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Monitoring nutrient balance, pH & EC in the root zone

The composition of the nutrient solution should be appropriate to the needs of the crop. One of the benefits of using Grodan stone wool substrates is that the nutrient composition is under total control of growers. This is because stone wool is inert and does not interact with individual elements during the cultivation cycle.

However, the uptake of particular nutrients varies constantly with growth and fruit load; therefore, the nutrient balance, pH, and EC in the root zone is also continually changing. Regular analysis of this solution in combination with daily monitoring of EC and pH allows you to exercise complete control over crop nutrition and plant development during the entire cultivation cycle. This will not only help avoid serious mistakes (plant & fruit quality) but will also enable you to grow with more precision, optimizing the input of water and fertilizer and minimizing unnecessary flushing of drain water to the environment.

Nutrient analysis of the root zone
To ensure regular growth and fruit quality it is important to maintain the correct balance between individual elements, particularly potassium to calcium (K+:Ca2+) and potassium to nitrogen (K+:NO3 - ). However, the root zone is a dynamic environment that is been continually modified by the development of the crop. Regular analysis of the nutrient solution will ensure that correct and timely adjustments to the drip solution can be made. It is advisable to do this once per month through phases 1 & 2, when only limited water is irrigated and thereafter every week.

Taking a sample is easy, but must be performed correctly to secure a representative sample to avoid making the wrong conclusions. For analysis at least 100 ml of solution is required. Sample flasks should be clean and completely filled to exclude air. For an analysis of the slab solution use a syringe to extract samples from 20 to 40 different slabs from within the irrigation or crop zone. Take an equal number from under and between the blocks taking care not to sample close to the drain hole. If the percentage of drain water being recycled is >30% a drain sample can be used instead of a slab sample. When interpreting the analytical results of the substrate solution growers should pay particular attention to the pH value.

Impact of pH
The pH is a measure of how acidic or alkaline the nutrient solution is. The pH of the nutrient solution will determine the availability of elements. Generally microelements iron (Fe) manganese (Mn), zinc (Zn) become less available as the pH is raised from 6.5 to 7.5 so it is always advisable to supply these in chelated form. At high pH levels phosphate (H2PO4-) will also precipitate as insoluble calcium phosphate, making H2 PO4 - unavailable to the crop and blocking irrigation drippers. A balanced uptake of each of the nutrient elements is only possible if the pH in the root zone is within the range 5.5 to 6.5. The target pH for the drip solution on the stone wool substrate should be 5.5 to 5.8. Never reduce the pH of the drip solution below 5.0. At these levels there is a danger that the acidic solution will break down the stone wool fibers causing the slabs to collapse 

During cultivation the pH in the substrate will fluctuate. For example with a greater uptake of negatively charged ions (anions) such as NO3 - , in response to strong vegetative growth the pH will increase (become more alkaline). With a greater uptake of positively charged ions (cations) such as K+ in response to a high fruit load the pH will decrease (become more acidic). To regulate pH it is important to maintain the balance between ammonium-N and nitrate-N (NH4 +-N and NO3 - -N). The managed changes of pH within the substrate should not be too large, for this reason pH should to be measured on a daily basis by doing so, trends can be followed by plotting results on a graph and growers can react in time by making informed adjustments to the drip pH, or by adding or removing NH4 +-N.

Taking a sample is easy, but must be performed correctly to secure a representative sample to avoid making the wrong conclusions. Simply extract a small quantity of solution from 20 to 40 slabs using a syringe, mix the sample (100ml) in a clean jar and test. However please note for measuring pH drain samples are not considered representative as the results are usually too high.

Impact of EC
EC is a measure of the total salts concentration in the nutrient solution. EC is therefore a mixture of essential (i.e. NO3 - , Fe3+) and non-essential (i.e. Na+, Cl- , SO4 2-) elements required for plant growth. EC is usually expressed in milliSiemens per linear centimeter (mS/cm) or microSiemens per linear cm (μS/cm) where 1 mS = 1000 μS. The higher the total salts concentration the higher the EC. As non-essential elements are not taken up by the plant they will accumulate in the root zone over time causing EC to increase. It is therefore important to undertake a regular nutrient analysis to determine the composition of solution and that essential elements are present in correct ratios.

On a daily basis EC can be used to influence plant development in line with incumbent weather conditions. For example when light levels are low it is possible to work with higher EC in the root zone to steer the plant in a generative direction. When light levels are high a lower EC steers crop development in a more vegetative direction. EC can also affect yield and fruit quality; in general a high EC lowers production but increases quality, maintaining the right balance maximizes economic return to the company. A perfect tool for the monitoring EC and stability of EC in relation to WC and light levels is the GroSens Multi system connected to the climate computer.

Alternatively the mulit-measurement function of GroSens hand held system can be used to gain an impression of EC from a greater number of slabs. When monitoring with a portable EC meter sampling should take place in a number of slabs so that a meaningful and accurate assessment can be made. Sampling is easy simply extract a small quantity of solution from 20 to 40 slabs using a syringe, mix the sample in a clean jar and test. The best time to sample using this method is when the slabs have started draining, because this is when you want EC to be under control from the irrigation strategy. 

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