Undercover Farming Magazine July / August 2022

all know people have to eat to live. The simple reasoning is to farm, to harvest and to supply people to feed his/her family. Currently though, agriculture as a whole is faced with several obstacles. The first item people talks about is input costs that has risen beyond belief. A grower is used to a two to three per cent increase annually, but the rate at which fertilizer, grow medium, crop protection product prices are increasing is unreal. General public is outspoken and even those within the current governing political support groups, that nothing is heard from the governments’ side over assistance to producers, lowering import tariffs and transport allowances to the food production sector. For these reasons many producers are holding back and some totally stopped production; some say the political interference from EU countries play a major role in this all. The news are spread that even our neighbours, Botswana and Namibia set a curb for imports of fresh produce from SA. This is to enable imports to these countries from EU and others. Several smaller fresh produce growers are being taken over or incorporated into the mega fresh produce farms. This is a dangerous situation for the man in the street as price fixing, even higher food prices and more empty shelves at supermarkets could be expected. Those that still export, find themselves with endless troubles at export facilities and harbours. Shipping and the time it takes for a product to reach its destination is one big nightmare. But you know what? Above all this negativity above, we still remain South Africans and have been recognized through the ages as a nation that can rise up from the ashes! I believe more small operations run by families will start up by young eager food producers to supply informal traders and corner shops and we still will have access to food. A great philosopher, Krishnamurti once said;” Fear twists our ideas and makes crooked the ways of our life; it creates barriers between people, and it certainly destroys love.” Let us keep on believing in our capabilities, make new plans, produce with what we have and live through these desperate times!!

GREENHOUSE TOMATOES START-UP –

An Example for New Greenhouse Produce Farmers

Hansvan de Arend started Tunnel Tomatoes in 2016 with the objective of growing yearround tomatoes on his farm, located in the Brits area. The product had to be of superior quality with no use of fossil fuels and as few inputs like chemicals, fertilisers and water as possible. When successful the project needed to be scalable; that is, easy to replicate and expand.

This starting point led Hans to construct the system and follow his method of production which he currently maintains.

As the method of production, Hans chose the high-wire tomato production with a pipe rail system utilising coco-peat substrate.

The coco-peat substrate is easily disinfected and can be replaced when needed. This guarantees his production to stay free from soil borne diseases. The substrate also allows him to check, register and control the root environment in order to create the best growing conditions for the roots and the plants. Run-off or drain water is collected, filtered, disinfected and recycled to save on water and fertilisers.

The pipe rail system has a dual purpose. It is an internal transport system for harvesting and crop maintenance and also used to heat the greenhouse during winter.

The original idea was to grow a high wire crop during a year cycle. That meant selected varieties grafted on a rootstock. The Brits summers seem to

be too harsh for this method with plants starting to deteriorate badly in summer. The method was therefor changed into a shorter production cycle with nongrafted plants. Plants are grown for an extended time in the nursery with two seeds per block. Plants are about 50 cm high with the first flowers opening when transplanted into the production greenhouse. This offers production within 6 to 8 weeks after planting.

According to Hans, an experienced greenhouse producer, planting 2 seeds per block requires good quality seed as both seeds need to germinate and develop equally. Also, the objective to produce superior quality requires good genetics. A variety needs to give wellsized, good-looking tomatoes that have a long shelf life. “Doufu RZ has those traits and suits us very well in our set-up and method of production,” Hans explained.

Reduced inputs

The goal of using as little inputs as possible leads to several advantages. All rainwater off the greenhouses is collected and stored to decrease the use of water from external sources and the quality of rainwater is always superior to that of other sources.

The greenhouses are naturally ventilated and do not have a pad and fan system as that system uses a lot of energy and water.

Find your variety Zwaan offers wide

“We recycle drain water out of the coco-peat slabs to save on water and fertilisers and also to prevent pollution to the environment. The controlled environment helps us to grow strong and resilient plants. These plants are better equipped to withstand attacks of pest and diseases therefor reducing the need for chemical intervention. When possible, we use natural enemies to fight occurring pests. The ground is covered reducing the need for weed control to a minimum,” Hans said.

