Uptake Concentrations of a Tomato Crop in Different Salinity Conditions

In the near future, it is likely that closed soilless growing systems will be increasingly adopted in Mediterranean countries because they reduce water and fertiliser use, and appreciably reduce nutrient loss to the environment. However, closed systems increase the salinity of the circulating nutrient solution through the progressive accumulation of ions not required by crops and/or applied in excess of crop requirements. Additionally, it is necessary that nutrients be added to the solution according to crop absorption to avoid fluctuations of nutrient concentrations in the circulating solution. To obtain information for the management of circulating nutrient solutions in soil-less systems in a Mediterranean environment, an experiment was conducted with tomato during the 2002-03 growing season. Five different salinity treatments (2.5, 4.0, 5.5, 7.0 and 8.5 dS m in the drainage solution) were compared. The first treatment was the control, the higher salinity levels were obtained by adding NaCl. Total aerial dry matter production, and the absorption of macronutrients and of Na and Cl were measured for each treatment. Uptake concentrations of the macronutrients, and Na and Cl were determined for each treatment. With increasing salinity, both dry matter production and nutrient uptake tended to decrease, while Na and Cl uptake increased. An antagonistic effect of NaCl on K and N uptake was observed. For the other nutrients, no antagonistic effects were observed with the decreases in uptake being proportional to the decrease in dry matter production. Uptake concentrations for each nutrient were almost constant with increasing salinity except for K, Na and Cl. With an increase of 1 mmol L of NaCl in the circulating solution, the uptake concentration of Na and Cl increased by 0.06 and 0.068 mmol L, respectively, and that of K decreased by 0.018 mmol L. INTRODUCTION The addition of fertilizers in soilless culture is based on concentrations of nutrients in the water added to the system. The use of nutrient concentrations rather than of absolute quantities is preferred because concentrations are more constant and easier to manage. To ensure relatively constant concentrations of individual nutrients, it is useful to know the rates of absorption of those nutrients. Crop absorption of individual nutrients can be evaluated relative to the absorption of water; this ratio is referred to as the uptake concentration of a nutrient (Sonneveld, 2000, p94). This ratio does not have a physiological basis but is a very useful concept for the practical management of nutrient solutions. In addition to being a management aid for essential plant nutrients, uptake concentrations can also be used for the management of potentially harmful ions, such as Na and Cl. By knowing the concentrations in irrigation water and the volumes of the added and absorbed nutrient solutions, uptake concentrations can be used to calculate accumulation in the growing system and to estimate the flushing fraction required to maintain the concentrations of potentially harmful ions beneath those where production may be affected. Several authors have published information about uptake concentrations for tomato (e.g., Sonneveld, 2000, p95; Malorgio et al., 2001; Stanghellini et al., 2003). However, very little information is available for greenhouses in the coastal region of southeastern Spain. The objective of the current study was to determine the uptake concentrations of nutrients for a tomato crop grown with different levels of salinity in a greenhouse in this region. Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. 697 ISHS 2005 366 MATERIALS AND METHODS The experiment was conducted in a multi-span greenhouse with polyethylene cover and passive climate control in Almería, Spain. Tomato (Lycopersicon esculentum Boludo) was grown from 10 September 2002 to 3 June 2003 at a crop density of 2 plants per m. The plants were grown in perlite (3-6 mm) in 28 L styrofoam containers located in gutters (4 plants were planted per container). Nutrient solutions were circulated in a semi-closed system with a drainage percentage of >90 %, which ensured that substrate and drainage solutions were very similar. Five different salinity treatments were compared. The electrical conductivity (EC) of the drainage solution of the control treatment was 2.5 dS m. The target ECs of the other treatments (4, 5.5, 7 and 8.5 dS m) were obtained by adding NaCl to the circulating solution to increase EC. This addition started one week after transplanting and was made gradually over seven days until the target ECs were reached. Replacement of water and nutrients absorbed by the crop was done by adding fresh nutrient solution, adjusted for crop nutrient absorption to the collector tank corresponding to each EC treatment. The circulating solution was flushed when its EC was consistently 0.5 dS m more than the target value, due to the accumulation of ions such as Na, Cl or Mg from the fresh water. The composition of the fresh water used in the experiment is shown in Table 1. Circulating and fresh solutions were analysed weekly. The volumes of each addition of fresh nutrient solution to each treatment were measured. The volumes and composition of the flushed circulating solutions were also measured. With this information, regular nutrient and water balances were calculated which enabled calculation of the quantities of water and nutrients absorbed by each treatment and of the uptake concentrations for each nutrient. Fresh fruit production and fruit dry matter content were measured on 4 replications of 16 plants each per treatment. Fruit dry matter production was calculated using these data. Shoot dry matter production was measured on 8 plants per treatment. The fruit and shoot dry matter data enabled the total aerial dry matter production of the crop to be determined. RESULTS The increase in the EC of the circulating nutrient solution by the addition of NaCl reduced total aerial dry matter production and macro-nutrient uptake (Table 2). However, the uptake of Na and Cl increased with increasing EC. There were positive linear relationships between dry matter production and total nutrient uptake for each nutrient examined (Fig. 1). For P, Ca, Mg and S, the straight line passed through the origin, whereas for K and N it passed below this point indicating a decreased absorption of both elements relative to dry matter production when salinity increases. There was a negative linear relationship between nutrient uptake and dry matter production for Na and Cl, indicating a strong accumulation of both elements in plant material with increasing salinity. Water absorption was decreased more than total dry matter production with increasing EC, which indicates an increase of water use efficiency referred to dry matter production with increasing salinity (data not shown). Uptake concentrations were more stable, with increasing salinity, than the absolute absorption of nutrients (Table 3). However, the uptake concentrations of Na, Cl and K were influenced by increasing salinity by the addition of NaCl. There was a strong positive linear correlation between the uptake concentration of Na and its concentration in the circulating solution (Fig. 2a). For Cl, the uptake concentrations were slightly higher than those of Na (Fig. 2b). In contrast, the uptake concentration of K decreased linearly with salinity. There was a clear negative linear correlation between the uptake concentration of K and the concentration of Na in the circulating solution (Fig. 3). With an increase of 1 mmol L of NaCl in the circulating solution, the uptake concentration of Na and Cl increased by 0.06 and 0.068 mmol L, respectively, and that of K decreased by 0.018 mmol L. The effects of increasing salinity from 2.5 to 8.5 dS m on the uptake concentrations of the other nutrients were very small or non-existent (Table 3). Those of Ca and Mg increased slightly with salinity, increasing from 2.6 to 2.8 mmol L, and from 0.9 to 1.0 mmol L, respectively. The uptake concentration of N decreased slightly from 10.9 to 10.6 mmol L. The uptake concentrations of P and S were unaffected by salinity, being