There's no single soil improver that does everything. The best results come from understanding what each one contributes and choosing the right combination for your soil type and growing goals. Here's an honest guide to the options worth knowing about.
What is Frass?
Frass is a natural by-product of insect farming. It is what remains after insects break down plant material, in our case, Black Soldier Fly. It's rich in nutrients, beneficial microorganisms and chitin - a natural compound that supports plant immunity, stimulates soil biology, and appears to be naturally off-putting to slugs and snails. It has a balanced NPK profile that releases gradually as soil life breaks it down, making it particularly effective for improving both fertility and biological activity at the same time. It's certified organic, completely circular, and one of the most considered soil improvers available.
How to use frass
For seed sowing, a small amount of organic frass mixed into your compost helps create a light, free-draining growing medium. Its natural nutrient profile and slow-release nature support steady, healthy development from the earliest stage - giving seedlings a considered start rather than a forced one.
For beds, borders and pots, frass soil improver can be sprinkled over the surface, lightly worked into the top layer and watered in. It releases gradually as soil life breaks it down, so results build over time rather than arriving all at once. Apply early in the season and reapply every few months as a top dressing to maintain soil biology through the growing year.
For slug-prone plants, applying frass around the base of vulnerable plants like hostas may offer a natural layer of protection. Slugs and snails appear sensitive to chitin, and while frass won't replace a broader approach to pest management, it's a considered addition to a garden that's working with nature rather than against it.
For lawns, apply after mowing and gently rake in for best contact with the soil. This helps the nutrients and microorganisms reach the root zone where they're most needed.
What is Biochar?
Biochar is made by heating organic material at high temperatures in a low-oxygen environment in a process called pyrolysis. What's left is a highly porous, carbon-rich material that creates a long-term habitat for beneficial soil microorganisms. This improves water and nutrient retention, and sequesters carbon in the soil for decades. Unlike most soil improvers, biochar doesn't break down - it remains active in the soil, steadily improving structure and biology over multiple seasons. It works particularly well when used alongside frass or compost, which provide the microbial life that colonises biochar's porous structure.
Biochar, coming soon to Shropshire Seaweed!
How to use biochar
Before applying, biochar benefits from being charged first - mixed with a liquid seaweed feed, compost or worm castings for at least 24 hours before use. Because of its highly porous structure, fresh biochar can absorb nutrients from the soil before releasing them, so activating it first ensures it works with your plants from the start rather than drawing from them.
For seed sowing, a small amount of pre-charged biochar blended into your compost improves aeration and moisture retention without compacting around delicate roots. Its porous structure creates the kind of open, well-structured environment that gives young roots room to establish properly.
For beds, borders and pots, biochar can be worked into the top 10–15cm of soil or mixed into compost at roughly 10–20% by volume. It doesn't break down in the same way organic matter does - it remains stable in the soil for centuries — so think of it as a long-term investment in your soil's structure and biology rather than a seasonal amendment. Apply once and it continues working.
For lawns, apply as a fine top dressing after scarifying or aerating, working it gently into the surface so it reaches the root zone. This is where its water and nutrient retention properties are most valuable, helping the soil hold what it needs through both wet and dry periods.
What is compost?
Compost is the most widely known, and used soil improver for good reason. Made from decomposed organic matter such as kitchen scraps, garden waste and cardboard, it improves soil structure, adds nutrients, feeds soil biology and increases water retention. Home compost varies in quality depending on what goes in and how well it's managed, but even imperfect compost is a valuable addition to any soil. For gardeners who make their own, it's also a free.
How to use compost
As a soil improver before planting, incorporate a generous layer - around 5–10cm - into the top layer of soil in beds and borders. This improves structure, adds organic matter and feeds soil biology ahead of the growing season. Avoid digging it in too deeply, as the most biologically active zone in healthy soil is the top 20–30cm.
As a mulch, spread 3–5cm over the surface around established plants and leave it to be worked in naturally by worms and rain. This suppresses weeds, retains moisture and steadily improves soil structure over time without any disturbance.
In raised beds and containers, compost forms the backbone of a good growing medium. Mix with other amendments - frass, biochar or leaf mould - for a well-rounded, biologically active growing environment.
In compost bays, adding frass to active compost heaps accelerates the breakdown process and boosts the finished product's nutrient profile. A light dusting between layers as you build the heap is enough to make a noticeable difference to the quality of the finished compost.
What are worm castings?
Worm castings, otherwise known as vermicompost, are produced when worms process organic matter through their digestive systems. The result is an exceptionally nutrient-rich, biologically active material that improves soil structure, introduces beneficial microorganisms and supports healthy root development. They can be made at home using a wormery or bought ready to use.
How to use worm castings
Worm castings are gentle enough to use in direct contact with seeds and roots without any risk of burning, which makes them one of the most versatile soil improvers available.
