Understanding Pukekohe Volcanic Soil

Shane Smith, Independent Agronomy Consultant NZ

Over the last twenty years tractors and the Horse power they produce has greatly increased and with it the misconception that deep cultivation is better. This has resulted in a poorer soil structure which is harder to work and has resulted in growers spacing cultivation tines out further in order to keep working the soil deep! Well I’m here to tell you that your Father and Grandfather did a better job!

In the last 15 years many growers in other countries have had to review their cultivation practices radically or risk going broke. Growing and compliance costs have increased dramatically without a lift in grower returns forcing cost cutting and better practices such as min till, GPS controlled machinery and better cultivation practices.

These cultivation practices involve less Horsepower doing more work more efficently and with less compaction while doing it!

Understanding the physics of soil cultivation and compaction is a good starting point because you don’t need a lot of horsepower to do a good job if you understand the problems and the process.

First of all after harvesting your crop the top 75mm is by far the most compacted area and needs time to recover and weather the most. The first mistake many growers make is to try and do deep cultivation straight away. This top 75mm takes a large proportion of horsepower to move through before even doing any good lower down the profile plus how do you move deep compaction when you have enormous pressure from the top of the profile pushing down? So the first thing to do is remove the surface compaction.

1) Using a number of options, Discs, Chisel plough, Cultivator remove the surface compaction 75mm roughly allowing the top of the soil profile to weather and breath. To do this depth control is critical and must be an option on each machine.

Now after leaving the soil to weather a little you need to concentrate your efforts on the deeper issues. Get your spade out dig down to the subsoil and establish how much and where your compaction lies above the subsoil ( In this situation Pukekohe Volcanic clay soils) Usually the mid soil profile is not too badly compacted and often there is a compaction layer just above the subsoil. This layer can vary from 50mm to 100mm depending on previous cropping and previous cultivation techniques.

Now that you have determined where you compaction is we can address it. Physics would say cultivating the mid soil profile before the lower soil profile would make things easier again but what happens is the mid profile becomes very aerated and loose which is good but it does not provide a good platform for the tractor to work on and traction is an issue. So leave the mid soil profile alone and attack the deep compaction at this point. Also a very important area to think about is the distance between tines on your cultivators. A good rule of thumb is to start at the top with tine spacing 225mm-250mm from point centre to point centre, then move down the profile using a tine that is 350-400mm and then finally a ripper or so call sub soiler at 450-500mm this way you’re not missing any areas of the soil profile or leaving wedges as I call them.

If your soil profile has good organic content and has been cultivated thoroughly and has little compaction then the close tine cultivator can be used to go deeper but do remember these type of cultivators are meant to work at speed and shatter clods, it’s not a ripper!

2) Using a chisel plough or Ripper with tines as close as possible 350-450mm drop the cultivator tine just below the compaction layer, try not to get into the clay layer as much as possible. To do this again we must have depth control and know our soil profile depth.

Also with this cultivation its best to take a shallow angle across the paddock so you are not following the previous cultivation straight.

For the third and final cultivation pass depending on your soil profile use a close 225-275mm tined cultivator at high speed to move through the mid soil profile area and bring any large clods to the surface to weather and break down.

3) Using a close tined cultivator 225mm-275mm move through the mid soil profile area at high speed preferable with a straight bar clod roller on the back for depth control. This cultivation should be done straight in line with the rows as the final pass before seedbed formation.

These three cultivation passes should have some time between them so that clods begin to break down and your next cultivation pass is more effective, this is not always possible.

Now your soil profile is without compaction aerated and loose. Depending on the next crop you can Power Harrow, or hoe to form beds or cultivate again with a close tined cultivator to produce a fine tillth seedbed. The less PTO powered cultivation use the better. This helps your long term soil health.

Each paddock and each soil type vary in how they compact with traffic and you must closely examine you particular situation. However the physics does not change and if you keep these in mind you won’t go far wrong.

In some situations where a soil has been compacted for several years above the clay subsoil, the clay itself will compact very slightly. In this instance running a subsoil cultivator 450-500mm tine with 50-75mm in the clay can be beneficial but generally there is no benefit in doing this. Pukekohe Volcanic soils are incredibly resilient and the clay drains very well. It breaks up very easily with dry and wet events. If the soil directly above the clay is maintained in good order and not compacted the clay does not need to be touched, you’re just wasting diesel!

Most New Zealand soils are very shallow 225mm-350mm in depth and therefore there’s no need for deep ripping machinery with a lot of Horsepower. Large tractors cultivating shallows soils is never going to be ideal so think carefully before you buy your next TOY!