Always read the label

Users of agricultural (or veterinary) chemical products must always read the label and any Permit before using the product, and strictly comply with the directions on the label and the conditions of any Permit. Users are not absolved from compliance with the directions on the label or the conditions of the Permit by reason of any statement made or not made in this publication.

Introduction

Vegetable growers can take steps to avoid a cadmium problem in the future or to manage an existing problem. This Agfact outlines those steps.

Cadmium (Cd) is a heavy metal that is taken up from the soil by plant roots. Cadmium in food is a potential threat to human health. For example, long-term accumulation in the body may lead to kidney damage. Health authorities have set an upper limit for cadmium in root, tuber and leafy vegetables. This is called the ‘Maximum Permitted Concentration (MPC)’ and is set at 0.1 milligrams per kilogram (mg/kg) of fresh weight.

Sources of cadmium

In most Australian soils cadmium occurs at very low levels. It is also present in some fertilisers and other products that are added to soils, such as:

• fertilisers containing phosphorus (P);
• by-product gypsum (phosphogypsum);
• certain zinc additives;
• biosolids (sewage sludge);
• manures and other organic wastes.

Cadmium added to the soil will be there for a very long time. It may take from 100 to 1000 years for the amount of cadmium in the soil to halve.

Cadmium is more available to plants grown in soils that are very sandy, acid and/or low in organic matter.

Reducing the risk of cadmium problems

• Use fertilisers low in cadmium.
• Do not overfertilise.
• Add lime to acid soils (pH (CaCl2) less than 6.0).
• Keep your soil’s zinc levels up.
• Choose plant varieties that take up less cadmium.
• Increase soil organic matter levels.
• Make sure irrigation water is of good quality.

Source: Managing cadmium in potatoes for quality produce (CSIRO 1996)

Steps to lower cadmium levels

Step 1	Is cadmium a problem in your vegetables?	Yes/unsure		Test plants for cadmium levels. (See advice below for Step 1.)
	No
Step 2	Do you use fertilisers, manures, and gypsum high in cadmium?	Yes/unsure		Ask your supplier for fertilisers and manures low in cadmium, and use only mined gypsum. (See advice below for Step 2.)
	No
Step 3	Are you overfertilising?	Yes/unsure		Test soil and apply fertilisers based on the results. (See advice below for Step 3.)
	No
Step 4	Is your soil’s pH below 6.0* (CaCl2)?	Yes/unsure		Test soil pH. If below 6.0*, add lime to obtain a pH of 6.5*, but do not overlime. (See advice below for Step 4.)
	No
Step 5	Are you growing crops prone to cadmium take-up?	Yes/unsure		Where possible, switch to crops less likely to take up cadmium. (See advice below for Step 5.)
	No
Step 6	Are your crops low in zinc?	Yes/unsure		Test plants. If levels are low, apply zinc at planting. (See advice below for Step 6.)
	No
	Reduced risk of cadmium problems

*All references to pH use the calcium chloride (CaCl2) method. It gives lower values, by an average of about 0.5 to 0.8, than those given by the water method or the small, commercially available kits.

If your response to any of the questions in the flowchart above is ‘yes’ or ‘unsure’, the following advice is provided for your assistance:

Step 1

Leaf and soil analysis kits provided by laboratories are available at most produce stores. The kits have detailed instructions on how to collect samples and send them away for testing.

Plant tests cost around $50–$70. Make sure the laboratory is accredited by the National Association of Testing Authorities.

Step 2

• Choose fertilisers with less than 100 milligrams of cadmium per kilogram of phosphorus (less than 50 mg Cd/kg P if available).
• Choose manures and composts with less than 1 mg Cd/kg of product.
• Use mined gypsum which usually has less than 1 mg Cd/kg.
• Avoid by-product gypsum which typically has 8–15 mg Cd/kg.

Step 3

Do not add phosphorus to soils with more than 150 mg P/kg (using the Bray or Cowell tests) for 1 year or longer.

Many NSW growers have at least twice as much phosphorus in their soil as their crops can use.

Step 4

Liming acid soils (low pH soils) will reduce the amount of cadmium that crops take up from the soil. A soil pH of about 6.0 to 6.5 (CaCl2) is recommended for most vegetable crops.

For potatoes, where common scab (Streptomyces scabies) problems may increase when pH is high, maintaining a soil pH of between 5.5 and 6.0 (CaCl2) is advised.

Step 5

There are large differences in the amounts of cadmium taken up by different vegetables.

Leafy vegetables like lettuce, cabbage and spinach take up more cadmium than root and stem vegetables like carrots and potatoes when grown in the same soil.

Fruiting vegetables like tomatoes and capsicum take up the least cadmium.

Step 6

A greater concentration of zinc in the soil means crops take up less cadmium.

Test plants for zinc at the same time they are tested for cadmium. If zinc levels are low, apply zinc sulphate at 20 kg/ha just before, or at, planting.

Soil and leaf sampling kits

Soil and leaf sampling kits, provided by testing laboratories, are stocked at many produce stores. There are a number of soil and plant testing laboratories listed in Yellow Pages phone books under headings including ‘Soil Investigation’ and ‘Inspection and Testing Services’.

Plant health diagnostic and analytical services

NSW Department of Primary Industries’ Plant Health Diagnostic and Analytical Services can be contacted at:

Menangle: (02) 4640 6428

Orange: 1800 675 821

Tamworth: (02) 6763 1100

Yanco: (02) 6951 2611

Alstonville: (02) 6626 2400

Wagga Wagga: (02) 6938 1999

Further information

Further information is available from: