How to Improve Garden Soil Quality

Make the most of your garden soil and keep plants thriving with this guide for enriching garden soil with fertilizer and mulch.
Organic Manure Promotes Microbial Activity in Soil

Organic Manure Promotes Microbial Activity in Soil

Organic manure promotes microbial activity in the soil as well as improving its structure, both of which encourage plant health.

©2011, Dorling Kindersley Limited

2011, Dorling Kindersley Limited

To get the most out of your productive crops and ornamental plants, you may need to improve the fertility or moisture-retaining capacity of your garden soil. There are many ways to go about this, following organic or chemical methods. 

Improving Fertility 

Some plants, such as bush and patio roses and many summer bedding varieties (fuchsias and petunias, for example), are particularly “hungry” and need regular feeding to keep them growing and flowering well, especially on naturally poor ground. You can feed border plants by adding well-rotted manures and garden compost to the soil around them or by applying granular or liquid fertilizers. Other woodland species, like Japanese maples, ferns, and deciduous azaleas, can survive with less feed, but the soil needs to be enriched with leaf mold or organic matter to create a suitably moisture-retentive rooting environment. 

Some plants have evolved to grow on relatively poor soils, and if concentrated granular fertilizers are applied around their shallow root systems, they scorch and suffer shoot die-back. A common example of this is boxwood (Buxus sempervirens), which is best fed with well-rotted manure or diluted liquid fertilizer. Hardy annuals and Mediterranean herbs and shrubs also thrive on nutrient-poor soils, such as sands, gravels, and chalks. 

Fertilizer Recipes 

Unless you enrich the soil organically, you will need to replenish lost nutrients using an appropriate artificial fertilizer, especially around flowering plants and crops. On the labels of most chemical fertilizers you will often see an “NPK” ratio, which shows the relative proportions of nitrogen (N), phosphorus (P), and potassium (K) in the mix. Other micro-nutrients may also be listed. Different plants have different requirements, so manufacturers have developed tailored formulas, such as rose, tomato, lawn, and orchid feeds. High-potassium feeds can help to condition cold-sensitive plants prior to winter and initiate flower bud production in shy-flowering shrubs and climbers. 

Green Manures 

A green manure is a crop grown and dug in before it sets seed and is used on vegetable beds to improve the soil. It can also be harvested and soaked in water to produce a liquid feed. Growing green manures improves the soil structure and can be used to help bind light soils or break up heavy clays. They can also draw nutrients from deep down and add atmospheric nitrogen to the soil. Examples of green mulch include clover, alfalfa and grazing rye grass.

Getting the Balance Right 

Follow instructions carefully for the amount and frequency of feeding. Too much fertilizer can be just as damaging as too little, and using the wrong type can also cause problems. Feeding flowering and fruiting plants with a high-nitrogen mix encourages leafy growth at the expense of blooms, and the resulting tall stems are more vulnerable to weather damage. Soft, sappy shoots are also more attractive to pests, such as aphids, and overfeeding lawns makes them prone to frost and fungal infection. 

Types of Mulch

Covering soil with a layer of organic matter, such as chipped bark, garden compost, manure, or cocoa shells (if a sustainable source can be found), is known as mulching. Materials, such as plastic sheeting, pebbles, or cardboard, can also be used. Mulches conserve valuable soil moisture and suppress weeds, and organic mulches also release nutrients as they break down, feeding the surrounding plants. To help break down woodier mulches like chipped bark, apply nitrogen fertilizer at the same time to promote the composting microbes. Mulches are applied to the soil surface, rather than dug in, as this encourages beneficial soil-dwellers such as earthworms to travel to the surface, improving soil aeration and drainage. 

  • Leaf mold makes an excellent soil conditioner. It does not require any special conditions and can be made by collecting leaves either in a wire cage or in black garbage bags, wetting them, and composting them for two years.
  • Farmyard and horse manure should always be well rotted before you add it to your garden. If not, the soil microbes that break it down will initially absorb nitrogen from the soil, defeating the point. Fresh manure can also scorch plants.
  • Garden compost should ideally be made up of three parts soft, green garden waste to one part woody waste. 
  • Mulching with garden compost will improve the soil structure as well as adding plant nutrients to the soil. Don't use a commercially produced soil improver without first checking its pH—most are alkaline.
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