Although most gypsum is used in the building industry, an increasing proportion is now being used in agriculture and horticulture. Here, it is very useful for improving the textural and drainage properties of heavy (clay) soils.
WHAT IS GYPSUM?
Gypsum is known to chemists as calcium sulphate di-hydrate (CaSO4 – 2H2O).
WHAT DO WE USE GYPSUM FOR?
Although most gypsum is used in the building industry, an increasing proportion is now being used in agriculture and horticulture. Here, it is very useful for improving the textural and drainage properties of heavy (clay) soils. Gypsum is also an excellent calcium and sulphur fertiliser. 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).
Winstone Gypsum™ make horticultural gypsum. This is a fine, off-white crystalline powder. The bulk density of Winstone’s horticultural gypsum is 908 kg/m3 (loose) and 1172 kg/m3 (compacted).
Speed of action is increased by reducing particle size. On the other hand, a fine powder is less easily handled by a mechanical spreader and requires calmer conditions.
To meet differing market requirements, Winstone Gypsum™ produce two, chemically identical, products – Soil Life® a fine ground form (more suited to the domestic market) and Gypsum Natural a coarser pulverised form (more suited to the commercial market).
For the particle-size distributions of the ground and pulverised products, see the graph below.
PURITY OF HORTICULTURAL GYPSUM
This product is of very high purity (93 to 98% calcium sulphate). The table below shows the mineral analysis of major and minor components of Winstone’s horticultural gypsum.
NB: Quantities are stated in ppm unless otherwise specified – indicates data unavailable.
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. It can, however, be applied at any time because gypsum does not damage plant tissues – even at high application rates.Calcium transport into fruit occurs mostly during spring. Where gypsum is being used to remedy a soil calcium deficiency, it is best to apply it well before bloom to ensure that the calcium reaches the feeder roots in good time.
Ground gypsum is quite fine and even the pulverised product contains a proportion of fine material. It should, therefore, be applied only in calm or at the most ‘light breeze’ (1-2 knots) conditions. Otherwise, substantial amounts of the product will be blown away from the target area and may cause offence to the neighbours.
See Gypsum Material Safety Data Sheet (PDF 158Kb)
GYPSUM & LIMESTONE
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.
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 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.
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.
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.
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.
Gypsum is a natural product
Gypsum is a naturally occurring mineral that complies with all current organic production regulations. Winstone Gypsum is a suitable product for organic gardening and farming.
Gypsum can safely be applied in almost all situations and even excessive applications are unlikely to be damaging. Because of its neutral pH (around 7) it can be used when large amounts of calcium are required without increasing soil pH.
A well nourished plant with a healthy root environment and a good root system is more resistant to pests and diseases. Composts are very important for organic growers as they build up and maintain soil structure and fertility. In addition to its beneficial effects on soil structure, gypsum improves the speed, efficiency and mineral balance of composts. This facilitates the quick return of nutrients and organic matter to the soil.
Levels of the heavy metals are very low in gypsum. The build up of heavy metal residues (especially cadmium) is already a problem in some regions in New Zealand. In addition to its general undesirability, heavy-metal build up can also limit root growth, reducing both nutrient and water uptake by plants. Gypsum is a valuable and very safe natural material that is especially useful for organic growers who wish to supply additional calcium and sulphur to their crops.
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Gypsum comes in three, chemically identical, forms.
ClayBreaker™ is a fine ground powder in bag sizes 8kg and 25kg.
Soil Life® is a fine ground powder in bags of sizes 25kg, 500kg and 1,000kg.
Gypsum Natural is a coarser, pulverised product in bag sizes 25kg, 500kg and 1,000kg or in bulk. The bagged products are consigned on 1-trip, non-returnable pallets.
Plants require substantial amounts of nutrients, including calcium and sulphur for growth. A fertiliser programme that maintains adequate levels of these minerals in the soil is necessary for continued cropping.
Gypsum is a fertiliser
As a fertiliser, gypsum is a good source of sulphur and, being more soluble than lime, it is also a good source of medium-term release calcium. It has the added advantage that it neutralises the by-products of organic decomposition. Some New Zealand soils, particularly large parts of Otago, Canterbury and Gisborne, are low in sulphur and traditionally superphosphate has been applied to amend this. The sulphur in gypsum is in a form that is readily available to plants. Superphosphate applications raise the concentration of phosphorous in the soil whereas gypsum allows the application of sulphur without input of phosphorous. Gypsum also alleviates problems of high aluminium and manganese in some clay subsoils.
Plants need Calcium
Calcium is a structural part of every plant cell wall and cell membrane. It also affects the activity of many plant enzymes. Calcium is taken up passively in the transpiration flow and moves in the xylem sap to high transpiring structures such as leaves and to a lesser extent fruit.
The calcium level in a fruit is usually much lower than that in the other organs (stems, leaves, roots etc.). As a result, the fruit of many species (particularly apples, tomatoes, capsicum etc) tend to suffer from specific calcium-deficit disorders such as bitter pit and blossom-end-rot. While the addition of luxury levels of calcium fertiliser will not solve these problems it is important that soil-calcium deficiencies are promptly corrected for these will only make matters worse.
