After rain should I irrigate?

Do I need to irrigate? After it had not rained from early October to mid-February (except a couple of 0.08 inch events in January), the soil here is dry.  I dug a post-hole and could not find moist soil even to a depth of 18 inches.  AND THEN it rained 0.66 inches as measured in my rain gauge.   So, how deeply did this moisten the soil?  In my soil I measured 5.5 inches of moist soil.  I used a “Brown Probe” developed by Dr. Paul Brown, USDA researcher in Montana, to help dryland farmers determine if they had enough soil moisture to grow a crop.  I also dug a hole and measured with a yard stick.  The measurements agreed.  The Brown probe is the easier way to measure the depth of moist soil.

When using a soil moisture probe such as the Brown probe, you push the probe into the soil and determine the depth it penetrated before stopping.  It will stop when it hits dry soil, a rock, a pipe, or a large root.  This is not as valid a reading in soil that has been rototilled or recently turned with a spade or garden fork.  To assure an accurate reading probe in several locations to be sure you didn’t  hit a rock, pipe, or root.

It is also important to measure in an appropriate area.  Near areas of roof runoff (water harvesting), recent irrigation, or low areas where water collects, the measured depth of moist soil.  This is good if that is where you are gardening, but if you want to know the benefit of the recent rain over the larger area, measure in an area where water does not collect from other areas and from an area that is not so steep that the water runs off before soaking in.

Now I know that I still need to irrigate.  At this time of year (February) in Central New Mexico tree buds are becoming active and stimulating root growth in preparation for the coming growing season.  I must moisten an appropriate depth for the trees.  I have also begun turning my garden soil with a garden fork and find it much easier to turn the soil if it is moist.  Dry soil is rock-hard and hard to turn.

Paul Brown soil moisture probe inserted into soil to determine depth of moist soil
Paul Brown soil moisture probe
Brown probe removed from soil next to inches scale to show depth of moist soil
Soil moisture depth 24 hours after first significant rain in 4.5 months
Meter stick in hole showing corroboration with Brown probe moist soil depth determination
Soil moisture depth by digging

The line between the lighter soil below the yard stick indicates the “dry line”.  The angle I had to hold the camera makes it look like the depth measurement is 6 inches, but that is a parallax error.  The true depth is 5.5 inches.

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