In the greenhouse the climate and irrigation are computer controlled. This gives better management control but also a good registration of what happens. Furthermore, it provides the opportunity to have total insight in the growing of the crop and find ways to improve it.

Tunnel Tomatoes tries to ban all fossil fuels in the production. Heating of the greenhouses in the winter is already fossil free. During the sunny and warm winter days, water is heated in large solar panels and stored in a tank. In the night the hot water from the tank is pumped through the pipe rail system to heat up the greenhouse.

“We currently have a small trial installation of a PV system to generate our own electricity. The plan is to go fully off-grid in the near future,” Hans said about future plans.

“As far as our employees are concerned, we try to make their work area as comfortable as possible. Normally it is cooler in the greenhouse than outside: the plants’ evaporation cools the greenhouse down, but it can still be very hot and humid inside. Employees have pipe rail trolleys to minimise carrying while harvesting. They also use these trollies to sit on while doing plant maintenance or use them to collect debris from the plants. Electric pipe rail trollies are used for work at the top of the plants (about 3 metres above ground level).

“Rijk Zwaan has a lot of experience with their varieties in many different growing environments. With that they can advise me on the varieties that will perform best in our circumstances. The experience also helps with finding solutions to problems we encounter and improve on our production” Hans concluded.

APPLYING THE BEST OF DUTCH HORTICULTURAL TECH IN SOUTH AFRICA

TheHorti Demo Centre on the grounds of the University of Stellenbosch was officially established on 11 May 2022 with the symbolic handing over of a tomato plant and cutting of a ribbon by Mr Guido Landheer, director-general at the Dutch Ministry of Agriculture, alongside Prof Nick Kotze and Prof Eugene Cloete of the University of Stellenbosch.

Components for the almost 3,000m2 large and six metre-high greenhouse have arrived in South Africa and construction has commenced so that the first plants, grafted tomato seedlings, can be planted at the start of the South African spring.

Consortium involved

A consortium comprising six Dutch companies (Delphy, Koppert, Rijk Zwaan, De Ridder, ControlUnion, Svensson), a South African company (Greener Solutions) and Stellenbosch University received 900,000 Euros (R16m) to build a showcase of what’s best and most cutting-edge in Dutch horticultural technology today. The partners are going Dutch in the agreement with half provided by the Dutch government and the other half by the consortium. Prof Nick Kotze, who holds the WinField United South Africa Chair in Plant Health in the department of agronomy at the University of Stellenbosch, says that the planned Horti Demo Centre is a major progressive step of the university’s current horticultural facilities for greenhouse management and vegetable production.

Principles of circularity

The Horti Demo Centre follows the principles of circularity: re-circulation of water (it will run on predominantly rainwater) and nutrients within the system, resulting, he says, in a 30% saving in both and moreover, eliminates the risk of environmental contamination. Inside, pest pressure and the concomitant use of chemical pesticides will be dramatically reduced by the insect netting at all openings.

“For students, both undergraduate but especially for postgraduate students, to rub shoulders with international technology places them in another league. I always tell my students: future industry experts have to come from among us, and you can’t really become an expert if you don’t have access to relevant first world technology,” Prof Kotze says.

Increased Fresh Produce Production intended

While he’s very excited about the impact it will have on the quality of their teaching, the project is intended to go beyond the campus to become a locus for farmer training taking place inside the fully automated, wholly climatecontrolled greenhouse. “I’m certain we could easily double production,” Prof Kotze said.

“We would like to show especially new or small-scale farmers the very best production techniques that are currently available in the world. I’m dead certain we could very easily double the average production of small-scale farmers. We

have set ourselves the target of doubling their production and I think we can achieve that.”

Seeing the technology within the greenhouse perform under South African conditions, where the focus is much more on cooling down than on heating up as it would be in a Dutch greenhouse, will be an immense learning opportunity for the members of the consortium too, he adds. A delegation from the Kingdom of the Netherlands at the future site of the Horti Demo Centre at the University of Stellenbosch

Potential site for independent trials

The commercial vegetable sector of South Africa, traditionally a fairly fragmented sector, can also expect to see the benefit through, for example, independent vegetable trials that generate local and nonpartisan data on vegetable varieties on behalf of commercial seed companies and administered by the University of Stellenbosch.

