Last week’s discussion of three differing fertilisers briefly mentioned that manures and manure-based products should not be thought of as complete fertilisers able to supply all nutrients in adequate amounts — and commercial suppliers of these do not make this claim.

Rather, such products are best regarded as boosters, in that they can be relied on to supplement, or boost, the amounts of the crucial macronutrients all plants need. But of the other nutrients, not so much. Other formulations and/or supplements will still be needed to overcome any deficiencies in soils or pots.

One advantage of commercial fertilisers is that they can be formulated to include any nutrient that is required, whether micro or macro. Another advantage is that these nutrients are applied in a state designed to readily enter soils and potting mixes for easy uptake by roots through the soil water.

Manures, as mentioned above, are not the best sources of all the essential nutrients. Another disadvantage is that the nutrients they do contain are ‘locked away’ in the organic matter that make up manures, and thus not as immediately available to plants as with the commercial fertilisers.

But where manures do have an advantage, is because of that very same organic matter.  For when earthworms and soil microorganisms get to work on breaking that material down and incorporating its components into soil, not only do those nutrients become available to plants, but soil structure also improves in a way not possible with manufactured fertilisers.

This is because manufactured fertilisers do not contain organic matter, and it is the earthworm and microbial activity on organic matter that is responsible for improved soil structure. As organic matter is broken down and worked into the soil, good ped formation and the amount of humus begin to increase. These are two very important components of good soil structure, and as structure improves, several beneficial flow-on effects become apparent, such as:

• enhanced root development — roots not wasting energy to push through poor soils can reach farther water and nutrient sources more easily,
• increased soil aeration and water retention in sandy soils,
• increased water penetration and reduced water runoff in clay soils,
• increased nutrient retention, and
• reduced watering needs

Overall, manures have a more important role as conditioners of soil rather than as fertilisers of plants.

Manufactured fertilisers are reliably the same with every batch, and the same measured applications can be relied on to deliver the same amounts of nutrients every time. Manures as a natural product are not so consistent, and even the animal they are sourced from will influence how they should be used. We shall cover those differences, and the pros and cons of these, next week!