Professional Soilless Media:
How to Make the Right Choice
Professional growers know and appreciate the many advantages that soilless media brings to bear in greenhouse and nursery operations. Soilless mixes are most commonly peat-based, with a wide array of amendments used to create the best media based on crop requirements and production techniques. Great uniformity and reproducibility across batches make growing in soilless media reliable, crop after crop. Adjusting water-holding capacity, pore space within the media, bulk density and even characteristics like pH are possible with the careful selection of media components and amendments.
What follows is an overview of common soilless mix components, to help you select the best product for your needs. If further assistance is needed, the GGSPro technical services team is available to lend a hand.
Sphagnum peat moss is the primary component of most soilless mixes, selected for its water-holding capacity, consistency and almost complete lack of insect and disease problems coming out of the peat bogs. Thanks to its comparatively low bulk density, peat-based soilless media is lightweight and relatively easy for employees to handle.
Peat moss, by nature, is hydrophobic, meaning it wants to shed water. High-quality surfactants overcome that characteristic and are blended into nearly every commercial soilless mix. Compressed bales of peat-based soilless media with an 80-20 or 85-15 ratio of peat moss to aggregate are mainstays in greenhouse production. Pro-Mix BX, Lambert LM-3 and Oldcastle C/20 are examples of high-quality, peat-based soilless growing media.
Composted pine bark is another common component of soilless mixes. Pine bark is favored in certain production situations, such as larger containers and outdoor production. In outdoor production, composted pine bark adds bulk density to a soilless mix, reducing the possibility the containers will blow over. As the percentage of composted pine bark in the media increases versus peat moss, the water-holding capacity of the media goes down. This can be another benefit for outdoor production, where heavy rains can otherwise lead to saturated soilless media.
Pine bark tends to decrease the overall media cost, which is beneficial when producing crops in larger containers. For the largest containers, the peat-based mixes may contain more pine bark than peat moss on a percentage basis! Premier’s Pro-Mix BK55 represents this type of soilless mix.
Pine bark can be hammer milled to various particle sizes to suit the needs of the grower. Those decisions are usually based on container size, with the largest particles being reserved for nursery mixes intended for the largest containers. Smaller pine-bark particles can be used in soilless mixes suited for containers ranging from 4-inch up through hanging baskets and color bowls. Premier’s Pro-Mix BK 25 and Lambert’s LM-16 represent popular soilless mixes for greenhouse production.
Properly composted pine bark has natural disease-suppressing qualities – an added bonus for growers. Pine bark is known to wet up and re-wet easily, which is particularly beneficial to long-term crops. Peat-based media that contains composted pine bark is less likely to shrink and pull away from the sides of the container when it gets dry. Soilless mixes containing composted pine bark aren’t compressed into bales because the pine bark won’t “spring back” to its original composition after being compacted, unlike sphagnum peat moss. Bark-based mixes are generally sold in 2.8 cu ft loose-fill bags or shipped in bulk.
Coir and more
Coir is made from the fibrous material found between the hard internal shell and the outer coat of a coconut. It’s ground up according to the specifications required by the soilless mix producer. Coir has several unique characteristics: It can be used to increase water-holding capacity while also providing air space. It’s fully decomposed, so it won’t break down further and sacrifice pore space over time, a nice feature for longer-term crops. Many growers report that, all factors being equal, peat-based soilless mixes with coir as an amendment tend to produce better roots.
With some exceptions, coir is typically added to make up 20-30% of the volume of a peat-based soilless media. Depending on how the coir is processed, it can arrive with high soluble salts levels. Reputable suppliers use pre-washed coir to alleviate that potential problem, as Lambert does with their PC-15 mix.
Aggregate of one kind or another typically makes up 15-25% of the volume of the peat-based soilless media. Perlite and vermiculite are expanded mineral products often used for this purpose. Perlite comes in various grades and sizes, and is used primarily to create pore space within the soilless media. Vermiculite adds pore space and water-holding capacity within its expanded plate structure. In certain cases, chunk coir (larger particle size, e.g., Pro-Mix HPCC) or parboiled rice hulls are used as a substitute for some or all of the perlite and vermiculite.
Seedling and plug mixes are specifically designed for seed starting and/or cutting propagation. These are peat-based soilless media utilizing smaller particle sizes to accommodate the smaller cells of plug and cutting trays, and screened for uniformity. Examples of this type of soilless media include Pro-Mix PGX, LM-18 and Oldcastle C/GP.
Organic and biological options
Organic soilless mixes are popular among some growers, especially greenhouse vegetable and herb producers. Most are peat-based soilless mixes, with changes made to some of the additives. Instead of the traditional inorganic fertilizer starter charges found in most soilless media, for example, organic-based fertilizers provide that charge. Non-organic surfactants are replaced with organic-approved products, generally based on a Yucca extract.
Organic soilless mixes and their traditional counterparts both use dolomitic limestone, a great source of calcium and magnesium, to adjust the soil pH. Many also add gypsum (calcium sulfate) to provide additional calcium without raising the soil pH. All-purpose organic soilless mixes include Lambert AFM-3 and Pro-Mix PG Organik.
Some soilless mix manufacturers enhance their products’ performance and value with added biological agents. Some contain biological fungicides such as Bacillus subtilis to provide weeks of prevention against significant root rots including Pythium and Rhizoctonia. (Example: Pro-Mix BX BIO) Some soilless mix manufacturers add Trichoderma-based biofungicides such as Rootshield G or Rootshield Plus G on a custom basis. These products can provide a couple of months of preventative control of different root rots, based on the selected strain(s). Mycorrhizae is a naturally occurring organism that grows in conjunction with the root system, enhancing the ability of many plants to extract moisture and micronutrients from the soil. This organism is credited with reducing transplant shock, too, especially on longer-term crops. Pro-Mix BIO + Myco contains the biofungicide and the mycorrhizae additives.