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.
Same site. These three 1/2 barrels were filled with MGM from the same large pile. Kale plants in the container on the right are much smaller than kale plants on the left. A non-uniform MGM could cause uneven growth!
MGMs are going to become more important as urban food production expands. Before making a purchase ask for test results and examine the product closely.