“We can add great value to the local vegetable industry,” Prof Kotze explained, “and we will be able to guarantee the independence of our trials.”

In conclusion, Professor Kotze said a close working relationship between academia and the commercial sector creates a platform for postgraduate students to address the research needs of the industry and provides useful contact between students and their future employers.

Source: SU

Professors Nick Kotze and Eugene Cloete receive a tomato plant from Mr Guido Landheer from the Dutch Agriculture Ministry at the laying of the cornerstone of a new Horti Demo Centre (photos supplied)

Representatives from the Dutch-South African consortium

A delegation from the Kingdom of the Netherlands at the future site of the Horti Demo Centre at the University of Stellenbosch

GRAFTED WATERMELON

Being on the forefront of technological advances in agriculture, Hishtil SA has been supplying grafted watermelon seedlings on a commercial basis for several years. Feedback from growers shows consistently that they are experiencing better disease tolerance as well as increased yields and fruit quality. This remains true as advances in genetics come to the industry on a regular basis.

Fusarium:

Due to increased cropping on soils, instances of Fusarium are on the rise in watermelon production. Clearing of new fields is becoming increasingly costly and alternative options should be explored. Extensive trials with grafted watermelon plants have shown a high level of tolerance to Fusarium on old watermelon fields. This reduces the amount of time needed for rotation back onto the old fields and is a much more economic option for farmers. Where traditional rotation of watermelon is between 7 to 10 years, rotation of grafted watermelon can be

reduced to between 3 and 4 years.

Reduced plant population:

When planting grafted watermelon, a greatly reduced plant population can be used without affecting the yield. Traditional non-grafted plant populations vary from 5500 to 6000 seedlings per ha. Grafted watermelon can be planted at 3300 plants per ha. Growers are reporting increased yields, better longevity of fields and therefore extended harvesting times.

Reports of increased fruit quality and shelf life of fruit is also common

amongst commercial growers of grafted watermelon. This has opened new markets for growers not previously available, including export.

Important to note is that cultural practices like irrigation and fertilization should be adjusted according to the needs of grafted plants.

Contact Hishtil SA for more information.

Tel: 015 395 4034 | e-mail: sales@ hishtilsa.com or visit www.hishtilsa. co.za

DISC FILTERS AND SOLUTION TO CLOGGING PROBLEMS IN WATER DISTRIBUTION NETWORKS

Themain objective of installing filtration systems is the protection of all the elements that make up the system. The installation of filter equipment enables the following:

- Improvements of hydraulic performance in the lines.

- Reduction in maintenance work for the control elements, protection and measurements installed in the water distribution networks.

- Reduction in the necessary maintenance for the safety filters.

- Elimination of particles, which are big enough to produce clogging in microirrigation systems.

For the suitable selection of a filtration system we need to know the origin and type of clogging that can occur:

TYPES OF CLOGGING

According to the size of particles that can cause clogging, these can be classified as follows:

- Particles with a direct clogging capacity, due to their size can block certain installation elements.

- Clogging caused by large particles can be prevented by means of installing filtration equipment that is suitable for the water quality.

- Particles with no clogging capacity, not initially at least however these can acquire this capacity if the conditions are favourable.

Clogging can also be produced by very fine particles that go through the filters and under certain conditions (prolonged presence of water in the interior of the lines and variations in the water circulation speed) can form larger sized particles.

To prevent this, filter dimensions should be suitable to ensure that the filter grade is correct.

On the other hand, according to the origin of the matter causing the clogging, it can be caused by physical, chemical or biological origin particles:

- Physical origin particles produced by particles of inorganic nature. Two different types:

Internal Clogging. Caused by physical matter (sand, silt, clay) present in the water source destined for irrigation.

External clogging. Produced by matter that is introduced from the exterior into the interior of the system through: line joints, air relief valves, cracks in the distribution network, emitters in negative pressure conditions, etc…

- CHEMICAL ORIGIN PARTICLES

Caused by precipitation in the interior of the installation containing fertilizers or substances, which are dissolved in the irrigation water that passes through the filters. There are also two types.

