OMAFRA Field Crop Report – August 20, 2020

Figure 3. Water infiltrates quickly and preferentially through continuous macropores created by daikon radish and oat cover crop roots.

Managing to get the most from available water

Water is the most important limiting input for crop production. In more than a couple of the past years, producers across the province have been forced to watch while their crops suffer from dry conditions in the heat of summer.

Other than irrigation (which is rare for field crops in Ontario), there’s really nothing we can do to manage how much water we get and when we get it. For that reason, it makes more sense to think about how we might lose it – and how we can keep it.

Striking the Water Balance

On the opposite side of the water balance equation from inputs (precipitation and irrigation) are losses – runoff, deep drainage, and evapotranspiration – and the soil storage component. The right lever to pull in this system will depend on the specifics of each operation and its soil landscape, so here are the broad concepts and some general recommendations for how to influence them to manage your risk for drought stress.

Deep drainage

As water moves through the soil, it eventually goes below the rooting zone and is essentially lost for crop production. This is an important natural process for recharging groundwater and the way that soils provide a water purification service.

Tile drainage essentially short-circuits this process, shunting groundwater into surface waterways. Water table management like this is critical for timely planting in most of the province and improves the drought tolerance of crops as it allows roots to develop deeper before dry conditions occur. However, we’re often left pining after that spring water once it gets dry in the summer. Controlled drainage systems allow for more refined water table management and allow us to keep water in the field when it’s needed. Once thought to be suitable only for flat fields, this technique is now also being applied on sloping terrain with drains installed on contour.

Evapotranspiration

Water evaporates from soil more quickly in hot, dry weather. It also evaporates more quickly from bare soil than from soil that is covered, either by residue or by plant canopy (Figure 1).  Bare soil competes with the crop for water just like weeds do. Overwintering cover crops can provide the best of both – transpiring excess water in the spring and providing mulch to cover the soil until the crop canopies. “Planting green” – into living cereal rye terminated soon after planting, for example – is an effective strategy for soybeans.

Figure 1. Relative evaporation rate from bare soil and residue-covered soil. (van Donk et al., 2010)
Figure 1. Relative evaporation rate from bare soil and residue-covered soil. (van Donk et al., 2010)

 

 

 

 

 

 

 

 

Runoff is a terrible way to lose water, because it usually also means losing soil. The erosive power of runoff is related to its speed, so anything that can slow the water down will help. Residue cover is the first step – 100% residue cover results in negligible erosion, but 50% cover still reduces erosion by 80%. This level of residue is usually achieved with no-till. A recent meta-analysis of runoff and tillage systems showed that no-till reduced runoff by over 20% on average compared with other tillage systems (Figure 2). The residue cover in a no-till system reduces crusting, and the lack of disturbance results in higher pore continuity, meaning more infiltration and percolation.

Figure 2. Change in runoff from no-till compared to reduced tillage and plowing. From Sun et al., 2015.
Figure 2. Change in runoff from no-till compared to reduced tillage and plowing. From Sun et al., 2015.

 

 

 

 

 

 

 

 

 

The devil is in the details, though. No-till fields may experience more runoff than tilled soils if they are left bare from residue harvest (i.e. straw removal or low-residue crops (e.g. soybeans) if their surfaces are smoother than tilled fields. This is where cover crops come in. Even in tilled systems, the pores created by cover crop roots can increase infiltration of a heavy rainfall event by 17% (Yu et al., 2016). Figure 3 shows a soil infiltration demonstration that provides a good visual of how coarse and fine root channels help move water through the soil. Cover crops with coarse roots (e.g. daikon radish) and those with very dense root systems (e.g. cereal rye) provide the most benefit.

Figure 3. Water infiltrates quickly and preferentially through continuous macropores created by daikon radish and oat cover crop roots.
Figure 3. Water infiltrates quickly and preferentially through continuous macropores created by daikon radish and oat cover crop roots.

 

 

 

 

 

 

 

 

 

 

 

Store it in the Soil

Making the best use of the water we get in a season means getting it in the ground, but also keeping it there and making sure it’s accessible to crop roots. A well-structured soil with a range of pore sizes will hold the most plant-available water. In a compacted soil, root access to water may be limited by soil resistance even before water levels fall below the permanent wilting point (Figure 4.)

Figure 4. The least-limiting water range (LLWR) is between field capacity and permanent wilting point in low-density soil, but soil resistance (SR, i.e. compaction) or aeration (AFP) may be more limiting in compacted soil. (from Keller et al., 2015)
Figure 4. The least-limiting water range (LLWR) is between field capacity and permanent wilting point in low-density soil, but soil resistance (SR, i.e. compaction) or aeration (AFP) may be more limiting in compacted soil. (from Keller et al., 2015)

 

 

 

 

 

 

 

If what’s noted above makes you think that you don’t need to worry about water because you are already no-tilling, think again. Tillage is a tool for dealing with compaction. If you take it out of the toolbox, extra care is required to avoid squeezing your soil of its water holding capacity.

Weather Summary

Weekly Weather Summary August 10-16

LocationDateTemperatureRainfallTotals
August 10 -16MaxMin(mm)RainGDD0GDD5CHU
 °C°C01-Apr01-Apr01-Apr01-May
Harrow2020311311307226616092447
201928132370224315792344
Ridgetown2020301216314217615222316
201928119514212314612209
London202030128338211214662230
2019311125452203013902130
Brantford202032120211214314952220
20193010N/A274210414492191
Welland202031130269216215162321
201928105362216514952286
Elora202030101281197513402078
20192982359167310831713
Mount Forest202030119396195213272086
2019289N/A129190312602001
Uxbridge202031118264200513802140
20192811N/A227189812491971
Peterborough202031914209202413762098
20192962334189212431901
Trenton2020291412276211514612275
201929119354209214202188
Kemptville2020311236261217115062272
2019298N/A203199913452037
Earlton202032106316185112411972
20192761349158910131653
Sudbury2020291323384187212562005
20192790351161010401689
Thunder Bay20202991181174611291814
2019276426715569681575
Fort Francis202028779289186212271947
20192760292168010611701
This table developed by OMAFRA using data from Agriculture and Agri-Food Canada and Environment Canada. Max and Min Temps show the extremes that occurred for the 7-day period.