For this soil assignment I have chosen a site in my backyard. This site originally contained an old apple tree, a nearby cotoneaster shrub and english ivy growing along the pevious fence. All were removed and a new fence was constructed. The intention is to prepare this site for a row of cordon fruit trees in the future along the fence line.
A plan of the site and its surroundings was obtained from the Hobart City Council (above). The boundary of the house is marked out in blue. Fine solid red line indicates sewerage, dotted blue lines indicate storm water and fine solid bue line indicates water supply.
The area in question has not been cultivated for many years. It has been overrun with weeds, in particular Malva sylvestris, Plantago lanceolata, capsella bursa-pastoris and sonchus oleraceus. For the last 12 months it has been left as a play area for dogs, further assisting in compaction of the soil profile.
When wet, the top surface of the soil forms a slippery layer of mud and when dry it crusts and cracks on the surface.
These are characteristics of a dispersible and reactive sodic soil, but further analysis is needed to confirm whether or not it is indeed sodic.
Good soil structure is really important. It enables plant roots to grow easily through the soil with the least amount of energy, allowing them to channel energy into shoot growth. In addition, good structure allows ample supplies of oxygen to the root zone; allows water to soak quickly into soil; and allows rapid drainage of excess water.
A soil is said to have good structure if aggregates are ideally 0.2-3mm in diameter; are bound together firmly and not disrupted by rain or during normal digging; small roots can still penetrate aggregates; and the arrangement of aggregates is such that there are many pores between them. Aggregates are held together by organic matter, secretions from living organisms (lignin), clay particles and a hight proportion of Ca ions relative to Mg/Na.
In my chosen site, the top layer of soil formed very large clods which are moderately well held together.
Texture analysis demonstrated a soil that was consistent with a sandy clay loam (forming a strongly coherent cast in which sand grains can be felt, with ribboning 25-40mm).
For further details on texture analysis refer to a previous post on this blog.
A soil profile depth of 600mm was dug with help of a shovel and a mattock - the soil became progressively harder to dig below about 100mm. It had rained lightly a couple of days beforehand, but the surface was quite dry. Gloves, eye protection, steel cap boots, full sleeve shirt and long pants were worn for protection against debris, sharp objects, insects and soil-borne disease.
The soil colour is hard to see clearly from the photo above, but in the top layers it is a dark brown, progressively gaining different shades of brown-reds until a layer of yellow was reached at about 500-600mm.
These colours mainly come from the different iron oxides. The deeper, yellow layers may indicate that these areas are more poorly drained. The top layers certainly smelt "earthy" whilst the deeper layers less so, but not a sour, anaerobic smell at least.
A small amount of earthworm activity was noticed in the top 100-200mm of soil.
Earthworms are beneficial to soils - they break up organic materials and mix them into the soil; the help to break up thick layers of leaf litter; they increase microbial activity, nutrients available to plants and the amount of water that can be held in soils; improve soil crumb structure; and overall allow better penetration of roots, oxygen and water. They can be protected by ensuring the soil is moist, providing them with plenty of organic matter and fertilisers, protecting the from the heat of summer with mulch, only using machinery to cultivate if absolutely necessary and not poisoning them with high levels of copper salts (from Bordeau mixture or copper oxychloride sprays).
(above) Deeper still, a number of bulbils of suspected oxalis pes-caprae were discovered. This weed is a problem in other parts of the garden. Cultivation of this site will likely disseminate these underground bulbils even further.
(above) other organisms found were red-headed cock chafers. These pests feed on root zones of plants.
(above) The top layers also had bits of plastic, broken glass and other bits of rubbish. This is fairly common for backyard suburban gardens that have been largely disused. These small fragments of inorganic material will need to be carefully removed during site cultivation.
The above set of four photos shows the soil profile at 200, 300, 400 and 500mm depth. It shows how the top layers are more porous, still have plant root activity, whilst the deeper layers become progressively more compacted and more poorly drained, with less visible organism activity. The deepest layers around 600mm were a heavier clay in texture, forming smooth plastic casts, like plasticine, with ribbons up to 50 -75mm in length.
