer application, usually based on chloride or sulfate salts, could contribute to soil salinization due to their high salt index (Mistrík et al., 1992; Taiz & Zeiger, 1991). Damage caused by soil salinity is not only due to the direct effects of the Cl- and Na+ ions, but also due to lowering soil osmotic potential, caused by salt excess in the soil solution. Soil water limitations and salinity effects are similar, leading to water deficits and plant growth reduction (Ayers & Westcot, 1991). Reduced plant water absorption and altered physiological processes caused by excessive use of fertilizer can appear due to excess of salts in the soil (Lima, 1997). Low soil water availability as caused by excessive soil salinity delays plant growth and restricts root development (Ayers & Westcot, 1991).
Potato crops are moderately sensitive to soil salinity (Doorenbos & Kassam, 1994) and since cultivation of this crop has expanding to regions where saline water is available while other water resources are restricted for irrigation, it is important to study the response of potatoes to soil salinity (Levy et al., 1993). Usually soil salinity slowed potato plant emergence, enhanced haulm senescence and reduced growth of both haulm and tubers (Levy, 1992). Applying saline solutions (electrical conductivity (EC) of irrigation water up to 7.0 dS m-1) to potato seed tubers delays root and shoot development and shoot emergence, which is likely caused by inhibition of the cell division and elongation of the sprout meristems (Levy et al., 1993). Hoorn et al. (1993) and Lima (1997) found that the potato yield began to decrease at levels of 1.7 dS m-1 soil solution EC. Potato tuber yield was reduced by 0, 10, 25, 50 and 100 %, as EC increased from 1.7, 2.5, 3.8, 5.9 up to 10 dS m-1, respectively (Doorenbos & Kassam, 1994). There is little information about soil EC behaviour as a function of K fertilizer applications, and their influence on the potato yield.
The objectives of this work were to evaluate the effects of K fertilizer rates on the soil K+/(Ca2++Mg2+)1/2 ratio and on soil electrical conductivity, and their consequence on the potato tuber yield.
MATERIAL AND METHODS
A field experiment was conducted on a Clay Tipic Ultisol, with 0-200 mm soil layer characteristics : coarse sand = 28 %; fine sand = 15 %; silt = 3 %; clay = 54 %; pH (H2O) = 4.7; P = 4.16 mg dm-3; K = 1.37 mmolc dm-3; Al3+ = 9.0 mmolc dm-3; Ca2+ = 1.7 mmolc dm-3; Mg2+ = 0.8 mmolc dm-3; H+Al3+ = 56.0 mmolc dm-3.
The soil was limed as recommended by Comissão de Fertilidade do Solo do Estado de Minas Gerais (1989). Later, K2SO4 was applied at the rates of : 0, 60, 120, 240, 480 and 960 kg ha-1 of K2O, in the furrow (100 mm deep). Each plot had four rows with twelve plants each, spaced 0.8 x 0.3 m. Plants used were only those of the central rows. Before planting, 300 kg ha-1 of (NH4)2SO4; 600 kg ha-1 of P2O5; 200 kg ha-1 of MgSO4; 15 kg ha-1 of borax and 15 kg ha-1 of ZnSO4, were also added to the furrows. Tuber seeds (uniform weight of 80 ± 5 g) were from the "Baraka" Cultivar (Solanum tuberosum L.). Ten days after plant emergence (DAE), before the plants were hilled, 700 kg.ha-1 of (NH4)2SO4 was applied to the soil. Throughout the growing season, the production system was managed according to the management practices recommended in the region, which included irrigation as needed.
Soil samples were collected from a layer of 0-200 mm at 20 and 48 DAE. Each composite soil sample was made up from two simple samples randomly collected in the planting rows. Samples were analyzed for pH and exchangeable K+, Ca2+ and Mg2+ contents according to Empresa Brasileira de Pesquisa Agropecuária (1979) and EC, using the saturated soil extract, according to Rhoades (1982). The soil K+, Ca2+ and Mg2+ contents were expressed in mmolc.dm-3, and used to calculate the soil K+/(Ca2++Mg2+)1/2 ratio.
Following natural senescence, the tubers were harvested.
The experiment was set up in randomized block, with four replication, in a factorial design (6x2) with six K levels and two sampling times. Analysis of variance and regression were used for the evaluation of the collected data. The best fitting model was chosen among the linear, quadratic and square-root models. To estimate the critical soil K+/(Ca2++Mg2+)1/2 ratios and EC values associated with maximum potato yield, the potassium rate associated with maximum potato yield was introduced into the best fitted model, previously established, which relates soil K+/(Ca2++Mg2+)1/2 ratios and EC to potassium levels. Based on these fitted models, soil K+/(Ca2++Mg2+)1/2 ratio and EC values were classified as very low (<80%), low (from 80 to <90%), medium (from 90 to <99%) and adequate (from 99 to 100% potato tuber yield).
