Hydroponics

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Hydroponics is cultivating plants without soil, using only aqueous solutions of nutrients. This method of agriculture does not require arable soils. Hydroponic growing media need only support the crop plants, and can be quite lightweight. This makes hydroponic systems easier to transport on spacecraft, and thus more desirable than soil-based agriculture for initial missions to Mars.

Because it is not dependent on containing soil and can support crops in almost any configuration, hydroponics is the quickest, simplest method of getting the maximum amount of produce from a given growing area.

Plant Nutrition

Most crop plants absorb the water and non-gaseous nutrients that they require through their roots. They also absorb a significant portion of the oxygen they need through their roots as well, and will not assimilate nutrients properly without this oxygen intake. These nutrients have to be in solution at the plant's root hair cell wall before they will be absorbed properly.

In soil cultivation, the soil provides both water and nutrients, which adhere to and are absorbed in soil particles. Organic material in soil provides the major source of nutrients, including minerals, and retains more water over time than the rocky component of soil. The rocky component of soil reduces the evaporation rate of the soil's water content, supports the plant, and also provides some mineral nutrients. The oxygen needs of the plant's root system are met by diffusion of gas through the soil, which is slow enough that decay of organic matter in the soil is delayed long enough for the plants to use it but is usually faster than what occurs by diffusion through stagnant water. Some crop plants can grow in stagnant water, or even water inundated soil (which transports less oxygen than stagnant water). However, most plants require the same level of oxygen available in arable soil to prevent their root tissue from dying.

The nutrients required by crop plants are frequently divided into macronutrients, of which the plants require relatively large quantities in steady supply, and micronutrients, which the plants require only in trace amounts and may only need at specific times during their life cycle. Both macronutrients and micronutrients must be supplied over the entire lifetime of the plant.

Macronutrients include:
Soluble nitrogen compounds (ammonia, nitrates, etc.)
Soluble phosphates
Soluble potassium compounds (carbonates, sulfates, etc.)
Soluble calcium compounds
Soluble magnesium compounds (carbonates, sulfates, etc.)
Soluble sulfur compounds

Optimum macronutrient concentrations vary from species to species. Because these compounds are often chemically reactive, excessive concentrations can damage plants. Appropriate macronutrient blends must be selected for each crop, and diluted using relatively large quantities of water.

Micronutrients include soluble compounds of:
Boron
Chlorine
Copper
Iron
Manganese
Molybdenum
Zinc

Most of these can be delivered in compounds whose concentrated forms are less caustic than the purified macronutrients, such as chelates. Although a plant’s use of micronutrients will vary over time, and will vary between species, the plants typically won’t be sensitive to excess amounts. The required concentrations are low enough that it will easier to use the same micronutrient mix for all plant varieties all season long rather than waste time trying to vary it.

Because sodium is not an important plant nutrient, many plants do not concentrate it in their tissues like animals do. Thus, crop plants alone will not supply sufficient salts for the crew, and an independent source of sodium will be necessary. This is true of many other nutrients.

All of these nutrients can be produced from either naturally occurring minerals or from composted organic matter. A wet-type compost pile can be used to recover all of these substances from plant waste materials and other organic matter, in a liquid form immediately useable for hydroponic culture. Chemical concentrates can then be used to adjust the relative concentrations of that compost pile effluent for each crop. Using composting to recycle plant materials can greatly conserve supplies of chemical fertilizers.

Experiments have revealed that the effluent of aquaculture systems – which must be maintained at concentrations suitable for animal life rather than crop plants – is not sufficiently concentrated for direct use as a nutrient solution to grow food for human consumption. However, several species of aquatic plants edible to fish will grow in the dilute effluent of an aquaculture system, raising the possibility that, with an independent control system, an aquaculture system incorporating plants grown hydroponically to feed fish can approach the same self sufficiency as a hothouse capable of feeding a human crew. This is useful because fish do concentrate sodium and other vital nutrients in their tissues that plants will not, making fish raised in aquaculture tanks a useful food even if plants grown that way are not.

The plant nutrient solution must be kept within the proper pH range for each crop. The solution should be regularly monitored, both because crop plants tend to be sensitive to changes in pH and because pH changes indicate changes in the concentration of macronutrients. Attempting to grow plants using a nutrient solution with improper pH can devastate the crop. However, if evaporative losses are controlled, pH is always verified prior to use, and the nutrient solution is replaced frequently enough, then significant variations in pH during operation are unlikely to ever be encountered at all, much less become a problem.

Hydroponic Methods and Growth Media

File:Epcot Exhibit Hydroponic Greenhouse.jpg
An exhibition greenhouse, showing plants grown by various active hydroponic methods, allowing varying use of greenhouse space for each
File:Hydroponically Grown Miniature Wheat.jpg
Miniature wheat grown on multi-tiered NFT hydroponic arrays

Hydroponic methods are characterized as active or passive. Active methods actively circulate nutrient solution to and from the plant roots, and will not function without power input for pumps. Passive methods may also require power input for pumps and aerators, but keep their nutrient solutions in a stationary state for long periods, immersing roots in aerated tanks or in beds of gravel or other growth media that is periodically wetted with nutrient solution. Passive systems do not require constant flow, and can be operated manually.

