Greenhouse and Nursery - Direct Measurement of pH in Containers or Pots

There are a number of good quality pH and soluble salt meters equiped with durable probes that can be inserted directly in the growth media of nursery or greenhouse plants. The following are some thoughts on direct measurement of EC and pH from the New England Greenhouse Grower Update.

Use of direct measurement soil probes is a new approach to measuring EC (electrical conductivity or soluble salts) and pH of greenhouse media. Traditionally EC and pH have been measured by collecting media from pots and packs and then analyzing the samples using the 1:2 dilution or saturated media extract methods. However, there are durable soil probes designed for field use that can be inserted directly in the growing medium for quick, accurate, on-the-spot measurements. Along with having immediate test results, another advantage to the probes is that the growing plants are not disturbed and do not need to be sacrificed by sample collection.
Many local growers are trying the probes made by Spectrum Technologies and similar instruments are available from other manufacturers. Obtaining reliable EC and pH test results using the probes starts with proper calibration of the meters. Calibration instructions and solutions come with all meters and they should be carefully used before any crop measurements are made.

Measurements should be made when the growing medium is moist, say within an hour or two of watering. The probes will not be responsive if the soil is very dry and readings may vary between dry and moist containers.

The probes should be inserted at a consistent depth in the containers being tested; for 4- to 6-inch pots about 1" or 2"; deep, larger pots and nursery containers about 3" or 4" deep and for bedding plant packs probably ½" or 1" deep. The probe can be marked at the desired depth with tape or a felt pen. Remember that the EC is generally highest at or near the surface and lower deeper in the container. Measurements taken near the surface can overestimate EC and fertility. Also, readings taken at random depths could be quite different.

The probe should be held still until the meter reading is stable after it is inserted in the mix. The probe must remain in “intimate contact" with the mix and the soil solution while the reading is being made.

EC readings made with a probe will be fairly similar to those obtained in soil testing labs using the saturated media extract test (i.e., most commercial labs serving floriculture and university labs like UMass). Instructions with the Spectrum Field Scout meter say that the EC readings will be about 10-15% higher than typical saturated media extract tests. pH readings made with the probe will probably not differ greatly from current soil testing methods.

Modified from an article by Dr. Douglas Cox in the February, 2006 archives of the New England Greenhouse Update site http://www.negreenhouseupdate.info/greenhouse_update/?p=2428