Methods of maintaining soil fertility

Fertility is the term used to describe the potential capacity of a soil to grow crops.  This is a combined effect of the natural fertility and the conditions of the soil at the time.

The natural fertility of soil depends on several factors.  The composition of the soil, the slope of the land, which affects drainage, the climate and local weather and the ease of cultivation all affect the natural fertility of the soil.  There is very little or indeed nothing that a farmer can do about these factors.  However, good soil management can improve the soil conditions and build up soil fertility.

Factors that are within the farmer’s control are:

The organic matter content of the soil.  The proportion of organic matter can be increased by adding manure, compost and green material.

  • The amount of water available through drainage and irrigation.
  • The acidity of the soil through the use of lime.
  • The amount of available plant nutrients through the addition of organic matter, mixed cropping and artificial fertilizer.
  • The use of crop rotation. Rotating cereals with legumes can reduce the soil acidity.

Traditionally, shifting cultivation where the land is cleared, crops grown until the yields become too poor and then another plot of land is cleared has been a method of restoring and maintaining fertility.  Bushes help improve soil structure because their roots open up the soil so that water can infiltrate more easily.  Continual plant cover helps prevent water and wind erosion.

Population pressures and the insufficient availability of land means that this method of cultivation is no longer practiced in many parts of the world, so soil fertility has to be maintained in other ways. This is usually done through a combination of factors. Using artificial fertilizer is the quickest and most reliable way to boost crop production, but it can be very costly and can cause serious pollution if incorrectly applied.

A balance of the following methods can be used in place of the costly artificial fertilizer:

Manuring

Animal manures or farmyard manure (FYM).  This is the use of dung, urine and bedding materials (if the animals are kept inside).  FYM varies depending on type of animal, their age and condition, food consumed and how the manure is stored.

Characteristics of FYM include:

  • Low in mineral nutrients but high in organic matter
  • Often low in Phosphorus, may need to combine with phosphate fertilizer
  • Composition varied according to source

Animal waste.  Including offal, dried blood and bone, unused hoof and horn. Often used in temperate climates by gardeners, care must be taken not to spread disease.

Human excreta and faeces and food waste.  Latrine waste can be used after two years buried in a pit.

Crop residue and by products can be used as a mulch.

Green manure.   Some crops are grown to provide organic matter.  Leguminous crops fix nitrogen in the soil.  When cut and composted into the soil or cultivated when they are young, they increase the organic matter content, improve soil structure, make phosphorus and certain trace elements available to plants, check erosion and leaching and help control weeds.

Compost making

Composting is the rotting down of plant and animal remains before it is applied to the soil.  The compost should be mixed with available manure and allowed to decompose together for a maximum of two months before applying to the soil.  Composting is safer because the heat generated whilst the material breaks down kills diseases and weeds and seeds and the mixture has a better balance of all the soil needs.

Building a compost heap is done in four layers:

Bottom layer – the base is made of stones and sticks to allow drainage and aeration

  • Next to bottom – crop residues, usually the bulk ingredient
  • Next to top – can be made of animal dung, egg shells and cooking waste
  • Top layer – made of soil about 25mm deep with wood ash, nitro chalk and superphosphate if available.

The layers on top of the base need turning once or twice during the composting period.  Compost needs to be kept moist, either by rainwater or watering if necessary, but must not become too wet or it will rot.  Too much rain will also leach out the nutrients.

Mixed Cropping

Different crops are grown in the same field which helps prevent soil erosion and controls the spread of soil-borne plant disease.  Leguminous plants will add nitrate to the soil which improves fertility.  Deep rooted crops will help improve the soil structure.

Alley Cropping

This is where the crops are grown between rows of trees, some of which can fix nitrogen in the soil.  The roots fix nitrogen, then the trees are cut back with the leaves being incorporated into the soil as green manure.

A prime example of what happens when the soil is not managed effectively happened inAmericaon the prairie lands in the 1930’s.

A rush of expansion in cultivation due to an increase in agricultural prices meant that the land was ineffectively managed – or it could be said, not managed at all.

Agricultural practices were used that encouraged erosion.  Plowing the land removed the natural grass that kept the topsoil from blowing away.

Fields were left bare over winter when the winds were highest

After harvest, the stubble was burned depriving the soil of a natural cover and source of nutrients.

This followed years of extensive farming without crop rotation, cover crops, fallow fields or any other of the techniques used to prevent wind erosion.

The results were that the  soil blew away  and in place, the “Dust Bowl” appeared, causing major agricultural, ecological and economic damage to the USA.

The US government intervened in 1935 with the Drought Relief Service and a program to educate farmers on soil conservation and anti-erosion techniques.  By 1938, the conservation effort including paying farmers a dollar an acre to adopt these methods, had reduced that amount of soil loss by 65%.

However, 2.5 million people had migrated from the region as a result of the dust bowl, the value of the land plummeted and failed to recover and even the land that maintained some of the topsoil never recovered its original quality.

In today’s world of an ever expanding population and many more mouths to feed , it is vital to ensure that the land is utilized as effectively and efficiently as possible.

Good soil management is essential to maintain and improve fertility in the soil. Poor soil management is potentially catastrophic.

Written by Fiona Johnson

August 2011

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