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SOILLESS CULTUREof Greenhouse Vegetables

Almost all of the vegetables we find on grocery-store shelves are produced either directly orindirectly in open-field soil. However, soil itself isn'tnecessary for plant growth - only some of itsconstituents.

Field soil serves two basic purposes: it acts as areservoir to retain nutrients and water, and itprovides physical support for the plant through itsroot system. Artificial means can also provide theseimportant requirements for plant growth with equal(and sometimes better) growth and yield resultscompared to field soil, although at substantiallygreater expense. Well-drained, pathogen-free fieldsoil of uniform texture is the least-expensive mediumfor plant growth, but soil doesn't always occur in thisperfect package. Some soils are poorly textured orshallow, and provide an unsatisfactory rootenvironment because of limited aeration and slowdrainage. Pathogenic organisms are a commonproblem in field soils. When adverse conditions arefound in soil and reclamation is impractical, someform of soilless culture may be justified. This guidedescribes the materials used in soilless culture anddiscusses management practices in various soillesssystems.

DEFINING SOILLESS CULTURE

Soilless culture is an artificial means ofproviding plants with support and a reservoir fornutrients and water. The simplest and oldest methodfor soilless culture is a vessel of water in whichinorganic chemicals are dissolved to supply all of thenutrients that plants require. Often called solutionculture or water culture, the method was originallytermed hydroponics (i. e., "water working") by W. F.Gericke in the 1930s. Over the years, hydroponicshas been used sporadically throughout the world as acommercial means of growing both food andornamental plants. Today, it is used widely inresearch facilities as a technique for studying plantnutrition. Various modifications of pure-solutionculture have occurred. Gravel or sand is sometimesused in soilless systems to provide plant support, andretain some nutrients and water. The retention ofnutrients and water can be further improved through

the use of spaghnum peat, vermiculite, or bark chips.These are the most commonly used materials, butothers - such as rice hulls, bagasse (sugarcanerefuse), sedge peat, and sawdust - are usedsometimes as constituents in soilless mixes. Strawbales have been used as growing medium in Englandand Canada. Rockwool (porous stone fiber) is usedin Europe, but there is little experience with it in thiscountry.

Since the major constituent of the media inartificial growing systems may be solid or liquid, it isappropriate to use the term soilless culture inreference to this general type of growing system andreserve the term hydroponics for those in whichwater is the principal constituent. Soilless culturemethods may thus be classified as either solid- orliquid-medium systems.

TYPES OF SOILLESS CULTURE

Liquid-medium systems are further differentiatedfrom solid-medium systems by method of operation.Liquid systems are generally closed circuit withrespect to nutrient-solution supply: the solution isrecirculated from a supply reservoir eithercontinuously or intermittently for a period of days orweeks. The two most common liquid systems in usetoday are nutrient-flow technique (NFT) and gravel-bed culture.

An NFT growing system consists of a series ofnarrow channels through which nutrient solution isrecirculated from a supply tank. A plumbing systemof plastic tubing and a submersible pump in the tankare basic components. The channels are generallyconstructed of opaque plastic film or plastic pipe(fig. 1); asphalt-coated wood or fiberglass also hasbeen used. The basic characteristics of all NFTsystems is the shallow depth of solution that ismaintained in the channels. Flow is usuallycontinuous, but some systems are operatedintermittently by supplying solution a few minutesevery hour. The purpose of intermittent flow is toassure adequate aeration of the root systems. Thisalso reduces the energy required; but under rapid-growth conditions, plants could experience waterstress if the flow period is too short or infrequent.

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Therefore, intermittent-flow management seemsbetter adapted to mild-temperature periods or toplantings during their early stages of development.Capillary matting is sometimes used in the bottom ofNFT channels, principally to avoid the side-to-sidemeandering of the solution stream around young rootsystems, but it also acts as a reservoir by retainingnutrients and water during periods when flow ceases.