- Direct chemical clogging. Those generated by precipitate in-situ.

Indirect chemical clogging. Clogging generated by particles which come from the detachment of chemical precipitates generated above the point where the clogging has finally taken place. Their behaviour and treatment is the same as for inorganic particles.

- BIOLOGICAL ORIGIN PARTICLES

Causes by organisms or organic remains: These can be classified in two groups:

Inorganic particles with no proliferation capacity. Vegetal and animal organic remains in suspension

in the water flow. Their behaviour and treatment is the same as for inorganic particles.

- Particles with proliferation capacity. Organisms present in the water (small insects, algae, micro-organisms…) that initially do not have direct clogging capacity but it is acquired due to their proliferation (multiplication or development) inside the flow and distribution networks.

CHOICE OF SYSTEM

When choosing between different filtration systems, one should evaluate the water flow, properties of the water to be filtered and the quality sought; as well as subsequent cleaning operations and maintenance that will have to be carried out, the intrinsic characteristics of each type of filter, the filtration system safety and maintenance throughout its useful lifespan.

The number of filters and the size, with the chosen filtration system and necessary filter grade, must be suitable to be able to cover the requirements in periods of maximum demand. One must take into account that these periods normally coincide with the lowest levels of water quality.

APPROPRIATE FOR SYSTEM

For this reason, and before the existence of different filtration systems, we must check whether the chosen system is appropriate for our installation system and complies with the following characteristics:

1. High safety filter quality faced with variable conditions of:

Water quality: types of solids in suspension.

- Circular water flow.

Work pressure.

- Existing differential pressure.

- Frequency of maintenance work.

2. Efficiency of backwash system in the automatic equipment.

and cleaning.

4. Long-term stability of properties.

5. Easy maintenance work.

The disc filtration systems provide the solution to problems that can come up in the distribution network due to solids that the water carries in suspension. The main features that highlight these filters are as follows:

These retain all types of particles, no matter their nature (organic or inorganic), as long as their size is greater than the filter grade.

These carry out filtration in depth. Not just the particles that have a greater size

than the filter grade are retained by the filter element, but a high percentage of smaller sized particles can also be retained. If a particle that has not been trapped on the surface enters the disc channel, the probability of it being retained depends on the following:

Size and form of the particle in relation to the dimensions of the channel: transversal area (directly linked to the filter grade) and its length.

Nature of the particle.

Existence of other particles already retained inside the channel that DISC FILTERS

11 DISC FILTERS

act as an obstacle for the rest and provide retention. In other words, an increase in efficiency is achieved as the number of particles retained rises (pressure differential increases), therefore, we can assure that as the grade of silting increases there is an increase in the quality of the filtration.

Resistance of the filtering element under high differential pressures levels, without causing its breakage. This is due to the high resistance carried by the disc set that is compressed and housed in the interior structure.

EASY TO ADAPT TO WORK CONDITIONS

Faced with changeable conditions in water quality or in the use and destination of the filtered water, through grooved discs the filtration system can allow for the change of the filtration grade easily, quickly and economically, carrying out this operation without using tools.

MAXIMUM SAFETY THROUGHOUT ITS USEFUL LIFESPAN

Unlike other filtration systems, this system does not reduce the quality of filtration even when working under pressure differential at all times, or handled improperly, or the duration of use.

Pic: Detail of HELICAL ELEMENT silting delayer. Situated in the base of the disc cartridge, it creates a helical effect in the water that takes the particles away from the filter element, thus, noticeably delaying its silting.

Reduced water consumption during the backwash process compared to other filtration systems:

One must take into account, that it is essential to know the “useful capacity” of the filter when purchasing filtering elements of the same nature and it is important to consider the existence of auxiliary elements that improve the effectiveness and efficiency equal to the surface use. These elements are the so-called “silting delayers”, that are able

to reduce the frequency with which cleaning has to be carried out, whether it is manual or automatic.