A continual supply of calcium in the soil solution is required as calcium is not mobile within plant tissues, so is not reallocated to the growing tips.
Deficiency symptoms include:
- Distortion of young leaves,
- Curled margins, and tips hooked back on young leaves
- Irregular or ragged shape of young leaves
- Brown spotting or scorching of young leaves
- Death of terminal buds
- Poorly developed root systems
- Fruit quality problems e.g. bitter pit in apples and blossom end rot in tomatoes.
Symptoms of calcium deficiency in tomato
Symptoms of calcium deficiency in cucumber
Plants need Sulphur
Sulphur is necessary for proteins, vitamins and the synthesis of some plant hormones. It is essential for the production of healthy, green leafy material. Without adequate sulphur, chlorophyll is not stable and plants suffer from chlorosis (lack of chlorophyll). Like calcium, sulphur is not mobile within plant tissues, so a continuous supply is needed for growing plants.
- Young leaves are pale green or light yellow without spots (similar to nitrogen deficiency, except nitrogen deficiency shows up primarily in older leaves as it is reallocated to new growth).
Sulphur deficiency symptoms in grass (right side) compared to a healthy plant (left side)
The soil is the fertile skin of the earth and it is home for almost half of every green plant – the roots. A good soil environment makes a huge difference to the growth and health of the above ground parts that we see.
Heavy, compacted and waterlogged soils slow plant growth and can cause disease problems. These soils can be greatly improved by applications of gypsum. Gypsum will improve soil structure and drainage through the profile and thus aeration. This will allow plants to explore the soil more deeply, thereby giving them the edge in times of drought.
Even quite heavy applications of horticultural gypsum have no negative side effects for it is a natural mineral that is certified for organic use. Gypsum is also a source of the essential plant nutrients – calcium and sulphur.
What is soil?
Soil is the biologically active material that covers much of the world’s land surface to depths ranging from a few millimetres to well over 1 metre. It is home to the below-ground parts of plants (the roots) and also to a host of micro-organisms (fungi, bacteria, protozoans etc) and small animals (earthworms, arthropods etc).
The non-living components of soil, in varying proportions, are: the highly weathered parent rock material, water, air and decaying organic matter. Soil is usually formed over a very long period of time, and its composition is dependent on the material from which it has formed and on the dominant local processes involved. Soils are very diverse and have different physical and chemical properties depending on their composition and location. Terms used to characterise soils include texture, consistence and structure.
Texture refers to the size distribution of the mineral particles that make up a soil. These are defined as the sand, silt and clay particles.
Coarse-textured soils have relatively high proportions of sand (particle sizes between 0.02 and 2 mm). These soils absorb and drain water easily but they hold relatively low amounts of water and they also leach nutrients very easily. These soils are favoured for viticulture as they control excessive vigour.
Medium-textured soils have higher proportions of silt particles (0.002 to 0.02 mm). These soils are more generally desirable for plant growth where high vigour is desirable.
Fine-textured soils have higher proportions of clay particles (less than 0.002 mm). Clay soils are usually poor draining and are difficult to manage when wet. When dry they become dense by shrinkage and therefore hard. On the other hand, the small particle sizes of clay gives it a very large surface area allowing good retention of nutrients. These are also held in such a way that they can be readily available to be taken up by plants. Much of our productive pasture is on these soils.
Consistence refers to the way moist soil deforms or fragments in response to an applied force. Soil consistence can range from very friable (aggregates are easily crushed in the hand), to very firm (very hard to crumble in the hand). Moderately friable soil that crumbles when squeezed is most desirable for agricultural and horticultural use.
Structure refers to the way and extent to which soil particles aggregate into larger units. A well-structured soil has aggregates that pack to create many pores. Pores are essential for free drainage, for aeration and for root growth. Weakly structured soils are prone to wind and water erosion, as well as to compaction. They are also likely to crust after heavy rainfall so that water will take longer to enter the soil bulk and will tend to run off if the surface is sloping. These factors all tend to reduce plant growth and yield. An important element of structure is structural stability.
Gypsum improves soil properties
The calcium component of gypsum encourages the soil clay particles to flocculate (group together) thereby improving soil structure. Flocculation occurs because the fine components of the soil (clay and organic matter) normally carry negative charges and attract calcium, which carries two positive charges. Water molecules act as a bridge between the two, so the clay particles become linked together. The calcium component also encourages the growth of soil organisms that, in turn, help create/maintain soil structure and stability. Improvements in structure have many benefits for the soil.
Flocculation of small particles into larger aggregates facilitates the easier movement of water and nutrients into and through the soil profile. Surface crusting is often a problem with fine-textured soils where raindrops and sprinkler splash destroy the structure of the surface layers. Surface crusting results in run off, reduces water infiltration and soil-air gas exchange, and also restricts seedling emergence. Gypsum can reduce crusting by restoring structure and maintaining more stable aggregates.
