Managing Fertility After a Drought
By: Thom Weir, Senior Precision Agronomist, Farmers Edge
We are all aware of the impact of Drought on crop yields. Drought also causes several soil processes to slow down or virtually stop.
The two most pronounced effects of drought are:
- Reduced crop growth and nutrient uptake but greater nutrient concentration in the plants.
- Reduced soil physical, chemical and biological activity. For example, mineralization, denitrification, and assimilation.
What do these reductions mean?
- Residual nutrients, especially nitrogen, were not utilized because there was poor or reduced growth.
- Similarly, some or most of the applied fertilizer may have remained unutilized and, depending on application methods, may be stranded in the surface soil horizon, as no water was moving downwards or caused it to be lost in the air through denitrification.
- Occasional rains created flairs of microbial activity and some re-growth.
What do these changes result in?
- Nitrogen soil test levels will be higher than normal. These levels can persist for a few years as shown in the two examples below illustrating the impact of the drought in the late 1980’s1:
- Variability of soil test levels increases. Soil texture and moisture tend to vary considerably across a field in drought years. Crop growth and nutrient uptake will also vary. These facts magnify the benefits of zone sampling. When composite samples are made from a number of locations, they may contain sub-samples with extremely variable levels and the average becomes less meaningful. Samples taken from like areas that make up a zone will result in a much clearer vision of what nutrients remain in the different areas of a field.
- You may notice a decrease in soil pH that can be followed by an increase in EC (salts) and available P – these will correct themselves in time (see examples below)1.
- Crops will yield grain with higher than average protein2:
What Actions Should You Take after Drought?
Step 1 – Soil Testing
- Review the zone maps with this year’s imagery. Modifications may be required to account for the impact of this year’s drought.
- Remember “high variability.” Increase the number of samples that you take per zone. Strive to take 14 – 16 or more samples per zone.
- When taking soil samples, make sure that the probe is full of the sample, as sandy soils when dry, tend to be loose and soil might fall off the bottom of the probe.
- Because there are reduced biological activity in dry soils, soil sampling and banding can begin earlier than usually recommended. However, if rainfall occurs before the soil temperature reached below 10oC (50 oF) these processes may give an inaccurate fertilizer recommendation or convert products into forms that can be lost.
Step 2 – Recommendations
- Next year’s fertility requirements (especially Nitrogen) are likely to be less than what has been recommended for the past decade. Be leery of 0 recommendations with such great variability in soil test values. Set a logical lower limit for recommendations.
- Be suspect of decreased P recommendations. They may be due to a decrease in soil pH. This will be temporary. Use fertilizer additions and crop removals over a 4-year period to come up with a sustainable P fertility strategy balancing applications to removals.
Step 3 – Product and Timing Selection
- Avoid “new products” with “silver lining” promises!
- Products applied to the soil surface have a higher likelihood of being “stranded” than products banded 5 – 7.5 cm (2-3 inches) below the soil surface.
- Fall banding has been shown to have a greater advantage over other applications in dry conditions as compared to moist conditions.
- Urea and Anhydrous Ammonia are the best choices for fall applied fertilizers.
- Remember anhydrous ammonia is absorbed quickly by the soil even when the soil is in air-dry condition2 and the early ammonia application on well drained soils application after the middle of September leads to the same yields as those of late fall and spring applications3.
Source: R.E. Karamanos, 1996. Recent trends in soil testing nutrients. Proceedings of the Soils and Crops Workshop ‘96, University of Saskatchewan, Saskatoon, Saskatchewan.
 M. B. Benke1, T. B. Goh, R. Karamanos, X. Hao, and N. Lupwayi. 2010. Retention of injected anhydrous NH3 fertilizer as affected by soil physical and chemical properties. p. 68 in Proceedings Transfers and Transformations: Our Evolving Biosphere, University of Saskatchewan, Saskatoon [Online] Available: HERE
 Karamanos, R., Hanson, K. and Hultgreen, G. 2010, Effect of time and rate of application of anhydrous ammonia and urea with or without a nitrification inhibitor on the yield and quality of a barley-wheat-canola rotation, Proceedings Soils and Crops 2010, Extension Division, University of Saskatchewan, Saskatoon, SK