An old rule of thumb was that gypsum was best used to improve soil structure in alkaline (high pH) soils whereas lime was best used to improve soil structure in acid (low pH) soils. Over the years, research has shown this to be oversimplified. Certainly it is true that gypsum, being pH neutral, is better than lime where soil structure improvements are required but where it is undesirable to raise an already-high soil pH. Also, that lime will neutralise soil acids and so will tend to raise an over acid soil pH. However, the pH of an acid clay soil can also be raised indirectly by gypsum through improvements in soil structure, mainly through improved aeration and drainage. Meanwhile, lime being water insoluble, is usually very slow to raise soil pH at any depth. In fact, more recent research has shown that lime+gypsum mixes can offer benefits that are unavailable with either lime or with gypsum if used on their own.
Root zone depth
It is generally true that pasture and crop productivity depend on the availability of soil water and especially so during the summer months. For this reason, the depth of the root zone becomes critical because the amount (
V) of water available to a growing plant is proportional to its root-zone depth (
D) times the soil's water-holding capacity (
W) - this idea can be expressed algebraically as
V = D x W.
Depending on soil type, on plant species and on climate, the root-zone usually lies between a depth of about 5 cm and 50 cm measured from the soil surface. Sometimes a few roots will go down deeper to about 100 cm. The root zone's upper boundary (~5 cm) is limited by extremes of temperature and/or dryness in the soil's surface layers which kill any very shallow roots. Meanwhile the root zone's lower boundary (~50 cm) is usually limited by adverse subsoil conditions of one sort or another.
With some crops it may be appropriate to lay a mulch on the soil surface (organic or plastic etc) which buffers the temperature and dryness extremes and so raises the upper root-zone boundary closer to the soil surface. However, more important to the plant's water economy in a drought is deepening the root-zone's lower boundary. If the lower boundary can be deepened, it will allow the crop access to significantly more stored soil water. Therefore, it is important to identify and to take steps to remedy any adverse subsoil conditions which may be limiting the depth of this lower root-zone boundary.
What are these adverse subsoil conditions? The common ones are:
- Poor subsoil drainage renders the subsoil anaerobic when it is wet and anaerobic soil conditions quickly kills the roots
- Compaction physically slows root extension or prevents root entry to the subsoil layers altogether
- Mineral imbalance which slows root growth physiology and extremes can also be toxic
- High acidity affects the roots directly, it also affects mineral nutrient availability and uptake, it also affects the level of exchangeable aluminium which can rise to toxic levels in an acid subsoil
Lime
Lime is effective in raising the pH of an acid soil and it is also a source of calcium. However, these pH benefits are gained mostly close to the surface and not in the subsoil. Lime is water insoluble so it is relatively immobile in the soil. It must react chemically with other soil components before it can gradually migrate downwards and so penetrate to the subsoil layers. This takes a very long time. Liming can also have negative effects as it can interfere with the infiltration of surface-water increasing runoff and so reducing soil water re-charge after rain.
Gypsum
On the other hand gypsum is water soluble and so has much greater soil mobility. Therefore, gypsum offers its benefits more rapidly, and at greater depth, and without any negative surface effects on rainwater infiltration. But gypsum is significantly more expensive than lime.
Mixes
When lime is applied along with gypsum in a mix (e.g. lime:gypsum 60:40) water penetration is facilitated by the gypsum component which also offsets some of the other limitations of a pure lime application. While lime:gypsum mixes are less suitable for higher pH soils (you may not want to further raise an already high soil pH) they do offer a broad spectrum of benefits for the more acid soils, improving plant health, nutrition and crop yield mainly via their effects on soil structure (better aeration and drainage), root-zone depth (ameliorating adverse subsoil conditions) and by raising soil pH (improving nutrient availability and uptake and lowering levels of exchangeable aluminium).
In New Zealand, lime is generally about 30% of the price of pure gypsum (the lime price is variable around the country because it depends on distance from source) and a 60:40 lime:gypsum mix is about 55% of the price of pure gypsum.
Agriculture and forestry contain many cases where a pure gypsum application would bring about a useful improvement in root-zone soil quality and a corresponding gain in production but where the relatively high cost of gypsum limits its economic usage (e.g. where a particular land use means that the $/ha/year returns are too low to justify the $/ha/year investment). In cases where, on an economic basis, an application of pure gypsum is judged uneconomic (or only marginally economic), use of a 60:40 lime:gypsum mix should definitely be considered. Talk to your adviser.
Application Rates
To improve soil structure in heavy soils, gypsum should be applied at rates between 2000-4000 kg/ha while a 60:40 lime:gypsum mix should be applied at around 5000 kg/ha. Ploughing or deep ripping after application will always speed the response to these applications.