NFT channels are frequently designed for asingle row of plants with a channel width of 6 to 8inches (15 to 20 cm). Wider channels of 12 to 15inches (30 to 38 cm) have been used to accommodatetwo rows of plants, but meandering of the shallowsolution stream becomes a problem with greaterwidth. To minimize this problem, small dams canbe created at intervals down the channel by placingthin wooden sticks crossways in the stream, or by theuse of capillary matting. The channels will need tobe sloped 4 to 6 inches per 100 feet (10 to 15 cm per30 in) to maintain gravity flow of the solution. Flowrate into the channels should be in the range of 1 to2 quarts a minute (I to 2 liters a minute). Channellength should be limited to a maximum of 100 feet(30 in) in order to minimize increased solutiontemperature on bright days. The ideal solutiontemperature for tomatoes is 68o to 77oF (20o to25oC). Temperatures of 59o or 86oF (15o or 3OoC)have been shown to decrease growth and yield oftomatoes. Black plastic-film channels will causesolution temperature to increase on sunny days.During cloudy weather, it may be necessary to heatthe solution to the desirable temperature. Solutiontemperatures in black plastic channels can bedecreased by shading or painting the surfaces whiteor silver. Cooper (1979) provides greater detail onNFT management.

Gravel-bed culture utilizes a waterproof troughfilled with pea gravel (or some other inert material ofsimilar size), which is plumbed to a nutrient solutionreservoir (fig. 2). Gravel particles retain very littlewater and nutrients, so the system must recirculatesolution from the supply tank to the beds severaltimes a day by means of a time clock andsubmersible pump. Some gravel systems aredesigned to be fed from the surface throughperforated pipes, and drained at the base of thetrough through a slitted drain line; others are bothsubirrigated and drained through a single pipe at thebottom of the bed. The advantage of the two-pipesystem is that any root growth into the drain line willnot interfere with the uniform distribution of nutrientsolution to the bed. In either case, however, rootgrowth will eventually clog the drain line and rotarycleaning equipment must be used to remove it.

Gravel-bed troughs are generally 24 to 36 inches(60 to 90 cm) in width and 8 inches (20 cm) deep.Pea gravel must be thoroughly washed before use toremove particles of soil or other material that mightclog the drain line. Care should be used in selectinga gravel supply that is free from pathogenicorganisms. Treatment of the gravel by steamsterilization or an appropriate fungicide is a wisepractice when condition of the material is uncertain.

The nutrient-solution supply tank should be largeenough to hold a volume of solution about twice thatrequired to fill the beds; this provides a good marginof safety.

The plumbing-system lines and submersiblepumps should have the capacity to fill the beds inabout 15 minutes, and allow complete drainage in 30to 45 minutes.

When managed properly, NFT and gravel-bedsystems are capable of growing good crops, but thereare some disadvantages that should be taken intoconsideration. The nutrient concentration of arecirculated nutrient solution is in a continuous stateof change because plants are removing elements atdifferent rates. Therefore, some means ofmonitoring and replenishing must be used to avoiddeficiencies (and perhaps toxicities from excesses ofsome elements). This increases the cost ofequipment and laboratory analysis. Recirculated

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systems are power dependent. If electrical energy isdisrupted, there is little reservoir of water andnutrients to protect the plants from stress.Recirculation of the solution is an ideal means ofspreading any pathogenic organism (such as water-mold fungi) that may be inadvertently introduced tothe system. For these reasons, more managementcare, experience, and capital will be necessary forsuccess with recirculated liquid-medium systems.

Solid-medium soilless culture may employ anyone of many types of suitable media in various typesof containers. Basic requirements are a material ofuniform texture that drains well yet retains somenutrients and water, a container in which thematerial is confined, and a means of supplyingnutrient solution. A well-drained sandy loam couldbe used as a growing medium, but a supply of veryuniform soil in the volume required may be difficultto find, and the weight of soil is much greater thanother types of material. Sand has been used insoilless systems in which the entire floor of agreenhouse is filled a foot or more in depth, but it israrely used in container systems because of itsweight. Where sand is used, particle-sizedistribution is an important consideration in order tomaintain a good balance between drainage (aeration)and nutrient and water retention. Particle sizesshould be in the range of 0.1 to 1.0 mm with anaverage of 0.25 to 0.50 mm.

