Nutrient uptake of cucumbers depends on light intensity and air temperature

When it comes to fertigation programmes or recipes, most commercial growers have a good indication of what is required. The nutrient program is either obtained from the fertilizer company or it is a tried and tested custom mix. Quite often what these recipes do not indicate or provide, is the variations of nutrients required within a 24 hour cycle. If you have a big greenhouse full of cucumbers it can make a difference at the end of the year in terms of fertilizer cost. Nutrient absorption is not a continuous linear process. Since most nutrients are absorbed and translocated throughout the plant through water absorption, the water balance and movement through the plant will have a significant effect on nutrient translocation. Nutrient absorption and requirements are influenced by:

• Plant age
• Air temperature
• Humidity
• Microclimate
• Light intensity
• Nutrient concentration and ratio (EC).

The factors that have an influence on the short term are air temperature and light intensity.  It is obvious that these two change throughout the day. Air temperature with humidity changes during the night. So if these two factors have an effect on nutrient absorption, it will have an influence on growth. It has been found (ADAMS 1980)¹⁾ADAMS, P. 1980. Nutrient uptake by cucumbers from recirculating solutions. Acta Horticulturae. 98: 119-126., especially with cucumbers, that water uptake peaks right after light intensity peaks (see image below). Since nitrogen and potassium uptake is directly correlated with water movement in the plant, these two elements will be affected.  So expect a higher uptake during the later warmer parts of the day.

Peak water uptake is usually just after peak light intensity. This implies that nutrients that rely on water transport will be affected the most during these periods.

The uptake of nitrogen and potassium can be seen in the graph below. The uptake pattern corresponds well with the solar radiation intensity or light intensity and water uptake patterns of cucumbers during the same periods. It is interesting to note that phosphorus is not that affected by light intensity or air temperature but rather by the temperature of the water solution. The average uptake of nitrogen and potassium during the night is ±6 and ±6.5 mg/plant/h which is significantly lower than during the day. Similar patterns can be found in tomatoes, however, cucumber water consumption is usually double that of tomatoes.

Nitrogen and potassium uptake during the day with Cucumbers. Peak uptake is in the afternoon.

In the figure below it can clearly be seen how the light intensity affects water, nitrogen and potassium uptake. The ideal fertigation program should take into consideration the light intensity effect to be anywhere near optimal.  Attaching your fertigation system to a light intensity meter (drip irrigation systems only) is one of the best methods to help the plant and reduce fertilizer wastage.  With gravel culture systems the only way to help the plant during high water requirements is to lower the EC slightly (this will help cooling the plant down by providing more water).  With drip irrigation systems the higher the radiation the shorter the irrigation cycles become therefor providing water (with nutrients) that is more available (gravitational water vs capillary water). So irrigation cycles can be calculated using total water requirements for a given cycle (12hr or 24hr period, whichever suits your system design), or it can be done according to climatic demand. More info on irrigation cycles can be found at the article: Calculating the correct flow rate for open hydroponic systems.

Uptake of nitrogen and potassium at different light intensities on cucumbers. Water uptake is also affected.