Getting at the heart of the matter

MEASURING AND BUILDING SOIL ORGANIC MATTER IS IMPORTANT ON EVERY FARM

crop farmers have many tasks, but chief among them is looking after their soil. Maintaining soil organic matter, especially if you have no access to manure, can be challenging.

The first step is to get a measurement through the analysis of a soil sample from a six inch depth,” says Christine Brown, OMAFRA’s nutrient management specialist. “You have to ask for it and it does cost a few dollars more than just the basic soil test, but it’s an important baseline.” The test measures the most stable organic matter – the form that doesn’t increase or decrease quickly. “It’s the most reliable test in terms of ease-of-sampling and cost that we have, but keep in mind that variation in the soil from one part of the field to another does result in different organic matter levels in different areas,” Brown notes. “For example, typically in sloping fields, the organic matter levels on eroded knolls will be lower than levels in the field depressions.”

Once you have your test results, you must examine them in terms of your soil type (see table 1). “It’s about context,” says Brown. “It would be difficult to get much higher than three percent organic matter in a tobacco sand soil, while three percent on a heavy clay soil would not be enough.”

TABLE 1: SOIL TYPE

Clay content (%) 10% 
(sand soil)
20% 30%
(loam)
40% (clay) 50% 
(heavy clay)
Ideal
ORGANIC
MATTER
content
(%)
3 4 5-6 6-7 9
Source: Agriculture and Agri-Food Canada publication “Health of our Soils”

taking action
Keep in mind that building organic matter levels in the soil takes a long time, but the good news is that there are a variety of choices available. “In addition to manure and cover crops, there are sewage biosolids, pulp and paper biosolids, digestate from anaerobic digestion systems, and biosolids pellets, as well as municipal green-bin compost and N-Viro,” notes Brown. “Each of these amendments has a slightly different composition, with different benefits and challenges.”

A DETAILED LOOK AT SOIL ORGANIC MATTER SERVICES IS AVAILBALE.

While Brown’s not sure a farmer could ever have too much organic matter, she says adding it in large amounts (e.g. 100 tonnes per hectare) is not a good idea. “Applying too much at once makes it difficult to incorporate uniformly and, depending on the material, there could be some soil fertility and carbon to nitrogen ratio (C:N) imbalances, leading to potential nitrogen deficiency,” she says. “The organic material you’re about to apply may look nicely composted, but you need to get an analysis done that includes the C:N ratio, as a high C:N ratio means that some N may need to be added to avoid nitrogen deficiency in the next crop,” (see table 2).

TABLE 2: CARBON NITROGEN RATIOS

Carbon:Nitrogen ratio of various organic materials  (Source: OMAFRA Publications)
Material C:N Ratio
soil microbes 4:1 to 9:1
soil organic matter 10:1 to 12:1
solid cattle manure 20: 1 (light bedding) to 40:1 (heavy bedding)
horse manure 27:1 (straw bedding to 60:1 (sawdust bedding)
solid poultry manure 5:1 layers; 10:1 broilers and turkeys
liquid hog manure
liquid dairy 15:1
legume residues 20:1 to 30:1
corn stalks 80:1
wheat straw 80:1
sawdust 500:1
pulp & paper biosolids 25:1 (nitrogen added during process) to 200:1 (little or no nitrogen added)
biosolids pellets 7:1 to 9:1
municipal green-bin compost 12:1
N-Viro 21:1
digestate from anaerobic digestors 3:1 to 20:1 depending on inputs & if liquid or soil
sewage biosolids 8:1 to 13:1 (liquid); 25:1 to 50:1 (solid)

With regard to protecting your farm against soil organic matter loss, Brown believes the most serious threat is not paying attention. “The process of reducing organic matter levels happens so slowly that it often goes unnoticed for a generation,” she says. “Add your amendments, use forage-based rotations and/or rotations that include cover crops, and keep as much residue as possible on the field every year.” Brown notes that, for example, if straw is removed from the wheat crop, the amount of residue returned over the rotation would be about one quarter of what it would be if the straw stayed put. “Another way of looking at it is to compare how long it would take to build soil organic matter in this situation to a given level,” she says. “Leaving the straw on could mean reaching the goal decades sooner.” 

In addition, Brown highly recommends using reduced tillage practices and watching erosion levels. “Be on the lookout especially for the occurrence of sheet erosion, since it takes soil from large areas unnoticed,” she concludes. •