Assessing current soil-test based fertilizer recommendations

While soil tests are recommended to determine the amount of nutrients available for the upcoming crop, growers often question their effectiveness and the economics of the recommendations. Some also wonder if tests are accurate for a zero tillage system and for farms that no longer use summerfallow.
This study investigated the effects of different soil-test-based fertilizer rates and seeding systems (direct seeding, DS and conventional tillage, CT) on canola and cereals (wheat or barley). The study ran for six growing seasons and compared four fertilizer rates – 0, 60, 100 and 140 per cent of the soil-tests-based recommendation – in a canola-cereal rotation.  Both canola and a cereal crop were grown each year on adjacent areas. Fertilizer recommendations were based on annual soil tests for depths of 0-6 inches and 6-12 inches. The amounts of nitrogen (N), phosphorus (P), potassium (K) and sulphur (S) nutrients were calculated for each treatment.

Low spring soil moisture and very low June and July precipitation in 2010 were suspected to reduce wheat yield and  canola was damaged by frost, which was then mowed down. The site received adequate moisture during 2011 to 2013. The 2014 was very dry and 2015 was dry in the early growing season.

Results

Results suggest that soil test recommendations are fairly accurate and provide good information to growers in both direct seeding and conventional tillage systems. In addition, nutrient availability data from the Plant Root Simulator soil probes supported the accuracy of soil test results. A fertilizer rate near 100 per cent of the recommendation was able to achieve optimum canola yield.

Soil tests were found to be responsive to the changes in growing conditions and yield of the preceding crop. For example, lower fertility rates were recommended for canola crops that followed a dry year and a low-yielding cereal crop, especially for the higher fertilizer rate treatments of 100 per cent  and 140 per cent. Then despite the low fertilizer rates applied to some of the of 100 per cent and 140 per cent fertilizer rate treatments, higher crop yields than the lower fertilizer rates resulted. These results indicated that the crops effectively used the residual nutrients detected by soil test results (and that using soil tests can save on fertilizer costs due to avoiding excess applications).

The relatively small changes to the P, K and S recommendations may indicate the lower sensitivity of soil tests for these nutrients as well as the relatively large amounts of total P and K in the soil compared to the amounts applied using fertilizers.

Use of direct seeding was shown to improve aggregate stability compared to conventional tillage. From 2011 to 2015, the differences between DS and CT were not consistent across all years, but an overall increase of 0.022 tonnes per hectare per year in canola yields was observed for the DS over the CT system.

As expected, increased fertilization correlated to increased canola yield and there was a diminishing response to fertilization at higher rates. Fertilization also improved canola root growth in early growing season, water uptake from soil during growing season and stability of soil aggregates.

Interestingly, the canola seed yield response to fertilizer did become larger with the passage of years. Compared to the zero per cent fertilizer rate treatment, canola seed yields at the 140 per cent fertilizer rate were 1.35, 2.34 3.00 2.20 and 3.25 times in 2011, 2012, 2013, 2014 and 2015, respectively. Apparently, repeated useof fertilizer enhanced the response of canola yield
to fertilization.