Agronomy Library > Soil Conservation

Do you plan on Fallowing this Spring?
Author: Don Wentz
Date Created: May 04, 2004
Last Reviewed: February 16, 2007

Producers who summerfallow have several reasons why they leave their land fallow instead of planting a crop. Some fallow for weed control, others for moisture retention and of course we hear some for recreation. Whatever your choice, base the decision on good management practices. With the herbicide choices nowadays, most weeds can be controlled simply by spraying rather than with tillage. Summerfallow is an inefficient method of storing moisture. Research from the Lethbridge Research Station in the past showed that only 25% of the moisture that falls during the 18 month fallow period is stored in the root zone and used by the crop. The rest runs off, evaporates (especially when you use tillage for weed control) or worse yet, moves deeper into the profile to create a water table. Water tables are associated with the recharge/discharge ituations that cause dryland salinity. As for recreation, try golf.

In the 1960’s, Dr. Paul Brown from Montana developed and patented a probe that was designed to help producers make better decisions when deciding to fallow. The probe is a little over 1 meter (4 feet) long. It is marked in 30 cm (1 foot) increments. The rod is made of rolled steel and coated with a non-rust material. A T-handle is welded on the top. The bottom has a ball bearing and a 2.5 cm (1 inch) long drill bit brazed onto it. The ball bearing helps to pull the probe out of wet soil. The drill bit is used to determine if the probe is in dry soil. The probe is designed to be pushed into the soil as far as moist soil goes and stop. Give the probe a couple of turns to bring up a small sample of the soil to insure you have reached dry soil and not a rock. Many farmers with a Brown Probe put a ribbon around it, when they push it into the soil the ribbon automatically stops when the probe stops. This way they know the depth of moist soil when the probe is removed.

Knowing the depth of moist soil will allow you to determine the plant available water in the soil. To calculate this you have to determine the texture of your soil. Generally if you know your soil is fine, medium or course, that will be sufficient. A clay soil (fine) holds about 2 inches per foot, a loam soil (medium) holds about 1.5 inches per foot and a sandy (course) soil holds about 0.75 inches per foot. There are many charts available that gives more accurate information on plant available water per foot. It is recommended that several sample areas be taken and the results averaged. It is not uncommon to have a wide variation across a field. Once you know the stored soil moisture level, there is also data available that gives growing season precipitation. Alberta Agriculture, Food and Rural Development has just published “Agroclimatic Atlas of Alberta, 1971 –2000, Agdex 071-1. Page 55 gives average growing season precipitation from May1 to August 31. If you divide mm by 25, it converts closely to inches. For example, just east of Calgary, the average precipitation is between 225 and 250 mm. Divide by 25 gives 9 to 10 inches. To determine what moisture level is available for crop growth; add plant available water from the soil and average precipitation.

The next step is to determine what yield level you are comfortable with. Dr. Brown also published a paper called “Grain Yields Related to Stored Soil Water and Growing Season Rainfall”. He has a high and low probable yield formula for winter wheat, spring wheat, barley, oats and safflower. For spring wheat, the high formula is Y (yield in bushels per acre) equals 5.1 times total water available minus initial yield point (3.8). The initial yield point is the level of moisture needed to get a crop up and out of the ground before any yield is produced. So an example of how to determine a potential yield of spring wheat, if you have a loam soil and the moisture is 2 feet deep, you have 1.5 times 2 equals 3 inches of available water. For the example east of Calgary where there is 9 to 10 inches of growing season rainfall, being conservative gives 12 inches of total moisture for crop use. Yield equals 5.1(12 – 3.8) which is 41.8 bu/ac. This yield level makes the assumption that fertilizer is adequate to meet the needs of the crop. I will publish Dr. Brown formulas on the WEB site under southern region. Check it out if you wish. It should be noted that there is risk associated with using average rainfall levels. If rain comes in small quantities it may not be beneficial to crop growth. As well, rain that comes late in the summer may be too late to help a matured crop. However, average rainfall combined with stored soil moisture calculations can also help in determining proper fertilizer levels to apply to your soil. It is one management tool amongst many. Check the AAFRD WEB site for AFFIRM, a more in depth fertilizer calculation formula to determine yields.

(1) Soil Water Guidelines and Precipitation Probabilities in Montana and North Dakota. Montana State University, Extension Service Bulletin 356, Reprinted January 1988.
(2) Agroclimatic Atlas of Alberta 1971- 2000. Alberta Agriculture, Food and Rural Development, Agdex 071-1
(3) Grain Yields Related to Stored Soil Water and Growing Season Rainfall, Montana State University, Special Report 35, November 1990.

 Also visit "Summerfallow and Soil Conservation"