How to read a soil test

It’s a lot of graphs, in­gre­di­ents and chem­istry that at first glance makes lit­tle sense, but read­ing a soil test is im­por­tant farm work.

NZ Lifestyle Block - - Contents - RUTH REN­NER

For the last three weeks I have been on an ex­pe­di­tion through a pile of books on soils. It has been far more ex­cit­ing than I had imag­ined and I'm sorry I've now reached the end in terms of writ­ing about it.

Our ap­proach to fer­tiliser on our farm is usu­ally de­cided af­ter we get the re­sults of soil tests. The ad­vice I've re­ceived gen­er­ally goes some­thing like this:

"Your ph is low, your CEC is good, but your Base Sat­u­ra­tion is a bit low and look, you ob­vi­ously need phos­phate."

I didn't un­der­stand much of that and al­ways knew I should.

A mea­sure of acid­ity Range: 0 -14

The lower the num­ber, the more acidic the so­lu­tion be­ing mea­sured. Most liv­ing things do best when ph is near neu­tral (7.0). NZ soils usu­ally tend to the acid side of neu­tral be­cause of our soils' history and our gen­er­ally wet cli­mate.

For pas­ture, we want to get the ph up to some­where be­tween 5.8 and 6.2. Within that range, most nu­tri­ents our pas­tures and an­i­mals re­quire for their best growth and health are most avail­able which is why it be­comes cru­cial to ad­dress the ph level of your soils. Com­mon prac­tice is to ap­ply both lime and fer­tiliser con­cur­rently to grad­u­ally ad­dress all mea­sured nu­tri­ent de­fi­cien­cies, but the pos­i­tive ef­fect of any fer­tiliser or ‘bi­o­log­i­cal soil im­prover’ won't be fully re­alised un­til the ph is cor­rected.

Agri­cul­tural lime is ap­plied to farm­land to ad­dress soil acid­ity, see page 34.

CEC: Cation Ex­change Ca­pac­ity A mea­sure of the fer­til­ity of your soil

How nu­tri­ents are held or made avail­able in the soil is all to do with elec­tro­static charges as­so­ci­ated with the chem­istry of the el­e­ments.

Soil par­ti­cles have both pos­i­tive and neg­a­tive charges, but more of the lat­ter, giv­ing soil a net neg­a­tive charge. These

charged sites form the soil's ex­change com­plex, at­tract­ing op­po­sitely-charged nu­tri­ent ions. Ions with pos­i­tive charges are called cations (and will be at­tracted to the neg­a­tive soil sites) and those with neg­a­tive charges are an­ions.

Nu­tri­ents dis­solve in the soil so­lu­tion into their con­stituent ions with ei­ther pos­i­tive or neg­a­tive elec­tro­static charges. The soil so­lu­tion is the wa­ter layer around and be­tween the soil par­ti­cles – wa­ter we can't see, but which ex­ists in the soil at or­di­nary mois­ture lev­els. For ex­am­ple, when you dis­solve or­di­nary ta­ble salt (sodium chlo­ride) in wa­ter it be­comes sodium ions and chlo­ride ions.

The more CEC sites there are, the higher the po­ten­tial fer­til­ity of the soil. The higher the num­ber, the more nu­tri­ent-hold­ing ca­pac­ity in that soil.

CEC lev­els are pre­sented in milli-equiv­a­lents/100g, a mea­sure of the charged sites in 100g of soil.

Or­ganic mat­ter and some clays have very high CEC lev­els, whereas sandy soils with low or­ganic mat­ter are usu­ally very low. The in­flu­ence we can have on that as farm­ers is in en­hanc­ing the amount of or­ganic mat­ter con­tent. For­tu­nately that can be as sim­ple as main­tain­ing per­ma­nent pas­ture cover, which con­stantly cy­cles dead or­ganic mat­ter back into the soil.

Base* Sat­u­ra­tion *in this con­text it means al­ka­line, the op­po­site of acid Mea­sures the pro­por­tion of the CEC oc­cu­pied by the ba­sic (al­ka­line) cations: Cal­cium (Ca), Mag­ne­sium (Mg), Potas­sium (K), and Sodium (Na)

Hy­dro­gen is also a cation (pos­i­tively charged ion), but its pres­ence makes the soil more acidic. Hy­dro­gen ions are ex­changed by plant roots for the (al­ka­line/ ba­sic) nu­tri­ent cations. The plant gets its nu­tri­ent and the soil be­comes slightly more acidic.