Full-floor sand culture has been successful forvegetable culture in greenhouses and is considered agood means of providing plants with a uniform,well-drained rooting medium (fig. 3). Installationrequires excavation of the greenhouse floor to theintended fill depth, and grading (about 4 inches per100 feet [10 cm per 30 m]) for drainage. First, thegraded area is covered with 10-mil plastic sheetingto prevent root penetration into the underlying soil.Then a system of drain tubes at the spacing of theplant rows is laid out on the plastic and connected toa common drain at the lower end of the house. Sandis then filled to the intended depth over the plasticsheeting and drain lines. Be careful to select sandaccording to its particle-size distribution, and itsfreedom from pathogens and constituents that mightbe toxic to the crop plants. Because of thepermanent preparations for full-floor sand culture, itis recommended that sand intended for use in thesystem be given a growth test in containers beforeactually filling the greenhouse to determine if itmeets the basic requirements.

Sand-culture systems for tomatoes or cucumbersare typically irrigated and fertilized by trickleirrigation. The nutrient solution should be supplied

at each irrigation because of the relatively lownutrient retention of sand. Irrigation frequency willvary with the crop, its growth stage, and thetemperature, but will range from two to several timesa day. Depending upon plant size and temperature,tomato and cucumber plants will require in the rangeof 1/2 to 4 quarts (1/2 to 4 liters) per plant a day.

CONTAINER GROWING

Soilless culture in bags, pots, or troughs with alightweight medium is the simplest, mosteconomical, and easiest to manage of all soillesssystems. The most common types of media used incontainerized systems of soilless culture are peat-lite(Boodley and Sheldrake 1977), or a mixture of barkand wood chips. Container types range from longwooden troughs in which one or two rows of plantsare grown, to polyethylene bags or rigid plastic potscontaining one to three plants. Bag or pot systemsusing bark chips or peat-lite are in common use

throughout the United States and offer some majoradvantages over other types of soilless culture: (1)the medium materials have excellent retentionqualities for nutrients and water; (2) containers ofmedium are readily moved in or out of thegreenhouse whenever necessary or desirable; (3) theyare lightweight and easily handled; (4) the mediumis useful for several successive crops; (5) thecontainers are significantly less expensive and lesstime-consuming to install- and (6) in comparisonwith recirculated-hydroponic systems, the nutrient-solution system is less complicated and lessexpensive to manage. From a plant-nutritionstandpoint, the latter advantage is of significantimportance. In a recirculated system, the solution iscontinuously changing in its concentration and itsnutrient balance because of differential plant uptake.

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In the bag or pot system, the solution is notrecirculated. Nutrient solution is supplied from afertilizer proportioner or large supply tank to thesurface of the medium in a sufficient quantity to wetthe media. Any excess is drained away from thesystem through drain holes in the base of thecontainers. Thus, the concentration and balance ofnutrients in solution fed to the plants is the same ateach application. This eliminates the need to sampleand analyze the solution periodically to determinethe kind of necessary adjustments, and avoids thepossibility of solution excesses or deficiencies.

In the bag or pot system, the volume of mediumper container varies from about 1/2 cubic foot ( 14liters) in vertical poly bags or pots to 2 cubic feet (56liters) in lay-flat bags. In the vertical-bag system, 4-mil black poly bags with pre-punched drain holes atthe bottom are common. One, or sometimes two,tomato or cucumber plants are grown in each bag(fig. 4). Lay-flat bags accommodate two or threeplants (fig. 5). In either case, the bags are aligned inrows with spacing appropriate to the type of cropbeing grown. It is good practice to place verticalbags or pots on a narrow sheet of plastic film toprevent root contact or penetration into theunderlying soil. Lay-flat bags, which have drainageslits (or overflow ports) cut along the sides an inch(2. 54 cm) or so above the base, would also benefitfrom a protective plastic sheet beneath them.Greater detail on lay-flat bag culture is provided byBauerie (1984).

