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.
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.