A lower Base Sat­u­ra­tion means a higher pro­por­tion of the ex­change sites are oc­cu­pied by Hy­dro­gen ions, so you have a more acidic soil.

The ideal Base Sat­u­ra­tion per­cent­ages (of the CEC) for pas­ture are: • Ca, 50-75% • Mg, 5-15% • K, 2-5% • Na, 1-2%

The op­ti­mum pro­por­tions of Mag­ne­sium and Cal­cium may change slightly with dif­fer­ent soil types, be­cause of their ef­fects on phys­i­cal soil prop­er­ties.

Both the ph level and the Base Sat­u­ra­tion of Cal­cium should be used to as­sess the need for lime in your soils. For ex­am­ple, if your ph looks rea­son­able at 5.8, but your Ca per­cent­age is un­der 50%, you will need lime.

A soil with a high CEC will re­quire more lime to raise its ph than one with a low CEC, but once the ph is in the re­quired re­gion, a high CEC soil will be more re­sis­tant to acid­i­fi­ca­tion again. You'll want to en­sure it is main­tained where you need it, but there is more ca­pac­ity for Hy­dro­gen ions to be held by the soil as a lesser per­cent­age of all the cations than if the CEC was lower.

Think of it like adding cor­dial to wa­ter. If you have a glass and put a mea­sured amount of cor­dial con­cen­trate in and add wa­ter, it will be sweeter than if you put it in a big­ger glass with more wa­ter. The cor­dial is Hy­dro­gen, the glass is the CEC of the soil (ie, how much ca­pac­ity it has in terms of the num­ber of neg­a­tive­ly­charged sites in its struc­ture), and the wa­ter is all the other cations. There's only a cer­tain ca­pac­ity and the big­ger the glass, the more cor­dial you can put in with­out mak­ing it sig­nif­i­cantly sweeter. Trans­lated: the big­ger the CEC, the more acid­i­fi­ca­tion can oc­cur with­out im­pact­ing neg­a­tively on the plant grow­ing en­vi­ron­ment.

Lime, Cal­cium Car­bon­ate Di­rectly im­pacts the acid­ity (ph) of the soil

Es­sen­tially, Car­bon­ate takes away the Hy­dro­gen from the soil CEC sites (and the soil so­lu­tion) and leaves Cal­cium ions in its place.

Cal­cium pos­i­tively af­fects soil struc­ture by fa­cil­i­tat­ing soil ag­gre­ga­tion or the way soil par­ti­cles come to­gether to form a good crumbly struc­ture. Good soil ag­gre­ga­tion means bet­ter aer­a­tion and bet­ter wa­ter-hold­ing and drain­ing ca­pac­ity.

The tem­per­a­ture of the soil, im­por­tant to both the or­ganic and the chem­i­cal ac­tiv­ity, will be warmer in win­ter and cooler in hot sum­mer tem­per­a­tures. A well-struc­tured soil drains well, but holds mois­ture when the weather is dry.

Lime rock varies in qual­ity depend­ing on its source, with dif­fer­ences in how much Cal­cium Car­bon­ate it con­tains or how finely ground it is.


Anion re­ten­tion ca­pac­ity may be pre­sented in your soil test and refers to a soil's po­ten­tial to re­tain phos­phate and sul­phate. Soils with high re­ten­tion re­quire a lot of Phos­phate to raise their mea­sured lev­els, but will hold it and buf­fer lev­els if fer­tiliser ap­pli­ca­tion is in­ter­mit­tent.

Mea­sur­ing Phos­phate is done in dif­fer­ent ways. Most com­monly you'll see the Olsen or Resin P tests in test re­sults. The Olsen test was de­vel­oped for use with soils fer­tilised with Su­per Phos­phate. The Resin P test is ap­pro­pri­ate for soils where RPR (Re­ac­tive Phos­phate Rock) has been used, and means you are able to eval­u­ate the yet-to-be-avail­able stores of Phos­phate which have been ap­plied.

The P level re­quired de­pends on how and what you're farm­ing in terms of pas­ture types and stock­ing rates

Here the soil seems nicely struc­tured, but the pas­ture com­po­si­tion tells me the fer­til­ity is not op­ti­mal. Some­thing is lim­it­ing the growth of the grasses I'd like to see here.

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