Waterborne Application - Gypsum New Zealand

By: Gypsum  06-Dec-2011
Keywords: Soil Organic

Managing Soil

Almost half of every plant lives underground. Because we don't see the roots, it is easy to underestimate the importance of the below ground environment. To achieve the high levels of production essential in modern production, close attention must be paid to the soil. Compared with many nations, New Zealand enjoys excellent soils and a climate that engenders rapid growth. Together these translate into record-breaking production.

New Zealand's rural industry has seen a recent shift in emphasis from extensive, land uses such as pastoral agriculture, to intensive ones such as perennial vine/ treecrop horticulture (kiwifruit, winegrapes, pipfruit, stonefruit etc). If this change is to be economic, significant productivity gains (biomass/hectare/year) must be achieved. In most cases this requires capital improvement to the soil. This upgrade remedies any shortcomings so as to lift soil properties closer to those identified as ideal for the new crop.

Many soils tend gradually to lose condition. This process is greatly speeded under intensive production. Their physical condition is degraded by tillage, the use of machinery etc; their chemical condition is degraded by the crop's removal of minerals, by leaching etc; and their moisture content is depleted by transpiration - plants remove water from the soil at a rate proportional to their growth. To achieve and maintain high rates of production, it is basic that a soil will require ongoing maintenance of its: (1) physical condition (structure, organic content, aeration/ compaction); (2) chemical condition (pH, mineral excesses/deficits/ balances) and (3) moisture content (water excesses/ deficits).

It is usual to manage these separately with practices such as manuring (to raise soil organic content) and tillage/ ripping (to reduce weed competition/ increase soil aeration/ break up pans). Fertilisers are broadcast (to raise soil-nutrient levels/ adjust soil-nutrient balance/ pH). And, drainage is installed and irrigation applied (to remove excess water/ minimise water deficit). Sometimes, however, these activities are combined.

Aspects of the soils (1) physical condition can be addressed at the same time as aspects of its (2) chemical condition. An example is in the use of gypsum - both a remedy for soil-structure problems and a calcium/ sulphur fertiliser. More recently, (2) mineral nutrients have been added to (3) irrigation water. Fertigation is on the increase as it offers a number of advantages (plus a few disadvantages). Less common, but now being considered by some, is the combination of a practice that addresses aspects of a soil's (1) physical condition with ones that addresses its (2) chemical condition and also its (3) moisture condition - namely the addition of mineral gypsum to irrigation water.


Horticultural gypsum serves both as a

and as a soil

. At low rates (200-1,200 kg/ha) it is used, along with other mineral fertilisers, to support the calcium/ sulphur requirements of the crop. At higher rates (2,000-4,000 kg/ha) it is used to remedy soil-texture, aeration and drainage problems in heavy (high clay) soils where it flocculates small particles into larger aggregates. The first usage is an annual one needed to replace minerals continuously lost from the site (removed with the crop or leached below the root zone). The second usage is repeated every few years to develop and maintain good soil structure.

Irrigation water is applied to most fruitcrops during the summer months to supplement rainfall. The amount of water applied, and the frequency of its application is very dependant on the soil type/ the crop/ rooting depth/ the weather. For many fruitcrops it would be common to apply around 400 mm (4,000 m3/ha) of water each year. This amount accounts, roughly, for the excess of evapotranspiration (Et) over precipitation (P) during the summer months (see Fig. 1).

Figure. 1. Averaged values for evapotranspiration (Et) and precipitation (P) in New Zealand. During the five cooler months (April-August) P>Et and approximately 200 mm of excess water is lost due run off/ infiltration. During the seven warmer months (September-March) Et >P and this creates a summer water deficit of approximately 400 mm. To a variable, but always rather limited extent, storage of water in the soil buffers the seasonal water excesses and deficits.

Texts give the solubility of CaSO4∑2H2O (the dihydrate of calcium sulphate) as 0.241 g /100 cm3 of cold water. This solubility equates to 2.41 kg /m3. is almost pure CaSO4∑2H2O. This should allow the application of about 10,000 kg gypsum /ha/year via an irrigation system (4,000 m3 x 2.41 kg /m3 = 10,000 kg). However, the practical upper limit for gypsum's solubility is much lower than the saturated value - it takes a long time and a careful laboratory procedure to make up a saturated solution. For all practical purposes, you will have either a true solution of gypsum that is quite dilute or you will have a more concentrated solution but that also contains, in suspension, a significant proportion of solid gypsum particles.

