How Compost Help Garden Soil

Compost is finished when it’s a dark, rich color, crumbles easily, and you can’t pick out any of the original ingredients. It should have a sweet, earthy smell. If it’s too stringy or lumpy, it may need more time. If this is the first time you’ve tried making compost, keep in mind that the amount of time can really vary. It can take anywhere from three to 12 months to produce compost. Decomposition depends on a number of things including temperature, what organic matter you’ve filled your bin with, type of compost bin used, how fine the waste material was chopped, how often you’ve turned it, and more (see The Biology of Composting).

Once you have achieved finished compost, you can add it to the soil any time of year without the fear of burning plants or polluting water. The benefits of using compost are numerous. It builds good soil structure; enables soil to retain nutrients, water, and air; protects against drought; helps maintain a neutral pH, and protects plants from many diseases commonly found in the garden. It also feeds earthworms and other microbial life in the soil. In general, it doesn’t matter what kind of soil you have. All soils can be improved with the addition of compost.

One easy way to apply compost is to mulch with it. Spread the compost in a thick layer on top of exposed soil. Worms and other creatures will help the compost meld with the soil. Mulching is not only an easy way to apply compost but also keeps down weeds and helps your soil retain moisture. Don’t have a garden? You can also use compost when potting indoor plants. Use seven parts soil to three parts compost to two parts sand.

• Compost contains nutrients that your plants need for optimum growth, such as nitrogen, phosphorus, and potassium. And it’s an especially good supplier of micronutrients that are needed in small quantities and are sometimes overlooked by gardeners, such as boron, cobalt, copper, iodine, iron, manganese, molybdenum, and zinc. The more varied the materials used to make the compost, the greater the variety of nutrients your compost will provide. In some situations, you will not even need to fertilize soil enriched with compost.
• Nutrients are released at the rate your plants need them. In early spring, as your plants are slowly starting their growth, the microorganisms in compost are slowly releasing nutrients. As the weather warms up and your plants begin rapid growth, the microorganisms also work faster, releasing more food for your plants. Isn’t nature wonderful?

• The organic matter in compost binds with soil particles (sand, silt, and clay) to form small aggregates, or crumbs. Crumbly soil is said to have good structure, as opposed to sand, which has poor structure because it’s too coarse to form aggregates, or clay, which can act like cement when wet. These aggregates hold water on their surfaces, making it available to your plants as they need it. As aggregates form, more spaces are created for oxygen, an essential for good root growth. At the same time, the soil spaces form channels for excess water to percolate through the soil, improving drainage.

• Increases water-holding capacity of soil. Compost can hold an amount of water equal to 200 percent of its dry weight, compared to 20 percent for a low-humus soil.

• Acts as an inoculant to your soil, adding microorganisms and larger creatures such as earthworms and insects, which are nature’s soil builders. The compost environment is teeming with life, and all soils can benefit from such a rejuvenation.

• Neutralizes various soil toxins and metals, such as cadmium and lead, by bonding with them so they can’t be taken up by plants.

• Acts as a pH buffer so plants are less dependent on a specific soil pH. The earthworms in the compost help in this process, because in passing organic matter through their bodies they modify the pH of the soil. And you can lower the pH of your soil by adding compost made from acidic raw materials, such as oak or beech leaves, sawdust, and pine needles.

Nutrient deficiencies

If plants fail to thrive, despite adequate soil preparation, watering and mulching, it may be a sign of a nutrient deficiency. Fruit and vegetables are particularly vulnerable, as are containerised plants and those growing in very acid or alkaline soils. Yellow or reddish coloured leaves, stunted growth and poor flowering are all common symptoms of nitrogen, magnesium or potassium deficiency

Quick facts
Plants affected All plants
Main causes Very acid or alkaline, or thin sandy soils; poor growing conditions
Timing Usually spring or summer

What are nutrient deficiencies?
Some garden soils and potting composts suffer from a lack of nutrient content, leading to deficiency symptoms in the plants growing in them.

Plants can also suffer deficiencies where the growing conditions are poor and the plants are unable to take up nutrients present in the soil. Very acid or alkaline conditions, dryness and waterlogging can all make it difficult for plants to take up soil nutrients.

Nutrient deficiencies cause symptoms such as leaf yellowing or browning, sometimes in distinctive patterns. This may be accompanied by stunted growth and poor flowering or fruiting.