A sample of soil was taken at 100mm and also at 600mm. A dry aggregate of soil from each sample (about 6mm across) was placed in distilled water to test for its sodicity. For further details on this investigation see a previous post on this blog.
(above) both samples showed slaking (aggregates fall apart, but remain where they fall without dispersion causing no colouring of the water). This indicates that gypsum would be of little value in improving the structure of this soil. Rather, plenty of organic matter will be the key.
pH testing indicated that both samples were neutral to only mildly acidic (pH approximating 6.5 - 7.0). In the photo below it is hard to see this. Again, for further details on pH testing refer to a previous post.
Salinity in suburban soils in Hobart is rarely an issue and so was not tested. Indications of saline soils include dieback or burning of tip growths and leaf margins. Salinity meters are used to measure the amount of ionised particles in soils. This involves the 1:5 method (20g soil per 100mL of distilled water) and pre-calibrating the meter.
Overall, this soil has some issues that impact negatively on its fertility, especially when it comes to growing fruit trees, which require generally deep well drained soils. The level of surface compaction from human and pet traffic needs to be relieved. Also, the medium to heavy clay sub-soil will impact on drainage.
To address this, the plan would be to double dig the area to relieve this impervious sub-soil.
This involves digging a trench to spade depth and forking the lower surface of the trench and incorporating matured compost. The top layer of soil from the second trench goes over the forked layer of the first trench - see pg. 33 of Gardening Down Under, Handreck.
Following this, a crop of potatoes will be planted to assist in creating sub soil channels for greater aeration and promote microbial activity. Potatoes generally prefer a soil pH of 5.0 -6.0, so the addition of either sulphur or iron sulphate would help to lower pH - see case study below. Harvesting the crop will also aid in breaking up the soil into finer, more crumbly aggregates. The weed issue is of concern, and perhaps a border of white clover, Trifolium repens, might act as a competitor against weeds, as well as being a nitrogen fixer for the soil.
Apples and Pears, Peaches and Nectarines tend to prefer a pH of 6.5, citrus around 6.0 - 6.5, and apricots 6.5 -7.5.
CASE STUDY QUESTIONS :
1. Raising the pH of a soil from 5.5 to 6.5.
Apply ground limestone (CaCO3), builder's lime (calcium hydroxide) or ground dolomite (CaCO3, MgCO3) to the soil surface. Dig it in wherever possible. The quickest result is with builder's lime (about two months compared to ground limestone which may take up to one year). The change in pH occurs in the top 10cm of soil. The amount to use depends on the scale of the pH shift needed (eg. more is needed to shift from 5.5 to 6.5 compared to 4.5 to 5.5) and the soil texture (less for sandy, more for clay loams).
examples of plants tolerating this pH - Lavandula stoechas hybrids, Lavandula dentata, Erigeron karvinskianus, Rosa hybrids.
2. Lowering the pH of soil from 7.5 to 6 - 6.5.
Use sulphur as the cheapest option. For a drop of one pH unit in the top 10cm of soil apply about 25 gm per square metre to sandy soils and up to 100 gm to clays. Sulphur is converted by bacteria to sulphuric acid, increasing the amount of hydrogen ions and hence lowering pH.
Changes in pH by 1.0 can take up to 3-4 months.
Iron sulphate can also be used, but the amount will need to be doubled to get the same change in pH. Salinity can be an issue, so only one third of the total amount should be added at a time, water heavily, wait a week and check pH before applying more.
The addition of organic matter can also lower pH.
examples of plants tolerating this pH - Camellia sasanqua and C. japonica, most Australian natives, including Banksia marginata, Hardenbergia violacea
Ref : Gardening Down Under, Kevin Handreck
notes provided by Marcus Ragus
For added durability and lower maintenance, a sealing product can be used, which will keep the sand between the joints in place and help prevent weed growth. A concrete toe is used to hold the two edges of the path together as a restraint.
