Most farmers are familiar with the notion of soil testing. It’s the
creed for most agricultural extension workers.
But surprisingly, many producers, including organic farmers, do not
regularly test their soil.
There are several explanations.
A few farmers do not see the potential economic benefit, and therefore
view soil testing as an additional expense. Many find it hard to
interpret soil test lab reports, while many organic producers find
conventional soil test lab reports frustrating because they are not
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practical for their production systems.
Similarly, many long-term organic producers find conventional soil
tests to be a superficial assessment of soil health and a poor
indicator of crop production potential.
The current draft of the National Organic Standards requires soil
testing every three years. A link to the National Organic Standards
can be found at www.producer.com under news links.
Soil testing is mandatory for some organic certifying bodies, while
others require it less frequently, such as when first applying for
certification.
- What is needed to take a soil test?
Typical equipment includes a soil probe or shovel, a bucket preferably
made of plastic or non-galvanized metal, boxes or bags that are often
provided by soil test labs, and a waterproof marker for labeling.
Ziploc bags are suitable if boxes aren’t available.
- How many samples are enough?
Composite samples of several individual probes within an area are a
good way to obtain representative samples. Collect cores from 15 to 20
random spots for every 20 acres. Avoid sampling near roads, fence rows
and highly eroded areas. Do not include soil from high-yielding and
low-yielding areas in the same composite sample, because
recommendations for these areas would vary. This may seem tedious, but
remember that a one kilogram sample represents more than 20 million kg
of soil.
Sampling depth is linked to field management. If only the top five
centimetres of the field are to be worked or the field is pasture, then
a 10 cm depth is adequate.
For soil that is worked or tilled, the depth would be about 15 cm. Soil
will generally have less fertility the deeper you go. Samples taken too
deep may make it appear the land is less fertile than it is.
- When and how often do you take samples?
A soil sample every three or four years may be sufficient to provide
general indication of fertility, but will not provide detailed analysis
for specific management decisions.
More frequent soil testing is needed to monitor soil improvement of a
particular area or produce a high quality product.
Frequency of sampling also depends on soil type, crop rotation and
climatic factors. Sandier soil has greater fluctuations in nutrient
levels and requires more frequent tests, as do crops such as silage
corn and alfalfa that remove a lot of potassium, and irrigated crops.
Also, droughty soil requires frequent soil testing because of the
variability in the uptake of available nitrogen.
Soil can be tested at any time, but is done primarily after harvest.
Organic producers often alter their crop rotation or choose to include
green manure and underseeded crops in response to soil tests.
- How do you interpret results?
The standard soil test provides data on soil nutrient levels including
nitrogen, phosphorus, potassium, calcium and magnesium, and a few soil
chemical characteristics such as soil organic matter, pH, base
saturation and cation exchange capacity. Cation exchange capacity is
related to the soil’s ability to store minerals.
These parameters form the basis of fertilizer and liming
recommendations for a subsequent crop. However, translating the results
into useful organic fertilizer recommendations is not easy. The United
States has several private labs and a few public ones that issue
organic recommendations or formulas to help convert conventional
recommendations into organic management.
The Prince Edward Island Department of Agriculture and Forestry has
started a program to help organic producers develop a nutrient
management plan based on fertility recommendations using only certified
organic amendments.
Many organic farmers view standard soil tests as “treating-the-symptom”
management, and desire better evaluation tools to help assess whether
their practices are leading to a healthy soil system.
Increased awareness of the importance of measuring non-chemical
parameters has led to several on-farm test kits and testing equipment.
In the U.S., kits are available that can assess soil respiration,
infiltration, bulk density electrical conductivity, soil pH, soil
nitrate, aggregate stability, soil slaking, earthworm activity, topsoil
depth, compaction, root development, water salinity and nitrate-nitrite
levels in water.
The diversity of these parameters shows that crop productivity is not
the only factor affected by soil health. The soil’s ability to resist
erosion, buffer pollutants and cycle nutrients is also important.
Increasing farmer knowledge about parameters such as biological
activity and soil structure may lead to more innovative farming
practices that promote a well-balanced soil system.
The writer, Av Singh, is the extension co-ordinator at Agriculture
Canada’s Organic Agriculture Centre of Canada. He can be contacted at
902-893-6275 or by e-mail at asingh@nsac.ns.ca
