We all know that soil and fertilizer are interrelated. Potting media and fertilizer are also interrelated, both chemically and physically. The performance of one affects the performance of the other. Not infrequently, when I see situations where nursery crops are not growing correctly, the problem is really potting media or irrigation – related, and not a fertilizer performance issue.
I’ve been asked many times to help design potting media for nurseries, from foliage farms in Jamaica to cannabis operations in Colorado to cactus nurseries in Texas. The growers often want to start with pH and nutrient adjustments, but that is not the way to do it. You have to get the physics of the media right first, and then deal with the pH and nutrient levels. If the physical characteristics of the media are not correct for the crop and the type of nursery, then the fertilizer is not going to perform at an optimum level.
Growers talk a lot about “drainage” and “light” and “heavy” mixes. The reason we have to have artificial media for container production is that a pot is basically an artificial hardpan on all sides and the bottom of the soil. Field soil that would grow perfectly good trees will not work when placed in a container because of the physical restrictions of the pot. Container media must be much more open, with larger amounts of pore space and drainage characteristics.
Have you noticed that the majority of potting media used in nurseries today are made up of 3 to 4 ingredients? Have you thought about why that is? The reason is that media made with multiple ingredients have a wider variety of different types of pore space than those made from just what are 2 ingredients. What happens is that the different types of components fall together differently in the blend, creating different physical air and water options for the roots to grow in. Effectively, media made from multiple components create different types of pore space options, giving the roots choices as to where they want to grow, based largely on physical factors.
Have you also noticed that many times container plants will have a concentration of roots at the edge of the pot, where the media touches the container? Believe it or not, one of the reasons for this is diurnal temperature fluctuations. During warmer day temperatures, the plastic expands just a little bit, creating additional pore space and airspace for the roots to breathe. At night, the plastic contracts just a little. This doesn’t negatively impact the plant, as roots take in very little moisture at night. Also, especially in warmer climates, the highest concentration of root growth can be observed on the north side of the pot.
When growers talk about “drainage” and “light” and “heavy” mixes, what they are really talking about is moisture holding capacity. Even bad potting media are going to drain eventually. What is critically important is how much moisture is left in the media after drainage is complete. The bulk density (weight per unit volume) or light or heavy really doesn’t matter much.
I often recommend to growers what I call a “forgiving” mix, one that won’t hold excessive amounts of moisture in case of rainfall or excess irrigation. Irrigation is almost never perfect, so a forgiving mix can help compensate for those imperfections. Media with lower moisture holding capacity will often perform more consistently as we try to irrigate rectangular blocks of plants with circular overhead sprinkler patterns.