By Emily Seed, Executive Director with the Northern Ontario Farm Innovation Alliance, Regional Communication Coordinator for Northeastern Ontario Soil and Crop Improvement Association

The West Nipissing/East Sudbury Soil & Crop Improvement Association held a field day in August focused on soil fertility. The day featured Colin Elgie, OMAFRA’s Field Crop Soil Fertility Specialist.

Elgie began at the basics, taking the audience through a breakdown of soil. Soil is 45% minerals, 20-30% water, 20-30% air, and 5% organic matter. Air and water are the components that allow soil to have life. The organic matter is what supports nutrients in cycling through the soil.

When looking at soil fertility, it is important to understand the crop intended to be grown. The most important plant nutrient is whatever nutrient is needed most. For example, adding more nitrogen to soil already high in nitrogen won’t necessarily help yields. A gain in crop yields will only be seen when the limiting nutrient (the one with the lowest nutrient level) is taken care of.

Nutrients in the Soil:

Understanding nutrients is key to understanding soil fertility. Primary (or macro) nutrients include Nitrogen, Phosphorus, Potassium, Carbon, Hydrogen and Oxygen. These nutrients are required by plants in the highest amounts. Secondary nutrients include Calcium, Magnesium and Sulphur. Micronutrients include Boron, Chlorine, Copper, Iron, Manganese, Molybdenum and Zinc. These micronutrients are not any less important, just required at much smaller amounts. 

These nutrients in the soil need to be accessible to plants, so looking at nutrient mobility in the soil will help to understand the availability of nutrients. Nitrogen can have a medium to high mobility in the soil, which means that it can easily leach from the soil. This is why nitrogen applied in the fall usually doesn’t last all the way to spring. Mobility of nutrient within the plant itself also needs to be considered. 

Nutrient mobility also affects where the nutrients should be placed. For example, if a field is low on phosphorus (which has an incredibly low soil mobility level), the phosphorus should be applied in bands where the plant roots can actually access it. There is a very small chance that phosphorus applied more broadly would move to where the roots can access it.

Soil Testing

“Soil testing is the most important dollar you’ll spend”, says Elgie. There are many different methods of soil testing, including composite sampling (the most traditional method), grid sampling, zone sampling, and sensor-based sampling. Typically, 6’ cores are taken. 20+ cores are mixed together to create one 1-lbs sample to be sent to the lab. Soil sampling should be done at a minimum of every 3-4 years. The soil sampling method used should be matched to the field variability.

Field variability can be identified by looking at what changes across the field, such as soil type, nutrient levels, water holding capacity and yield potential. Variation in a field can be caused by:

• The parent material 
• Topography or drainage 
• Climate 
• Field management 

Field variability can also be identified by:

• What is currently/previously done in a particular field, and what is planned 
• Bare ground imagery 
• NDVI imagery 
• Yield data 
• Your experience 

Composite or bulk sampling is the simplest method of soil sampling and can be done very easily across a consistent field. Point or grid sampling divides a field boundary into sections of 5 acres or so, taking samples in each “grid” square and GPSing where that specific sample was taken. 

Finally, sensor-based sampling is a newer method that is still being learned about. The sensor can measure electrical conductivity, topography and slope. 

Soil sampling results can be used for variable rate applications, such as lime, manure, nutrients, seeding populations and tillage.

Target Soil Fertility Levels:

Target soil fertility levels may differ based on the crop, yield goals, management (e.g. tillage, planting, crop protection, soil type, economics, and the short-term vs. long-term outlook).

Moderately high fertile soils typically have a soil pH of 6-7.5, a phosphorus level of 15-20 ppm and a potassium level of 100-120 ppm.

Nitrogen and Sulphur Fertilizer Approach:

Nitrogen and sulphur are highly mobile nutrients in the soil, so soil reserves cannot be built up by fertilizing. It is important to soil test for nitrogen and sulphur, but there is no real benefit to doing this test in the fall, as the levels will not be the same in the spring. Because it is difficult to know the exact levels, nitrogen and sulphur fertilizers should be applied based on desired crop response each year to get the best “bang for your buck”. A dollar more of nitrogen will typically result in a dollar more in crop yield to a certain extent.

Phosphorus, Potassium and Magnesium Fertilization Approach:

For phosphorus, potassium and magnesium, the sufficiency approach can be used. Ask yourself, “will I get a response to additional fertilizer this year”? Rented ground or a crop that may not see a big response may change your answer to the question.

The other approach is to build up and maintain soil nutrient levels. This focusses on more of a long-term outcome that can allow you to cut back on fertilizer in future years when prices are high. By applying fertilizer, you can build to a responsible level to maintain the highest yield potential. Building soil test levels requires a lot of inputs, as the added nutrients need to be greater than the nutrients lost during crop removal.

Soil pH & Neutralizing Soils:

Soil pH is an indication of how acidic or basic the soil is (or the hydrogen concentration of the soil). The pH level impacts soil organism activity, performance and breakdown of pesticides, nitrogen fixation by legumes and the availability of nutrients. Nutrients are most available around a pH of 6-7.5. Crops also have varying sensitivities to soil pH levels.

Raising soil pH is relatively easy and cheap, whereas lowering soil pH is difficult and very expensive. When applying lime, it is important to consider the product, as there are several products but not all have the same effectiveness. 

By taking into consideration soil test results, field variability and nutrient interactions, soil fertility can be managed using the 4R approach – right source, right place, right time, right rate.

The article was condensed for this publication; for the full article, please visit: https://membership.ontariosoilcrop.org/soil-fertility-day-in-west-nipissing-east-sudbury-2/