Fertilisers For Dummies

August 7, 2020

There are 14 essential nutrients required by plants. Both carbon and oxygen are absorbed from the air but all other nutrients must be provided by the soil.

The primary macronutrients are nitrogen (N), phosphate (P) and potassium (K). These are the major nutrients that are required for plant growth and are normally the limiting compounds in soils. All fertilisers are categorised according to their nutrient composition. This is normally clearly labelled on the fertiliser products so that buyers are able to assess the actual costs of the nutrients. The ratings are specified as percentages of each of the primary macronutrients and are listed as percentages in order of the amount of Nitrogen, Phosphate and Potassium that are contained within the blend. This is often referred to as the N, P ,K, rating of the product. Whilst these blends may contain other nutrients the main feature of most fertilisers is their ability to provide these macronutrients.

The nutrients Calcium, Magnesium and Sulphur are commonly referred to as secondary macronutrients are these are also essential for plant growth and health. They are often also listed as a rating on bags of fertilisers. The other essential nutrients are referred to as micronutrients or trace elements and these include the elements of Boron (Bo), Chlorine(Cl), Nickle (Ni), Copper (Cu), Zinc(Zn), Manganese(Mn), Iron (Fe) and Molybdenum(Mo). The difference in these categories is that plants absorb and consume higher quantities of macronutrients and these are found in higher concentration in the plant whereas the micronutrients are adsorbed at lower rates.  Low analysis fertilisers such as superphosphate based products tend to contain higher concentrations of micronutrients than high analysis fertilisers.  Note that in human health terms these elements are most often referred to as minerals. Other fertiliser types may also contain traces of these ingredients.

common fertilisers and their ratings
Common fertilisers and their ratings

So, urea which is a nitrogen-rich compound fertiliser contains no phosphate, potassium or sulphur and this is reflected in the NPK rating.  Often additional figures are also included after these and these refer to the calcium(Ca) and magnesium(Mg) content of the fertiliser blend. 

The nutrient requirements for a plant is very complicated and the manner in which nutrients interrelate with each other in the soil effects plant uptake. These relationships are critical and therefore it is important that a balance is achieved to optimise plant responses. However, in a home lawn situation, the key characteristics revolve around plant health, plant density and plant colour rather than growth optimisation.  The type of fertiliser or fertilisers applied will have a major influence on the level of benefit achieved for each of these outcomes.  

Fertiliser products can be divided into broad categories which are standard release inorganic fertilisers, slow-release inorganic fertilisers, organic fertilisers and bio-stimulants.

Standard Release Fertilisers

The standard release inorganic products are generally highly soluble nutrients that become available for uptake by plants rapidly after contact with water in the soil.  Most agricultural fertilisers fall into this category.  The plants will luxury uptake these nutrients and store them in their leaves leading to rapid growth which is great if you want to feed pastorally grazed animals but not so great if you have to frequently mow a lawn that is flushed with growth by these nutrients!  The other aspect to consider is that by harvesting the cut litter with each mow you are indeed removing the vast majority of those nutrients and confining them to the compost heap, which tends to defeat the objective which is to increase the base fertility of the soil.  In a pastoral situation, up to 90% of these compounds are returned to the soil as dung, whereas patently this does not apply to a lawn unless a mulching mower is employed.  The result is that these products have a limited response period in a lawn situation and therefore they need to be applied more frequently to achieve the appropriate result or improvement in appearance.

Slow Release Fertilisers

These inorganic slow-release fertilisers come in a number of forms some of which develop solubility over time such as phosphate rock products and others become increasingly soluble as a result of the degradation of coatings that surround the soluble nutrients.  These are more generally referred to as controlled-release fertilisers. The most common method employed to reduce the solubility of fertilisers is to cover them with a polymer or a sulphur-polymer coating. In this manner, the coating needs to either dissolve or be attacked by microbial activity in the soil to allow the release of the active ingredients. The greater the thickness of the coating the longer it takes to release those nutrients.  Many of these coatings are reliant upon the activity of micro-organisms in the soil and their activity is determined by soil temperature.  The warmer the soil is the higher the rate of microbial activity. Conversely, the cooler the soil is the lower the rate of microbial activity. Indeed, once soil temperature decreases below 7o- 8oC microbial activity reduces markedly. Therefore in very cold conditions, these slow-release fertilisers become less effective and it is advisable to use a different form to deliver the appropriate nutrients.  

