Special Series in the rootzone #3: Control in the rootzone

May 18, 2010
Written by Andrew Lee
In the third of six articles for Greenhouse Canada, Grodan® Crop Consultant Andrew Lee provides an insight into the 6-phase model® which describes plant development and explains how through using this knowledge, growers can form coherent strategies in respect to rootzone management throughout the duration of the cropping cycle.

Picture 1.0 - Pin roots in propagation block.

I have already described transpiration and how this process is influenced by the aerial climate (Greenhouse Canada December issue). In the last article, I also defined key substrate functionalities and why these are important when it comes to substrate design and the fundamental thought processes behind an irrigation management strategy (Greenhouse Canada February issue). Now I will introduce the 6-phase model®. The model describes each development stage of a crop (tomato, cucumber, pepper, eggplant and roses) and defines targets for crop and rootzone management, in response to changing weather or plant development.

Growers worldwide have to react to developments in the market. Today’s immediate challenges for high-tech horticulture are doing more with less in a better way in response to increasing costs and decreasing product prices. This does not necessarily mean an ad-hoc approach to cost cutting. Doing more with less in a better way in this context means becoming more efficient at what a grower does in daily practice. The first thing he needs is a growing plan, a strategy that provides a structured approach to the way of growing. In essence, the structure should have four key pillars:
  • Strategy
  • Uniformity
  • Strength
  • Balance
A strategy is simply taking a global overview of the production cycle and market. It is invariably a paper exercise conducted between grower and consultant that will consist of choosing the right variety for the right retail outlet which, when planted at the right time and density, will achieve the right fruit size, yield and quality. Uniformity in both climate and rootzone environments is also an important consideration because without uniformity, the grower will be unable to grow to the defined strategy. This ultimately comes down to infrastructure and tools at his disposal, i.e., the quality of greenhouse construction, vent calibration, heat/fog distribution, substrate quality, evenness of the irrigation delivery system, etc. The grower must also consider the quality and uniformity of the young plants received from propagation. If these are of uneven size and development, it will result in many compromises to your strategy. On a seasonal basis, maintaining strength in the crop throughout the production cycle is also important because without strength, it is impossible to achieve the right crop balance on a daily basis (for example, a weak crop cannot be steered generatively and production will be compromised).

As cultivation systems become more high-tech and greenhouse businesses get larger, the requirements for structured growing plans are more important than ever. These global blueprints are the basis for ensuring optimal crop performance. Growers can become more focused at producing high quality produce for minimal cost. The 6-phase model takes all four previously described pillars into account and is now accepted worldwide by growers, consultants and seed companies. The model describes specific changes in crop development and consequently defines targets and objectives for each phase of growth (Table 1.0). It therefore provides a structured plan to follow over the course of the year, because if a grower has objectives and targets over the long-term, decisions on a daily basis become easier and more informed. Moreover, the model is applicable worldwide – wherever the location, the objectives and targets will be the same, only the weekly numbers related to the start and end of each growing phase will change.

The following part will focus specifically on the rootzone environment within each growing phase and further expand on the objectives taking the example for tomatoes described in Table 1.0.

If a grower opts to stand the plants on the side of the slab (Phase One-A), he should understand that during propagation the block was probably steered between 30 and 80 per cent WC.  This strategy should be continued with the objective of developing more pin roots within the block (Picture 1.0 illustrating pin roots tomato in propagation block) and allowing him to find the right balance between generative and vegetative growth. This will ensure the plants continue to grow in a controlled, uniform way. Remember that uniformity of WC and EC over the height of the propagation block increases the period of steerability while the plant is standing next to the plant hole. So, if growers opt for this strategy, it is an important feature of block quality. A typical strategy during this phase is summarized in Table 2.0. Start times can be determined by block weight (g) or water content (WC). The delta EC is a term used to describe the difference between slab EC and drip EC.

