Uptake of Water and Nutrients from Recirculating Nutrient Solutions

Recirculating nutrient solutions provide a convenient method for studying the uptake of water and certain nutrients by mature plants. For this purpose, the system must not leak, should be well covered (not sealed) to exclude light and render evaporative losses negligible, and must be free of algae. The uptakes are calculated from the volume and concentrations at the beginning and end of the period under study and from the additions of water and nutrients during that time. Since nutrient uptake is a function of the nutrient concentration, and often of pH, the examples given were obtained in a system in which the volume, nutrient concentrations and pH were held constant throughout the period of study.

Water usage by a tomato crop, monitored over a period of nine months, showed very close agreement with that predicted from solar radiation integrals. The daily uptake of water by the plants was highly correlated (r = 0.93; P < 0.001) with light intensity.

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Much detailed information has been obtained about the relation between nutrient uptake and both the stage of plant development and the environment. For example, the uptake of water and nutrients by tomato plants with the first truss in flower varied considerably with time of year, but the ratios at which nutrients were absorbed was not affected. The rates of uptake increased, however, as the fruit began to swell. The rates of uptake of nitrogen and phosphorus had declined again when the ninth truss was in flower whereas the rate of potassium uptake had doubled, reflecting the heavy fruit load (Table 1). This trend was confirmed by studies of the weekly uptake of nitrogen and potassium throughout the season. During the early stages of growth, nitrogen and potassium were absorbed at a K:N ratio of 1.25:1 (by weight). As the load of fruit on the plants increased the rate of vegetative growth was checked. This reduced the rate of uptake of nitrogen whilst that of potassium continued to increase in response to the demand by the fruit, and the ratio at which potassium and nitrogen were absorbed increased to 2.5:1. Later, when the lowest trusses of fruit had been picked, the plants resumed more rapid growth. The rate of nitrogen uptake, therefore, increased again, and the K:N ratio declined and remained at about 2:1. In consequence, failure to adjust the K:N ratio in the nutrient solution to match plant requirements results in rapid depletion of potassium in systems controlled by salinity monitors.

Table 1. Average daily rates of uptake of water, nitrogen, phosphorus and potassium by indeterminate tomato plants.

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Using refined techniques, the rates of nutrient and water uptake have been measured over short intervals (90 minutes) throughout the day and night. Water, nitrogen and potassium were absorbed at very low rates during the night, and these increased to their maxima during the brightest part of the day, after which they declined again. In contrast, the uptake of phosphorus increased to a maximum after the rates of water, nitrogen and potassium had begun to decline, and appeared to be closely related to solution temperature. Although nitrogen and potassium uptake were highly correlated with water uptake (r > 0.90), water uptake tended to increase and to decrease slightly more rapidly than the uptake of these two nutrients.

The rates of water and nutrient uptake are very low at night, but the ratios at which nutrients and water are absorbed (i.e., K:water, N:water) are at their highest. These ratios decline as the rates of uptake increase and reach a minimum during the afternoon. For the same reason, the ratios at which nutrients and water are absorbed vary as the light intensity changes during the year (Table 2).

Table 2. Maximum rates of uptake of indeterminate tomato plants under glass at low and high light intensities and the ratios at which nutrients and water are absorbed.

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The uptake of chloride by tomatoes generally exceeds that of sodium. For example, when the plants had been supplied with water containing 14 mg/L Na and 25 mg/L Cl for 12 weeks, the recirculating solution contained 150 mg/L Na, but only 5 mg/L Cl. Similarly, when 500 mg/L Na was maintained in the solution by addition of NaCl, the ratio of Cl:Na found was 0.9 - 1.1:1 as compared with 1.54:1 in a fresh solution of sodium chloride.

Good aeration is essential for healthy root growth and for efficient uptake of nutrients. Measurements of the oxygen content of the nutrient solution in the root mat showed that the oxygen concentration declined to a minimum during the brightest part of the day. This response was closely related to solution temperature. Under similar environmental conditions, the oxygen content of the solution was reduced more seriously by cucumbers (2.7 mg/L; 34% saturation) than by tomatoes (6.7 mg/L; 84% saturation), reflecting the thicker root mat formed by the cucumbers.

Conclusion

Recirculating nutrient solutions provide a useful system for studying the effects of the stage of plant growth and of the environment on the rates of nutrient uptake by mature plants and on oxygen depletion in the root mat.

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