RESULTS AND DISCUSSION
The application of K fertilizer increased potato tuber yield, reaching the maximum of 732.5 g plant-1 (30.5 Mg ha-1) with 353.4 kg ha-1 of K2O (see Fontes et al., 1996), which was higher than the average Brazilian yield, of 14.6 Mg ha-1 (FNP Consultoria & Comércio, 1998).
Potassium fertilizer levels and sampling time did not affect soil pH. Although decrease in soil pH with the application of fertilizers was expected, it did not occur in this experiment. Average soil pH was 4.69.
Soil K content increased with increasing levels of K fertilizer (Reis Jr et al., 1997), while exchangeable soil Ca and Mg contents did not. Soil Mg content decreased (p£0.01) from 1.23 mmolc dm-3 at 20 DAE to 1.03 mmolc dm-3 at 48 DAE. Average soil Ca content was 3.95 mmolc dm-3.
Soil K+/(Ca2++Mg2+)1/2 ratio increased with increasing levels of K fertilizer (p£0.01) and there was a significant interaction between K fertilizer application and sampling time (p£0.01) (Figure 1). Since the application of K fertilizer increased soil K content (Reis Jr. et al., 1997) and did not affect soil Ca and Mg contents, the increase in the soil K+/(Ca2++Mg2+)1/2 ratio was expected with fertilization. Critical soil K+/(Ca2++Mg2+)1/2 ratio associated with maximum potato tuber yield was 1.79 at 20 DAE and 1.91 at 48 DAE.
Figure 1 illustrates that application of potassium fertilizer caused a linear increase in the soil K+/(Ca2++Mg2+)1/2 ratio at 20 and 48 DAE. At 20 DAE, the K+/(Ca2++Mg2+)1/2 ratio increased from 0.84, in the treatment without K, up to 3.43, with application of 960 kg ha-1 of K2O. At 48 DAE, the soil K+/(Ca2++Mg2+)1/2 ratio increased from 0.04, in the control with no K fertilizer, up to a value of 5.13 with 960 kg ha-1 of K2O. TABLE 1 shows that soil K+/(Ca2++Mg2+)1/2 ratio lower than 0.85 was associated to less than 80% of maximum potato tuber yield at 20 DAE.
Soil K+/(Ca2++Mg2+)1/2 ratio decreased between 20 and 48 DAE at the lowest K fertilizer levels, while this ratio increased between 20 and 48 DAE at the highest K fertilizer levels (Figure 1). This was caused by the reduction of soil K content between these two sampling times at the lowest K application and an increase of soil K content at the highest K fertilizer levels (Reis Jr et al., 1997).
The application of K fertilizer increased soil EC (p£0.05) (Figure 1). However, there was no significant interaction between K levels and sampling time. Critical soil EC associated with maximum potato tuber yield was 1.60 dS m-1. This value agrees with the maximum limit of soil EC proposed by Hoorn et al. (1993), Doorenbos & Kassam (1994) and Lima (1997) to avoid potato tuber yield reduction. Figure 1 illustrates that the application of potassium fertilizer caused a linear increase in the soil EC. The soil EC increased from 1.37 dS m-1, in the treatment without application of potassium fertilizer, up to a 2.00 dS m-1 with 960 kg ha-1 of K2O. TABLE 1 shows that soil EC higher than 2.23 dS m-1 was associated with less than 80% of maximum potato tuber yield and the potato yield reduction in relation to the increase of the soil EC from 1.6 to 2.0 dS m-1 was of 10 %.
Potato tuber yield began to decrease after 353.4 kg ha-1 of K2O. Potassium toxicity is a hypothesis that should be rejected, then others factors led to tuber yield reduction. Among them, soil K, Ca and Mg ratio and EC could have contributed to this yield reduction. Although the results found in this paper were predictable, this information is useful to evaluate how much the soil K, Ca and Mg ratio and EC changed with higher K doses. And this information should be used to guide the K fertilizer program in potato crop management.
The critical levels found in this study were for the Baraka variety, therefore, it will be necessary to verify the validity of these findings for other varieties.
The application of K fertilizer increased soil K+/(Ca2++Mg2+)1/2 ratio and electrical conductivity (EC). Critical soil K+/(Ca2++Mg2+)1/2 ratio associated with maximum potato tuber yield was 1.79 at 20 DAE and 1.91 at 48 DAE. Critical soil EC associated with maximum potato tuber yield was 1.60 dS m-1.
The highest soil K+/(Ca2++Mg2+)1/2 ratio and EC were obtained with the highest application of potassium fertilizer.
As potato crops receive greater amounts of fertilizers, growers should give more attention to the effects caused by the high input of these fertilizers. Inadequate ratios between nutrients in the soil and soil salinity problems may be avoided because they might lead to potato yield reductions.
We would like to thank Domingos Sávio and Janie Jasmin for assistance with chemical analysis.
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Received October 22, 1998
Accepted May 20, 1999
1 Presented at the XXXVI Brazilian Vegetable Crop Congress, Rio de Janeiro, 1996.