Soil testing

I must admit that it has taken me a long while to be convinced of the benefits of soil testing. When I started out, most of the established gardens were resisting the obsessive wave of soil analysis which swept across the agricultural and commercial horticultural landscape. There were scientists and graduates from the universities wanting to help farmers and growers by analysing the chemical and physical properties of their soils. Several times each year, technical representatives from the emerging chemical and agricultural corporations would visit major farms, sporting grounds, and established gardens throughout the country.

Around this time, I lived and worked on a private estate and was on one occasion harassed by a foreign gentleman seeking to evaluate the nutrient composition of our soils. With no appointment, he proceeded to remove a hand auger and various sampling apparatus from his illegally parked vehicle. I correctly informed him that he would need to seek permission from the landholder. He provided me an expression of disgust interspersed with several volleys of debate. I directed him towards the gate and he drove off in an unnecessarily excited state. The following morning, I happened to be draining a pond near the boundary of the estate when I captured a glimpse of his vehicle partly concealed behind a mature stand of oak.

I eventually located him collecting soil samples among our native ornamentals. I enquired if he had contacted the landholder and he grunted in the affirmative. Upon checking, this was patently incorrect. I was subsequently advised to contact the police and have him removed as a trespasser. That’s just personal anecdote, of course. Perhaps somewhat irrationally, this incident prejudiced me against soil testing and soil testers for many years. More recently, I have encountered numerous scientists and technicians who undertake soil analysis and provide recommendations in a form which is entirely compatible with my organic and holistic approach to gardening.

Many gardeners find it helpful to access details about the chemical and physical properties of their soil. Basic soil testing kits can be purchased from garden retailers although the laboratory based testing conducted by professional firms will provide significantly more information. In addition to pH analysis, these are likely to include data relating to the soils texture, amounts of organic material present, availability of major and minor nutrients, plant toxins plus a schedule of recommendations and corrective measures. Whenever possible, it is better to choose an analysis that is specifically intended for organic growers. This type will concentrate on the quality of the organic material (humus) and the health of soil organisms required for optimum fertility. They should also provide recommendations based on correcting problems or deficiencies with organic fertilisers and supplements.

Soil testing and analysis is necessarily complex due to the range of physical and biochemical properties which influence fertility and manageability of the various soil classifications. I’m informed that each component of a comprehensive soil test has certain limitations and a predictable margin of sampling error. Most of the conventional sampling techniques require small amounts of soil to be physically removed from their original locations. By removing soil, there is disruption to the structure. This disruption inevitably contributes to alterations in particle distribution, temperature and moisture profiles, along with the complexity and distribution of organic substances and microbial organisms. This means that accurate analysis depends upon comparing results from many individual tests and removing extreme measures to arrive at an averaged or approximate view of soil condition. Understandably, this is an area for university graduates or gardeners particularly keen on mathematics and statistics.

The pH analysis of soil provides a measure of its tendency toward acidity or alkalinity. This characteristic varies from region to region depending on local soil chemistry, rainfall and climate patterns. Most soils in coastal rainfall regions are naturally acidic. Samples from these districts typically indicate a pH between 4.0 and 6.5. Drier inland plains and semi-arid zones are more likely to have neutral or alkaline soils with pH readings between 7.0 and 8.5.

Vegetables, fruit and most other plants prefer to grow in soils with a pH range between 5.5 and 7.5. There are some exceptions, and a number of common species like sweet pea and gladioli prefer more alkaline soils. In general, the symptoms of excessively high or low pH include yellowing leaves, lack of flowering, retarded growth and, in some instances, death of the plant. The health of beneficial micro-organisms and availability of essential plant nutrients are both influenced by the pH characteristics of the soil. When a soil is too acidic or alkaline, plant nutrients become insoluble and cannot be absorbed at the roots. Under such extremes of pH, the soil bacteria required for plant growth are unable to multiply in sufficient concentrations and may in fact be replaced by organisms which release toxic elements.

When assessing soil pH, it is once again important to obtain a representative sample. This generally requires an average of several independent measurements and the discretion to avoid inaccuracies arising from sites where the soil pH has been influenced by specific garden applications. These include samples taken from recently fertilised beds, around woodpiles, compost heaps, incinerators or distinctly burnt areas.