In principle it should be possible to make up, and to distribute, a watery mixture (a slurry) that contains a very substantial proportion of fine gypsum particles in suspension. In this way it would be possible, greatly to increase the irrigation water's gypsum content to a level far above that of a true saturated solution. However, the likelihood of the gypsum particles settling out and causing blockages in less-turbulent parts of a distribution system are extreme. These blockages would be difficult and expensive to removed. So far, research has not developed a practicable system for this, so we are left with the use of true solutions.

Because true solutions, close to saturation are difficult to make up and to manage, practical recommendations for horticultural dissolutions of gypsum in irrigation water commonly suggest rates lying between 15 and 30% of the saturated value. This makes dissolution faster and more straightforward and also reduces the risk of blockage from settlement of undissolved solid in pipelines and outlets. Unfortunately, the use of a dilute solution (say, 20% of saturation) reduces the amount of gypsum that can be applied via an irrigation system to about 2,000 kg/ha/year (20% of 10,000 kg/ha/year).

Clearly there is a little flexibility to raise this maximum - either by increasing the volume of irrigation water applied or by increasing the concentration of gypsum closer to the saturated value. Conversely, for a fruitcrop in which significantly less irrigation water is usually applied (e.g. winegrapes commonly receive only about º of this amount, say, 100 mm per year) the amount of gypsum able to be distributed is correspondingly reduced.

These maxima also assume that all irrigations are carried out with water having a significant dissolved-gypsum content. This requirement will almost certainly create difficulties of incompatibility (precipitation) with other nutrient materials that must be distributed via the same system. Therefore, a practical upper limit of 1,000 kg per year is suggested (allowing for half the irrigations to be gypsum ones).

This amount of gypsum (1,000 kg/ha/year) compares with the usual dry-application rate for gypsum when used as a fertiliser (200-1,200 kg/ha/year). It is definitely low when compared with the gypsum amounts required to achieve a significant improvement in soil structure (2,000-4,000 kg/ha).

We may fairly conclude that waterborne applications of gypsum at rates sufficient to ameliorate a soil's physical properties are not feasible - the amounts of gypsum able to be applied falling well below those usually required to obtain significant benefits.

Feasibility is not advisability, however, and we must consider all aspects of the technology and, especially, its relevance to the New Zealand situation.

Some (promotional) literature claims a number of benefits for waterborne gypsum applications. It is important to recognise that these do not necessarily apply to the same extent, or even at all, in New Zealand because of marked differences (compared with the USA, the Mediterranean basin, Australia etc) in our:

  • Politico/ economic environment (no subsidies/ tax breaks to distort the economics of capital-intensive technologies)
  • Climate (mild, maritime, temperate)
  • Rainfall (about 800 mm per annum and distributed fairly uniformly throughout the year. Our rainfall is also lighter and more frequent - more wet days)
  • Soils (often heavy, rarely sodic)
  • Irrigation water (generally high quality/ plentiful)

Some proponents of waterborne gypsum application include in their lists of benefits, ones that are generic to all applications of gypsum. That is, they include those also obtainable from dry-broadcast gypsum applications. While in a sense this is valid, it is important not to imagine that all the benefits claimed as associated with waterborne applications are necessarily peculiar to them.

Some issues to consider..


Published material promoting proprietary gypsum products for waterborne distribution tends to focus on their benefits as calcium/ sulphur fertilisers rather than on their benefits as soil improvers.

This emphasis is fair, in view of our conclusion regarding the maximum application rates for waterborne gypsum. However, some promotional material seems to claim that the 'fertiliser effect' is in some way better achieved with their waterborne product than with a dry-broadcast one. This idea could be misleading, especially in the New Zealand's climate. Once in the soil where they have their effect, calcium and sulphate ions are, after all, just calcium and sulphate ions!

In Australia, and in some low-rainfall areas of the US, it is not uncommon for waterborne gypsum to be used in the reclamation of sodic soils (characterised by high exchangeable sodium and high pH).

New Zealand has few areas with sodic soils, so this application is largely irrelevant here.

Water quality (salinity)
The cation balance of a soil tends to equilibrate with that of the irrigation water. For example, irrigation water having a high Na/Ca ratio tends to degrade the soil. Gypsum applied along with poor-quality irrigation water can be beneficial.

By and large in New Zealand, we are not forced to irrigate with poor quality water, so this is less relevant.

