Soils - roots and their properties
There's a number of factors to be taken into consideration when deciding on what trees are suitable to the location. For example, sandy soils at the beach front behave totally differently to heavy clay soils in low lying boggy areas. And so do the trees that live in each area so knowing what you have is a good place to start.
There is also the counter argument that the existing soil could be excavated and a better soil laid down or built up. Whilst there is merit in this argument and it is a landscaping technique to plant non-endemic species in a location it is not the scope of this page to cover that scenario ... however, you could apply the same principals gained from here to know the blend of soil you want to import to the site.
Soil structure and texture
On one side of the texture scale with a large particle size is sand decreasing to a smaller particle called silt down to the smallest being clay.
Soil structure is how these particles are arranged into lumps, called PEDS. You may recall last time you dug in your garden of how the soil lumped together. There's some basic terminology for the sizes and shapes of these PEDS but at this level we want to stay with the basics. The preferred structure is like small crumbs loosely adjoining other PEDS. This soil is therefore porous and permeable, yet retains moisture. It is the most ideal soil structure. The most ideal texture is called loam and is 40% silt, 20% clay and 40% sand.
Improving clay soils
Changing the soil texture is a difficult task whilst changing the structure is somewhat more manageable. For example you may think that adding sand to a heavy clay soil will help ... wrong, in fact you may turn the soil into a solid cement style mixture.
Clay is actually high in nutrient, moisture retention and low in leaching so it's not always a bad thing however it's downside is that it compacts, can be boggy drowning trees, can form a hard crust stopping water penetrating (hydrophobic), expands and contracts breaking finer roots and is low in air pores (roots need air).
To improve it you need to add organic matter, compost and mulch.
Avoid working the soil when it's extremely wet as you'll compact it and when it's really dry as it will turn to dust and be almost impenetrable. In farming the practice of rototilling clay soils has long gone, the preferred method is ripping. A tractor with hook shaped tines rips the soil to a depth of about 300mm, this breaks up the PEDS and increases water penetration. Now this would be impossible to do around established trees without destroying their roots. We have a device that is a probe with air injection so once the probe has entered the soil it can blast compressed air into the hole fracturing soil aggregates and aerating. The probe is less than 10mm diameter therefore minimizing compaction around the entry point.
The gypsum myth
No doubt you would have heard or read about using gypsum to break up clay soils, it's even written on most packets. Gypsum can only improve clay soils that have sodicity. The sodium particles are displaced by gypsum from the clay. This is different to salinity where chloride is present. Only if it were that easy to just sprinkle some gypsum and problem solved, but farmers have long ago invested in ripping and organic green crops to till organic matter back into the soil. You can do a sodicity test, called the Emerson Dispersion Test to see if in fact your soil is sodic.
Improving sandy soil
Some of the positive things about sandy soils are good drainage, good air pores so roots can go deeper and a reluctance to compact. The negatives are poor moisture and nutrient retention (leaching), tendency to become acidic, erosion and summer dehydration - evaporation.
To improve a sandy soil add organic matter (spent mushroom compost is ideal), if possible fork it over and through the top 150mm layer, if not a liberal top coating along with a mulch.
So as you can see the common denominator to improving both types of soil structures is organic matter and mulch. This in turn will attract worms which aerate the soil and the right bacteria, fungus and humus to help your trees grow and stay healthy.
Soil pH - acidity or alkalinity
There are simple testing kits available at most nurseries and garden centre's to test the soil pH. It is wise to test at various depths and locations. A pH of 6.5 to 7.0 is considered neutral.
Trees need to be within their own pH range to take up nutrients. We have diagnosed some common errors where customers have literally been throwing money away on fertilizers as the soil was out of range, plants were yellowing and chlorotic, we adjusted the pH back into range and health was restored within weeks.
To bring an acidic soil (pH number below 6.5) back toward neutral you need to add lime. To bring an alkaline (pH number above 7.0) soil back to neutral you need to add sulphur. What you also need to consider is why the soil was out of range. For example, trees close to concrete will tend to get a lot of alkaline water run off as concrete is extremely alkaline, so perhaps adding a drain or changing the way water runs off will prevent the soil becoming alkaline. Also consider what the natural pH of the soil in the region is, trying to change that over the long term would be near impossible as the soil will always try to revert to it's natural pH. Some regions are limestone base resulting in alkaline soils whilst others may be more volcanic with granite or alluvial soils. Either way, pH testing is fast and cheap.
Tree roots and their requirements
The function of roots is to transport water and nutrients, anchorage and storage of food reserves.
There's basically 2 types of roots, woody and non-woody. The primary purpose of woody roots is support and anchorage of the tree. They also provide water and mineral transport as well as carbohydrate storage. Non-woody roots are mainly found in the top few centimeters of soil and are often called feeder or hairy roots. Their primary purpose is to absorb water and nutrients.
Tree roots need moisture, nutrients and air. So now you can see the relationship between soils and roots. A compacted clay soil is low in air so aeration and decompaction will help. Roots will then be able to spread and grow easier. This would be one of the most common problems. The root area can spread 4 to 7 times the drip line of the tree so keep this in mind when mulching or using those selective turf weed sprays. The fine roots do have to compete with turf and other plants so be thoughtful when designing your landscape for root space.
Beneficial mycorrhizal fungus
First you need to understand that there's 3 types of fungi.
Saprophytic, which means the fungus consumes dead plant or animal tissue to gain it's nutrients. They cannot photosynthesize or produce chlorophyll and actually help break down dead matter.
Parasitic, which means the fungus relies on other plants to gain it's nutrients and in doing so harms the host. Often seen as fruiting bodies or bracket fungi on living trees.
Symbiotic, these are the good guys to have in your soil. They have a give and take relationship with the host. They coat the fine roots and increase the surface and absorption area feeding the tree ... in return they consume some of the carbohydrates the tree makes. Now there's thousands of different types which suit different trees, soils and locations.
Rhizosphere
Tree roots themselves plus the beneficial fungi also secrete and shed organic matter into the soil. Add to this the
leaf matter falling from the canopy, water, organic matter - compost and mulch etc as described above and you have a special micro environment. Think of forests and how you'd expect the soil to be
different and teaming with life unlike that of a hard pan leveled house block. This zone surrounding the roots of trees and plants is called the rhizosphere.
Rhizospheres can be quite unique, different fungi for different species of trees and soil conditions.