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Are you achieving the full benefit afforded by fallow moisture?

Inadequate soil N can prevent full moisture benefit from being realized!

March 5, 2008  By John Harapiak*

Figure 1. Historical trends in Saskatchewan stubble and fallow soil test N levels. Soil N levels tend to increase during drier periods. Note the gradual downward trend in fallow soil test N levels during the period from 1966 to 1997. Information courtesy of Rigas Karamanos, Westco Fertilizers.
Figure 2. Yield response of Roblin wheat grown on chemfallow to increasing rates of seedrow applied urea-N using openers with SBUs of 10, 20 and 45 percent. This striking response to fertilizer N was almost equal to the N response achieved on an adjacent trial located on stubble. Courtesy of Westco Fertilizers.
Figure 3. Average yield and protein responses of six 2004 wheat (AC Eatonia) trials on fallowed fields. Three sites located in Alberta and three in Saskatchewan. Note the excellent average yield response to added N fertilizer in situations that growers normally assume do not require N fertilization. Courtesy of Rigas Karamanos, Westco Fertilizers.
Figure 4. Average yield and oil response of six 2004 canola (Liberty Link ‘Invigor 2733’) trials on fallowed fields. Three trials located in Alberta and three in Saskatchewan. Note the excellent average yield response of the canola to added N fertilizer. Canola is a crop that typically responds well to higher rates of N application. Courtesy of Rigas Karamanos, Westco Fertilizers.

In a recent column in this magazine, I discussed the fact that water was usually the most important crop input in the arid prairie region (December 2007 issue). Within these drier areas, additional water can be accumulated in the soil through the practice of summerfallowing. However, this added soil stored moisture comes at the cost of a lost crop, so it is important that a grower fully captures the additional potential yield that this opportunity presents. The same article points out that an adequate supply of nutrients, especially N, is essential if the crop is to efficiently convert the water into grain.

Water supply is the key to yield variability

Within the drier portions of the prairies (Brown and Dark Brown soil zones), the practice of summerfallow is used to conserve additional soil moisture. This should not be surprising since in these regions 50 to 60 percent of the variability in wheat yields can be attributed to fluctuations in the supply of moisture available to the crop. However, in many cases, the supply of N can also have a significant role in explaining variability in crop yields.

Risk of low fallow N increasing
As illustrated in Figure 1, in Saskatchewan, there has been a slight downward trend in the long-term amount of N that is annually mineralized during the fallow period. Rigas Karamanos of Westco indicates that this decline in the ability of fields to generate an adequate release of N will be most evident in the areas where fallowing has been carried out most regularly and where soil organic matter levels are the lowest.

Decline related to frequency of fallowing
In 1999, while still with Westco, Kevin Erickson arranged to have fall soil samples collected within the Brown soil zone near Frontier, Saskatchewan from six fallowed fields that had been in a continuous fallow-wheat rotation for a minimum of 40 to 50 years. He also had four fallow fields sampled that had only been broken out of native grassland for an average of eight years. The average available N content in the top two feet was 58lb/ac for the recently broken fields versus 37lb/ac for the older fields. This difference clearly establishes why fields that have historically been maintained in a fallow-crop rotation are now often requiring fertilizer N to fully capitalize on the extra soil stored water.


N release associated with incubation demo
In 1999, Thom Weir who was also still with Westco, conducted some short-term incubation trials to demonstrate the difference in the ability of soils from the Yorkton region to
mineralize N. He compared soil samples collected from a field broken from virgin grassland one year prior and from a field with a history of limited fertilizer application that was fallowed every third year. Both fields were being fallowed at the time of sampling. During the incubation period, the sample from new breaking released 41lb/ac of N while the other field only mineralized 16lb/ac of N.

Westco field trials confirm significant N requirement
The response of HRS wheat grown on chemfallow in the Moose Jaw region in 1994 using seed openers with a range of SBUs is illustrated in Figure 2. Interestingly, with the 45 percent SBU opener, the response to seedrow N applied at a rate 54lb/ac was 26.6bu/ac. This N response was similar to the yield gain in an adjacent study conducted on stubble. Chemfallowed fields tend to mineralize less N than is the case for conventionally fallowed fields. You will note that 1994 corresponds to a year of relatively low fallow N levels in Figure 1.

Westco researcher assesses N on fallow
For two successive years starting in 2003, Karamanos established six sites (three in Alberta and three in Saskatchewan) within the drier region to test the need for using fertilizer N on fallow for growing wheat and canola. The 2004 wheat data is illustrated in Figure 3 and the canola data in Figure 4. This information clearly establishes that in many cases the addition of fertilizer N can be essential for achieving the additional yield potential that is associated with the extra moisture that can be found in fallowed fields of the Brown and Dark Brown soil zones.

For fallow crops to flourish they must be nourished
There is no question that fallowed fields have the potential for achieving above average crop yields, but only if the normally expected increase in soil N does in fact materialize. More and more often, that simply is not the case. According to Karamanos, “Responses to N fertilization of crops grown on fallow fields can be significant even in drier years and reflect the lower soil test N levels (that are becoming increasingly common on fields that have been frequently fallowed in the past).” Karamanos goes on to point out that an increased emphasis on soil testing of fallowed fields will provide growers with the tools for predicting and correcting the unnecessary yield loses that are associated with these N deficiencies.

Soil testing fallow should be a priority
In fact, it is this writer’s opinion that within the Brown and Dark Brown soil zones, it is actually more important to soil test to assess the N status of fallowed fields than stubble fields. I say this because the fields that are being re-cropped are usually predictably low in available N. Increasingly, we are finding that the N requirements of crops grown on summerfallow are much more difficult to predict. A significant economic concern is the fact that higher yield potential that is associated with fallowed fields may not be realized due to an unexpected shortage of mineralized N. -end-

*John Harapiak has more than 40 years of western Canadian based fertilizer related experience. He will continue to contribute stories to Top Crop Manager.
He can be contacted by e-mail:, telephone: (403) 281-3132.


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