Plant nutrition begins with the condition of the soil in which it is planted, unless it is planted in another growing medium. SA Smallholder has explored soil health and the soil food web in a feature on soil health.
However, soil degradation is an alarming problem all over the world. According to the Food and Agriculture Organisation (FAO), ⅓ of the soil on the planet is classified as moderately to highly degraded. (The Status of the World’s Soil Resources)
This is due to due to loss of topsoil, erosion, salinization, compaction, acidification and chemical pollution of soils. This has been brought about by intensive agriculture, deforestation, desertification and industrial activity. Climate change has led to extreme weather events, such as storms, flooding and fires, all of which affect the condition of the soil.
There are different approaches to building up the quality of the soil. Healthy soil has the correct balance of minerals, water, air and organic matter.
We know that in terms of mineral nutrients, plants need a balance of the big three, namely nitrogen (N), phosphorous (P) and potassium (K). They also need small amounts of magnesium, calcium and sulphur. Necessary trace elements include iron, boron, manganese, zinc, copper and molybdenum.
Some small farmers apply synthetic fertilisers. However, these do not always provide the correct balance of the major nutrients and sometimes will not include the minor and micro nutrients. Where the balance is not correct these fertilisers sometimes cause toxicity, incorrect pH balance and a break down in the humus.
Makers of organic fertilisers strive to create the correct balance of major, minor and micro nutrients. They also stimulate soil life, by activating beneficial bacteria, micro-organisms, fungi and soil insects.
Organic fertilisers can be sprinkled on the soil surface and worked into the soil. They can be spread through irrigation – this is called fertigation. Some smallholders apply organic fertilisers to compost heaps, where they promote the composting process and add nutrients to the organic matter.
Some organic fertilisers are slow release, so they only need to be applied every four months.
Fertilisers come in liquid, granular or meal forms.
A soil amendment is any material added to soil to improve its physical properties. This includes water retention, permeability, water infiltration, drainage, aeration and structure. The goal is to provide a better environment for roots.
A soil amendment is not a fertiliser, which provides nutrients for the plant.
There are inorganic amendments which are either mined or man-made, such as pea gravel, perlite, tyre chunks, vermiculite and sand.
Organic amendments come from something that was alive. They include compost, grass clippings, manure, sphagnum peat, straw, sawdust, wood chips and wood ash.
Growers need to mix soil amendments thoroughly with the soil. Amendments also need to be carefully chosen, based strictly on the requirements of the soil.
This is part four of a series on seeds and plant nutrition. To ensure you don’t miss out on the rest of the series, subscribe here to receive our Feature Newsletter at the end of the series.