Soil Testing for the Organic Farmer
Organically farmed soils are tested to measure the soil quality & nutrient levels and to test for contaminants.
About this Testing
Organic Farmers must check to see that their land and, in some cases, the food they produce does not exceed the guideline levels set by the certifying organisation. The test results also provide ongoing feedback that their practices are effective. Soil nutrient and organic matter testing is a requirement for approved organic farming and growing systems. Organic producers aim to maintain soil nutrient status, maintain or improve soil physical quality and manage the soil environment to achieve efficient utilization of soil nutrients.
Plant tissue (leaf) tests are used to measure the nutrient status of plants (major nutrients and trace elements) as an indicator of the effectiveness of the nutrient management programme.
Assessing New Organic Properties
To assess the soil quality on a new property it is helpful to start with a Basic Soil Profile, Organic Soil Profile and heavy metal soil test. Based on the test results and consultation with the certifying organic organisation, subsequent testing can be carried out.
When assessing a property for organic production, one of the main factors to consider is the land use history. For example if the land was previously used for intensive horticulture, a multi-residue test (which includes DDT) is generally required due to the wide range of chemicals that potentially could have been used. For land used for pastoral farming, a DDT test is required due to the widespread use some 30 to 40 years ago of the persistent DDT insecticide. For organic compost, a herbicide residue test and heavy metal test are generally required.
The main areas of soil testing for Organic Farmers are:
- Nutrient Status
- Pesticide Residues
- Heavy Metals
Organic Farming Soil Test Options
For organic soils a Basic Soil Profile, sulphate-S and Organic Soil Profile are usually recommended. Where reactive phosphate rock (RPR) fertiliser has been used, the Resin P test should be added.
- The Basic Soil Profile includes: the soil pH, Olsen P, exchangeable cations potassium, magnesium, calcium and sodium, Cation Exchange Capacity (CEC), Base Saturation and Volume Weight.
- The Organic Soil Profile: includes the total nitrogen, available nitrogen, organic matter (total carbon), and the total carbon:total nitrogen (C:N) ratio of the soil.
Other soil tests include organic sulphur, total sulphur, aluminium, boron, total phosphorus, total selenium, EDTA trace metals, Mehlich 3 trace metals and more.
Why Test Soils
The first step towards management of the soil nutrient and organic matter resources is to conduct tests that quantify these assets, this soil testing enables changes from season to season to be monitored. Soil test information can then be used to optimise the nutrient status and quality of the soil and crops grown through good organic management practices and application of permitted fertilisers, composts and other soil amendments.
Soil testing takes the guess work out of nutrient management, allowing the most cost effective fertiliser programmes to be produced. Herbage tests of plants complement soil tests and allow major and trace nutrients to be adjusted for improved plant growth.
As an independent lab, we leave the fertiliser recommendations to competent advisor's who know the property and the organic farmer's objectives.
Taking a Sample
For soil samples avoid dung and urine spots and other non typical sample areas and do not sample within three months of applying fertilisers. Use a 15cm soil auger for horticulture and cropping sampling and a 7.5 cm soil auger for pastoral sampling. Taking a soil sample in a diagonal line across the block can be helpful and samples should be taken at the same time each year for best nutrient monitoring. To obtain a representative soil sample, collect many (preferably 20) sub samples from the block. For compost heaps, ensure hand “grab” samples are included from the centre of the heap. The sub samples should be placed in a soil test bag to give a representative 500g sample to be sent to the lab.
- For Tree Crops, take two 15 cm soil cores from the root zone of ten trees from two diagonal lines through the sample area.
- For Row Crops like Kiwifruit or Grapes, takes twenty 15cm soil cores from the plants root zone across the sample area.
- For Vegetable and Arable Crops, collect twenty 15 cm soil soil cores across the sample area.
- For Pastoral samples take twenty 7.5 cm soil cores across the area.
Sampling at the same time of year in the same way will give better nutrient monitoring
Guides and Supporting Information
Freephone 0508 HILL LAB (44 555 22) or email Ag.CSM@hill-labs.co.nz