Weeds contain high amounts of key minerals

ECHO South Africa Research Update

Do weeds merit attention as a source of fertility? A key component of conservation agriculture systems such as zai and Foundations for Farming is that of planting pits into which fertilizers and seeds are placed. Could weeds be added to manure piles to improve the nutritional concentration of composted manure?

To help answer this, we collected fresh, green, leafy material of several commonly found weeds growing around the research farm where we are located in South Africa. A few of these were dayflower (Commelina benghalensis), wild hibiscus (Hibiscus sp), and khaki bush (Tagetes minuta). We also sampled leaves of a few trees including moringa (Moringa oleifera). Samples were dried, ground, and then sent to a laboratory for mineral analysis. Below are a few highlights (values are on a percentage dry weight basis):

Phosphorus was highest with wild hibiscus (0.44%), khaki bush (0.43%) and moringa (0.40%).
Potassium was highest with dayflower (3.58%) and khaki bush (2.07%).
Dayflower contained the most iron (623 ppm).
Zinc, which can be limiting in S. African soils, was highest with African wattle.
Moringa contained, by far, the most nitrogen (4.67%). It even contained more nitrogen than legumes, which have typically contained up to 3.5% in our trials to date. Moringa also contained the most magnesium (0.43%) and sulfur (0.96%)
So, yes, weeds do contain high amounts of some minerals. When working with legumes, we tend to focus on nitrogen to the exclusion of other plant-essential nutrients. Considering their ability to grow in poor soils, which is an indicator of their ability to scavenge nutrients, perhaps we should take a look at the potential of weeds as sources of nitrogen as well as other nutrients like phosphorus and potassium. A future post will show the effect of these weeds when added as green material to manure compost piles.

A few photos are below::

Khaki bush (common alongside roads and in fields; be careful of adding weeds to compost piles that have already gone to seed like this—ours had been sampled prior to this stage)

Dayflower, another very common weed