Understanding soil pH

1 To understand soil acidity you need toappreciate that soil is made up of variouscomponents which determine its include mineral particles (sand, silt and clay,which give soil its texture), organic matter (livingand dead), air and water component is where pH is measured,where dissolved chemicals cause the soil to beacidic or acidity and alkalinity are measured in unitsof pH. The pH scale is from 0 (most acid) to 14(most alkaline) and a pH of 7 is soil pHLEAFLET pH of a soil will change over timeinfluenced by factors including parent material,weathering and current agricultural practices. Itwill also fluctuate through the year. soil pH willaffect how plants soil pHSoil pH can be measured in the field using atest kit or by sending a sample to a laboratory formore accurate results. The standard depth ofsampling is 10 cm. soil pH in the field can be measured using asimple test kit based on a colour-card methodavailable from agricultural supply stores calledthe Raupach soil pH kit.

2 The kit gives the soilpH on the water scale (see later) and should beused only as a guide to soil pH. Analysis in a laboratory provides the mostaccurate measurement of soil pH. It is the bestbasis we can have when deciding whether or notto start an acid soil management strategy such further details on collecting a sample andwhere to have it analysed see Acid soil ActionLeaflet No. 1. Plant growth and pH (CaCl2) sensitive barleystruggling in acid soil next toa healthy tolerant lupin too acidfor sensitive plantsIncreasing alkalinity leads to some plant nutrientsbecoming unavailableFIGURE 2. Optimum pH (CaCl2) range up to pH for a number of crops and pastures. Above pH theacidity/alkalinity is no longer the principal factor that controls growth. Other factors such as phosphorus,zinc, cobalt or boron deficiency or the sodicity of soil are most likely to affect production. CROPS pH (CaCl2) PASTURE34 56789 Barley: Schooner & Yerong O Conner & Skiff BrindabellaClover: Balansa, Berseen & Persians Red, Caucasian & Kenya Haifa white & SubterraneanCanolaCereal RyeChickpeasFaba BeanLinseedGrasses: Cocksfoot Couch Buffel Kikuyu Tall wheatgrass red grass (Wagga) wallaby grass (D.)

3 Linkii) consul love grass perennial ryegrassLupins: Narrow leaf Broad leaf (Albus)LucerneMaizePhalarisMilletSerrade lla (Yellow & Slender)TrticaleOatsSorghumSoybeansWheat : Durum Rosella & Janz DiamondbirdThe two main laboratory methods used inAustralia use calcium chloride or pH in calcium chlorideThis is the standard method of measuring soil pHin all states other than Queensland. An air-dry soilsample is mixed with five times its weight of adilute concentration ( ) of calcium chloride(CaCl2), shaken for 1 hour and the pH is measuredusing an electrode. The results are usuallyexpressed as pH(CaCl2). soil pH in waterDistilled water is used in place of calciumchloride, and results are expressed as pH(w).The pH(CaCl2) test is the more accurate of thetwo pH tests, as it reflects what the plantexperiences in the soil . The values of pH(CaCl2)are normally lower than pH(w) by to Auseful, but not consistently accurate, conversion isto subtract from the pH(w) to obtain apH(CaCl2) value.

4 The difference between themethods can be significant when interpretingresults and it is important to know which methodhas been used, especially if pH figures derivedsome years apart are being compared to assess anypH soil pH RESULTSSoil pH effect on plant responseA soil pH(CaCl2) of to provides optimumconditions for most agricultural plants (Figure 1).All plants are affected by the extremes of pH butthere is wide variation in their tolerance of acidityand alkalinity. Some plants grow well over a widepH range, whilst others are very sensitive to smallvariations in acidity or alkalinity. Figure 2 providesa guide to the preferred pH(CaCl2) for somecommon crops and activity in the soil is also affected bysoil pH with most activity occurring in soils of to Where the extremities of acidity oralkalinity occur, various species of earthworms and nitrifying bacteria disappear. Legume rootcolonising bacteria (Rhizobia) vary in theirsensitivity to soil pH and have preferred ranges inwhich they are effective.

5 In some crops andpastures ( faba beans and lucerne) the Rhizobiaspecific to these plants are more sensitive than theplant pH effect on availability of soil nutrientsSoil pH affects the availability of nutrients andhow the nutrients react with each other (Figure 3).At a low pH, beneficial elements such asmolybdenum (Mo), phosphorus (P), magnesiumNITROGENPHOSPHORUSPOTASSIUMSULF URCALCIUMMAGNESIUMIRONMANGANESEBORONCOPP ER & (CaC12)STRONGLY ACIDMEDIUM ACIDSLIGHTLY ACIDVERY SLIGHTLY ACIDSTRONGLY ALKALINEMEDIUM ALKALINESLIGHTLY ALKALINEVERY SLIGHTLY ALKALINEFIGURE of pH (CaCl2) on the availability of soil nutrientsThe effect of aluminium toxicity on theroots of Condor wheat. From left to right,the plants were grown in solutionscontaining 0, 10 and 20 ppm leaflet is one of a series on Acid soil Management, prepared for the New South Wales Acid soil Action Program by Belinda Lake, Acid Soils Project Officer, Yanco, June 20002090 DISCLAIMER: The information contained in this publication is based on knowledge andunderstanding at the time of writing (June 2000).

6 However, because of advances in knowledge,users are reminded of the need to ensure that information upon which they rely is up to dateand to check currency of the information with the appropriate officer of New South WalesDepartment of Agriculture or the user's independent adviser.(Mg) and calcium (Ca) become less available toplants. Other elements such as aluminium (Al),iron (Fe) and manganese (Mn) may become moreavailable and Al and Mn may reach levels that aretoxic to plants. The changes in the availability ofnutrients cause the majority of effects on plantgrowth attributed to acid soils. Sensitive crops suchas barley and lucerne can be affected by smallamounts of exchangeable , knowledge of the soil pH andassociated aluminium toxicity is vital beforeplanning to sow crops or contrast, when the pH(CaCl2) is greater , calcium can tie up phosphorus, making it lessavailable to plants. Additionally, alkaline soilscause zinc and cobalt deficiencies that lead tostunted plants, poor growth and reduced yields insome crops and pH trends over timeMonitoring pH changes over time is an importantmanagement tool.

7 By comparing past and presentsoil tests, it is possible to see if the soil acidity isincreasing over time and, if it is, to altermanagement methods to prevent this trend INFORMATIONFor further information on Understanding soilpH, see the NSW Agriculture publication soil acidity and liming,and Acid soil ActionLeaflets 3 to 6, and the chapter on soil chemistry inSoils: Their Properties and Management,P. E . and Murphy (eds), second edition,(Oxford University Press, Melbourne).Belinda LakeYanco Agricultural InstitutePhone (02) 6951 2629 June