m 



NOTES 



Air Is the Secret 



Many growers have recently been discussing 

 soil mix and how porosity fits into the 

 equation. Porosity is "the ratio, usually ex- 

 pressed as a percentage, of the volume of a 

 material's pores to its total volume." It's better 

 to look at the amount of air space a soil mix 

 has. This is defined as "the amount of air a 

 media has after it had reached container capac- 

 ity"— or, simply put, how much air is left after 

 the media is saturated. 



A good soilless mix should have a total po- 

 rosity of 75-80% and air space of 20-30%. How- 

 ever, in peat-based mixes, these numbers are 

 short-lived. Most peat-based mixes will shrink 

 by 25% after one or two waterings — and as 

 much as 50-60% over the life of the crop. This 

 shrinkage will reduce your air space by up to 

 40-50% 



A way to reduce this shrinkage is to reduce 

 the amount of peat and add either aged pine 

 bark or coconut coir. Both have less shrinkage 

 than peat (coir has 50 % less). 



D.R. Fonteno from North Carolina State Uni- 

 versity has devised and standardized a test to 

 detect the total porosity and the amount of air 

 space in mixes. Why this standardization is im- 

 portant is that these numbers will vary accord- 

 ing to the size of the pot and how the mix is 

 handled. The height of the container will dic- 

 tate the air space of the mix. Going from a 

 four-inch pot to a six-inch pot increases air 

 space by 50%, simply by increasing the vertical 

 column of soil and gravity. Try this with a wet 

 sponge. Lay it flat and let water drip out. If 

 you turn it on edge, more water will drip out, 

 making more air available. 



Why is all this important? Plants root where 

 there is air, not water. Water is important in 

 keeping cuttings or seedlings from drying out, 

 but air is the secret. Mixes that are more com- 

 pacted or excessively wet allow little air space 

 and, of course, little rooting. 



Jim Zablocki, technical manager of the Northern 

 Horticultural Group, Scotts Company, can be 

 reached at 60^-224-^58^. 



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