EFFICIENCY

Most importantly, remember to not cut corners when planning the filtration section of your irrigation system. As mentioned, the success of filtration will determine the efficiency and sustainability of the entire system. Do not allow your filters to cause a weak link in your system. By I. Reubin

HOW CANNABIS PRODUCTION BUSINESS CAN BE A SUCCESS IN AFRICA

Wesley Petzer is born and raised in East London, and as agriculturist is overseeing Cannavigia’s business in Africa, starting at its most southern point. He explained why cannabis business can be a success in Africa.

“BeforeI tell you what is right with Africa and why people should grow cannabis here, let me start off with getting two big problems out of the way,” says Wesley, “and these two problems are very important to understand. Some people have been growing and then they were surprised when there were problems when they wanted to export. Then they blame the governments for what is happening.

Before you grow in an African country there are two things you have to make sure of: Firstly, are there any sanctions against this country, this government or certain of its representatives? Secondly, and also very important: What are the existing trade agreements this country has signed on for? Once you have done your homework and gotten those things out of the way, your life will be a lot easier.

Wesley gives us a list of why your cannabis business can be a success in Africa and especially in South Africa.

South Africa Is the Hub South Africa is the country with the closest to an industrialized economy in Africa. This means the systems work similar to those in Europe, allowing for familiarity and the ease of doing business. Not only does it serve as an entrance into Africa, but the rest of Africa is also looking at how South Africa is doing things.

Good Climate

This doesn’t really need a lot of explaining, as any person who has every holidayed in South Africa will tell you. Good sunshine, enough rain, temperate climate – all the things needed for a

successful grow.

Potential for Green Energy

Growing cannabis takes a lot of electricity. Coal is going out of fashion. Africa has the potential for huge amounts of renewable energy. It has the sun, it has the wind and it has the water. There are large empty tracts of land that can be used for this. In South Africa, the whole Northern Cape Province is ideal for being developed for this purpose.

Access to Water and Space

These are the other two things that cannabis growers need. Depending on the location and country that you are looking to grow, water resources and space will not be a problem.

Mine Rehabilitation

A lot of mines in South Africa have land that’s no longer being used. According to South African law, land needs to be rehabilitated before they can sell it, or return it to the rightful owners. This means that there are huge tracts of land lying empty in agricultural areas that need to be fixed up. And it’s not only cultivation that can happen there, but also the facilities, like processing hubs.

New Revenue Streams Sought

The price of gold, diamonds, tobacco and various other commodities are dropping due to a changing world economy. African governments are looking to replace those income streams and the growing of cannabis fits into the new way of thinking. In Botswana, the government is looking to diversify its income from just diamonds and new industries like cannabis are perfect for that. With the world changing its stance on tobacco, countries like Zimbabwe are looking to transition their tobacco farmers into cannabis producers.

Labour force available

Growing in Africa is cheaper than anywhere else because of affordable labour. Agriculture is a tradition in Africa and people are therefore pre-disposed to it. It is also easy to up skill workers to

become more specialised in this field for a very low cost.

License made simple

The rules are clear. If you understand the bureaucracy or have the capital, you can get a license in most African countries. Everybody wants this to work, so even if it’s not perfect, people are aspiring to streamline it. So, as the industry’s knowledge of the bureaucracy grows, so does the bureaucracy’s knowledge of the industry. Both are learning and working towards making it easier and more transparent in the future.

Open doors

The African governments are eager to get involved in this new way that can generate cash and they have very much a “what can we do to make it work” attitude. They are prepared to listen to the needs of the industry and be shown what is needed. They want to play and that makes it easier to get things to be the way the industry wants it to be.

“Europe”, says Wesley, “need to get more involved with the creation of growing standards in Africa. This will help both sides move forward – Africa with the new knowledge and Europe with a new place to grow.”

By: Deon Maas

GREENHOUSE SANITATION BEFORE NEW SEASON PRODUCTION: ALL-IMPORTANT IN MANAGING PESTS AND PATHOGENS

Organic plant debris that is left discarded in an uncovered container may be a breeding ground for pest and pathogens that could potentially infect your crop.