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Irrigation Systems

Nutrient solution is delivered to the containers bysupply lines of black polyethylene tubing to spaghettitubing, spray sticks, or ring drippers in thecontainers. Application devices have differentwetting patterns and are available in different flowrates. The choice of application system is importantin order to provide proper wetting of the medium ateach irrigation. Texture and porosity of the growingmedium, and the surface area to be wetted areimportant considerations in making the choice.Spaghetti tubing provides a point-source wettingpattern, which might be appropriate for fine-texturedtypes of media and allows water to be conductedlaterally with ease. In lay-flat bags, single spaghettitubes at individual plant holes will provide goodwetting of peat-lite media. In a vertical bagcontaining porous medium, a spray stick with a 90-degree spray pattern will do a good job of irrigationif it is located to wet the majority of the surface.Ring drippers are also a good choice for vertical bagsalthough somewhat more expensive. When choosingan application system for bag or container culture,remember that the objective of irrigation is todistribute nutrient solution uniformly so that all ofthe medium is wet. Since a root system cannotfunction in dry medium, dry medium is wastedmedium.

Growing Media

The growing medium used in container culturemust have good nutrient- and water-holdingcharacteristics, and provide good aeration to the rootsystem. Light weight is another importantconsideration so that filled containers can be easilyhandled. Growing media should be free ofpathogenic organisms and substances that are toxicto plants. The principal materials that meet theserequirements are peat moss, bark, shavings, sawdust,vermiculite, bagasse, and rice hulls. Table 1provides a summary comparison of thecharacteristics of these materials. Some should notbe used alone, but have one or more characteristicsthat make them valuable constituents when used in amixture. Bagasse is low in porosity and high inwater-holding capacity, which would lead to pooraeration and drainage if used alone. Because ricehulls have low water-holding capacity and high porespace, plants would be vulnerable to water stresswhen rice hulls are used alone. Both vermiculite and

sawdust are poor choices as sole constituents becausetheir high water-holding capacities can lead tosaturation and poor aeration if over-irrigated.Vermiculite particles also tend to collapse with time,resulting in compaction and volume loss. Sawdust(except for cypress and redwood) and bagasse havehigh carbon: nitrogen ratios (C:N ratios), andrequire extra nitrogen fertilizer to avoid thecompetitive demand for nitrogen betweenmicroorganisms and the plants. Bagasse, rice hulls,sawdust, and vermiculite possess usefulcharacteristics when used in mixes with othermaterials in the range of 20 percent to 50 percent ofthe total volume. Because of their high C:N ratios,bagasse and most sawdust material should be limitedto no more than 20 percent of the total volume of amix.

Care should be used in the kind of wood materialselected for soilless culture. Cedar, walnut, andeucalyptus may have components that are toxic toplants. Fresh redwood also affects the growth ofsome plants, but this effect becomes negligible withaging and leaching. The causes of toxicity fromwood materials is not clearly understood, butprobably varies with the type of plant being grown,and the type and age of wood being used. Woodmaterials are generally acidic and any toxicity fromtheir use may be due to the effects of acidity on theavailability of some nutrients to the plants. Inredwood, the toxic component is transient because itdecomposes or is leached away during composting.Materials such as pine sawdust decompose rapidlybecause of the high C:N ratio and, if supplementalnitrogen is not provided or is present in insufficientamount, the deficiency that develops may give theimpression of toxicity. The barks of pine, fir, andredwood (and possibly others) can be safely usedwithout growth-retarding effects, but cedar andwalnut should be avoided. Sawdust and shavings ofpine, fir, and redwood can make good, safeamendments when composted with nitrogen at 13pounds per cubic yard (8 kg/m 3) for 2 to 3 months.

