Maximum Yield USA 2012 September | Page 86

hOW TO SUCCESSFULLY CHOOSE GROWING... should also be heavy enough to provide physical support to the plant.  Growing medium and production system compatibility  It might be surprising, but in order to choose the best medium, the first thing you should do is consider the production system’s specifications. These factors include the type of irrigation technique (dripper’s density and discharge), container size and container shape. These specifications and the growing medium must be compatible in order to obtain effective and uniform distribution of the irrigation water.    Porosity and water-holding capacity Each growing medium has a characteristic particle-size distribution. The spaces (pores) between the solid particles can be filled with either air or water, and are referred to as “total porosity.” Each medium contain pores of various sizes. Smaller pores can retain water with more force than larger ones, and a large pore cannot hold water against gravity. As such, the higher the pore is positioned in the container, the smaller it has to be in order to retain water against gravity. So, pores at the top of the container that are too large to hold water against gravity are empty; therefore, the top of the container will always be dryer than the bottom. At the bottom of the container, all pores—including the largest—are filled with water, making the bottom layer saturated. Let’s visualize... Pores in a growing medium can be viewed as a series of capillaries. In our model (figure one), the width of the column represents pore size and the capillaries have been “ironed out” so they are straight and can be easily compared to each other. It is easy to understand why the bottom of the medium is always saturated, while the top of the medium contains less water and more air. There are forces that make the water climb in the capillaries against gravity—however, these will not be discussed here. We will only mention that they are called cohesion (which is the affinity between water and particles Figure 1 84 Maximum Yield USA | September 2012