TROPO's Organic Info Library

Soil Tests

This summary was compiled by Graeme Eggins, based on research and experience of Wollongbar TAFE lecturers Alan Coates and Dave Forrest.

A SOIL test is vital before you use any land for horticulture, specially so if you intend growing organically. It can help you decide if the soil is suitable for your planned crop and, if extra nutrients are needed it will give you some idea of how much to apply. If you are an established grower, regular soil testing is a tool which helps you manage your fertilizer program in the most effective manner. Separate tests are needed if you want to find out if the soil has been affected by overuse of farm chemicals in the past.

Taking soil samples

To find out the situation on your property you first need to collect soil samples. All you need is a clean - preferably sterilized - spade and a couple of containers. Plastic buckets are excellent. Take about 30 samples per hectare - the more you take, the more accurate the final test result. Walk in a zigzag or diagonal pattern across the paddock, stopping at regular intervals to take a sample. Clear away any vegetation and dig down about 15cm (6in). Make a neat hole and then cut a thin sliver from the side. ( If you take only the first couple of centimetres of soil you will get an unusually high pH result because topsoil contains most humus.) Put the sliver in your bucket. Continue across the paddock taking samples. Avoid obviously a typical spots like gates, around fertilizer or manure dumps, on cow pads or along fence lines. Dry all the samples in a shady place, mix together thoroughly and then weigh out at least a kilo for analysis. North Coast rural supply stores sell kits on behalf of soil testing labs. The kits contain full instructions on how to label and pack samples for testing.

Understanding the test results

After sending off your sample(s) and waiting several weeks back will come the lab report. Making sense of the numbers need not be hard. ( Important: The guide below only applies to the land on the NSW North Coast.) Now take your lab report and using the tables given, convert each result into the words VERY LOW, LOW, MEDIUM or OPTIMAL. Write these words next to each figure on the report. For example, if the report showed a pH of 4.8 write in next to it VERY LOW. Similarly if the cation exchange capacity is given as 8, write LOW. By doing this you will make the report much easier to understand. You can see immediately what elements are deficient in your soil and where levels are OK.

Major soil measures.

pH: The pH result is frequently given as being taken from a ratio of soil and distilled water of 1:5. A 1:2 ratio, which is sometimes quoted, gives a higher pH result. A 1:2 ratio using Calcium chloride solution instead of distilled water gives the most accurate reading. Most North Coast soils are acidic, with a pH around 5.0. Soil pH influences the availability of several important plant nutrients. Here a pH of 6.0 is fine for most crops. Farmers find it very hard to acheive - and hold - higher levels in our subtropical environment. Some tests include a figure for Buffer pH. This is a esoteric reading that even some lab technicians admit has no practical use to many growers. Ignore it.

Organic Carbon: To find the percentage of organic matter in your soil, multiply the organic carbon percentage by 1.8. The answer should be a minimum of 5% but preferably approaching 10%.

Nitrate Nitrogen: Nitrate nitrogen is one form of nitrogen, formed from the ammonium nitrogen released when organic matter decomposes in the soil. It is mobile and transient in soils so results of a test can quickly become out of date. Although plants take up more nitrogen than any of the other nutrients - it is essential for leaf growth and creating new plant matter - you have to be careful not to over apply.

Sulphur ( Phosphate extractable): Plants need sulphur to manufacture protein. It is also essential for the production of vitamins.

Potassium: All North Coast crops need potassium. It is a major nutrient that increase vigor and helps plants fight fungal diseases while also increasing fruit quality.

Calcium: Essential for early vigorous growth. Soils in the TROPO area are often low in calcium.

Magnesium: Magnesium acts as a catalyst in the formation of compounds within the plant and also helps distribute water throughout the plant structure. While you are reading your soil test results, check the ratio of the figures for calcium and magnesium - the ratio should be between 3:1 and 4:1.

Sodium: Although essential in small amounts, it stunts plant growth in large concentrations.

Chloride: Chloride is one of the anions which contributes to the total soluble salt content. Conductivity is a measure of the total soluble salts in the soil solution. High levels of soluble salts can prevent strong plant growth.

Trace elements: Copper, zinc, manganese, iron, molybdenum and boron are called trace elements or micronutnents. Plants need them in very small quantities although their exact role in many plants is still being investigated. Far North Coast soils are often low in copper, boron, molybdenum and sometimes zinc, but results can vary dramatically between properties. Most organic experts recommend treating soil low in trace elements holistically - i.e. by increasing organic matter and general nutrition through slow release fertilizers rather than trying to apply doses of specific elements. Tests for manganese are often inaccurate unless done in the field. Levels are usually high on the North Coast.

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