Agood and easy first step in greenhouse sanitation is to physically remove all crop debris. Weeds, plant debris and unsalable plants can serve as sites for insects and mites to live and for diseases to develop, progress or spread. Remove all weeds and crop debris and place them in a tightly sealed, covered garbage bin so that pests and pathogens are not able to migrate out of the trash and into your crop (Photo 1).

Remember to remove the trash daily. Also, remove spilled media because organic residues from plants and growing media reduce the effectiveness of disinfectants (Photo 2).

If you ultimately compost the removed organic material, make sure the

compost pile is at least 10 metres away from the greenhouse and not uphill or downwind from the greenhouse (Photo 3). Situating the compost pile in this manner will prevent pests from migrating from the compost pile back to the greenhouse.

When removing diseased or infested plants, discard them into containers or bags that are immediately adjacent to the plants and seal the container or bag to transport them out of the greenhouse immediately. In this way, inoculum or insects are contained and not spread throughout the greenhouse during transport.

Once all surfaces are free of organic matter, consider power-washing structures and walkways with soap and water when in between cropping cycles.

Before putting new plants in the greenhouse after the previous season, following these simple sanitation protocols may help greenhouse businesses prevent pest outbreaks.

Then, clean those same surfaces with a disinfectant. Make sure propagation or pruning tools such as knives, scissors, etc., are properly disinfected after being used on each bench or each variety or cultivar.

It is also best not to reuse pots as some diseases are capable of surviving on recycled trays or grow bags and thus infecting new crops. However, if you must use recycled containers, be sure that all organic matter is removed from these first and then disinfect them properly with a bleach, quartinery ammonium or peroxide product (Photo 4).

Some of these cleaning products have a short life span, so you may need to create new batches of disinfectant frequently while you are using it. For further information about disinfectants, consult your local co-operative or greenhouse advisor.

Also, remember to train employees to keep all irrigation hose nozzles off the ground. In fact, any tool that touches the floor should be disinfected before it touches any growing surfaces. While it may be tempting to do so, do not reuse growing media that has fallen on the floor during potting or during container filling operations due to the potential for contamination.

Removing plant debris and spilled media from this stone greenhouse bench will make disinfectants more effective.

Once the greenhouse is sufficiently sanitized, to maintain the cleanliness, consider restricting entry into growing areas to necessary personnel and supervise all visitors. Consider requiring everyone to clean their shoes before entering growing areas by first removing any soil on the shoes and then using a foot bath or foot mat that contains disinfectant.

Remember to change the disinfectant daily in these foot baths or mats. Also, make sure employees wash their hands thoroughly with warm, soapy water and perhaps other sanitizing products as well before work and at intervals throughout the day before handling plants.

Consider having employees use protective clothing, such as disposable or sterile coveralls (Photo 5), gloves or shoe coverings, during potting or transplanting, when taking cuttings, or when rouging diseased plants. Remove and dispose of used protective clothing before working with healthy plants.

Now that proper hygiene protocols are in place, be sure not to introduce new problems from plant materials being brought into the greenhouse. Purchase high quality certified, diseasefree stock whenever possible. Consider arranging a visit to your plant supplier to see their sanitation program. When new plant material is brought into your greenhouses, quarantine it in a separate area at first to inspect for disease and insects.

By Kristin Getter, MSU

Consider having employees and visitors, such as this greenhouse manager, wear protective gear when entering sensitive propagation areas of the greenhouse.

A compost pile that is situated too close to the greenhouse may allow pests and pathogens to migrate from the pile back into the greenhouse.

While it is best to not reuse plant containers because of potential disease carry-over, removing all organic debris from the pots and soaking them in a disinfecting solution will help minimize this disease risk.

Waterand nutrients can be saved when nutrient solutions are recycled in hydroponic plant production units. However, this was not the only reason why the Dutch spent millions on research to be able to recycle. They started to recycle

when strict laws were enforced in the year 2000, prohibiting the release of nutrient-rich water from production units into drainage systems or rivers. The Dutch researchers solved their problems as described in the following two paragraphs:

1) Sodium (Na+) and chloride (Cl ) accumulation

Feeding water with very low Na and Cl levels is needed in order to recycle nutrient solutions without the danger that these ions may accumulate to toxic levels. These levels should be lower than the levels that can be absorbed by the specific plant in production. A saline tolerant crop such as tomatoes can remove 16 ppm Na and 32 ppm Cl. Thus, tomatoes can be recycled with feeding water containing these, or lower, Na and Cl levels.

