There is no use buying the most wonderful plant if the pot is filled with a bad substrate later. The quality of the culture medium has a direct impact on implantation, growth, beauty, and productivity, as well as the longevity of the specimen. Here we tell you what qualities a good substrate must-have.
The main objective of the substrate is to provide suitable physical support for good development of the root and consequently of the plant. It must offer a good anchorage and favor the growth of the root system. This implies a number of qualities:
- It must have a sufficient density for the adequate support of the plant.
- Be homogeneous and stable both physically and chemically.
- Be free of pathogens (soil fungi, for example, which can cause root rot), heavy metals, weed seeds, etc.
From the point of view of physical properties, it must offer:
- Good water retention capacity, the key to storing irrigation water and making it available to the roots as the plant needs it. The greater your capacity to store water, the better you will withstand drought conditions and the less the need for irrigation.
- The water retention capacity must be accompanied by a good drainage capacity, that is, ease of absorbing water and releasing excess water. This property contributes to the distribution of the water throughout the pot or container and at the same time avoiding waterlogging and therefore possible root rot.
- Adequate porosity, understanding porosity as the space that it incorporates internally and that is occupied by air or water. In this way, the water, not being retained in excess, can coexist with the air that occupies the pores.
- A good aeration capacity, that is, it must be spongy to avoid excess compaction and thus be able to house oxygen, essential for the development of the roots.
- A homogeneous and spongy structure that does not decompose or degrade too quickly, thus avoiding compaction and therefore the reduction or disappearance of air spaces.
- Ideally, it should contain particles of different sizes in a homogeneous distribution. If there is an excess of very tiny particles (dust), they would occupy the small spaces and block the release of excess water, leaving no room for oxygen.
A good substrate must ensure:
- A good cation exchange capacity (CIC). It is the ability of soil or a substrate to retain nutrients and exchange them with the plant. When it is good, the nutritional contribution that the plant receives is better and more stable. However, even if a substrate has a high CEC, it cannot store enough nutrients for a long time, hence the importance of frequent fertilization.
- An adequate pH. In general, the values that provide the optimal conditions for the use of micronutrients are between pH 5.5 and 6.5. However, acidophilus plants, such as hydrangeas, camellias, heather, azaleas, rhododendrons, and many others, require a more acidic one, around 4.5 and 5.0.
- Low salinity. A substrate must not contain an excess of salts, which can be the product of improperly selected raw materials or an excess of fertilizer.
What a good substrate is made of and what each ingredient is for
- Peat Sphagnum: traditional raw material, excellent as a growing medium for cleanliness and water retention capacity, drainage, ventilation, and so on.
- Composted pine bark: raw material with a long history and very good properties, always after proper composting that provides stability. Proper particle size classification optimizes retention, drainage, and aeration capabilities.
- Coconut: Raw material with a good trajectory and good properties as long as it has been subjected to adequate washing and stabilization, as well as the classification of the size and shape of the particles – ground coconut, coconut fiber, chips – for greater control of the drainage and aeration.
- Wood fiber: Raw material of very recent incorporation in substrates for horticulture. One example is Pindstrup’s Forest Gold, which is produced by a unique thermomechanical process from FSC certified wood chips. Mixed with Sphagnum peat, it provides excellent structural stability allowing better control of aeration and drainage. It also improves water distribution throughout the root ball.
- Granulated clay, an additive that enhances the water retention capacity. Its high cation exchange capacity also contributes to higher retention of fertilizers.
- Sand, an additive that provides a higher dry bulk density and better drainage. It is washed to eliminate possible salts and it is sieved to obtain particles of a suitable size.
- Humidifying agents, which lower the surface tension of the water, allowing it to more easily penetrate the substrate.
- Perlite, an expanded mineral of natural origin that increases the aeration and drainage capacity due to its porous structure and because it helps to avoid compaction.
- Nutrients. Quality substrates sold at garden centers typically contain base or quick-release compost. There are different sources (organic or chemical) and different duration and form of release. It is important to look at the substrate container to see from what moment it will be necessary to start adding fertilizer to the crop.