Root zone
When gypsum is applied along with the irrigation water, deposition will be localised to the root zone, therefore (it is argued) less gypsum is required. This benefit is most relevant in low-rainfall climates where roots congregate around the dripper/ sprinkler outlets and are relatively scarce elsewhere in the soil volume - infrequent rain = dry soil = few roots. There is little point in applying gypsum where there are no roots!

This is less of an advantage in New Zealand where, because of a relatively high rainfall, crops are less dependent upon irrigation water and their roots are not as localised around the irrigation outlets.

A 'super-fine' gypsum product is claimed to offer the advantage of rapid dissolution.

'Enhanced' uptake
Some promotional material implies that very tiny, undissolved, particles of 'super-fine' gypsum (i.e. solid CaSO4∑2H2O) are small enough to be somehow taken up by the plant more directly than are the component Ca2+ and SO42- ions when dissolved in the soil water.

It is difficult to identify a scientific basis for the claim.

Soil water movement
A downward flow of water into the soil (infiltration) is required to carry the dissolved gypsum from the soil surface down to the root-zone (generally from 50 to 750 mm below the surface) where it will have its main beneficial effect. Under low-rainfall conditions, there is reduced tendency for surface-applied gypsum to move into, and down through, the soil and thus a potential benefit for waterborne gypsum applications.

This benefit of waterborne applications is less relevant in New Zealand where there is sufficient rainfall to dissolve and move up to 16,000 kg /ha/year of gypsum down into the soil (a calculated value based on the solubility of gypsum and an annual rainfall of 800 mm). The rate of movement of a dissolved solute in the soil is very much affected by the soil's ion-exchange properties.

Water quality (hardness)
Gypsum when dissolved in hard water (water containing significant levels of HCO3-) can cause a precipitation of lime scale (CaCO3) in the irrigation pipes and nozzles, leading to their eventual blockage.

Much of our apple and winegrape production in the Hawkes Bay is irrigated with 'hard' water.

For waterborne distribution, the gypsum product must be of high purity (possibly at higher cost) to avoid the possibility of line/ emitter blockage by insoluble minor contaminants.

Waterborne applications of gypsum require the installation of expensive equipment (extra capital cost) and this equipment requires careful attention to maintain cleanliness, to clear filters and to attend to minor blockages (extra operational cost).

Any undissolved particles of gypsum escaping into the reticulation system1 will tend to settle out wherever water flow is slow or less turbulent (e.g. in the main lines or in the emitters). Dissolution of these particles will be slow2.

A solid deposit, building up with time will gradually impair system performance (reduced/ unbalanced flows). If the gypsum solution is substituted for pure water from time to time the deposits will, presumably, dissolve away but if a line were to become completely blocked, correction would be much more difficult.

Keywords: Soil Organic

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Product Info - Gypsum New Zealand

To meet differing market requirements, Winstone Wallboards produce two, chemically identical, products - Soil Life a fine ground form and Gypsum Natural a coarser pulverised form. Its special benefits are that horticultural gypsum is fast acting and pH neutral (contrasting with other calcium fertilisers that are slower and either raise or lower soil pH).


Applying Gypsum - Gypsum New Zealand

Gypsum relies upon rainfall to solubilise it and so move it into the soil profile where it has its effect, it is therefore best applied in early spring or after harvest when rainfall can do its work. Gypsum is also useful as a carrier to assist in the uniform application of small quantities of zinc, manganese, boron and the other trace elements.


Subsoil Compaction Affects Wheat / Barley Yield

Soil acidification is an inevitable consequence of intensive cropping with silage crops having greater acidifying effects than grain crops due to the removal of the basic cations contained in the straw. Normal practice is to apply lime and to cultivate this into the topsoil to achieve appropriate pH and calcium levels based on annual soil tests made prior to planting.


Subsoil Compaction Affects Maize Yield

While management of just the 0-20 cm topsoil layer is sufficient to meet the crop's mineral nutrient requirements, to provide it with sufficient water in summer requires either regular irrigation or root zone access to water contained in a much deeper soil profile - roughly from 0-100 cm.


Effect of Dairying on Pasture Soils

The pasture growth reduction can be offset to some extent by increased fertiliser usage but this tends to decrease soil pH and to increase nutrient loss so they are not remedial in their effects - more of a 'band-aid' approach. In recent years we have seen good growth in the New Zealand dairy industry associated with a rise in dairy cow numbers along with higher stocking rates and increased use of nitrogen fertilisers.