Should water with higher Na and Cl

levels be used for a limited recycling period (until the red lights start flashing) saline sensitive crops can locally be flushed as soon as root zone levels reach 69 ppm Na or 107 ppm Cl. Most greenhouse crops will be unaffected with Na levels below 115 and Cl below 178 ppm.

Saline tolerant crops such as tomatoes will be able to withstand levels of up to 184 and 284 ppm Na and Cl respectively. Should these levels be exceeded, yield and quality losses will occur.

Special care is needed for low EC crops. As an example, roses only absorb 5 ppm Na and 11 ppm Cl. These are the highest levels that may be present in the feeding water to grow roses in a 100% recycled (closed) system.

The Dutch use feeding water with relatively high levels of Na and Cl. Since NUTRIENT SOLUTIONS RECYCLED?

Tanks for fertilizer mixes at a large greenhouse facility.

accumulation of these ions occur in closed systems that may not be released with the nutrient-rich water (flushing is a crime in the Netherlands), they are forced to lower the Na and Cl levels by adding rain water to their feeding water.

This is one of the reasons why the roofs of all Dutch greenhouses are used to catch rain water, to be stored in plasticlined reservoirs. They lower the Na and Cl levels in their feeding water by diluting it with rainwater.

2) Sterilization options for closed (recycled) systems

The following organisms should not be present in nutrient solutions:

Nematodes Special filters can be used to eliminate harmful nematodes.

Fungi are much smaller than nematodes, ranging from 3 to 50 μm. A slow sand filtration system may control fungi but with clear drainage water, ultra violet (UV) radiation is more effective.

Bacteria are small, difficult to control

and usually in the order of 1 μm. Apart from entering a greenhouse via the feeding water, these pathogens may also be carried on the shoes of visitors, pruning knives, seed or seedlings.

Viruses are extremely small (0.03-0.3 μm), usually insect transmitted and only visible with an electron microscope. Some viruses are effectively transmitted with recycled water.

Once in the recycled nutrient solution, bacteria and viruses can infect (wipe out) all the plants in the production unit. UV sterilization is commonly used in Europe where drained solutions are clear, due to the use of inert rockwool growth slabs.

The concentration of phytopathogens in some rivers and dams has increased in certain areas over the past few years, forcing growers to sterilize feeding water drawn from rivers. These growers use chlorination, as used by most South African municipalities. The drain-to-waste system is mostly used in South Africa, limiting the spread of

water-borne diseases between plants in a production unit. The drained solution should not be allowed to pollute rivers, but rather be used to fertigate field crops.

Recycled nutrient solutions should be chemically monitored and managed.

As a crop develops, or climatic changes occur, nutritional needs change. Mineral malnutrition can not effectively be corrected by using leaf analyses or petiole sap measurements. The best procedure is to analyze the drained nutrient solution and to identify nutritional imbalances, even before it is reflected in the leaves. By doing this, yield losses can be prevented.

This procedure is described in the book: ‘Nutrient solutions and Greenhouse management’ only available from the Combrink family trust: E-mail: njjc@sun.ac.za

ART OF GREENHOUSE VENTILATION MANAGEMENT

InSouth Africa we tend to focus on removing heat out of our greenhouses. But ventilation has multiple effects, which must be considered when making a management decision. During the course of the day, and depending on climatic conditions, a grower will have different objectives they are trying accomplish with ventilation. Ventilation takes place at different periods of the day.

• Morning

• Day – Warm

• Day – Cool

• Pre night - Night

Morning

When the sun rises the sunlight stimulates transpiration. This will cause the humidity in the greenhouse to rise – the rate of the rise will be determined by canopy leaf area. The crop and its fruit are still relatively cool in temperature coming from the night. As the moisture content of the air raises (increasing humidity) the risk of condensation forming on the relatively cool crop increases, particularly the

fruit, which heat up slowly in the morning because of their mass.

