Fallow is a term used to describe a farming technique of leaving a field unplanted for a growing season.  Anciently, fallowing land once in every seven years was a tradition practiced by the Hebrew people to “rest the land”.  In modern times, fallowing is still used primarily as a moisture conserving technique in semi-arid cropping areas.  It is usually called “summer fallow” and is done to conserve sufficient moisture in the soil in order to grow a reasonably high yielding crop the following year.  It is not uncommon to have a two-year crop-fallow rotation in these dry areas. 

Summer fallowing has also been used in the Northern Great Plains (NGP) region, over the past years, as a means of controlling weeds, and mineralizing nutrients, primarily N from soil organic matter and crop residues to aid in crop production.  The advent of selective herbicides for weed control, and use of inorganic fertilizers as a nutrient source have reduced summer fallowing use for weed control or for supplying nutrients.  For example, in the NGP use of summer fallowing peaked in the early 1960s at up to 30% of land cropped to annual crops, and has been reduced to a present level of 11.5% (12.5 million acres) of total land used for annual crops (109.5 million acres) as reported in the latest available census data.  For the states and provinces in the NGP, the percent of land used for annual crops that is summer fallowed is 27, 3, 3, 16, and 9% respectively for Montana, North Dakota, Manitoba, Saskatchewan, and Alberta.  The driest areas where summer fallowing is used for moisture conservation are in northern and eastern Montana, southeast Alberta, and southwest Saskatchewan.  In these areas, summer fallow is regularly used and it is well integrated in to the existing cropping systems.

However, when a farmer is forced to summer fallow land due to adverse weather conditions there may be some questions as to what to plant for the next crop and how nutrients should be managed.  Some growers in eastern Saskatchewan are considering planting winter cereals, mostly winter wheat, even though they would normally plant spring seeded crops.  Also, there is a question as to what rate of fertilizer to use for the subsequent crop after the unplanned year of summer fallow.  Most of the farmers in the affected area apply fertilizer for their crops in two different ways.  First, around 20% of them fall-apply N in the form of anhydrous ammonia injected to about a 4 in. depth in bands, and in the spring at planting apply the remaining P, K, S, along with some N, and any other required nutrients in the seed-row or starter blend.   Secondly, around 70% of farmers apply all the fertilizer at the time of planting in a precision side-band operation about 1.5 to 2 in. to the side and about 1 in. below the seed row, usually using no-till or reduced tillage. 

Under normal annual cropping, the rates of fertilizer used are based on crop removal nutrient amounts minus regular residual plant-available nutrients as determined through soil sample analysis or from area experience, plus nutrients mineralized from soil organic matter and previous crop residues.  Thus, the nutrient rates in applied fertilizer are used to supplement crop nutrient requirements, so yields are not limited by nutrient availability deficiencies.  However, when an unplanned year of summer fallow occurs there is some uncertainty as to what rates of fertilizer to use for the next planted crop. 

The most appropriate way to assess residual nutrient levels is to take soil samples and have the samples analyzed.  This is especially important on fields that have experienced excessive rainfall.  Both N and S are very mobile in soil as the nitrate (NO3-) and sulfate (SO4-2) ions, respectively.  These ions move with water that is leached through saturated soils, and can be moved either deep within the rooting zone of crops, or even below the normal rooting depth.  This then requires that soil sampling depth be greater than normal.  For example, usual soil sample depths are 0 to 6, or 0 to 12 in., with some sampling procedures including an additional depth of 6 to 24, or 12 to 24 in. respectively.  However, after excessive moisture it may be helpful to sample to the whole rooting depth of the subsequent crop in three increments, i.e. 0 to 6, 6 to 24, and 24 to 48 in. 

Another reason to accurately determine residual nutrients – especially N – is that under excessively wet soil conditions gaseous losses of nitrate-N can occur as soil bacteria requiring oxygen, normally obtained from air in the soil, use the oxygen in the nitrate ion and the remaining N is emitted from the soil into the air mostly as nitrogen gas (N2), along with some as nitrous oxide gas (N20).  This loss process is called denitrification.  These denitrification losses of N can be quite large.  For example, if the farmer had applied the majority of fertilizer N in the fall previous to the planned planting, e.g. 90 lb N/A as anhydrous ammonia and then the next spring planting was not possible due to excessive rainfall, the majority of the applied N could have been lost as described above.  The only way to accurately determine to what extent the N was lost and what proportion remains is through soil sampling and analysis.  There is potential to add excessive N and other nutrient fertilizers if soil residual nutrients are not considered.

Unplanned summer fallowing due to excessively wet conditions during planting is an unfortunate event, but it does happen occasionally in the NGP.  It is important to accurately assess residual nutrient levels before deciding on nutrient rates when fertilizing for the subsequent crop.