The advantages of this type of fertiliser are that they are easy to use from a management perspective, they offer reduced toxicity, therefore, providing a higher degree of safety than other fertiliser types and lastly they are more environmentally beneficial because of reduced volatilisation losses and reduced leaching into the surrounding environment.  As a result of these advantages, they have become increasingly common for use on home lawns. The disadvantages are that they are more expensive to use and there is a risk that the granules can be picked up by the lawnmower when the lawn is subjected to low mowing heights. This is less of an issue with soluble fertiliser granules as the granules dissolve rapidly.

Organic Fertilisers

Organic fertilisers come in a number of different forms and include;

  • Chicken manure
  • Duck manure
  • Horse & pony dung
  • Sheep manure
  • Composted materials
  • Biowaste 

These are all naturally occurring organic materials and they are not manufactured products as such. The composition of these products is variable and this is determined as much by the variety as the source of the products although most suppliers will be able to specify the common composition of the products they are supplying. Some of these are also available in a more processed state such a palletised products.  The advantage of the processed versions is that the common malodorous features of these products are often greatly lessened. Singularly this is probably the greatest reason why these products are not always viewed favourably in a residential environment. The application of chicken manure in particular in an urban environment can lead to less than harmonious relationships with surrounding neighbours as their nostrils are accosted by the powerfully acrid aroma of the fresh chicken manure! However, in the twenty-first century marketplace where homeowners are now more aware and more concerned about the use of inorganic chemical products in close proximity to their home I can envisage a greater sympathy for these styles of products in the near future.  Secondly, a greater range of these semi-processed (and less smelly) products are beginning to become available in the marketplace.  They are particularly useful for lawns that have been established on soils that have very little or no organic matter. This tends to be an issue on sandy soils or in beachside suburbs in particular. The addition of these types of fertilisers will aid both the water holding capacity and nutrient holding capacity of these soils. Additionally, as these organic substrates decompose they will release plant-available nutrients to the soil in a process known as mineralization.    

Bio-stimulant fertilisers

For the purposes of convenience, I have grouped together humic acid, seaweeds, phosphate and potassium humates and plant hormones under this heading.  These are being increasingly sold as “cure-all” recipes but in reality, they play an insignificant role in the care of the average residential lawn. That aside, there are specific circumstances where these substances could be useful.  In making this statement I am particularly mindful of inner city lawns that may be established on shallow substrates.   Increasingly some inner city lawns are being established on shallow profiles that have been created over non porous substances such as concrete. These are commonly built over the top of underground car parks or on cantilevered balconies. In these circumstances the total soil profile is often only 150mm – 200mm and sand is commonly used as the substrate to ensure reasonable drainage characteristics.  In this circumstance, the use of substances such as humic substances or other soil amendments can be useful to aid plant health. I suggest that an agronomist or soil scientist should be employed to assist in recommending the most appropriate soil amendments. However, for the average garden lawn, they are probably best ignored. Essentially, the degree of response gained from the application of any one of these products will always be limited by the availability of macronutrients so you are better off meeting those macronutrient requirements.  

Organic matter does play a series of important roles within the soil. It acts as a buffer for soil pH, an exchange site for the transfer of nutrients and through the process of mineralisation releases plant available nutrients.  It can also influence the physical properties such as the soils inherent water holding capacity. These characteristics are best addressed by incorporating compost or other organic material into the soil at the time of construction.  Alternatively, when low levels of organic matter in the soil is a real issue (for instance on very sandy soils) it can be added afterwards by using the types of organic fertilisers that have already been discussed. 

As a cautionary note, I add that there appear to be a great many claims made by fringe fertiliser companies that are commonly harping on about soil biology, soil conditioners and growth hormones.  If these claims are to be believed they are miracle cures that prevent everything from turf diseases to drought and even tuberculosis! ..... OK...... maybe not the last one but you see my point! Most of these products contain little in the way of macronutrients but many contain some nitrogen and plant hormones such as auxins, cytokinins or even gibberellic acid.  The response one sees when using these products are generally the manifestation of these ingredients rather than some miracle biological discovery.  

So, when it comes to fertilising your lawn, I suggest sticking to the basics and don't waste your precious time and money taking a punt with some of these new and in many cases, scientifically unproven products.

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