When constructing a building the contractor will put a lot of effort into constructing a solid foundation from which to build on.  The heavier the building, the stronger that foundation has to be.  The same goes for growing plants, we can't expect to get big healthy plants by ignoring what is going on in the soil.

This blog post is going to be focusing on indoor gardening for the sake of simplicity.  The concepts are mostly applicable when growing outside in a natural setting, the unique point being that the soil quality can be quite variable and controlling the soil environment can be a lot more difficult. 


Pot Size

The general rule of thumb here is that a foot (30 cm) of vertical growth will require a minimum of 1 gallon (4 liters) of soil.  If there is too little medium then the roots will have nowhere to grow and it will become root bound.  When this happens its very hard for water to penetrate and so it will drain out the bottom of the pot which deprives the plant of water and nutrients. While the plant could be saved using greater amounts of water all of the excess will wind up evaporating raising humidity which in turn provides a better environment for powdery mildew (Podosphaera).


Over rooted plant

Caution must also be taken with an over sized pot growing inside as it will take longer to dry out and increase the risk of root borne pathogens and so flooding the pots with large amounts of water isn't recommendable.  Growing in containers outside is a little different since hot dry weather will use up water reserves very quickly and so a larger than normal pot size should be used.

The other thing to keep in mind is that a pot will offer stability.  A tall plant grown in a small pot will be top heavy which increases the risk of toppling over and breaking foliage.


Rows of plants in pots


Clean media

This point should be common sense.  While it's tempting to reuse media such as peat or coconut choir to save a few bucks but this is risky to be sure.  First of all just because there was no sign of insect or fungal pathogens during the first crop it doesn't mean that a population isn't there waiting for conditions to be right to infect a crop.  

In addition there could also be residual salts in reused media which could increase the salinity.  

I don't want to sound like used media is dangerous or anything like that.  Using it to build up gardens and flowerbeds will work wonders for vegetables and ornamentals, this point really only applies to high end crops.  

Reusing other equipment is perfectly okay as long as everything is cleaned and disinfected between crops.  Keep in mind when using sanitizers, the longer an object is exposed to the agent the better the chance is of killing the unwanted organism.


Water levels

Like so many things in growing this is a balancing act.  Too little water will dry out the plant which will cause stress, and in more extreme cases kill it entirely. Over watering will result in not enough oxygen being available to the roots, and encourages the growth of root born pathogens like pythium of fungus gnats.  Unless growing in a hot dry environment, a number of small doses of water are better than one large one. 

*Did you know?*

Pythium (root rot) is a widespread fungal pathogen infecting many different kinds of plants.  Unlike most types of fungi as part of their life cycle they develop what is known as a zoospore.  These specialized reproductive cells have a tail like structure known as a flagellum allowing them to swim.  Although they are microscopic the zoospore is able to swim several meters in search of new roots to infect.  For that reason over watering can be dangerous as it provides an environment that they can easily move around in.

Pineapples infected with root rot
Pineapples infected with root rot


Soil Temperature

Like any other living thing roots have an optimum range they operate in and it's pretty close to the exact same temperature humans prefer, 20-23 degrees Celsius (70-75 F).  While they can tolerate and even thrive in higher or lower temperatures than this pests are more likely to attack in the higher ranges and while low temperatures will inhibit pathogens it also slows plant metabolism as well.  Also keep in mind that large fluctuations in soil temperature will also stress the roots. 


Soil pH

This was discussed in some detail in the previous blog post nutrient lock out for dummies part 2 so we won't go too deep into it.  Roots are made to absorb nutrients and a less than optimal pH will keep this from happening.  



Roots absorb water and nutrients by maintaining the correct balance of salt and water.  When salt levels in the soil are too high this can't happen and water will be more difficult to absorb resulting in dehydration.  Growing in pots its relatively easy to remedy by simply flushing with water.


Saline Soil
Saline Soil

*Did you know?*

Ancient Mesopotamia was the area of present day Iraq between the Tigris and Euphrates rivers.  Irrigation was used extensively for thousands of years and over time the salt left behind from the irrigation water built up in the soil.  Initially farmers switched from wheat to the less desirable barley as it a lot more salt tolerant.  As more and more salt was deposited even barley was unable to grow.  This was a leading factor the eventual collapse of the civilizations centered in this area. 



The area surrounding the root is inhabited by microbes such as fungi and bacteria.  While many live independently, others live cooperatively with the plant.  The plant provides nutrients such as sugars to the microbes and in exchange they can provide many benefits.  These include fixing nitrogen from the atmosphere, breaking down minerals into nutrients usable by plants, and protecting the plant against attack from pathogens. 


Trichoderma a beneficial fungi under magnification
Trichoderma a beneficial fungi under magnification



There is a misconception that because plants produce oxygen that they don't require it.  Overall they produce way more oxygen than they require but the vast majority is from the leaves meaning that the roots need to be exposed to the atmosphere to have sufficient oxygen. Soil texture is the key here, in large scale agriculture clay soils under produce since they compact making it difficult for air to penetrate.  Peat, coconut choir, and perlite all offer excellent aeration.  



This is more of a sub point of both the water levels and oxygenation.  Having somewhere for excess water to go with keep the water levels from going to high as well as give an area from which oxygen can reach the roots.  For potted plants it's relatively easy to achieve simply by adding some pebble sized rock to the bottom of the pot before adding the medium.