Soil amendments, and which one your garden actually needs

Start with what your soil is actually doing

If one bed sulks all year while the bed beside it does fine, the difference is nearly always under your feet rather than anything you sprayed on top, and what fixes it is a soil amendment rather than another round of fertiliser. An amendment changes how the soil behaves, so it either holds water for longer or lets water through faster, and it feeds the microbes that make nutrients available to a root, which is a different job from feeding the plant directly.

Work out which problem you have before you buy anything

There is no all-purpose amendment, so the first job is to dig a hole about a spade deep and have a proper look at what comes out, because sand that drains in seconds and clay that stays wet for days want opposite treatments, and picking the wrong one costs you a season. Tip a couple of litres of water into the hole and watch what it does. If it vanishes almost straight away then you are short of organic matter, and if it sits there in a puddle while the sides of the hole stay slick, you have a clay or a compaction problem instead.

Gypsum only works on some clay, and there is a test for it

Gypsum gets recommended for every heavy soil in the country, and it genuinely only works on dispersive clay, which is clay held together so weakly that it falls apart the moment it meets water, so it is worth spending one night finding out before you spread a 20kg bag over the whole yard. Take a dry crumb of your soil about the size of a pea, drop it into a glass of clear water, and leave it alone overnight without stirring it. If the water around it goes milky and the crumb slumps into a cloudy halo, that clay is dispersive and gypsum will pull it back together into crumbs that water and roots can move through. If the water is still clear in the morning then gypsum will do nothing for you at all, and what that soil actually wants is organic matter and mulch, plus a fair bit of patience.

The ones worth knowing

• Compost. The all-rounder, and the one to reach for when you are not sure. It adds organic matter, which holds moisture in sand and opens up clay, and it feeds the soil life that does most of the work for you.
• Blood and bone. Slow-release nitrogen and phosphorus, so it feeds over weeks instead of in one hit. Dig it in before you plant and keep it off seedling roots.
• Mulch. It is not really an amendment on the day you spread it, but it breaks down into one, and in the meantime it keeps the surface cool and stops the top 50mm drying out between waterings.
• Seaweed concentrate. Not a fertiliser, whatever the shelf makes you think. It is a root tonic, and it earns its place at transplanting time or after a hot week.
• Worm juice. Diluted down to about the colour of weak tea and watered on, it is a gentle way to get microbes into a tired bed without burning anything.
• Gypsum. Only on dispersive clay, and only once the glass test above has told you so.

Putting them on

Most amendments belong in the top 100mm or so, because that is where the feeder roots live and digging any deeper mostly wrecks the structure you are trying to build. Work the compost and the blood and bone in before you plant, then mulch over the top and leave a few centimetres of clear ground around every stem so nothing sits wet against it. Twice a year is plenty for a bed that gets used hard, and if you only manage it once a year you will still be a long way ahead of where you started.

Shop related products

• Brunnings Ultimate Garden Soil 30L. Bagged organic matter for topping up a tired bed.
• Brunnings Blood and Bone Based Fertiliser 5kg. Slow-release feed to dig in before planting.
• Seasol Seaweed Concentrate 1L. The tonic for transplanting and for hot weather.
• Neutrog Seamungus Liquid Concentrate 1L. Seaweed and manure in a liquid, for a general pick-me-up.

Did you know we also stock gypsum in 10kg and 20kg bags, bagged compost, lucerne and sugar cane mulch, pea straw bales and worm juice in the garden centre, and most of that is not on the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

More from the garden centre

All of this comes out of the garden centre, and if you are not sure which bag you want, bring a photo of the bed in and we will talk it through with you in the shop.

Keep reading

• Compacted soil, what it does and how to fix it
• How to compost kitchen scraps for a veggie garden
• Homemade plant food, what actually works and what does not

I dig a hole and have a look before I believe anything a bag tells me, and I'm still learning, same as everyone.

Candeece