Why Your Plants Can Starve in Nutrient-Rich Soil (and What to Check Before You Feed Again)

You’ve composted. You’ve fertilised. You’ve added seaweed.

There should be plenty of food in that soil.

So it’s pretty frustrating when the plant sitting in it still looks pale, slow and generally miserable.

The obvious response is to feed it again.

But before you do, there’s another question worth asking.

What if the nutrients are already there, but the plant can’t get to them?

Because having nutrients in your soil and having nutrients available to your roots are not the same thing.

Sometimes the problem isn’t a lack of food.

It’s the soil system that’s meant to deliver it.

Nutrients don’t just jump into a plant

Roots have to be able to grow through the soil, breathe and interact with the biology around them.

If any of those things aren’t working properly, nutrient uptake can slow down even though there’s plenty sitting in the soil.

That’s why continually adding more fertiliser can become a bit of a trap.

The plant looks hungry, so you add food.

Nothing changes, so you add some more.

But if the problem is happening around the roots, another handful of fertiliser isn’t necessarily fixing anything.

There are three things I’d look at first.

1. Is the soil compacted?

Roots don’t grow through solid soil.

They grow through the spaces between soil particles.

Those spaces also hold the air and water roots need to function.

When soil becomes compacted, those spaces get squeezed out. Roots stay shallow, water moves poorly and the amount of soil the plant can explore becomes much smaller.

So you can have plenty of nutrients sitting 10 or 20 centimetres away from a root that simply can’t reach them.

One of the easiest checks is the screwdriver test.

Wait until the soil is moist, then push a long screwdriver into it.

Don’t force it.

If it slides in reasonably easily, that’s a good sign.

If you’re leaning your body weight on it and struggling to get through the first few centimetres, imagine what a tiny new root is dealing with.

Compaction is common in beds that get walked on, clay soils, areas that have been worked when wet and soil that has been left bare and exposed to heavy rain.

If it is compacted, I wouldn’t start by digging the whole bed over.

Push a garden fork into moist soil and gently rock it backwards and forwards. You’re trying to open some pathways without completely turning the soil profile upside down.

Then keep the surface covered and get plants growing through it.

Living roots and soil organisms can do a lot of the rebuilding once you give them somewhere to work.

2. Can water actually move through the soil?

The second thing I’d look at is water movement.

Not whether you’re watering enough.

Whether the soil can actually accept the water and move it down through the root zone.

A simple way to test this is with a large tin, piece of pipe or other sturdy container with the bottom cut out.

Push it a few centimetres into the soil so water can’t escape across the surface.

Pour a measured amount of water into it and time how long it takes to disappear into the soil.

You don’t need to obsess over an exact number.

What’s more useful in a home garden is comparison.

Do the test in one of your best-performing beds and then do it again in the problem area.

If water disappears quickly in one but sits around for ages in the other, you’ve learned something useful.

Poor infiltration can point towards compaction, crusting or weak soil structure.

And at the other extreme, if water races straight through very sandy or tired potting soil and the soil is bone dry again shortly afterwards, that can be a problem too.

Good soil needs both drainage and water-holding ability.

Roots need water, but they also need oxygen.

When the soil stays saturated for long periods, those air spaces fill with water and roots start struggling to function.

That’s why a plant sitting in wet soil can sometimes look strangely thirsty.

The roots are surrounded by water, but they aren’t working properly.

Adding more fertiliser at that point won’t fix the drainage.

3. Is there much biology working around the roots?

This is the part most gardeners never get told about.

A lot of the nutrients in soil aren’t sitting there ready for a plant to instantly absorb.

They’re tied up in organic matter, compost, dead roots and other materials.

Soil organisms help break that material down and cycle nutrients through the system.

Bacteria, fungi, protozoa, nematodes and other organisms all play different roles.

Mycorrhizal fungi also form relationships with roots and extend their reach through the soil, helping plants access nutrients such as phosphorus and water beyond the immediate root zone.

Soil biology is also involved in building structure. Fungi, roots and microbial compounds help bind soil particles into aggregates, creating the pore spaces that allow air, water and roots to move through soil.

You can’t identify all of that biology just by looking at the surface.

That’s what I use a microscope for.

But there is a very simple backyard test that gives you an indication of biological activity.

Bury a pair of 100% cotton underpants.

Yes, seriously.

Cotton is mostly cellulose, and soil organisms break cellulose down.

Bury them in the topsoil, mark where you put them and leave them for several weeks.

Better still, bury one pair in your best-performing soil and another in the area you’re worried about.

When you dig them back up, compare what’s left.

If one pair has been heavily broken down and the other still looks like it could go back in the washing machine, there’s clearly a difference in biological activity between those two soils.

It won’t tell you which organisms are there.

But it can tell you whether much decomposition is happening.

So what should you fix first?

That depends on what you find.

If the soil is compacted, start opening the structure and stop walking on the bed.

If water isn’t moving properly, work out why before adding more fertiliser.

Keep the surface covered, add organic matter where it’s needed and keep living roots growing as much as possible.

And if biological activity looks poor, start creating the conditions that allow soil life to rebuild.

That means organic matter, living roots, less unnecessary disturbance and a soil environment with enough moisture and oxygen for aerobic organisms to function.

You can also introduce beneficial biology directly.

That’s where RootWise Balance fits in.

It’s not another fertiliser.

It’s designed to put beneficial fungi and bacteria back around the root zone, alongside humic acid, fulvic acid and seaweed extract, to support the biological side of the soil system.

It doesn’t replace good structure, drainage or sensible soil chemistry.

Those things still matter.

But if the biology is the missing part, simply adding more fertiliser won’t replace it.

What about pH?

pH absolutely matters too.

If your soil is very acidic or very alkaline, some nutrients can become harder for plants to access even when they’re present.

So if the three checks above look pretty good and your plants are still showing nutrient problems, pH is one of the next things I’d test.

A basic home soil test can give you a useful starting point.

The important bit is not to guess.

Because this whole problem usually starts when we see a struggling plant and immediately assume we need to add something.

Sometimes we do.

But sometimes what the plant really needs is for the soil underneath it to work properly.

The short version

If you’ve fed your plants and nothing much has changed, don’t automatically feed them again.

Check the soil first.

Can the roots move through it?

Can water and air move through it?

Is there much biological activity happening around those roots?

Because your soil can contain plenty of nutrients and still grow a hungry plant.

The problem might not be what’s missing from the soil.

It might be whether the soil can actually deliver what’s already there.

John

Living Earth Biology

FAQs

Can plants really be nutrient deficient when the soil contains plenty of nutrients?

Yes. Nutrient availability depends on more than the amount present. Root growth, oxygen, water movement, soil biology and chemistry can all affect whether plants can access those nutrients.

How do I test soil drainage without digging a hole?

Use a sturdy cylinder such as a large tin or piece of pipe with the bottom removed. Push it a few centimetres into the soil, add a measured amount of water and time how long the water takes to infiltrate. Comparing a healthy area with a struggling area can be particularly useful.

How do I know whether my soil is compacted?

Try pushing a screwdriver into moist soil. If it is very difficult to push in, roots may also be having trouble moving through the soil.

What does the cotton-underpants test tell me?

It gives you a rough indication of decomposition and biological activity. It does not tell you which microorganisms are present, which requires more detailed analysis.

Does RootWise Balance replace fertiliser?

No. Fertiliser supplies nutrients. RootWise Balance supports the biological processes around the root zone that help cycle nutrients and maintain a functioning soil system.

Should I test the pH as well?

If structure, water movement and biological activity appear reasonable but the plants are still struggling, pH is worth checking because it can affect nutrient availability.