Be current and be adventurous about your soil testing

Saturday, August 8, 2015

The results of any soil test are a direct reflection on how you have been managing your soils. Are you curious enough to want to know?

by DALE COWAN

We are approaching that time of year when we need to implement post- harvest activities. One of the many tasks to do is proper soil sampling.

Soil sampling is one the simplest things we can do to give us a predictive tool for assessing soil fertility needs. It is important to make nutrient management decisions with current soils test of no more than four years old. Just as you would not make a financial decision today based on a balance sheet from 2001, you shouldn't do that with a soil test either.

The results of a properly taken and analyzed soil sample will indicate the soil's reaction (pH) and ability to supply essential nutrients. This should lead to a nutrient management strategy for addressing any shortfalls. Multiple years of soil tests can indicate trends and reflect how the fields have been managed.

For example, if soil levels of phosphorus (P) and potassium (K) have been declining over time, then the removal of nutrients in the harvest portion of the crop is greater than the rate applied. Conversely, if the soil test levels are increasing and are into the high range, then applications of nutrients have generally exceeded crop removal.

There are two soil sample test ranges that you want to avoid – having soils test Low (L) or Very High (VH). If they come back rated L, you may not be maximizing yield due to insufficient nutrient levels and not returning enough soil-building residue. Ratings of VH may indicate you are applying more than required, thus increasing the size of your environmental footprint and not optimizing your nutrient expenditures.

Both situations warrant attention because they affect the environment, cash flow and return on investment. Yield matters – the more you grow, the more you have to sell at any price. This also requires matching expenditures to outcomes. Soil testing is one of the main activities of the 4R Nutrient Stewardship Program.

With greater emphasis on soil health, should we consider broadening our outlook on soil testing? Plant growth reflects an environment that encompasses three main areas of soil productivity – chemical, physical and biological. Plants grow in these environments concurrently and sometimes the results may differ from expected outcomes indicated by the current soil test.

The implementation earlier this year of the Great Lakes Agricultural Stewardship Initiative (GLASI) in southwestern Ontario provided a template approach to assessing farming practices and gaining insight on how these practices affect soil health and water quality. Invariably, the analysis reveals the benefits of crop rotation, cover crops, reduced tillage and residue management.

The question often asked by farmers is whether there is a soil health test to measure changes in soil's physical and biological properties. The short answer is, maybe yes. Currently, there are commercial laboratories offering various test packages. But along with a new soil health test comes new terminology, new sampling and testing methods and different interpretations.

The physical tests have been around for a long time. The determination of bulk density, hydraulic conductivity (infiltration rate of water), and air-filled porosity and percentage of sand, silt and per cent clay. Biological tests offer such things as Solvita soil respiration, mineralizable nitrogen and soluble carbon, to name a few. All of them indicate a biological activity based on the evolution of carbon dioxide by bacterial respiration or conversion of organic nitrogen (N) to nitrate N.  

The real question is, what do the numbers mean on the report? Do they reflect the results of an action you implemented? Do they indicate a long-term change? How often should I test? As with most new tests, what is often required is a paired sampling approach. By collecting a "bad" area and a "good" area, you can do a direct comparison.

Any new test offered should be used with caution. You should take time to think critically, ask questions of the service providers and seek professional opinions before taking any major changes in farm practices.

Most results are qualitative at this point on paired samples. However, if we take an approach of doing paired samples and work with knowledgeable people, we will create the demand for more knowledge on soil health tests that sends a clear signal to the research community and funders to get busy. Current nutrient soil testing has had an 85-year head start.  

So keep your nutrient soil testing practices current, be adventuresome and try a paired sample technique for a soil health test.

Remember, the results of any soil test are a direct reflection on how you have been managing your soils. Are you curious enough to want to know? Do you need to change? BF

Dale Cowan is a Certified Crop Adviser in Ontario and the Senior Agronomist and Sales Manager for Agris and Wanstead Co-operatives, located in southwestern Ontario.