Almost every week I get a phone call from someone living in Eastern Australia asking the same question, “What`s the best fertiliser for my pasture?” Most are surprised when instead of directly answering, I fire off a few questions of my own. “Where are you? What is your basic soil type? How much rain do you get? How many animals do you run? How many animals would you like to run? What is the soil pH? Have you done a soil test? and if so, what is the PBI?…….” “The what?’
If you ask this same question to someone in the ag retail sector, the answer will very likely be DAP, and to be fair, in some certain circumstances, they will be right. DAP certainly offers phosphorus at the lowest price per kilo, and it comes with a healthy wedge of nitrogen as well, so that`s all good, BUT, what if your soils aren`t in the pH goldilocks zone? “Doesn`t really matter…….does it?”
A visual prompt on how quickly P availability reduces as pH decreases
Redrawn from “Balancing risk and reward with high nitrogen and phosphorus costs” GRDC 2022
Unfortunately, it kind of does matter. The thing that lots of people choose to forget in this situation is that soils with pH`s below 6 and above 7.5, that also have a high clay content, begin to exhibit this characteristic called phosphorus tie up. Throw in high levels of iron and/or aluminium and it gets worse again. What this means is, that if your soil fits this description it might only allow your DAP a few short months to have it`s cheap phosphorus available to the grass. What is not used during this time (The experts say it`s between 70 and 85%) is tied up. Locked away where it can no longer be used. Not so cheap anymore?
But how will I know?…….. enter the PBI. This little calculation, that stands for Phosphorus buffering index, should be included in any good soil test. What it tells us is how aggressively your soil will steal your hard earned phosphorus fertiliser away from the pasture. And it`s so simple, if it`s below 140 there is nothing to worry about, from 140 to 280 you will need to alter your strategy, and above 280 you have got some serious thinking to do. The old wisdom when dealing with high PBI soil was to simply put more phosphorus on. Instead of 20Kg/Ha, make it 40, or 60, or even 80Kg/ha. This strategy however, has two problems, firstly it is incredibly expensive and wasteful, and second, in many parts of Queensland that are subject to reef regulations it is now against the law.
The good news is that if you do have soil with a high PBI there are ways to mitigate the problem. The first thing to do is educate yourself on the subject. There is heaps of information available that will help you understand how this all works, and armed with that knowledge you can then make better informed choices. From my viewpoint though, there are two main options, one is to change the number of DAP/Superphosphate applications per year, from one big one to four or five smaller ones. Option two is to choose a product that is less influenced by these curious soil forces; and they do exist.
Some natural products like Guano for instance, will dissolve in soil over long periods of time, kind of like a natural slow release process, that will supply the grass with a slow and steady amount of phosphorus. Coincidentally, this is just how perennial pastures like their nutrient delivered. Not feast and famine, but steady and even, and by any measure this must surely result in more of the phosphorus you pay for actually making it into the pasture. Of course guano is not the only product that can help, and it`s not the full picture by itself either, but it is an example of how we can do better.
So before you make your next fertiliser purchase, do some reading, ask some questions (of us at Katek, or others as well) and look beyond the idea of measuring fertiliser value simply by the price.
Our Sales Manager Andrew Weeks would love to answer any questions you may have about our products and help you choose the right product for your fertiliser program or customise a blend to suit your soil needs.
For more information or to chat about product options,
GIVE ANDREW A CALL ON 0427 223 307.
