Soil amendments and fertilisers: are they the same?

The terms ‘soil amendment’ and ‘fertiliser’ are sometimes used interchangeably, but in fact mean quite different things. However, some substances such as compost and aged manures can function as both.  

Amendment

Soil amendment refers to a substance that conditions the soil and thereby influences plant growth. Amendments do not necessarily contain much nutrient but do improve the biological, chemical and physical properties of soil including making it more water retentive.

Soil amendments include substances of low nutrition such as mulch when dug into the soil, leaf mould, sawdust, coconut coir, wood chip, biochar, gypsum, straw, perlite, vermiculite, lime, sand, and even some types of clay. These can improve drainage, moisture retention, aeration, change the pH, add organic material making the soil more friable and provide for microbial activity.

Fertiliser

Fertiliser refers to the application of substances containing additional nutrients that influence plant growth.

The 17 essential nutrients for plant growth

Fertilisers include N:P:K which contains the 3 elements most often required by plants – nitrogen (N), phosphorus (P) and potassium (K). NPK is usually inorganic. Organic sources include compost and well aged manure, fish meal and emulsion, as well as blood and bone, and bone meal. These supply plants with the nutrients vital to healthy plants. Be aware that high nitrogen products such as fish and blood products should be used conservatively to prevent overgrowth of leaves at the expense of fruit.

There are 11 essential nutrients which are required in smaller amounts. These are calcium, magnesium, sulphur, and trace elements including boron, chlorine, copper, iron, manganese, molybdenum, nickel, and zinc. (Note that Banyule soil is deficient in boron and molybdenum). These nutrients are all taken up through the roots of the plant and delivered to the stems, branches, leaves, flowers, fruits and seeds but only when there is sufficient water in the root zone as they are water soluble. If in doubt, apply seaweed solution to your soil and plant foliage. Seaweed is an excellent source of trace elements.

 There are 3 more essential nutrients – carbon, hydrogen and oxygen. Carbon is absorbed from the air through leaf stomata (pores), and hydrogen and oxygen from water.  

 Root depth and watering

The roots of most vegetables only extend 30cm beneath the surface and citrus trees are quite shallow rooted. Nevertheless watering needs to be deep rather than shallow to sufficiently provide for the transport of nutrients to the above ground parts of the plant. If in doubt, dig down 30cm close to a plant and see if the water is reaching the 30cm mark. Other fruit tree roots extend considerably deeper, and deep watering is essential to support the health of the tree, fruiting, and spur formation for next season’s fruit immediately after fruiting. This includes stone fruits, apples and pears, figs, pomegranates, olives and other large fruit trees.

Both soil amendments and fertilisers are important for productive plant growth. By being clear what constitutes each, you will be able to work on both fronts to achieve optimum results.