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Soil Preparation in Landscaping


Successful planting depends on more than just choosing the right flower or shrub. In this article, the Blue Mountain Nurseries experts take you through everything you need to know about soil preparation


A successful planting job requires the right plant for the site, climate and soil conditions. Soil provides all the elements that plants need to grow, such as nutrients, organic matter, air and water. Soil also provides support for plant roots.


When considering soils, the chemical properties like pH and nutrient levels can be adjusted relatively easily. On the other hand, soil's physical properties are much more challenging. Soil textures are classified by the amounts of sand, silt and clay within them.

Sandy, or ‘light’, soils tend to be loose, warm quickly, are easily worked, free draining, prone to drying out and do not hold nutrients well. Clayey, or ‘heavy’, soils can be hard, dense, poorly drained, difficult to work and slow to warm – but are very good at storing nutrients. Depending on the type of clay, they can also be very sticky and shrink and swell. Soils with a good mix of sand, silt and clay are the happy medium that are good at holding and supplying both nutrients and water. These are termed ‘loams.’


Don’t change the soil, change the structure!

The only way to make big changes to the soil texture is to physically remove the soil and replace it. This is often impractical in a planting job; however, improvements can be made to the soil structure relatively quickly. ‘Structure’ refers how the sand, silt and clay particles are arranged. Soils with good structure have the sand, silt and clay particles bound together in granules, but even soils with a high clay content can be relatively loose and well drained if they have good structure.


If you need to improve structure, aeration, drainage and fertility, use organic matter. In the soil, organic materials break down into compounds that bind soil particles together.

Examine the soil at the planting site in more detail. Sandy soil is free-draining, but often lacks nutrients so you may need to dig in some well-rotted compost to increase the organic matter content. Adding organic matter to clay soil makes it more workable. Depending on the condition of the soil in the planting site, dig in any well-rotted compost and/or potting mix or sand that you might be using, but do not add more than a 5cm layer of organic material.


Some common organic matter additives include:

Plant materials: These include well-composted leaves, straw and grass clippings. If it is not composted, you will need to work this material into the soil several months before planting to allow it time to decompose. Winter is the ideal time to do this.


Manure: Do not use fresh manure as it can damage plants and introduce diseases. Only use composted manure and incorporate it into the soil well ahead of planting.


Compost: Compost consists of decayed plant materials. Work it into the soil before planting.

Sawdust: This also needs to be composted before adding it to the garden. Do not use un-composted sawdust because it will rob the soil of nitrogen and starve the plants of this essential nutrient.

Green manure: Depending on the timing of the job, it is possible to plant rye or oats in the autumn and then rotary hoe or spade it under in the spring. This technique won’t be suitable if an autumn garden is planted.


How to work with your soil


Depending on the condition of the soil in the planting site, dig in any well-rotted compost and/or potting mix or sand that you might be using. Deep tilling will loosen the soil and allow roots to go deeper. Turn each spadeful of soil completely over when soil is moist but not wet, as working the soil when it is too wet can cause it to become rough.

Ideally, spade the soil in the winter to prepare for spring planting. Be sure all plant material is turned under the soil. If organic material is added before planting an autumn garden, it should be well-rotted, such as compost. Instruct clients to add organic matter each year to build and maintain the soil.


Forming a layer of mulch over the soil and above the roots is an excellent way to conserve soil moisture during hot, windy periods. Form a 5cm layer of material over the roots, at least 1.5m in diameter. Do not put mulch up against the trunk of the plant as this can rot the bark. Mulch not only saves soil moisture but; it also acts as a buffer to soil temperature extremes, it also controls weeds and replenishes organic matter and nutrients in the soil. Many types of mulch can be used, including bark waste, tree chip and pea-straw. Make sure the bark mulch and wood chips are well composted. Saw dust and grass clippings are not recommended unless composted first.

Do not use weed matting in any form (eg, old carpet). Over time, this is certain death to the soil, and eventually, the plants.


Air


Plants need air, both above ground for photosynthesis and in the soil. Air in the soil holds atmospheric nitrogen that can be converted into a form that can be used by plants. Soil oxygen is also critical to the survival of soil organisms that benefit plants.

Good soil provides just the right space between its particles to hold air that plants will use. Silty and heavy clay soils have small particles that are close together and have little air. Sandy soils have the opposite problem – their particles are too big and spaced out. The excessive amount of air in sandy soil leads to rapid decomposition of organic matter.

Adding organic matter, especially compost, will help balance the air supply. Try not to step in planting beds or use heavy equipment that can compact the soil and avoid working the soil if it is very wet.


Water

When talking about water, there is a difference between water-holding capacity and water-supplying capacity. Sands and potting mixes have lots of large pore spaces and are very good at supplying water to plants, but they are terrible at holding water and dry out quickly. Clays are really good at holding water with lots of small pore spaces, but do not like to give it up and, therefore, are poor at supplying water and tend to stay wet.


Silts and loams are good at holding and supplying. When organic matter is added to sand, it generally improves the water-holding capacity and the water-supplying capacity to a lesser degree. Adding organic matter to clays improves the structure and therefore the water-supplying capacity

In soils with too much pore space, such as sandy soils, water quickly drains through and cannot be used by plants. In dense, silt or clay soils, the soil gets waterlogged as the pore space fills with water which suffocates plant roots and soil organisms.


Clay soil is prone to waterlogging and has nutrients, but they are bound up. If soil is crumbly and very light in colour, well-rotted compost dug through the planting site will help.


Good garden soil with proper moisture will not form a hard ball when squeezed in the hand. It should crumble easily. It should not crack or crust over when dry. Moist, dark coloured soil with earth worms is ideal.




Article prepared by Robert McBride (PhD Soil Science) and Rebecca Hughes (B.Sc Botany) from Blue Mountain Nurseries. www.bmn.co.nz

Rebecca Hughes has an honours degree in Botany and a background in education as National Training Manager for the Horticulture Industry Training Organization (ITO) and General Manager for the Community Support Services ITO.


She currently works with Blue Mountain Nurseries, a family-run wholesale and retail plant nursery based in Tapanui with a passion for plants since 1932.

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