There are several different composts that are available to the buyer. Peat-based composts are non-renewable because it takes years for peat to form. We cannot keep using it at the current rate because of the aforementioned point. Peat grows in low-land raised bogs. Peat is obtained from partially decomposed plants. These bogs provide homes for flora and fauna so it is environmentally controversial to use peat. Another drawback of using peat-based composts is that peat has little nutritious value. It can, however, be argued that this is beneficial to the horticulturalist who can manipulate nutrients more precisely. Peat based composts support acid loving plants well because peat has a pH of 3.6 to 4.5. Lime may need to be added to such composts if the plant to be grown in it does not like acidic conditions.
A major advantage of using peat-based composts is that they have excellent water retaining capacities. Hence, plants that are rooted in such composts do not need to be watered as often. Small hanging baskets need to be watered less frequently if water retaining composts such as peat based composts are used. Peat provides a good environment for the roots of plants and plants can readily draw water from it. In the same way roots can easily draw oxygen from peat based composts because peat has a high porosity. Peat is light to work with which we appreciate as gardeners.
John Innes compost is an example of a compost that has peat in it. The main ingredient of John Innes compost is loam so we should not describe this as a peat based compost.
There are several peat-free composts available on the market. Peat-free composts are made up of composted bark, coir, woodfibre and green manure and inorganic materials. The inorganic materials that are mixed in are materials such as grit, sharp sand or perlite.
The majority of coir that reaches the UK is from Sri Lanka. Coir is a ‘waste product’ from the coconut industry. Using what would be a waste produce for constructive purposes is of course undeniably ecologically sound. Coir has an excellent water holding capacity. In fact this type of compost holds water more effectively than peat based compost. It says on the finegardening.com website:
Coir can absorb water at a rate of about seven times its dry weight. This is almost 30 percent more water than peat can hold. However, it must be noted that even though coir holds more water than peat it does not hold it as long. So plants cannot be left unattended in coir for as long.
Large amounts of air are held within coir even when it is saturated. Fine and coarse fibres successfully hold air in pore spaces. This can provide plant roots with much needed oxygen. Coir holds an enormous amount of air providing plant roots that are within it with enormous amounts of oxygen. Furthermore,. High levels of the macronutrient potassium are present in coir. Coir also has significant levels of various micro-nutrients in it.
A disadvantage of using coir is that it does not hold nutrients very well. High levels of soluble salts are more likely to accumulate in coir based composts than peat based composts. Coir based composts do not support acid loving plants as well as peat based composts because the pH of coir based composts is between 3.6 and 4.5. It can be argued that using coir is not very environmentally friendly because of the distance that it needs to be transported to be used in the UK. However, according to the guardian about half the peat we use is transported to the UK from elsewhere. Coir tends to lock up nitrogen. In other words nitrogen becomes trapped in soil organisms so that plant roots cannot gain access to it. Coir dries out at the top quickly, whilst maintaining water lower down so this often leads to overwatering. A study at Utah University discovered that plants tend to grow less well in coir than in peat.
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