Manufactured Growing Medium (MGM) is the stuff urban gardeners and farmers often use when soils are degraded, compacted, infertile, or missing in action. Leafy greens can be produced in 100% compost or leaf mould (as long as they are watered and fertilized as needed) but most other crops grow best in a blend (MGM) of two or more ingredients such as compost, soilless “potting mix,” and topsoil. MGMs vary based on the type and amount of individual ingredients.
I visited a small urban grower this week who was seeing some growth variation in leaf crops grown in a compost-topsoil blend. The transplants were uniform in size and appearance, and grown in a commercial greenhouse. Weeks later the grower noticed that all plants had good color but some were smaller in size and these seemed to occur randomly. Soil pH was 7.0- no problem there. Drainage was ok and there were no signs of root or stem diseases, or insect feeding.
The MGM contains small pieces of bark and wood, along with pebbles, clumps of clay, and partially decomposed organic matter. The physical, chemical, and biological properties of the MGM may vary enough to cause growth differences depending on the location of the plant and its root system in the beds. This problem will probably smooth out over the next year with cultivation, freeze/thaw cycles, addition of fertilizers, and further breakdown of the organic components.