Horticultural Soils 101

Horticultural Soils 101

I took soils in college and I learned about alfisols, mollisols and tons of nitrogen per acre.  I also took horticultural production classes that discussed crop pH, peat moss and slow release fertilizer.  Neither went into depth on the implications of soil, or soilless media in horticulture.  Some important concepts were taught, but I have since had to piece together various resources to bring me to my current understanding of horticultural soils.  It seems the more I learn, the more complex it becomes.  After many explanations and talks about horticultural soils I am compelled to spell it out here to bring any who desire my take on this topic and what I feel is important topics in the overall scheme of hort soils.  This page will pull ideas from various posts and will evolve to hopefully be a succinct explanation of hort soils.


What does a growing media contribute to plant growth?

• Air
• Water
• Structure
• Nutrient Reservoir

What is the IDEAL mix

• It all depends.....
• Fit your growing style or site specific conditions
• All growers are different and have different styles for growing.  Some prefer growing dry, some wet.  Some operations have more wind exposure, others have flood floors or benches.  Each grower specific characteristic can help determine what type of mix is best suited for that situation.
• Similarly landscape sites have certain localized rainfall, foot traffic.  They may be designed for certain plants.  Maybe it is an organic garden.  Each of these as well will dictate what type of soil to utilize.
• Produce quality plants with limited management
• The goal of most horticultural soils is to preform the function of plant growth.  So really the ultimate goal is to grow good plants, but for an ideal mix, it needs to require less soil and plant management.
• Consistent and predictable
• If there is one thing that is a must, it needs to be consistent.  It is often said that a good grower can grow in anything.  That is if they understand what they are dealing with and it doesn't change.  
• Cost effective
• This seems obvious, but is often neglected in discussing horticultural soil research.  Yeah it may be a good product, but if producing it, hauling it costs too much, then it really isn't going to work.

PHYSICAL PROPERTIES

Macropores vs Micropores (and mesopores if you want to get technical)

Macropores is a measure of air porosity, Micropores hold water after free water drains

The porosity of a media is a measure of voids.  In the greenhouse world, many of the media suppliers will tell you, what they are selling you is air.  When the total porosity, or total voids of a greenhouse mix is 80%, then that means in a given pot, only 20% is solids.  The rest is made up of Macropores and Micropores.  Macropores are the voids spaces created in a media allow for water to flow freely from them by the force of gravity, making them the air content of the soil environment.  Micropores on the other hand will hold water against gravity and be a source of water for the soil environment.  Micropores can be additionally broken down into Mesopores and Micropores where the distinction is that Mesopores account for the plant available water and water is held so tightly int he micropores that it is unavailable to plants.
There is a measurement of the size of the pores and at what vacuum pressure the water is held, but that is not important for the discussion at this time.



Capillary Action and Zone of Saturation

Capillary action refers to the phenomenon where water (in this case) is sucked up the pores against the force of gravity.  This is a combination of cohesion of the water and adhesion of the water to the sides of the pores, or in this case the soil. (Capillary action wikipedia)
Well why is this important?  In our case the types of materials and the macro and micropore structure will impact the impact of capillary action acting on the mix and will hold water against gravity to a certain level.  This creates what we call a Zone of Saturation.

Artistic rendering of the zone of saturation. 

These zones will stay wet and will be devoid of oxygen and roots may have difficulty penetrating that part of the pot.  The zone's height doesn't change much from one pot to another.  In the diagram above, the area below the line is the zone of saturation, and as you can see there is little impact on a large pot, where the zone takes up say 5% of the soil volume, where the smallest pot it takes up 50% of the volume.  This is an example of how there can be more forgiveness in larger pots than in small plugs.  

If you were to add a drainage aggregate like stone to the bottom of the pot, the only effect would be to raise the level of the zone of saturation because as was stated, capillary action holds water against the force of gravity, and where the soil stops and drainage holes or drainage aggregate starts doesn't matter.

                                                         

The force of gravity is an important consideration when looking at the porosity of a given media.  The classic example of this is if you take a soaked and hold it sideways.  A certain amount of water will drain out.  Then if you hold it vertically, more will drain as well.  This is water being pulled down by gravity.  In mixes the actual porosity of a given container will depend on the size of the container.  In a large container the air porosity might be 45%, where in the smallest container that same mix may only be 10%.  Many measurements for porosity are done with some sort of standard container size.  The Horticultural Substrates Lab at NCSU has a porometer method which is the standard across the industry for the most part.  If you evaluate the porosity of a mix, keep in mind that any testing of mixes needs to be done using the same method and container size to get accurate comparisons.  A value of 30% air porosity is a relative value, and can't stand alone. That 30% may be compared to another mix using the same method that has an air porosity of 10%.  At that point you can say the first has much more air space.  Whether that is a good thing depends on what you are trying to achieve.

Mix Bulk Density

Bulk Density is a measure of how heavy something is in a given volume.  Water for instance weights 1kg/liter, which is a really clean value, or about 8.33 pounds per gallon.  Soil mixes on the other hand don't come with a set value.  The grower, or user of the media will determine the bulk density by how it is handled.  Loosely filled pots will have a lower bulk density than those that are crammed in to the pot and pushed down as plant is planted.  Bulk Density can be related to air porosity in that the heavier the bulk density, the lower the air porosity. There are many exceptions, but when looking at a similar mix it works.  As we discussed plant roots need air, so when potting up plugs or liners, if you cram the plant in there and push it down, you are squeezing out the air, so you are not doing yourself any favors.

The physical properties of a mix are important and can be very complicated.  The balance between water holding capacity and air space of a mix is a tricky and important part of growing crops.  You don't want the crops staying too wet because then you get disease, and you certainly don't want plants drying out and wilting either.  So the real take home message is that once a plant is potted up, you really can't do a whole lot to improve to physical structure of the media, so do what you can ahead of time to make sure you don't have to worry.  

CHEMICAL PROPERTIES

comming soon