The improvement in soil structure following gypsum application also involves the creation of pores having a range of different sizes. These control the balance between the important properties of free drainage on the one hand and water holding capacity on the other. The larger pores allow water infiltration and drainage, while the small ones hold the water so as to provide water storage for the plants. A good range of pore sizes also means that the soil is well aerated allowing the roots and soil organisms to ‘breathe’ better. Lastly, a porous soil allows easier root penetration so plants can better explore the soil volume in search of both minerals and water. This leads to better growth and improved drought avoidance.
Soil compaction is a common consequence of the intensive management systems that are employed in modern agri/horticulture. It changes the distribution of pore sizes, increasing the number of small pores. Soil compaction can be alleviated by mechanical means (e.g. ripping), but gypsum alone will also perform this function to some extent. In conjunction with mechanical methods of alleviation gypsum helps to create soil aggregates that tend to maintain a structure that is less likely to compact. A less compact soil allows easier root penetration and is also easier to manage. Gypsum also helps to reduce compaction in subsoil layers, and in ‘cultivation pans’ as the relatively high solubility of gypsum allows it to move down through the soil profile.
Clay textured soils tend to swell when wet, and to shrink when they dry. This causes large cracks to appear at the surface. Gypsum reduces the amount of shrink and swell by moderating change in water status. The amount of cracking is therefore reduced.
The calcium in gypsum also increases the activity of soil organisms. Soil microbes break down dead plant material and other organic matter. This process produces organic ‘glues’ that bind soil particles together and stabilise soil structure. Decomposed organic material is also bound to clays at a molecular level via calcium. Earthworm activity is increased by calcium, and this brings with it many benefits including increased aeration, increased mixing of organic matter, and macropore formation.
Gypsum is a fertiliser
As a fertiliser, gypsum can be used to increase and maintain levels of both calcium and sulphur in the soil. Gypsum is relatively soluble, much more soluble than lime. It is therefore a good source of medium-term release calcium which has reasonable mobility through the soil profile. Maintenance of calcium supply is essential for plant growth, and unlike some other nutrients, it does not move easily within the plant from, say, older leaves to the growing tips where it is needed. Calcium is especially important for the horticultural crops (apples, kiwifruit, winegrapes, tomatoes, capsicum) as it is important in fruit development. It is well known that fruit with a higher-than-average calcium status are less likely to suffer physiological and postharvest storage problems. The advantage of gypsum, over lime, as a calcium fertiliser is that it has little or no effect on soil pH. Application of minerals (e.g. lime) that raise pH, make trace elements less available and tend to reduce plant growth.
New Zealand soils are commonly low in sulphur. Traditionally, superphosphate has been applied to amend this. Gypsum is a good source of sulphur, the sulphur being in a form that is readily available to plants. Superphosphate applications raise the concentration of phosphorous in the soil, whereas gypsum allows the application of sulphur without further input of phosphorous. Plants require sulphur for protein synthesis.
Gypsum is better than lime for relieving problems associated with acidity at lower soil depths. Calcium associated with sulphate in gypsum moves down the profile faster than calcium associated with bicarbonate in lime. Gypsum will also alleviate problems of high aluminium and manganese concentration in some clay subsoils.
High, soil-sodium content has severe detrimental effects on soil structure. Sodium causes loss of aggregation and reduces pore spaces, and this may increase pH in severe situations. The calcium in gypsum restores aggregation and pore space by displacing the sodium from the soil. This is more of a problem overseas than it is in New Zealand.
Not all soils respond to gypsum. Those that respond best have high clay contents (30% or more), high sodium contents (exchangeable sodium > 5%) and low organic matter. As a rule volcanic soils respond to gypsum more quickly than alluvial soils and soils from sedimentary parent material.
Many of the soils of New Zealand’s 10m ha of pasture and 0.2m ha of arable farmland are classified as ‘heavy’. These soils tend to suffer structural problems that would benefit significantly from applications of gypsum. However, there is an economic barrier to the use of gypsum to remedy these problems. Basically, the cost of remediation is too high taking into account the overall profitability of the low-input, extensive production systems we employ. This does not preclude the use of gypsum to smaller areas of this farmland to remedy very local problems (see Pastoral Farming and Arable Cropping).
Most of the 45,000 ha used for horticultural production in New Zealand have soils possessing good drainage characteristics. However, even those regions with the best soils include areas where soil drainage is poor. Also, a significant proportion of our horticultural production comes from regions with soils that have less-than-ideal drainage characteristics. The latter include some heavy clay soils in the Kerikeri and South Auckland districts, patches on the Gisborne plains, the so-called Yellow Grey Earths of the Manawatu and Wairarapa, southern Yellow Grey Earths of the Canterbury plains and Central Otago and the Moutere hill soils in Nelson.
Gypsum can be widely used to improve the fertility and functioning of productive soils. It:
• Improves soil structure, aeration and drainage
• Reduces soil compaction and cracking
• Improves root penetration
• Increases soil calcium balance without changing pH
• Increases available sulphur in the soil
• Stabilises the organic components of the soil
McLaren RG and Cameron KC (1990) Soil Science. OUP (available from educational booksellers for around NZ$80). This book provides an excellent introduction to the properties and management of New Zealand’s soils.