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How to improve soil structure and fertility

Compost is both the best soil amendment and the best fertilizer
Photo: kristyhall
  • Compost
  • Living soil.
  • Aerating the soil

Most soil in Quebec is rich in clay or sand. In general, clay soil (more than 25% clay) is a rich environment that retains water and nutrients well. However, it is often poorly aerated and drained. This type of soil is difficult to work, warms up slowly in spring and compacts easily. Sandy soil, containing mostly coarse sand, is easy to work and warms up quickly in spring. It is well aerated and drained, but prone to leaching water and mineral salts. This type of soil also tends to be acidic and lacking in nutrients. Whether you have clay or sandy soil, you can amend it by aerating it if it is compacted, topdressing it with compost to improve its structure, correcting the pH and fertilizing it as needed.

Aerate compacted soil

Aerating involves making holes in your lawn with a tool that removes small plugs of soil to allow a better flow of air, water and nutrients to the roots. This stimulates root growth and prevents thatch build-up. You need to aerate when the ground becomes hard and compacted. Try to poke a pencil 10 to 15 cm into a moist lawn. You’ll know that your lawn needs to be aerated if you meet much resistance.

If your lawn is small, you can use a manual aerator, a fork with hollow tines that you plunge into the lawn to remove plugs of soil. If you have a large property, it is better to rent a mechanical aerator or to hire a landscape professional. Drench the lawn the previous evening to make it easier for the tines to reach deeply into the soil. You can expect good results if you make holes about 15 cm deep, removing from 45 to 90 soil plugs per square metre. Work across the lawn in a criss-cross pattern. Go back and forth in one direction, then at right angles. Leave the soil plugs on the lawn and rake the surface to break them up and distribute them evenly. The micro-organisms they contain will help break down any thatch.

Because aerating creates quite a few openings in the lawn, it is best done from mid-August to mid-September, when lawns are less susceptible to weeds and the ground is moist without being drenched. You can also aerate in spring when the ground is cool but not waterlogged.

After using organic lawn care for a while, you will find that you need to aerate less frequently, because earthworms are good aerators. Play and foot traffic areas will probably still need to be aerated once or twice a year.

Topdress with compost

Compost is both the best soil amendment and the best fertilizer. It adds micro-organisms to the soil, giving them both shelter and nutrients, in addition to improving the soil structure, balancing the pH and supplying essential nutrients for plant growth. You should apply a layer of compost once every three or four years in spring or early fall, when the lawn is in active growth.

Use mature compost (rich, loose and dark), that is finely textured and doesn’t contain any weeds. Avoid compost with large amounts of topsoil or peat moss, both of which decompose too slowly. Some professionals prefer to use a blend of 60% compost and 40% coarse sand—because it is heavier than pure compost, it will move more easily through the grass to the soil.

After aerating, apply a thin (0.5 cm) layer of compost, about 50 kg/100 m2. Use a leaf rake to spread it evenly across the lawn and work it in. Once you’re finished, the grass should be standing up through the compost, not bent over or buried.

Apply lime, if necessary

Soil acidity (pH) affects the availability of nutrients. When the soil is too acidic or alkaline, plants can’t absorb all the nutrients they need, even if they are present in the soil. Some elements, like aluminum, iron, zinc and manganese, can even become toxic. Improper pH levels also affect the structural stability of the soil, promote fungal disease and can decrease the population of beneficial organisms.

Most types of grass seed grow best in relatively neutral soil (pH of 6.5 to 7), typical of most of the clay soil in the St. Lawrence Valley, where it is not usually necessary to correct acidity levels. Elsewhere in Quebec, soil tends to be too acidic and needs to be amended with horticultural lime in spring or fall. To find out how much lime to apply, have your soil analyzed. This will tell you how much calcium it needs.

Dolomitic lime is usually recommended because it contains magnesium. If you need to apply more than 20 kg/100 m2, it is best to do so in two stages, once in fall and again in spring. If possible, aerate before applying the lime, so that it will penetrate deeper into the soil.

Fertilize in moderation, with a natural product

Unlike a wild meadow, which is self-sufficient, a lawn is a plant community that is subject to very difficult growing conditions: for one thing it is constantly mown and trampled. Moreover, we often tend to expect our lawns to be greener and lusher than the soil can support. In most cases, this means that we need to fertilize.

Natural fertilizer may be either organic (plant or animal waste) or mineral (crushed stone), but either way it has not been chemically processed. In order to release its nutrients, most natural fertilizer has to be broken down by organisms living in the soil. In addition to feeding the plants, it will promote biological life in the soil. In addition, it is not as likely to leach out or to burn the roots. Most natural fertilizer is slow acting and remains available over time in the soil.

Natural lawn fertilizer contains the three essential nutrients your grass needs: nitrogen (N), phosphorous (P) and potassium (K). The three numbers on the packaging indicate the percentage of each element. For instance, fertilizer marked 8-2-4 contains 8% nitrogen, 2% phosphorous and 4% potassium. You also need to consider the ratio between the three elements. You can figure it out by dividing each number by the smallest number. For example, fertilizer marked 8-2-4 has a 4:1:2 ratio. Lawns need large amounts of nitrogen, so it is best to use a fertilizer with an N-P-K ratio ranging from 3:1:2 to 5:1:2.

As a general rule, you can go by the amount of nitrogen your lawn needs in deciding how much fertilizer to apply. If you are using organic lawn care, you will need to reduce the dosage recommended for conventional techniques to 0.75 to 1.0 kg of nitrogen per 100 m2 per year. The usual dosage is 10 kg fertilizer/100 m2/year—this corresponds to 0.8 to 1.0 kg of nitrogen, for most commercially available natural fertilizers.

Reduce the dosage for a shaded lawn or one containing grass that doesn’t feed as heavily as Kentucky bluegrass (the most common grass), but increase it for a lawn established on poor soil or for one used as a play area.

Fertilizer is usually applied once or twice each year. If you are applying it only once, do so in spring, around mid-May. Otherwise, fertilize once in May and again in mid-August. Never exceed 10 kg of fertilizer/100 m2 (1.0 kg nitrogen) in a single application.

Synthetic (chemical) fertilizers vs. natural fertilizers

Synthetic fertilizers are made from chemically transformed substances. Their use is not recommended for ecological lawn maintenance. Here’s why:

  • Synthetic fertilizers do not stimulate biological life in the soil, because the elements they release can be assimilated by plants immediately. They can even be harmful to beneficial micro-organisms.
  • Because synthetic fertilizers are soluble salts, using them exclusively and repeatedly can cause salinity problems in your soil.
  • The risk of leaching (pollution) and burning roots are higher with synthetic fertilizer.
  • Manufacturing synthetic fertilizer requires the use of non-renewable resources (fossil fuels).

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