Mixes should not be made merely to takeadvantage of availability or low cost, but shouldconsider the basic factors of weight, nutrientretention, water-holding capacity, pore space, andC:N ratio. Mixtures of spaghnum peat andhorticultural vermiculite (peat-lite) have all of therequired characteristics and make an excellentgrowing medium. Proprietary peat-lite mixes areavailable, or growers can prepare their own supplyfrom the basic components (Boodley and Sheldrake

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1977). Barks from pine, fir, cypress, and redwoodhave been used successfully as a growing mediumfor greenhouse cucumbers and tomatoes. Particlesizes of bark range generally from 1 to 10 mm indiameter; and the distribution of particle sizes inmost mill-run material provides good aeration, andgood water- and nutrient-holding characteristics. A

bark medium can be used for several successivecrops without a significant reduction in volume dueto decomposition. A supply of bark withpredominantly large particle sizes should beamended with a material such as sawdust, shavings,or bagasse in order to improve water-holdingcapacity.

FERTILIZATION

In field culture, the clay fraction of soils can beexpected to supply adequate amounts of at least someof the nutrients required by plants, especially theminor elements.

Fertilizer programs for soilless-culture systemsmust supply all nutrients required by the plants.Carbon, hydrogen, and oxygen are provided fromwater and carbon dioxide in the air. The grower willsupply nitrogen, phosphorus, potassium, calcium,magnesium, sulfur, iron, boron, copper, zinc,manganese, molybdenum, and chlorine. Mostmedium materials contain small amounts of theseelements; but they should not be considered inplanning the fertilizer program because they are asmall proportion of the requirement, or they may bein forms not readily available to plants.

Liquid-medium systems - such as NFT andgravelbed culture - use complete nutrient solutionsprepared from soluble inorganic salts containingvarious elements. Proprietary mixes of all requiredelements are available, which are simply dissolved in

water to prepare the nutrient solution. These mixesare available in various concentrations and ratios ofelements. Nutrient solutions can also be prepared bythe grower using readily available soluble salts.Many complete nutrient-solution formulas have beendeveloped and used successfully. All contain thesame elements and are generally prepared from thesame compounds, although in somewhat differentproportions. No one formula is necessarily the bestfor all plants, but all are capable of providingadequate nutrition. Special formulas are oftenrecommended for a particular crop plant based uponresearch under the prevailing climatic and waterquality conditions at a specific location. Theseformulas are soundly based for those conditions, butnone should be construed as being the bestuniversally under all conditions. They are goodpoints of departure in developing a feeding programfor the crop for which they are recommended, andmay be well suited for use without alteration.Successful managers seek as much information aspossible from reliable sources to develop a soundunderstanding of plant nutrition and inorganicchemistry before attempting to alter publishedformulas for their imagined or perceived needs.

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Improper alteration of formulas can lead to seriousadverse effects due to excesses or deficiencies. It isrecommended that growers either utilize preparednutrient mixes obtained from reliable manufacturersor, if preparing their own, follow recommendedformulas carefully. Competent assistance should besought before making changes. Solid-mediumsystems - such as bark or peat-lite- can be providedwith nutrients by three methods: (1) entirely from acomplete nutrient solution; (2) from a combinationof premixing some elements in the medium andsupplying others by liquid feed; or (3) premixing allelements in the medium. The complete nutrient-solution method is commonly used in sand cultureand for various mixed media or bark. Nutrientsolution is applied up to several times a day tomaintain the medium in a moist (but not saturated)condition. This system requires either a supply tankfor the nutrient solution or a ratio feeder or fertilizerproportioner that prepares solution upon demandfrom stock nutrient concentrates. For smallgreenhouses, the frequent chore of replenishing thesolution supply in a tank may be more attractive thaninvestment in a fertilizer proportioner. When usingthe supply-tank method for a recirculated growingsystem, the tank should be large enough to hold avolume of solution about twice that required to fillthe system. This provides a safe margin of nutrientsupply.

Nutrient-solution Formulas

Formulas for several nutrient solutions are givenin the Appendix, along with methods of preparation.While they differ in concentrations of the elements,all have been used successfully by commercialgrowing operations, principally for the production oftomatoes. Formula I has been widely used inresearch greenhouses as a general nutrient solutionfor a wide range of plants, and is a good formulachoice where more specific information is not knownfor a particular crop. It is possible that adjustmentsin concentrations of some elements (particularlynitrogen, phosphorus, or potassium) may bebeneficial to the yield or quality of a given crop.Until research is clear on this, however, it is best toadhere to the basic formula.

