Substrates for Soilless High Tunnel or Greenhouse Production

In a previous article ‘Opportunities in Hydroponics’ (VCH 609) we discussed two types of Hydroponics, solution culture and medium culture. In this article we will focus on Growth Substrates (media), which form an integral part of medium culture.

Growth substrates can be divided into two groups, organic and inorganic media. Inorganic media can be further divided into natural and synthetic. Media included under inorganic and natural are sand, gravel, rockwool, perlite, vermiculite, pumice, expanded clay aggregate, zeolite and volcanic tuff. Inorganic and synthetic media includes foam mats (polyurethane) polystyrene foam, oasis (plastic foam), hydrogel, and Biostrate felt®. Included under organic media is pine sawdust, pine bark, wood chips, peat moss, coconut coir, and rice hulls. The number of substrates available are not limited to this list. Using growth substrates instead of soil gives the grower several advantages:

No need for arable land. Soilless growing media (substrates) is lightweight and can be mixed according to plant needs. Aeration and drainage concerns can be addressed effectively by choosing the appropriate substrate or substrate mix that is compatible with your crop and irrigation practice.

Total control over root environment. Medium culture allows the grower to have total control over the root zone. Meaning the grower has control over what the plant gets and when it gets it. Having control of the root zone permits the grower to apply exact amounts of fertilizer and water on a daily basis to each plant. Since we are now growing in a media other than soil, it is possible to use a complete, balanced, crop specific nutrient solution that fulfills every nutritional need the crop might have. The pH and electrical conductivity (EC) of the mineral nutrient solution can be adjusted instantaneously to address root zone or plant growth issues. This allows the grower to plant at higher planting densities, which results in higher yields.

Precise irrigation scheduling. Growing media has a lower water retention than soil and can therefore be irrigated more frequently, allowing for a more frequent supply of nutrients to the roots. Therefore the grower can accurately schedule a precise pattern of irrigation that will have a positive effect on growth and productivity based on the crop grown and the age of the crop, the climatic conditions at your location, the growth substrate used, the texture of the substrate, and the volume of available growth substrate per plant. However, in order to have control over and manage the fertilizer and water needs of your plant you need to frequently sample, measure and keep complete records.

Improved crop uniformity. Managing every need of the plant makes it much easier for the grower to harvest produce that is uniform in weight, size and texture. Yields are more predictable, which allows for more consistency in marketing.

Limits root disease. The possibility of root disease is limited by using medium culture. This is not a guaranteed solution for root disease since disease can still enter the production system through the water or any other unhygienic production practices. However, root disease can easily be prevented by disinfecting your water source and improving hygiene before and during plant production. The use of containerized substrates reduce the turnaround time between crops. During the cleaning process all containers with substrate (including roots) are easily removed. Afterwards the growing space can be washed down and sterilized in a very short amount of time.

Environmental impact lower. Using a substrate gives the grower an opportunity to collect the nutrient rich drainage water from the greenhouse. The drainage water can be reused in the greenhouse or can be disposed of in an environmentally friendly manner. It is important to note that disinfection needs to be part of the production process when the nutrient solution is recycled.

What constitutes a good growing media?

  1. Physical and Chemical properties
    • Nutrient retention. The growth medium must have a relatively low soluble salts content, but need to have an adequate cation exchange capacity to retain and supply the necessary mineral elements for sustained plant growth. The pH should be between 5.0 and 6.5.
    • Gas exchange/aeration and porosity. The total porosity of a medium is the sum of all the space in the macro-pores and micro-pores. A medium with good porosity will allow for efficient gas exchange in the root zone. Usually gas exchange takes place in the large pores or air spaces in the growing medium. Aeration is known as the percentage of pore space that remains filled with air after excess water has drained away. However, a substrate composed primarily of large particles will have more aeration and less water holding capacity than a substrate with smaller particles. The invert applies to substrates that is mostly made up of smaller particles.
    • Water retention and drainage. The growing media needs to be porous and well drained, but must also be able to retain enough moisture between irrigations to satisfy plant water requirements.
    • Biologically and chemically stable. Organic substrates need to be well composted. This will ensure that there is no nitrogen negative period at the onset of production. Some substrates like rice hulls are very resistant to decomposition and do not pose a serious problem with nitrogen depletion. Inorganic substrates that are inert and well composted organic substrates work best.
    • Standardized and uniform. This will allow the grower to use standardized production practices, such as fertilization and irrigation, with every crop.
    • Free from harmful soil pathogens. Make sure that substrates are free of harmful soil pathogens. Inorganic substrates like rockwool and perlite are sterilized by virtue of the production process, but gravel and sand, for instance, need to be well cleaned and sterilized.
  2. Cost and availability
  • Substrates should be readily available and inexpensive.

In the next article about substrates we will focus on the characteristics of rockwool, perlite, vermiculite, coconut coir, and peat moss.

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