Good quality fruit.
When slab contact is made, usually when the first cluster is flowering (Phase One-B), the objective is to get the roots quickly established into the substrate. In this phase, it is important to maintain good block/slab contact by minimizing the difference in WC between the two components (Table 3.0). This practice will ensure increased DM per cent in the growing crop and improve the resilience to disease, particularly Botrytis, in later phases.

This phase comes to an end when the roots have penetrated into the substrate by several centimetres and water uptake no longer depends on the moisture content in the propagation block.

The choice of substrate, made as part of the overall strategy, will now influence dramatically what can and cannot be done in terms of rootzone management. I will use typical strategies employed when using a dual density rock wool slab in the subsequent growing phases. But remember, whatever the choice of substrate, the goals and objectives (Table 1.0) will remain exactly the same.

It is essential to form a large volume of roots in this growing phase as this forms the basis for crop growth and quality. Irrigation in relation to crop activity will encourage the roots to “look” for water and nutrition in the substrate. Steering in the rootzone on a daily basis should also be adjusted to the required direction of crop development (vegetative or generative) in order to keep the crop in balance. For maximum generative steering, your objective should be to gradually lower day level WC from 85-90 per cent to 45-50 per cent by the time the sixth cluster is flowering (Table 4.0). However, please remember the control range water content of some slab types will not allow such aggressive actions.

As the fruit load increases, the quantity of assimilates available for the development of shoots and roots will decrease. It is important that a grower distributes the available assimilates evenly between fruit, shoot and roots. Good crop registration will supply the required data as to how to keep strength in the crop with adjustments to the 24-hour temperature in accordance to the daily light sum. The growth of new roots and root tips should be encouraged by adjusting the start and stop times of irrigation, especially on darker days. On bright days, the WC can be steered upwards (Table 5.0). In combination with a targeted irrigation strategy, this will facilitate maximum growth and development of the crop. The drip EC is an important steering tool to maintain balance and fruit quality in this phase. Ideally this should be maintained sufficiently high which in combination with large dripping sessions especially on dark days, provides maximum generative development for a strong crop.

After the first harvest, the fruit load should be in balance with crop growth. The crop will assimilate all radiation into re-growth and weekly production will start to increase. However, re-growth should not be explosive or jerky. Targeted start and stop times between bright and dark days (Table 6.0) will ensure a good balance between vegetative and generative growth and will prevent fruit quality issues such as Blossom End Rot (BER), soft fruit and uneven colour. I will deal with these fruit quality issues in a subsequent feature.

The water content meter is a good tool to use in this respect. I like to see at least 1.5 to 2 per cent decrease in water content between sunrise (SR) and the start of irrigation on dark days (Table 6.0).

During this phase, the crop will have the highest production capacity, however, it is also when most fruit quality problems related to lack of water and nutrient uptake occur, i.e., BER. It is during this phase that the root system created and maintained by having a good rootzone management strategy through Phases Two-Four really pays dividends. Vegetative steering will encourage the crop to grow optimally for maximum production (Table 7.0). The start time of irrigation should now be related to light sum (J/cm2) or intensity (W/m2). It is also important to keep the decrease in water content overnight between 8 and 12 per cent. A larger decrease will impact negatively on production, as the fruits will become “dry” at the end of the day. This normally occurs if the stop time is targeted too early. I therefore like to stop irrigation when the outside light intensity is around 150 to 200 W/m2.

This phase starts when the growing heads are taken out, leaving 7 to 8 trusses on the plant. However the objective is still to continue producing high yield of high quality produce (Table 1.0). Fruit quality will be assured by adjustments to the irrigation strategy, which is aimed at preventing root necrosis and at maintaining the correct nutritional balance in the rootzone by ensuring adequate drain over 24 hours (Table 8.0).

The 6-phase model is accepted worldwide as a method of describing crop development. Using this framework, growers and consultants can plan climate and rootzone strategies over the duration of the cropping cycle. Having long-term objectives facilitates better decision-making in short-term when reacting to daily changes in weather conditions.

In the next issue of Greenhouse Canada, I will describe how to interpret information from a water content meter (substrate WC/EC). ■

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