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What’s so hard about soil?

When is soil just dirt, why should you care, and what can you do about it? Arborist Mark Roberts digs into this important topic

When I was a young horticulturalist, each and every soils teacher seemed to say something along the lines of ‘soil is soil and dirt is what you get under your fingernails’. Some of them would go on to say that they were taught that, but it ‘reflected their sediments exactly’ – and they wondered why their soil classes were never well attended…

While their approach may not have endeared some horticulturalists, it turns out they were right about one thing – soil science is actually quite interesting and knowing about soil is equally important. I recall an arboricultural conference where a good portion of the international brains trust was gathered, debating whether 92% or 95% of all plant issues could be directly attributed to the soil. Either way, it’s a lot.

So how can this be? How can almost all plant issues be directly attributed to the soil? The answer is water and air – yes, water and air, not fertilisers and soil amendments.

It turns out that almost every physiological process in a plant requires water, and almost all of the water in a plant enters via the roots. Functioning roots need to breath (more or less), therefore roots need air as well. Without water and air in the soil, roots can’t grow and the plant can’t function, it is as simple as that. Without water and air in the soil, fertilisers and soil amendments can’t and don’t

function either – hold that thought.

Back to soil. The ideal soil is made up of 50% solids and 50% pore space, providing space for water and air. A less-than-ideal soil has fewer and/or smaller pore spaces, but the volume of solids may not have necessarily changed. A soil with few and/or small pore spaces is called a compacted soil. Generally speaking, as soil compaction increases, plant function decreases.

How are soils compacted?

There is a certain amount of natural or background compaction that goes on, called soil ‘settling’ if you like. There is also a certain amount of natural or background de-compaction going on, in healthy soils at least. Soil compaction in urban areas is often caused by the use of heavy equipment during construction and constant traffic (foot or equipment). In agriculture, you can add in livestock and repetitive cultivation as probable causes and, in both cases, the rate and depth of compaction can be intensified when the soil is worked on when it is very wet.

How can you tell if your soil is compacted?

Water will drain through an ideal soil (with or without the presence of plants), so compacted soil will generally be an issue in areas where water ponds or is slow to drain.

Where plants exist, signs of compacted soil are slower seedling emergence, slower growth rates/shorter growth increments, shorter plants, uneven coverage, off-colour or purple leaf discolouration.

To give you an indication of soil compaction, try and push a pencil into the soil with a normal, steady force. If the pencil goes in reasonably easily, the soil is not too compact. If you have to push hard to make the pencil go in, then you’ve probably got compaction issues.

A more accurate approach would be to use a penetrometer or have the soil tested for ‘bulk density’.

A penetrometer is a relatively quick and easy option involving a long steel rod that is driven vertically into the ground – the greater the resistance the more compacted the soil is. Resistance can be measured by hammering the rod in (by dropping a set weight onto the rod, from a set distance) or using a penetrometer with a built-in resistance gauge/measurer.

Bulk density is a measurement of dry soil weight divided by volume and measuring it involves taking soil samples of a known size, baking them so they are completely dry then weighing them. Most soils sit between 1.1 to 1.6 g/cm3, while less compacted soils generally having a have lower bulk density (ie, 1.2 to 1.3 g/cm3).

How do you de-compact a soil?

1. Prevention is the best cure

This is one of those ‘prevention is the best form of cure’ things – effectively de-compacting a soil isn’t quick or easy and beware of products that claim otherwise. The first thing to do is get off it and the second thing is not to make mud (working in mud would be the worst thing). Diving in with a rotary-hoe may sound like a good option, but cultivation is also a causal agent and the act of tilling may dice up plant roots and things in the soil you want to keep.

2. Formula applications

Applying calcium, gypsum or some other silver bullet fix formulation will only work if there is sufficient moisture in the soil to enable chemical reactions to take place – so those sort of things possibility work in soils that are not too compacted, which isn’t much help if the soil is quite compacted. It’s also worth noting that chemical reactions also tend to require warmth, so there is little point in using those types of product in the winter months or indeed applying fertilisers or most soil amendments.

3. Soil manipulation options

Forcing air into the soil can be fun, but it is also incredibly messy. You can drill holes in the ground, then fill them with short bursts of high-pressured air – the burst of air fractures the soil and there are ways and means of keeping the fracture-lines open. The theory behind it is good but, in my experience, the results are not great. I believe that the same sort of thing can be done with high-pressured water, once the water drains the fracture-lines are left open – but again, I’d question the long-term effects.

There is a tool called an air-spade, which uses high-pressured air to literally blow away the soil. An experienced operator can work around large roots and cause minimal damage to them. Once removed, the soil is then put back. As de-compaction tools go, air-spades seem to actually work.

On the flip-side, there is a machine called a hydro-vac which sucks soil – in my experience, this is not an effective soil de-compaction tool and is an effective destroyer of roots.

4. Soil amendments

The addition of soil amendments is probably the easiest and best long term method of de-compacting a soil. I’m not talking calcium, gypsum or some silver bullet fix formulation, and I’m not talking about fertilisers – the best way to de-compact a soil is to get and keep the soil healthy.

Get some organic matter on or ideally into the soil (compost, leaf mulch, wood chips, manure etc), get the soil alive and let the things living in it do what they do best – mix, turn and aerate. If the soil is really compacted to start with, you may need to mix some sand in there to assist the process. The downside of soil de-compaction using soil organisms is the time – think in years, not months or weeks.

Once your soil is alive – and only once your soil is alive – can you or even should you consider fertiliser. And never add fertiliser to a soil until you have had the soil tested (chances are it probably doesn’t need any, and the addition of fertiliser will do more harm than good).

So, soil shouldn’t be too hard. But if it is, it’s probably compacted, in which case it’s just dirt.

Mark Roberts is a qualified arborist and tree risk assessor with 25 years’ experience. Mark is also former President of the International Society of Arboriculture, and a past President of the NZ Arboricultural Association. Mark now heads an arboricultural collective based in Dunedin:

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