The formulas in the Appendix list amounts ofindividual salts to be dissolved in 100 gallons (378liters) of water.

This prepares the nutrient solution in the form to besupplied to plants from a storage tank. When afertilizer proportioner is used, the amount of eachsalt must be adjusted to account for the dilution rateof the proportioner. By this method, liquidconcentrates of the salts are prepared that will bediluted for the final nutrient solution as they are fedthrough the proportioner. For example (in formula1), to calculate the amount of potassium nitrate toprovide 50 gallons (190 liters) of concentrate to beused with a 200: 1 proportioner, divide 95 grams by100 to obtain grams per gallon (or by 378 for gramsper liter) in the final nutrient solution as shown, thenmultiply by 200 to obtain the amount in I gallon (or1 liter) of concentrate, and finally multiply by 50 (or190 liters) for the amount required for 50 gallons(190 liters) of concentrate. The amount of potassiumnitrate required for 50 gallons (190 liters) of 200: 1concentrate is 20.9 pounds or 9550 grams. Whenpreparing concentrates for proportioners, twoseparate concentrates are required to avoidprecipitates. One contains only calcium nitrate andthe iron compound; all other ingredients are in theother concentrate. The concentrates are kept inseparate tanks and must be used in conjunction witha twin-head proportioner. When activated, theproportioner draws equal volumes from each tankand mixes them with an appropriate amount of waterto provide the dilute nutrient solution.When using a proportioner, it is good practice tomonitor its operation on a regular basis to be certainthat solution of proper concentration of elements isbeing provided to the plants. This should be done intwo ways. A water meter attached to the outlet sidewill record the volume of solution mixed for a

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particular period of time. The depletion ofconcentrate volume in each tank over the sameperiod of time and the volume of solution suppliedshould be in the same ratio as the dilution ratio ofthe proportioner. Another check on the system is tocompare periodically the total salt concentration of aphysical dilution of both concentrates with water.As an example, mix 200 ml of water with I ml(using a 1-mi pipette for measurement) of eachconcentrate. Salt concentration can be determinedby an analytical laboratory or by a portable battery-operated instrument that measures electricalconductivity (fig. 7). During preparation of theconcentrates, measurements of the individual saltsshould be made very carefully so that the finalsolution will contain amounts of individual elementsas intended in the formula. Mistakes can be made,however, and for this reason it may be wise tosample the final solution from the proportioner andhave a complete analysis made in a laboratory on aperiodic basis.

When premixing fertilizer materials with bark,or bark and sawdust, compounds that supplyphosphorus, magnesium, calcium, sulfur, and allminor elements may be added to the growingmedium prior to planting. A small amount ofnitrogen may also be mixed with the medium I butmost and sometimes all of this element is supplied inthe irrigation water. Nitrogen is generally suppliedin the nitrate form in the range of 100 to 200 ppm. 1

The ammonium form of nitrogen, if included, shouldnot exceed 10 percent of the total nitrogen supplied.Potassium also is supplied routinely with nitrogen inthe irrigation water at about 200 ppm. Anitrogen/potassium liquid-feed solution providing150 ppm nitrogen and 2 10 ppm potassium can beprepared by mixing 0.45 pound (208 gm) potassiumnitrate and 0.40 pound (182 gm) calcium nitrate in100 gallons (378 liters) of water. To premix theother elements in the medium per cubic yard (orcubic meter), phosphorus can be supplied assuperphosphate (0-20-0) at 2 pounds (1 kg), calciumand magnesium from dolomitic lime at 10 pounds(4.5 kg), and minor elements from trace elementmixes such as FTE 503 or Esmigran at 5 ounces (155 gm). Additional iron may be added in dilutedform at I ounce (28 gm) of 138 Fe.

