Soil Improvement


By Kate Wall

In general work on improving the soil or correcting soil pH should be done before planting takes place. When starting a new garden, understanding your soil will greatly assist you to choose plants well suited to growing in that soil. If you wish to grow plants with special needs such as the acid loving plants (including azaleas, camellias, gardenias, blueberries and strawberries), you may need to make adjustments to your soil before planting to give them a greater chance of success. Poor soil will affect plant growth, so taking some time to improve the soil before planting will result in greatly improved plant success.

Some soil improvement products, such as compost, are fine to use on existing plant beds while others such as gypsum need to be dug into the soil before planting. In most existing beds, a top up of compost is of great benefit on an annual basis, in addition to fertiliser applications, and fresh mulch.

Before starting a new garden bed it will be of value to understand your soil and what products may be most appropriate to assist you to improve that soil.

Gypsum and clay breaker products are highly valuable in opening up most (but not all) heavy clay soils. Dolomite is also effective as a clay breaker but does include lime so should not be used in alkaline soils. Dolomite is often recommended to be dug into the soil where a new vegetable patch is to be planted as most vegetables prefer a slightly alkaline soil, and will appreciate the magnesium contained in dolomite.

Clay soils will greatly benefit from the addition of organic matter to the soil, which may be in the form of homemade compost, or any one of a number of compost or manure products available in bags from the garden centre. If using a lot of bagged compost products, it is recommended that you do not rely on only one variety, as each is sourced differently it will have different nutrients to contribute to the overall soil health. Searle’s “5in1” soil improver is one of the best as it includes organic matter sourced from many different raw materials so has a more balanced nutrient mix.

Organic matter in the form of compost is not only beneficial to sandy soils, but is essential to build some structure into the soil to allow plants to do well in difficult sandy soils. The addition of water crystals at planting can help retain moisture at the plants roots in soil which is highly prone to drying out. Organic products such as coir fibre or vermiculite can also be used in highly sandy soils as water retention agents. Water crystals, coir fibre and vermiculite should all be soaked in water and allowed to swell before being incorporated into the soil.

Wetting agents are also soil improvement products. They are useful in soils which have become hydrophobic. This is when water does not penetrate the soil but runs off the top. They are most beneficial if used together with other soil improvement products which improve the structure of the soil.

Rock mineral products such as Earthlife's Garden Mate (available from speciality suppliers or online) are also soil improvement products as they contain no nitrogen. They are an excellent option for improving the structure of both clay and sandy soils, adding essential minerals to the soil and adding important soil microbes. This product can be a very cost effective way to eliminate the need for many other soil improvement products, as it performs the role of gypsum, dolomite, trace elements and wetting agents.

Soil pH can sometimes be out of range for healthy plant growth and need adjusting. Soil pH test kits are readily available and easy to use. They are worthwhile if you are planning new major planting beds, or if your existing garden is failing to thrive.

Correcting a low pH (acidic – below 6.5 – 7) can be done fairly easily with the addition of garden lime. Garden lime is a readily available product. Acidic soils are fairly common in coastal Australia.

Correcting high pH (alkaline – above 7) can be much more difficult. Alkaline soils are not as common naturally but are common in landscaping soils. They can be corrected through the use of powdered sulphur although this can be a slow process. Iron chelates can be of assistance in acidifying soil, and Yates Liquid Sulfur can also help speed up the process. You will need to repeat your pH test and repeat the treatment periodically as lowering soil pH is a gradual process. Watering the soil (not the leaves of plants!) with one cup of white vinegar to a full watering can will give fast but short lived results and should be done in conjunction with other measures.

When adjusting soil pH it is highly advisable that organic matter in the form of compost is added to the soil as this will greatly assist to buffer pH in the soil. Be aware that some organic products can alter the pH, mushroom compost is often quite alkaline, chicken manure can be quite acidic.