Managing the Root Zone in Soilless Culture
Author: Eyal Ronen Haifa Chemicals Chief Agronomist
In solid growing media, there are five important parameters that should be monitored around the root zone to optimize plant growth and yields. EYAL RONEN offers some guidelines on how to manage these parameters to prevent major crop problems, and explains the importance of measuring fertiliser solution at both the dripper and drainage points. General terminology Soilless culture, commonly referred to as ‘hydroponics’, is a cultivation technique by which plants are grown detached from the soil. Plants are cultivated in containers filled with several possible growing media. If these media are solid, the method is called ‘soilless culture’. If no medium is present and the plant roots are bathed in circulated nutrient solution, the method is called ‘hydroponics. If no medium is present and plant roots get their nutrients by frequent spraying or misting, the method is called ‘aeroponics’. The method we are concerned with in this article uses solid growing media.
Figure 1. Different soilless cultivation methods.
Soilless culture characteristics The limited volume of medium and water availability generally causes rapid changes in the status of water and nutrients. Changes in the medium solution, such as electrical conductivity (EC), pH and nutrients level, should be monitored for the efficient use of water and nutrients. Failures in the careful supervision of fertilisation and/or the accuracy of irrigation are likely to result in severe plant damage and reduced yields. Hydroponics, however, offers several major advantages in the management of both plant nutrition and plant protection, if the right tools are applied and careful management is carried out. There are five important parameters that should be monitored by the grower, employing simple devices and methods. However, the common perception of some of these parameters is not always correct and this can result in some major problems. There are two reference points of special importance used to determine the status of the medium. These are the ‘drip-line point’ (fertigation input) and the ‘drainage’ point (output), based on the understanding that the drainage point best reflects the condition in the active root zone. By monitoring these two points, it is possible to see what changes are occurring in the medium after fertigation. Changes in the medium environment Water functions as a source of some nutrients, and as a delivery vehicle to transport nutrients into the plant via the xylem vascular system. When plants are exposed to low relative humidity, they lose water by transpiration through their stomata. Water also evaporates from the medium. However, transpiration and evaporation can lead to a salt build-up in the medium if proper management practices are ignored. Although some salts are absorbed by the plant, there is a sharp increase in the concentration and a build-up of some undesirable salts. When growing in soil, root volume and soil space are large enough that salt accumulation does not interfere with plant growth as quickly. But in soilless culture there is no space to buffer this salt
build-up, and immediate action is needed to purge the medium and lower the concentration of these dangerous components by washing them away. To avoid this problem, the common practice is to supply extra water at every irrigation cycle to ensure sufficient drainage - irrigation water should pass through most of the medium volume and leach away high salt concentrations at the drainage point. Theoretically, a 10% increase in water volume during daily irrigation cycles should be sufficient, but practically, an extra 30-50% of water is used. When plants are supplied with mineral fertilisers, although some are consumed and some are lost by leaching, the medium solution electrical conductivity is increasing compared to the drip-line point. The accumulation is mainly of nitrate and...
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