Symptoms, cause and remedy
Nitrogen deficiency
Symptoms: Spindly yellow plants or yellow leaves, sometimes with pink tints.
Cause: Nitrogen promotes green, leafy growth and deficiency results in yellowing and stunted growth. Nitrogen is very soluble, so is easily washed out of the soil in winter rains, leaving the soil deficient in spring, just when plants are putting on new growth. Nitrogen deficiency is a common cause of yellow leaves in spring.
Remedy: In the long term, mulching with organic matter (such as well rotted garden compost or manure) provides a steady trickle of nitrogen to stabilise levels. In the short term, applying high nitrogen fertilisers such as sulphate of ammonia or poultry manure pellets will remedy the problem.
Potassium deficiency
Symptoms: Yellow or purple leaf-tints with browning at the leaf edge and poor flowering or fruiting.
Cause: Potassium is needed for controlling both water uptake and the process allowing plants to harness energy from the sun (photosynthesis). Potassium promotes flowering, fruiting and general hardiness. Shortages are more likely on light, sandy or chalky soils where potassium is easily washed away. Clay soils, by contrast, hold potassium within their structure.
Remedy: Apply high potassium fertilisers such as sulphate of potash, tomato feed or certain organic potassium sources derived from sugar beet processing.
Phosphorus deficiency
Symptoms: Slow growth and dull yellow foliage.
Cause: Phosphorus is needed for healthy roots and shoot growth. Soil shortages of phosphorus are rare, but may occur in areas with high rainfall and heavy clay soil.
Remedy: Apply fertilisers such as superphosphate or bone meal.
Magnesium deficiency
Symptoms: Yellowing between the leaf veins, sometimes with reddish brown tints and early leaf fall. Magnesium deficiency is common in tomatoes, apples, grape vines, raspberries, roses and rhododendrons.
Cause: Magnesium is needed for healthy leaves and for plants to harness energy from the sun (photosynthesis). Soil shortages of magnesium are more common on light, sandy soils. Over-use of high-potassium fertilisers (such as tomato feed) can cause magnesium deficiency, as plants take up potassium in preference to magnesium.
Remedy: In the short term, apply Epsom salts as a foliar feed in summer. Dilute the salts at a rate of 20g of Epsom salts per litre of water (1/3oz per pint) plus a few drops of liquid detergent. Apply two or three times at fortnightly intervals, spraying in dull weather to avoid leaf scorch. In the long term apply to the soil around the roots either Dolomite limestone (calcium-magnesium carbonate) at 100g per sq m (4oz per sq yd) or Epsom salts (magnesium sulphate) at 30g per sq m (1oz per sq yd). Dolomite limestone will make the soil more alkaline, so should not be used around ericaceous (acid-loving) plants such as rhododendrons or camellias, or where the soil is already alkaline.
Manganese and iron deficiencies
Symptoms: Yellowing between the leaf veins with browning of leaf edges on acid-loving plants.
Cause: Manganese and iron are important for allowing plants to harness the energy of the sun (photosynthesis). Soil shortages are rare, but manganese and iron can be unavailable to plant roots in alkaline conditions. Ericaceous (acid-loving) plants are particularly vulnerable when growing in alkaline soils or potting composts.
Remedy: Apply chelated iron and manganese treatments, such as Sequestrene, to the soil around the plant roots.
Molybdenum deficiency
Symptoms: Elongated twisted leaves on cauliflowers or other brassicas growing in alkaline soil. Molybdenum deficiency is normally seen in cauliflowers and brassicas, particularly when growing in insufficiently alkaline soil.
Cause: Molybdenum is required for a variety of plant growth processes, but is needed only in tiny quantities. Soil shortages of molybdenum are rare, but it can be less available to plant roots in acid conditions.
Remedy: Treat with fritted trace elements (see our advice on fertilisers). Liming the soil will help in the long term, as making the soil more alkaline will help to make the molybdenum more available. See our advice on lime and liming for further detail.
Boron deficiency
Symptoms: Stunted growth and tip dieback on lettuce, brown cracks in celery; rotten swedes, turnips and celeriac; dimples in pears with brown patches underneath.
Cause: Boron is required for healthy plant cell formation. Soil shortages are rare, but this nutrient can be less available to plant roots in alkaline conditions.
Remedy: Treat with fritted trace elements (see our advice on fertilisers) or by applying borax (disodium tetraborate) to the soil before sowing vegetables or as a foliar spray feed applied to pear trees. Soil application rates for borax are: 35g per 20 sq m (1oz per 20 sq yd). Mix well with a large quantity of light sand before spreading so that the chemical is evenly distributed. Foliar spray application rates for borax are: 70g borax in 22-litres water (2½oz in 5 gallons), plus a few drops of detergent to act as a wetting agent, sprayed at petal fall.