The method of premixing all fertilizer materialsin the medium before planting may include the useof slow-release nitrogen materials such as Mag-amp,Osmocote, isobutylidene diurea (IBDU), or sulfur-coated urea (SCU). This method is not commonlyused but, when compared to the liquid-feed method,it is reported to produce equal or better yields oftomatoes (Sheldrake, Dallwyn, and Sangster 1971).The premix method offers important advantages byeliminating the need to prepare nutrient solutions,and the purchase and maintenance of a fertilizerproportioner. The potential disadvantage of slow-release fertilizers that supply all of the nitrogen andpotassium is that they may not be able to release theelements at the proper rate to satisfy the plants'needs. While slow-release fertilizers are widely usedin the ornamental plant industry, their success rateand cost-effectiveness on vegetables in soillessculture have not been adequately established. Untilmore information becomes available, it is suggestedthat their use be limited to medium amendments inproven fertilizer programs.

1ppm (parts per million) = mg per liter. Calculation example: toobtain 100 ppm N from potassium nitrate (14 percent N), 100 / 0. 14= 715 mg potassium nitrate/liter = 0. 715 gm/liter = 2.79 gmpotassium nitrate/gallon (1 gallon = 3.785 liters).

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Analysis of Solution, Tissue, and Media

Knowledge of the nutritional status of allcomponents of a soilless culture system is importantfor two reasons: (1) it is the only means of judginghow successful management practices have been inattaining the objectives of the fertilizer program interms of the availability of nutrients and the nutrientlevels in the tissue; and (2) it helps diagnose thecauses of any abnormal plant symptoms that mayoccur. The costs of this knowledge are a form ofinsurance toward success. Periodic laboratoryanalysis of the nutrient solution is imperative inrecirculated hydroponic systems, especially if use ofthe solution is extended over a period of weeks. It isequally important in solid-medium systems in orderto evaluate the operation of the fertilizerproportioner and the preparation of concentrates.

The nutrient solution in a recirculatedhydroponic system-such as NFT or the gravel-bedmethod-may be used on either a short-use orextended-use basis. For short use, a fresh solution isprepared every week or every 2 weeks with periodicreplenishment from concentrates. This systemassumes that, during the short-use period, nutrientremoval from the solution will not reach deficiencylevels as long as some fresh concentrate is addedevery few days. This is a workable procedure andhas been used with some success, but it is wasteful ofsome nutrients and, if the used solution is discardedon porous soils, it can be a source of groundwaterpollution. The alternative is extended use of thesolution for a period of several weeks or perhapsmonths. This method can also be successful butrequires close periodic monitoring to avoiddeficiencies or excesses of nutrient elements, eitherof which can have adverse effects on plant growth.Because the nutrient status of an extended-usesolution is continually changing due to plant uptake,frequent analysis is necessary to maintain thesolution close to the original concentration ofelements. At a minimum, a complete analysis iswarranted every 3 weeks with weekly analyses fornitrogen, potassium, and phosphorus. Dailydetermination of the total salt content will provide auseful estimate of the nutrient status of the solutionalthough this cannot substitute for complete analysis(Johnson 1980).

The constantly changing nutrient status inrecirculated solutions and necessity (and cost) forclose control through solution analysis to avoiddeficiencies and excesses of elements provide alogical justification for the preferred use of solid-

medium systems in soilless culture. In solid-mediumsystems, plants are fed a uniformly balanced solutionat each irrigation. Solution management problemsare reduced to measuring precisely when preparingconcentrates and monitoring the fertilizerproportioner to assure that it is in proper workingorder.

