Chapter 11

Growing Media for Greenhouse Crops

Grower-Mixed Media versus Commercial Media

Greenhouse growers have the option of either mixing their own media or purchasing a commercially-formulated media. Growers who mix their own growing media have both control and flexibility: control over the selection of ingredients and the quality of the final product, and flexibility to choose from many media recipes tailored to specific crops, container types, etc. Growers who purchase commercially-formulated media, although more expensive, do not need to invest in mixing equipment and have the added flexibility in purchasing from suppliers who offer a diversity of media mixes.

Commercial Media

Premixed media is a common sight in the greenhouse industry. Suppliers offer a diversity of mixes in either pre-packed (bags, bales, super sacks) or bulk forms. Some mixes are prefilled into cell packs, seed trays, or pots that are ready to be planted. Recipes are specially formulated for propagation, specific crops, or general crops. Most commercial growing mixes have a nutrient charge that is balanced for starting plant growth. Some soilless proprietary mixes are very sophisticated, containing peat moss, vermiculite, and perlite, plus a nutrient charge of nitrogen, potassium, phosphorus, dolomitic limestone, micronutrients, and a wetting agent with the pH adjusted to about 6.5.

Grower-Mixed Media

The decision to mix one’s own media, as opposed to purchasing commercially-formulated media, is basically an economic one. Growers can typically save 25 to 45 percent in material costs by mixing their own media. Costs include mixing equipment, raw materials, skilled labor, and quality control testing. Not only the cost savings, but the inherent flexibility to produce any mix formula as needed is usually another reason why growers mix their own media.

Media Mixing Methods

The mixing process is one of the most important steps in the formulation of custom growing media; the best quality components are of no use if the growing medium is improperly mixed. Improper mixing is one of the major causes of variation in container plant quality. Incorporation of fertilizers and supplements must be considered too. Preparation of larger batches requires motorized equipment.

Pad or Skid Mixing. Pad or skid mixing is usually accomplished by layering the raw materials to be mixed on a concrete pad (See Figure 11.9). A front-end loader is used to turn the material over and over, mixing the raw materials.

Batch Mixing. Batch mixing utilizes a rotary-type mixer, such as a converted-cement mixer, or a drum and paddle type mixer (See Figure 11.10). This method can be highly accurate when specific recipes are required. After the materials are placed in the mixer, the materials are tumbled or blended until consistency is achieved. Depending on the type of batch mixer, cycle times for consistent blending will vary.

Continuous Mixing. In large greenhouse a continuous-belt mixing system is most common (See Figure 11.11 & 11.12). Continuous mixing is accomplished by using material hoppers or feeder bins that layer material at set volumes onto a main conveyor belt located below the hoppers. The hoppers are large, 1- to 4-cubic-yard bins that dispense their contents, each holding one of the components of the mix (e.g., peat moss, perlite, or vermiculite). After the raw materials are layered, chemicals, nutrient charges, and other trace elements such as lime are also dispensed onto the belt. Each hopper is adjusted, either manually or automatically, to dispense its contents at a specific rate onto a conveyor belt. This belt deposits the media components into one end of a rotating drum to blend the mixture.