26 May 1985 Intensive cultivation could cause a deterioration in soil productivity

Intensive cultivation of some soils could cause significant deterioration of soil structure, increasing risk of reduced yields or even crop failure.

However, not enough is known about many soils in New Zealand to be able to say how these will react .

Continuous cropping might not be possible on those soils with inherently weak structures ; instead pasture-crop rotation might need to be used.

Careful monitoring of the soil's structure condition could help the farmer make a decision about what management system to use.

The physical condition of the soil can have an over-riding influence on crop growth.

However, the physical condition of the soil is often ignored.

Good soil structure is difficult to define, but must mean stable soil aggregates.

These aggregates should be of a size, shape and packing maintaining the necessary balance of air and water in the soil to allow ease of root growth.

Restricting the rate of root growth or the rooting pattern can limit crop yield.

Roots grow through existing cracks and pore spaces in the soil ; between the aggregates rather than through the aggregates.

Root growth will stop if aggregates are packed too tightly or the pores are too small.

Plough pans can severely restrict root growth and drainage on soils that have been intensively cultivated.

This is more likely to happen if the soil is too moist at the time of ploughing.

Plants on soil with too low a strength have poor anchorage, and may also suffer from drought.

Very fine seedbeds can blow away in high winds, and can form surface caps with only light rainfall.

Wind erosion is serious, resulting in the loss of many tonnes of fertile topsoil.

Erosion is not always a spectacular dust cloud ; much erosion is soil being lifted and rolled along the surface.

Yields are also affected when a surface crust forms on the soil, reducing plant emergence and restricting the growth of plants that do emerge.

A surface crust is also a barrier to water infiltration, stopping the plant getting the full benefit of rain or irrigation.

However, much more serious is surface run-off, leading to loss of crop and valuable topsoil, and to soil erosion.

Water erosion is worst on steep sloping paddocks, but can occur also in ridges and gullies in paddocks on the plains.

Roots and essential soil microorganisms need air to grow, but if soil is waterlogged the air cannot move around easily or exchange with the air in the atmosphere.

Soil drains at a rate controlled by the number of large pores in the soil.

If these pores are lost or blocked, drainage is restricted.

Soil needs a correct balance of large and small pores to allow drainage and water storage.

The form and stability of the soil aggregates control this balance.

Temperature of the soil also affects soil micro-organisms and plant growth, and spring temperatures are particularly important.

Growth and activity are slow if the spring temperature is below 5 degrees Celsius.

How quickly the soil warms in spring depends on how much water is in the soil ; well-drained, open-porous soils warm fastest.

Earthworms and bacteria have a big influence on the structure, formation and stability of soil.

In many paddocks earthworms will process the topsoil of an entire paddock in a year.

Soil organic matter is also very important for the stability of soils, and soils with less than 2 per cent organic matter are likely to be unstable.

However, if other soil-cementing agents, such as clay, iron and aluminium oxide and calcium, are dominant organic matter levels are less critical.

If fine sandy soils and silty soils lose organic matter these can become unstable, but heavier soils are less likely to lose stability.

But if clay soils are worked when too wet there can be smearing and compaction.

Many soils on the Canterbury Plains are low in iron and aluminium oxides, so organic matter is important for maintaining stability.

Obvious soil structure problems such as wheeling and pugging are obvious, and the effect on plant growth can be appreciated.

Considerable damage can be done by harvesting root crops on wet soil, and the effects can last for years.

Drilling in the spring can also damage soil structure if done on wet soil.

More subtle changes in soil conditions occurring over years present dangers, such as reduced yields, that are less obvious.

General rundown of the soil structure, with pans and surface crusts, may have more effect on plant growth in one year than another.

Cultivation breaks up and aerates the soil, increasing organic matter loss and reducing the stability of aggregates.

Heavy cultivation on Templeton Silt Loam in Canterbury caused more than half of the soil aggregates to become unstable and disintegrate in water.

This was shown in some limited results on the loam.

By comparison only one-tenth of the aggregates of the same soil under long-term grass was unstable.

Soils under light cultivation and direct drilling have more stable aggregates than under heavy cultivation, but less than under grass.

By returning to grass, levels of organic matter in the soil can be restored.
Different crops provide different levels of organic matter in soil, but root crops take a lot out of soil.

One year under grass will add 4000-5000 kg/ha of organic matter to the top 15 centimetres of soil.

This is at least as much as would be returned to the soil by adding 10 tonnes of farmyard manure.

Three-year grass returns 6000-8500 kg/ha, but potatoes return only 250 kg/ha and sugar beet 500 kg/ha.

Winter cereals return 2200 kg/ha, red clover 2000 kg/ha and spring cereals 1300 kg/ha .

Estimating the physical condition of the soil for plant growth can be done easily by a farmer or an
adviser.

Careful and systematic examination of the soil profile can help decide the best paddock for a particular crop.

This can be done by digging a goodsized pit, about 50 cm by 100 cm, to a depth of 50-60 cm noting how much digging is needed at each depth.

The pit face should be gently picked out, to remove the spade cut, and then examined from top to
bottom.

Root patterns in profile should be examined, and increases or decreases of depth noted.

Soil texture changes, presence and shape of aggregates and density of the soil should also be examined.

The soil profile should be divided into four layers : surface, that disturbed by normal cultivation,
that below the cultivated layer, and the subsoil.

The surface should be examined for surface crust, and the cultivated layer for dense layers or pans.

Most critical for crops is examination of the sub-cultivated layer which has the greatest structure problems.

Drainage conditions are most important in the subsoil and water or grey-blue coloured soil should be looked for ; mottles indicate periodic waterlogging.

Putting a relative value on the structure condition of the soil is useful in many situations.

Visual scales developed for cropping overseas could be adapted for New Zealand.

One of the scales is the Peerlkamp, and using this the cultivated layer is examined by taking a spadeful of soil.

After loosening by hand, the soil is given a value on a 1-10 scale for size and shape of aggregates, cohesion of soil particles, porosity of the aggregates and the entire plough layer, 'root development, and dispersion at the soil surface.

Using the test showed lowest crop yields on soils with the lowest structure index, and a value of at least five was required to ensure high yields.

Further information from Dr Keith Cameron, Soil Science Department, Lincoln College 8150