Tissue analysis is the best method to evaluate thenutritional status of the plants and the success of thefertilization program. Periodic sampling andanalysis of leaf tissue during growth of the crop willprovide information that can be used to makeadjustments in fertilizer practices, as well as to helpinterpret any abnormal plant symptoms. Wholeleaves, leaf petioles, or leaf blades are the plantsparts normally used, although desirable levels fornutrients vary depending upon the part sampled, itslocation on the plant, and the analytical methodsused. Desirable levels of nutrients have beenreported for tomatoes, cucumbers, and a wide rangeof vegetables (Ward 197 3; Wittner and Honma 1979; Lorenz and Tyler 198 3; and Johnson 1980).Growers who wish to include tissue analysis in theirmanagement program should seek assistance fromtheir local county-extension agent or farm advisor inchoosing an experienced agricultural analyticallaboratory. The lab can help to outline proceduresfor sampling and the handling of samples, so thatmaterial provided to them is from the proper locationon the plants, is representative of the growing area,and is in good condition upon arrival. Usually theyoungest, fully expanded leaf should be sampledfrom randomly located plants throughout the area;this is generally the fourth or fifth leaf from thegrowing point. An adequate sample size is 30leaves. A single sample will be adequate whereplants are uniform in vigor and appearance.Abnormal plant growth should be sampledseparately. Samples should be delivered as soon aspossible, in a fresh condition to the laboratory.

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sawdust for mulches and soil improvement." USDACircular 891.

Barragry, A. R., and J. V Morgan. 1978. "Effect ofmineral and slow-release nitrogen combinations on thegrowth of tomato in a coniferous bark medium." ActaHort.82:43-53.

Bauerle, W. L. 1984. "Bag culture productivity of greenhouse tomatoes." Special Circular 108. Columbus: OhioState Univ.

Boodley, J. W., and R. Sheldrakejr. 1977. "Cornell peat-lite mixes for commercial plant growing." InformationalBulletin 43. New York State College of Agricultureand Life Sciences.

Cooper, A. J. 1979. The ABC of NFT. London: GrowerBooks.

Cotter, D. J. 1974. "Yields of successive cropping oftomato in sawdust and bark media." Hort Sci. 9(4):387-388.

Furuta, T 1974. Environmental plant production andmarketing. Arcadia, CA: Cox Publishing.

Gartner, J. B., S. M. Still, and J.E. Klett. 1973. "The useof hardwood bark as a growing medium." Proc. lnt,Plant Prop. Soc. 23:222-230.

Ingratta, E J. 1979. "Soilless culture ofgreenhousevegetables." Agdex 290/5 18. Ontario, Canada: OntarioMinistry of Agriculture.

Jensen, M. H., and N. G. Hicks. 1973. "Exciting futurefor sand culture." Amer. Veg. Grower 33-34, 72-74,Nov. 1973.

Johnson, H., Jr. 1975. "Greenhouse tomato production."Leaflet 2806. Berkeley: Division of Agriculture andNatural Resources, Univ. of California.

Johnson, H., Jr. 1980. "Hydroponics: A guide to soillessculture." Leaflet 2947. Berkeley: Division ofAgriculture and Natural Resources, Univ. of California.

Johnson, H., Jr., and G. W Hickman. 1984. "Greenhousecucumber production." Leaflet 2775. Berkeley:Division of Agriculture and Natural Resources, Univ. ofCalifornia.

Jones, J. B., Jr. 1983. A Guide for the Hydroponic andSoilless Culture Grower. Portland, OR: Timber Press.

Larsen, J. E. 1973. "Nutrient solutions for greenhousetomatoes." Texas A&M Univ., College Station. Mimeo.

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UC Davis, Vegetable Research and Information Center Soilless Culture of Greenhouse Vegetables

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Appendix

Some useful formulas for complete nutrient solutions.1

UC Davis, Vegetable Research and Information Center Soilless Culture of Greenhouse Vegetables

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The author is Hunter Johnson, Jr., Extension Vegetable Specialist, Riverside.

To simplify information, trade names of products have been used. Noendorsement of named products is intended nor is criticism implied ofsimilar products which are not mentioned.

COOPERATIVE EXTENSION UNIVERSITY OF CALIFORNIA

This information is provided by Cooperative Extension, an educational agency of the University of California and the UnitedStates Department of Agriculture. Support for Cooperative Extension is supplied by federal, state, and county governments.Cooperative Extension provides the people of California with the latest scientific information in agriculture and familyconsumer sciences. It also sponsors the 4-H Youth Program. Cooperative Extension representatives, serving 56 counties inCalifornia, are known as farm, home or youth advisors. Their offices usually are located in the county seat. They will be happyto provide you with information in their fields of work.