Both types of system will recycle the same batch of nutrient solution until it becomes depleted or concentrated in one or more nutrients. Most literature on hydroponics recommends that batches of nutrient solution be replaced after use, and either disposed of or recycled separately rather than re-enriched and added back to the system.

Active hydroponic systems generally have the lightest weight, some consisting of little more than pumps and tubing to deliver the nutrient solution to the roots. Aeroponics delivers the nutrient solution to the plants in a fine spray while their roots are suspended below a wire frame or other support. Because the fluid flow rate is relatively low, aeroponic systems must also incorporate a means of periodically rinsing salt accumulation from the plant roots. The Nutrient Film Technique (NFT) allows the plant roots to extend down an enclosed plastic channel, receiving a shallow constant stream of nutrient solution as it flows down the channel. This method can use gravity fed flows, as can drip feeding, in which a flow of nutrient solution is directed down the plant roots as they are allowed to hang below the supported plant, with or without an enclosing channel.

Passive hydroponic systems are typically heavier, as they must incorporate either water-retaining growth media or sufficient liquid nutrient solution to enclose the plant roots. Raft cultivation and other water cultivation techniques involve suspending or floating the plant in a tank of aerated nutrient solution and suspending the plant roots in that liquid solution. This method requires constant aeration of the nutrient solution to provide enough oxygen to the plant roots, but only an air pump is required since the solution is not circulated. The Bengal method involves rooting plants in sand, gravel, or other medium for support, then feeding them by periodically flooding and draining the growing container. A portion of the nutrient solution remains in the container after each cycle, adhered to the growth medium, and nourishes the plant. This method can also be gravity fed.

Passive hydroponic systems are desirable not for their low power requirements, but for their low maintenance needs. Active systems must operate continuously under all conditions or the plants will dry out, while a properly maintained passive system can go days between maintenance cycles under the right conditions.

Seed Starting

Hydroponics is sufficient to support plants over their entire life cycle. Some techniques, such as the Bengal method and aeroponics, have sufficiently low flow rates and/or sufficiently dense growth media that they are suitable to root cuttings and start seeds without worries about how tiny seedings will interact with the nutrient solution flow. Other techniques, such as NFT, require separate starter beds for optimum performance with some crops.

Plants will readily reproduce with hydroponic cultivation. Plants that reproduce by runners should be checked regularly to prune or propagate them, as their runners will try to grow in the hydroponic system.

Thermal Considerations

Passive hydroponic systems typically have a great deal of thermal mass concentrated near the plants, and are just as effective at heat regulation as a soil gardening system. Active hydroponic systems typically have very little thermal mass concentrated near the plants, but can still have as much thermal mass as any passive system due to the bulk of their nutrient solution. Because they have more leeway to distribute nutrient solution around the greenhouse, active systems can use their nutrient solution as part of an active thermal control system.

The concentration of nutrients in a typical nutrient solution is not sufficient to dramatically alter its freezing point. The nutrient solution will exhibit the same thermal behavior as fresh water.

Crop Yields

Because the supply of nutrients is controllable and constantly available in a readily absorbed form, hydroponics can allow higher yields than soil cultivation when limited to the same growing area. Plants can simply be fed faster by nutrients delivered in streams than by traces of aqueous solution clinging to soil grains. Plants fed using hydroponics tend to grow faster and develop denser foliage than soil cultivated counterparts. Liquid delivery systems also allow use of vertical configurations and trellises for plants that could never be trained to those growing conditions using soil cultivation, making still higher yields possible through increased planting density.

Hydroponics offers less resistance to changes in salinity than soil cultivation, but this problem can be partially overcome through a combination of diligent monitoring and the use of compost-derived nutrient solutions, which tend to have lower initial salinity than those constituted from refined salts.

Hydroponics produces higher crop densities than soil cultivation. However, total yields ultimately depend on the area cultivated. The initial setup of systems using hydroponics tends to be expensive and labor intensive, while soil cultivation does not. If cultivation area is more readily available than construction supplies, then soil cultivation becomes preferable to hydroponics. Therefore hydroponics is most commercially competitive with soil cultivation when property boundaries, controlled environments or other enclosures limit the amount of area that can be cultivated.

Flavor

In light of popular misconceptions, a note must be made regarding the flavor of hydroponically cultivated vegetables.

Commercially available hydroponically grown crops are often criticized for their lack of flavor. However, this criticism is also equally applicable to many commercially available crops that are not hydroponically grown. While this effect is often real, it is not due to nutrient deficiency, as hydroponics actually supplies more nutrients to plants than soil agriculture. Lack of flavor in comparison to other vegetable products is more often a result of breeding and variety than plant nutrition. It is the varieties of plants chosen – those bred for high crop yields, size, or ease of shipping rather than for flavor and nutritional value – that is most often responsible for the relative blandness of a grower’s product. Often growers already lured to hydroponic cultivation by higher crop densities are willing to further boost production by using high yield varieties with inferior flavor.

Hydroponics or soil cultivation will have little effect on taste or nutritional value.

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