In potting mixes and seed compost, blend worm castings with compost at a ratio of roughly one part castings to four parts compost. The result is a nutrient-rich, biologically active growing medium that supports strong early root development without overwhelming young plants.
As a soil improver in beds and borders, work a handful or two into the planting hole before setting plants in, or sprinkle over the surface and water in as a top dressing. The microbial activity in worm castings gets to work quickly, making them a good choice when you want to support a plant that's establishing or recovering.
As a liquid feed (otherwise known as worm tea) steep a generous handful of castings in a watering can of water for 24 hours, stirring occasionally. The resulting liquid is rich in soluble nutrients and beneficial microorganisms. Water directly onto the soil around your plants every couple of weeks through the growing season for a gentle, consistent boost.
What are green manures?
Green manures are fast-growing plants sown specifically to be cut down and incorporated back into the soil. Different varieties serve different purposes, for example, legumes like clover and field beans fix nitrogen from the air into the soil, while mustard and phacelia add organic matter and suppress weeds. They're particularly popular with no-dig gardeners as a way of building soil fertility between crops without buying in additional inputs.
How to use green manures
Green manures are one of the most low-maintenance soil improvers available. Once sown, they largely look after themselves, and the benefits they deliver are considerable for very little effort.
Sow in autumn on ground that has just finished cropping, or in early spring on bare patches that aren't yet needed for planting. Scatter seed evenly over the surface and rake in lightly - most green manure seeds establish readily without much preparation. Keep the area watered until germination if conditions are dry.
Cut the plants down before they set seed, ideally when they're still young and leafy, as younger material breaks down faster and is easier to manage. Leave the cut material on the surface to decompose naturally, or lightly incorporate it into the top layer of soil. Either way, allow two to four weeks for the material to begin breaking down before planting into the area.
What is leaf mould?
Leaf mould is one of the simplest soil improvers available and it costs nothing but some time and patience to make. Collect fallen leaves in autumn, keep them damp in a simple wire cage or bin bag with a few holes, and leave for one to two years. The result is a dark, crumbly material rich in fungal activity that improves soil structure, increases moisture retention and feeds soil biology.
How to use leaf mould
Leaf mould is one of the few soil improvers that costs nothing to make and requires almost no effort beyond collecting the raw material. The main ingredient, fallen leaves, is available in abundance every autumn, and the process from there is largely a matter of patience.
To make leaf mould, collect fallen leaves in autumn and keep them contained in a simple wire cage, a wooden frame, or a bin bag with a few small holes pierced in the sides. Keep the leaves damp and leave them undisturbed. After one year you'll have a rough, partially broken-down material useful as a mulch. After two years the result is a dark, crumbly, fine-textured material ready to use as a soil improver or potting ingredient.
Understanding Your Soil Type
Before you start improving your soil, it helps to know what you're working with. The four main soil types behave differently through winter and spring, and respond to different treatments.
Sandy soil drains quickly, often too quickly. It warms up fast in spring, which feels like an advantage, but it struggles to hold onto moisture and nutrients, meaning plants can dry out and become deficient more easily. Sandy soil benefits most from organic matter additions that improve its ability to retain both water and nutrition such as compost and frass.
Silty soil is made up of finer particles than sandy soil and holds moisture better, but it compacts easily and can form a surface crust that makes it harder for water and air to penetrate. It's generally fertile but needs regular organic matter additions to maintain its structure and keep it from becoming dense.
Clay soil is the most challenging to work with in spring. It holds moisture well but drains slowly, and can become heavily compacted over winter. It's slow to warm up in spring and can be difficult to dig when wet. The good news is that clay is naturally nutrient-rich. The work is in improving its structure and drainage rather than its fertility. Compost, biochar and frass can help to improve clay soil structure.
Loamy soil is the ideal, a balanced mix of sand, silt and clay that drains well, holds moisture and nutrients, and is relatively easy to work. If you have loamy soil, spring prep is about maintaining what you have rather than correcting a problem. Feeding with our cold-pressed seaweed fertilizer is your best care option here.
Not sure which type you have? Take a small handful of damp soil and roll it between your palms. Sandy soil falls apart and feels gritty. Silty soil feels smooth and silky. Clay soil holds its shape and feels sticky. Loamy soil holds together loosely without sticking.
Which combination is right for your garden?
For most UK gardens, a combination of compost or frass for fertility and biology, biochar for long-term structure and carbon sequestration, and leaf mould or green manures for ongoing organic matter is a well-rounded approach. The exact combination depends on your soil type. Sandy soils benefit most from additions that improve water and nutrient retention, while clay soils need help with structure and drainage above all else.
Our organic frass soil improver and cold-pressed liquid seaweed fertiliser are both available in the shop to help you grow a resilient garden this season. Free UK shipping on orders over £25. Explore our organic soil improvers.