When the house is closed, the air is still and humid, so crop transpiration is at a minimum. This can result in leaf edge “burning” as the crop is not prepared for the rapid increase in transpiration with the increasing light intensity.

It is therefore important to open the greenhouse before sunrise to remove humidity and create air movement. This will stimulate transpiration and prepare the crop for the increasing transpiration. After sunrise, even on cool mornings, it is necessary to ventilate to remove humidity to prevent the risk of condensation on the crop. This can assist dramatically in disease prevention. Ventilation also assists in refreshing CO2 in the greenhouse, which can be rapidly depleted by a crop with a large leaf area on a sunny day.

Day – Warm

During hot high light intensity days heat builds up in the greenhouse. On these days it is necessary to ventilate to remove the heat. But when you open vents you also remove humidity and promote air movement, these actions

increase transpiration and increase water loss from the crop.

When crops transpire they cool and humidify the air, so be careful about ventilating too aggressively on warm days. It is actually possible to increase greenhouse temperature if you ventilate too much! How? Remember the crop does have the ability to cool the air around it, if you are ventilating too much (replacing air too often) you are not giving the crop sufficient opportunity to humidify and cool the air. This applies if you are also using fogging systems.

Your fogging system and crop work hard to add humidity so don’t waste it! Also keep in mind, aggressive air movement and removal of humidity creates a high water loss environment for the crop, which can result in wilting or promote physiological disorders such as blossom end rot. Do not chase temperature only. Crops can tolerate significantly higher growing temperatures if humidity is preserved within reason.

Day - Cool

On cool, cloudy days it is tempting to keep your greenhouse closed to accumulate heat. But remember if

the house is closed humidity is going to increase. This increases the risk of condensation on your plants and disease. Also it can lead to depressed transpiration, which in turn limits nutrient uptake possibly resulting in nutrient disorders.

So even during cool times of the year it is better to ventilate during the day to stimulate transpiration, keep the crop dry and also to refresh CO2. Yes, you will lose some heat and crop development may slow. But a disease free crop is generally more productive than a diseased crop. Rather have a cooler, dry crop than a warm, wet crop.

Heavily diseased tomato crop due to condensation on crop

Pre-night - Night

This is one of the most important times of day to manage ventilation is pre-night. During the day the crop is transpiring adding a lot of moisture to the air, increasing the humidity.

As the sun set the crop starts to cool and the risk of condensation on the crop increases. The more humid the air at the time of sunset, the greater is the risk of condensation on the crop. For this reason it is advised to open up the greenhouse an hour or two before sunset and flush out that humid air. This will dramatically reduce the risk of disease by reducing air humidity and risk of condensation on the crop.

In areas with mild conditions you can actually consider leaving the greenhouse

vents open over night – depending on factors such as wind and rain. This will ensure good air movement, limit high humidity pockets and reduce the risk of condensation.

In colder areas where heating is being used, the crop can still release significant humidity over the night. If the greenhouse is closed the whole night the humidity will accumulate and increase the risk of condensation and high air humidity, both conditions bring about disease development. It is therefore necessary to ventilate over the night period, even on cold nights to push the warm humid air out the house and replace it with cool drier air. However, in these instances the vents only need be slightly opened or in the case of pad and fan, use only the small gable fans for a short period of time to remove humidity without losing too much heat energy.

Strategies differ

to ventilation management. Ventilation strategy will be influenced by greenhouse type, design, climate control technology and crop leaf area. Even in low technology greenhouse systems these principles can be applied to improve crop performance and reduce disease pressure, but the best results will be achieved when combined with automation and monitoring the climate. Once a grower starts to monitor climate: light intensity, wind, temperature, plant temperature and humidity, they can refine and optimize their ventilation strategy.

Always remember: Do not chase a single climatic variable, keep in mind that when we take an action there is normally more than one climatic factor influenced, which in turn influences the crop. Always think about what you want to achieve – crop or climate wise; how will different ventilation actions influence the climate and crop; and then finally