Farming & agriculture

Weeds and soil health

Californian Thistle flower

This article appears at Please visit for more info and weed identification

Rather than talk about a specific weed I thought it would be worthwhile to explore the underlying cause for weed establishment in pastures- unhealthy soils. Soil science is a complex subject and I’m more of a novice than an expert in the field yet what I’m learning through working as a Landcare Officer is the fundamental role soil health has in farming systems-particularly its role in supporting or suppressing undesirable plants, a.k.a weeds, therefore it’s a topic I cant ignore and nor would I want to.  It’s commonly believed that healthy soils support weeds and desirable pasture species equally well.  However it’s quite the contrary, in the same way that an insect infestation indicates unhealthy plants with a nutritional imbalance, a weed infestation indicates that something’s not right with the soil which in turn suppresses the growth of high producing pasture species and provides an environment favoured by weeds (Shepherd G. 2009).

In determining what’s not right with the soil its physical, chemical and biological components need to be properly assessed.  If thistles are your annoyance, a soil assessment may discover that the soil is deficient in the trace element Calcium and bacteria and high in Potassium and Sulphur-a profile typical of supporting thistles. An infestation of thistles would suggest soil conditions and fertility, (status of soil with respect to the amount and availability of elements to plants necessary for plant growth i.e. calcium, magnesium, potassium and sodium) are inadequate to maintain a complete, vigorous pasture cover. Competitive suppression by vigorous pasture growth plays a major role in preventing weed establishment. Once the underlying soil deficiency is known appropriate action can be taken i.e. the application of liquid calcium incorporating a form of organic carbon along with the addition of fertiliser with the right trace elements to help alter the soil environment in such a way that weeds don’t want to grow.

Changing the soil environment can successfully deal with any weed problem and provides for a more effective long term solution than the use of herbicides. However when faced with the initial problem of weeds, the incorporation of herbicides into a solution containing a pH modifier with ammonium humate, can provide good weed control. A mixture such as this enables the amount of herbicide used to be reduced by 25-35%, helping to buffer the effect of herbicide on soil life. Regular use of herbicides has a negative impact on soil microbes which are responsible for maintaining the nutrient balance and availability in the soil. The underlying message of this article is that whilst using chemical herbicides provides a quick-kill it only addresses the symptoms and does nothing to rectify the underlying cause.

References and additional sources

Baxter, M.N.  2001. Know Your Soils Agriculture Victoria- Bendigo. Centre for Land Protection Research. Part 1, 2 and 3.

Shepherd, T.G. 2009. Visual Soil Assessment. Volume 1. Field guide for pastoral grazing and cropping on flat to rolling country. Second edition. Horizons Regional Council, Palmerston North. 119p

Internet resources

Target 10 online Follow the prompts to Manuals, Soils and Fertilisers.

The Department of Primary Industries Type Soil Fertility into the search bar and follow the link.