There is a lot of talk about soil biology and soil organic matter currently.
Not so long ago, such topics seemed to agronomists to be part of the hippie world view, and hardly credible. There are now many startups, often established by scientists, with an aspiration to develop a line of biological products that are applied to seed or through irrigation to the soil. Many of these novel products address environmental issues as well as production issues. They may protect the plant against drought or high temperatures, and be linked to climate change, or reduce the need for fertiliser, and be linked to offsite damage to watercourses and inshore coastal waters, through eutrophication.
These products may or may not work, depending on the research supporting the product, and a host of site considerations such as soil texture, pH, crop etc. The proliferation of products broadens the farm manager’s awareness of factors that affect productivity and that is good. It also makes it easy to imagine that soil health is primarily a result of biological factors first and foremost. Its a view that can lead to further disappointment.
High soil health (for productivity) is the result of the balanced interaction between soil physical condition, soil chemical condition and soil biological condition. Each of the factors influences the other two in a dynamic way.
It’s essential to measure both soil physical condition and soil chemical condition, and optimise these factors before applying soil biology products, or at least at the same time as optimising them.
The application of ecology in agricultural pest and weed control has become embedded in US agricultural policy, resulting in management options that blend biologically and chemically-based strategies (Liebman and Gallandt 1997; Ehler and Bottrell 2000). In contrast, an integrated nutrient management strategy based on ecological concepts has yet to be broadly applied.
2007 Drinkwater L, and S Snapp, Nutrients in agroecosystems: Rethinking the management paradigm
Although farm managers routinely conduct soil sampling programs to guide their management of soil chemistry, it is far from routine to assess soil physical condition and soil biology. With a kit of cost-effective tools, the land manager can conduct soil sampling programs that quantify some the effects of human management on the soil, or natural limitations that often go unobserved but have real impacts.
A good starting point is to print off some maps of the farm. Choose a medium resolution size so that you can identify features in the paddock when you are out there. Plan sample locations based on your knowledge of the farm. In some regions, there are soil maps publicly available that would be a guide to soil types or classes. Keep in mind that a grid of samples is not an efficient plan. Soil varies across the farm, but not in a regular way, unless you are in a very favoured location. Aim to detect areas that are the edges of relatively homogenous zones, and have a sample or two within the zones.
It is essential to consider where human management may have caused changes to the soil. Are there locations where the topsoil is thin due to leveling? Areas at the bottom of slopes or areas where water can pond may have impacts such as high electrical conductivity or low pH. Ponding areas can be small. For example under tree irrigation may create ponded areas where the fertigation mix collects between trees. Even in a three metre wide row, the range of conditions can vary significantly.
The plant knows only its root zone and perhaps some signalling in the soil to similar plants nearby. Whether it thrives or not is down to local conditions. Average conditions across a farm block are not meaningful measures if there is significant variation. What degree of variation is significant is determined by the plant, and its symbiotic relationships with soil biology. Avocados are well known for their sensitivity to even a moderate electrical conductivity in the soil.
Since we are attempting to define the edges of soil zones and identify and quantify problem areas to drive better productivity, its worth collecting data and samples at at least 50 locations on a 100 ha farm. Some of these you will want to revisit in subsequent rounds of sampling to calibrate the effectiveness of your management strategies. Store a GPS location for each site. Most smart phones have a built in compass. The various app stores provide compass apps that show the GPS location.
Our Low-cost Soil Health Toolkit
The components of our kit include:
- soil penetrometer: to measure how far down roots are likely to penetrate, providing oxygen and nutrients to the soil biology
- electrical conductivity meter: measures the level of salts present in the soil sample or location. As EC increases, soil microorganism activity decreases, affecting respiration, residue decomposition, nitrification and denitrification.
- pH meter: The growth and reproduction of soil bacteria and fungi is reduced as pH declines below 6. Unless the specific crop requires a lower pH or higher pH, the optimal range is 6 - 7.
- temperature probe. Soil temperature can vary across paddocks due in part to the effects of the parent rock, or the orientation of the slope. Soil temperature and soil moisture levels are critical factors in soil biology levels. This explains why counts of bacteria and fungi go up and down. Knowing how your soil varies in regards to temperature and moisture can help calibrate your nutrition plans. Of course you cannot add more fertiliser to make up for less biological activity, based on soil conditions.
- portable soil moisture probe. The tool to quickly identify soil moisture levels. We might be able to guess at differences but this focuses the user’s attention.
A snapshot based on a single survey is useful, but building a history for each point adds more certainty. The trend may wobble a bit for various reasons but over time becomes clear. In fact, data is becoming a key asset of farms.
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