How chemistry influences soil physical properties.
To illustrate how chemistry influences physical properties, let’s consider a typical situation. A client has purchased land for redevelopment. The intention is to level the land in readiness for building construction, which requires significant cut and fill on a rolling hill landscape. Bulk earthworks commence, but midway through the process, there is a substantial 500mm rainfall event. Consequentially the following occurs:
1. Significant erosion
2. Soil surface seals and sets like concrete
The site is cleaned up, works continue, buildings are erected and remaining land is now ready for landscaping. Fortunately, the contracted landscapers know a thing or two about soil so they incorporate 0.1 kg/m2 of gypsum and cultivate the soil to relieve compaction.
Post-planting there is another rainfall event, this time only 100 mm. The following occurs:
1. Erosion
2. Soil surface seals and sets like concrete
3. Plants start declining with some mortality
Why does the soil keep dispersing, eroding and hardsetting?
The answer is soil chemistry. Soil chemistry affects the physical properties of soil. Sodium makes soils dispersive and erosive. Magnesium makes soil tough, hardsetting and causes the surface to seal over. In both situations calcium is required to amend the soil, either in the form of gypsum but also lime or dolomite if the pH allows (I.e. acidic soil). Calcium holds individual soil particles together and hence reduces dispersion, erosion and hardsetting behaviours. Calcium has a higher propensity to bind to soil, therefore will displace some sodium and magnesium ions. Irrigating the soil helps facilitate this process, by allowing the displaced magnesium and sodium to be leached out.
Figure 1. (a) High magnesium and sodium cause soil to become dispersive, erosive and hardsetting. (b) Well balanced soil with correct proportions of exchangeable ions (calcium, magnesium, sodium, potassium, hydrogen).
The landscapers in this scenario were correct by adding gypsum, however 0.1 kg/m2 was not going to cut the mustard. The soil chemistry in our case shows extremely low exchangeable calcium, paired with very high levels of exchangeable sodium and magnesium. To correct the soil chemistry, 1 kg/m2 of gypsum is required, 10 times the amount initially applied. My point is not to increase the amount of gypsum, but rather to emphasize the importance of soil testing to know the correct amount of gypsum to add. Adding excessive amounts of gypsum or any unneeded amendments can render the soil toxic and unusable, create environmental risks, and burn a large hole in your pocket.
On average, gypsum costs approximately $1/kg. On a 1-hectare site at an application rate of 1 kg/m2, the total expense would be roughly $10,000. However, if only 0.5 kg/m2 were needed, the cost would reduce to $5000, a significant financial reduction. Only a soil test and a qualified CPSS soil scientist or agronomist can correctly determine the application rate.
Plant mortality presents another hit to the pocket. Once plants are in place it can be challenging to cultivate and amend the soil. Gypsum can be surface applied and irrigated however if the soil has formed a crust, tillage is required to loosen the soil and allow the amendments to penetrate into the lower layers. Therefore soil testing prior to planting is essential.
To prevent future landscaping issues its advisable to consult a CPSS accredited soil scientist in the first instance. This will not only provide correct application rates but also provides insight into how the soil can be reused, whether imported soil is necessary, if the soil is suitable for your chosen plants, recommended soil depths and a wealth of other additional soil information.
Embarking on a new landscaping project? Call The Urban Soil Doctor today on 0473 505 550.
