Soil is essential for our survival; it supports and sustains life on Earth. We measure the quality of soil by its ability to perform its task, and for most of us that is to provide food and shelter. Of course, what soil works for one purpose is not so good for the other (strong and stiff for shelter/engineering and weaker and crumbly for food/plants).
In order to maintain a strong and healthy soil though we must also consider its other functions: (i) media for plants/trees aiding hydrological cycle and gas exchange; (ii) atmosphere modification through gas exchange; (iii) habitats for animals and organisms; (iv) recycling of nutrients from dead organisms; (v) engineering platform for roads, buildings, etc; (vi) absorption, hold/release, and purification of most terrestrial water (surface, ground, and soil); and (vii) filters water before it reaches an aquifer (very relevant for Waiheke).
Soils are basically weathered rock and Waiheke’s is the oldest known rock from the Auckland region. A predominantly yellow-brown earth on a greywacke base ranging from moderately well to poorly drained soil means we need to condition the soil to improve permeability and allow plant roots to access the wealth of minerals within.
For our gardens and lawns we must look at the structure – soil aggregation should be healthy which aids aeration and water infiltration. The soil holds water and air which carry minerals, nutrients, and gases to plant roots.
The depth of topsoil across the island is inconsistent and the argument that topsoil is needed to grow a beautiful garden is fallacious. Most of our native plants thrive in clay conditions (the addition of gypsum to the pit when planting or sprinkled on top of the soil can be the difference between success and failure). Conditioning soil with compost (homemade or available commercially) is the best way to boost the topsoil.
SOIL QUALITIES
Condition
Aggregates and water holding capacity (WHC)
Soil particles bind together to form aggregates; strong aggregates provide soil structure which is more resistant to degradation from erosion. Stable aggregates provide pore space for air or water in the soil to facilitate gas exchanges to and from plant roots and to aid nutrient flow or retain water depending on the pore size. Water tends to drain through macropores as gravity draws it downward whereas micropores have a much higher WHC. If the micropores are too small the water is held on to tightly and is not plant available. Conditioning soil so that the porosity has a healthy WHC is a balance of aggregate size and strength which can be improved with the addition of organic matter in the form of compost and gypsum to help clay aggregate. It is essential that water movement can continue through the soil profile; perforating the soil by standing on a garden fork and pushing it deeply into the soil and then moving it back and forth will assist both aeration and infiltration. Compost can be spread on top of the soil; there is no need to dig it in as this causes degradation and disrupts the microorganisms that are helping your plants access nutrients by creating pathways deep down in the soil.
Nutrient holding capacity and pH
Cation exchange capacity (CEC) is the capacity of soil to hold exchangeable cations. These are positively charged ions (eg aluminium, calcium, copper, hydrogen, iron, magnesium, manganese, potassium, sodium, and zinc) which are attracted to the negatively charged clay and organic material in the soil. Cations are weakly bound which mostly prevents them from leaching but makes them easily plant available.
Soil acidity (pH) is a measure of the concentration of hydrogen ions in the soil on a scale 0 to14. Neutral is 7 and the optimum range is 6 to 7 for plant available nutrients. Clay soils tend to be more alkaline. Acidic soils have high solubility rates for nutrients which can become toxic to plants particularly aluminium, iron, and manganese. There are some plants that thrive in acidic soils such as beans, broccoli, squash, blueberries, hydrangeas, camellias, and daffodils. Those preferring a more alkaline soil are cabbage, beets, celery, sweet potato, thyme, parsley, lavender, salvia, daylily, and geranium.
Recommendations
Work with what you have. Nature has provided us with a mineral rich clay and limited topsoil. Do not disturb soil where digging is not necessary, soil can be aerated with a fork and compost spread above ground (unless structure is very poor). For flower and vegetable garden beds dig pits for plants and leave the surrounding soil. When planting add Saturaid to reduce the need for watering. Applications of lime will alkalise soil; use elemental sulfur or iron sulphate to acidify soil. There is a simple vinegar/bicarbonate soda test to determine soil pH (or there are kits available). Take a handful of soil and place in a saucer, mix in distilled water to form a paste and pour a little vinegar (or sprinkle bicarbonate soda). A reaction from vinegar indicates alkaline soil, bicarbonate soda is acidic soil, and no reaction is a neutral soil.
Lastly and most importantly, mulch. The value of mulching cannot be underestimated. It conserves soil moisture, reduces erosion and weed growth whilst moderating soil temperatures, and encourages growth of beneficial soil microorganisms. Mulch can also stabilise eroded areas. Your plants will love you for it. For best results lay it at 100-150mm thick.
