Soil Amendments
You can't have a good lawn without good soil!
Soil Amendments
If you’re adding lime be careful not to overdo it. Making the soil for an ornamental lawn too alkaline is detrimental to the finer grasses and will promote weeds and weed grasses. The amounts should be given to you by the laboratory that did your soil analysis. When adding lime do so as evenly as possible otherwise you will have different pH values and therefore different growth properties throughout the lawn. Use ground limestone or dolomitic limestone only.
When adding soil or organics do so in stages and NEVER layer one soil on top of another without blending by digging or rotovating. The greater the quantity or depth of amendment will affect the steps you will take to incorporate it. This will also affect the pH of the finished job so it’s worth getting a pH for the new soil.
Adding 6” or 15 cm of new soil. This is unusual but may be required for very heavy clay areas.
- Split quantity into three lots
- Dig or rotovate lot 1 down to 20cm deep
- Add lot 2 and rotovate down to 15cm deep
- Finally add lot 3 and incorporate into the top 10cm only
From this you will see that there is a thorough mixing of the old and new soils, the old soil has been well broken up and the soil profile is light and freely draining at the surface with the moisture holding clay layer deep down. This is perfect for grass health and to promote excellent rooting.
For quantities that are less than this incorporate as follows:
- For 4” or 10cm of new soil split into 2 lots and start as for lot 2
- For 2” or 5cm of soil incorporate into the top 10cm only
- For organics or lime incorporate at 10 to 15cm as for or with lot 2
Cleaning the Seed Bed
The process of tilling the ground will have brought weed seeds to the surface which will germinate into your new lawn. You can do several things at this stage to reduce the impact:
- If you're laying turf, weed seeds will not be too much of a problem
- If you're seeding, increase the amount of grass seed to outcompete potential weed seeds
- If you have time to leave the prepared ground for a while to allow weed seeds to germinate then do so. You should then spray the area with a glyphosate weedkiller a week prior to seeding
Using Sand
The addition of sand or sharp sand to lighten heavy soil is possible but it does require to be very well blended with your existing soil. Heavy clay soils benefit the most from the addition of sand but they are also the hardest to blend. River sand is the best and builders or coastal sand a no-no!
I often see sand layered at about 1” deep on top of prepared ground prior to seeding or turfing. This is done supposedly to add a smooth finish and improve drainage. In my humble opinion this does nothing for drainage, may well affect the grass negatively and at best is a poor substitute for a properly prepared and smoothed surface. Admittedly, heavy clay soils that are wet are the devils own job to level and smooth so timing, hard work and proper amendment of the soil is required, not a layer of sand!
