Monitoring and managing soil pH

by David Vernon

Now some ideas to monitor and manage soil pH levels. Before any crop is established a soil sample should be analysed to determine the nutrient levels, soil health, and what may need to be added to maximize crop yield. For the cost of a few bags of fertilizer you can establish your soils ability to give you the best possible outcome - yet this is still not carried out enough.

Historically, adding lime to most horticultural soils was a standard pre-plant practice. Now, with the higher demands being placed on our soils productivity and the increasing salinity of our water sources, the addition of lime to balance the pH may not be necessary. One of the trends in intensive farming is that our soils are starting to become more saline, mainly from the residues of concentrated fertilizers, regular irrigation with increasingly salty water and infrequent rainfall. Lime or dolomite is used pre-plant to help bring the soil pH range to the desired level necessary for maximising crop production. It is also the most cost effective method for balancing soil pH, but its difficult to know what is required of your soil without a soil test?

When faced with a soil that is alkaline or neutral what can be done to keep the pH at the desired level and how do you monitor it?

 You can use one of two simple methods to measure soil pH during cropping, (both require the use of a quality pH meter regularly calibrated with standard solutions), 1) taking a soil sample from around the root area, or 2) collect a sample of soil solution from a vacuum tube placed in the root zone.

Soil samples are accurate but a little more time consuming. Take a soil sample (preferably air dry it overnight), mix it with de-ionised water at the rate of 1 part soil to 5 parts water (20g soil:100mls water), mix it and wait 30 minutes then measure with the pH meter.

With the soil extraction method you will need to install vacuum tubes in the crop at working root level. The tube is charged with a vacuum via a syringe at the commencement of the irrigation cycle and then collected the following morning from the tube with the same syringe. The solution collected is then measured with the pH meter. This is an emerging method for data collection and is yet to be widely accepted in the general Agricultural community, however early results are promising.

With high levels of fertilizer and other soil additives being used in cropping the eventual effects on soil chemistry can be varied. Most products we add to the soil have an effect on the soil chemistry, and to understand the changes that occur! The only way is to monitor.

Soil extraction tube

Pre-plant nutrition will do one thing, but many farmers now fertigate as a form of side dressing to supplement crop nutrient requirements during growth. So how can you determine what each effect will be?

By monitoring soil pH growers, can plan what action or requirements are to be carried out to keep the soil within the desired pH range for your crop. Your fertilizer company or crop agronomists should be able to tell you what the effect of adding different products will have on your soil, particularly if you have a recent soil test.

If you have a neutral or alkaline soil you may need to be adding acidic fertilizers. You could also be utilising inter-season cover crops to increase organic matter and endeavouring to establish farm practices that minimize delivery of any form of residual salt to your farm.

This topic is becoming more of an issue as farmers have greater demands being placed upon them to deliver cost competitive, quality produce, with water sources that are declining in quality. Infrequent rain events are now not flushing the soil profile as they once were. Plan ahead, use the experience of professionals in this field and do not skimp on regular soil testing and monitoring.

Two good references for further reading are - Understanding Soil Ecosystem Relationships - published and available from Queensland Department of Primary Industries, and the Fertilizer Handbook, first printed by Consolidated Fertilizers now available from the CSIRO as the Australian Soil Fertility Manual.