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Soil Improvement

SOIL IMPROVEMENT

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.

Soil properties
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.

Before   Gypsum Applied
 

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.

Which soils?
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.

Summary
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

Reading
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.

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