Designing for an Ecological Nutrient Management System – Joe Williams

Joe Williams is a Soil Health Specialist and the West Team Leader at USDA-NRCS.

He starts this presentation by asking us, “What functions do we want our soils to provide”. He reminds us that soil’s function is driven by soil organisms and that roughly 90% of soil bacteria in the soil are inactive. If the drivers of your soil are inactive, your soil is likely not functioning properly. To activate soil biology, you need ot create optimal conditions for their growth. Slide 8 explains how soil biology exists best with the pH near neutral, soil moisture levels at field capacity, moderate temperature, food (which is mostly carbon at a rate of 24:1) and good aeration. Your soil samples from the UH Crow Lab will give you a reading on the bulk density of your soil. This will give you an idea of how much pore space is in your soil and thus, how well it is aerated. During our visit with Kahuku Farms, seasoned farmers Melvin Matsuda and Clyde Fukuyama stressed the importance of having good aeration in your soil in order to maintain crop and soil health.

In slide 9, Joe reminds us that good soil structure and aeration improves soil moisture and temperature. Saturated soils caused by heavy rains and soils that do not adequately drain and high soil temperatures can cause beneficial soil microbes to become inactive. 

Slide 10 introduces us to the term ‘bio-turbation’ which is the impact of biology in the soil activating the system. This occurs as part of the biological decomposition process and through gas exchanges that occur within the pore space in your soil.

Slide 11 reminds us that having a diversity of soil particle size creates a variety of habitat types and a variety of biology. 

Slide 12 reminds us if we don’t have a healthy rhizosphere and healthy and diverse root growth, we won’t receive the nutrient cycling system in our soil will not be fully functional. Having roots that have a far-reach will extend the ‘sphere of function’. 

In slide 14, Joe introduces us to the hyphosphere, the zone influences by hyphae and fungi. Both from the mycorrhizae. Practices that increase the surface area of the rhizosphere and hyphosphere increase the amount of nutrients made available to your corps and increases nutrient cycling. The extension of roots is important not only for the health of your plants but also for the health of the soil ecosystem.

In slides 15- 16 Joe explains how the nitrogen cycle is fueled by bacteria and nitrogen-fixing plants that provide and process nitrogen. He describes bacteria and fungi as ‘little fertilizer bags’, describing how they’re consumed by nematodes and protozoa and excreted as plant nutrients. 

Slides 17-21 cover “life, death and mineralization in the rhizosphere.” In this slide, Joe reminds us that the C:N ratio of organisms all vary. Bacteria have a C:N ratio of 5:1.Bacteria feeding nematodes have a C:N ratio of 10:1.Since the ideal microbial diet has a C:N ratio of 24:1, the higher the C:N ratio, the slower decomposition occurs. Ideally, farmers want to to use cover crops and organic or carbon amendments that have a C:N ratio lower than 24:1 to prevent immobilization, which occurs when microbes compete with plants for nutrients. 

Slide 22 describes the C:N ratio of different types of cover crop. For instance, Harry Vetch has a C:N ratio of 11:1. This is lower than cover crops that are more fibrous or woody, such as rye. The timing of when you terminate your cover crop also determines the C:N ratio. For instance, when you terminate a rye cover crop before it flowers, it has a much lower C:N ratio than when you terminate it after it has flowered. During this part of the presentation, Joe also talks about how microbes impact the C:N ratio and cases when you amy want to slow the system down by having a higher C:N ratio (such as in orchard systems). 

As you may recall from our workshop at Thrive Farms in Haleiwa, Oahu, CTAHR extension agent Amjad Ahmad recommended using organic matter and organic amendments with a C:N ratio of 15:1. For example, he recommended adding nitrogen to lower the C:N ratio of carbon-heavy compost. 

Slide 23 Joe covers the nitrogen depression period and soluble nitrogen levels and residues. 

In Slide 24 Joe reminds us about the important role of the Phosphorus cycle. As he explains, phosphorus is needed for nitrogen fixation, root growth and photosynthesis. Joe gets technical on this slide and describes the process of adsorption, which is when phosphorus is bound to clay particles and not readily available to your crops and desorption. When biological makes phosphorus available. 

Many of the farms on Oahu’s North Shore are high in phosphorus. However, much of this is tied up. Having a pH that is too low or too high can tie up phosphorus and reduce the conditions needed to active soil biology. Since your ecological soil system needs phosphorus to function properly, using a phosphorus-fixing cover crop such as Buckwheat can help improve nutrient cycling. This is an example of why it is important for farmers to balance soil fertility and address any nutrient deficiency in order to have a properly functioning soil ecosystem.

Slides 25-26 address the importance of the water cycle in relation to nutrient management. In this part of the presentation, he stresses the importance of maintaining soil moisture in the soil profile and keeping evapotranspiration low. He reminds us that water infiltration is largely impacted by our management practices. He also stresses the importance of having good soil aggregate stability in order to facilitate the cycling of water and nutrients. 

Slides 27-29 describe some of the ways a conventional nutrient management strategy often doesn’t include the addition of carbon, which is the fuel for activating the ecological system and nutrient cycling in your soil. As Joe explains, the convetional system can be called a ‘Leaky System” as it is prone to leaching, runoff, and gaseous losses. In the dominant nutrient management strategy there is less mineralization that a system which focuses on feeding and activating soil biology. As he explains, adding too much nutrients to the soil solution could off balance the soil biology and excess nutrients ay favor an overgrowth of fungi or bacteria in the system. In this system, nutrients are rapidly release and all the nutirents can’t be consumed by the plants or feed soil biology. 

Although adjusting the application rate, range, time and placement, applying the concept of the 4 R’s to your nutrient management system, reduces the result of a “Leaky System”, it is still not ideal as this approach still uses rapidly releasing soluable fertilizers and is missing a carbon input. In this system the biological system is not activated and minerliazation is low. The Nitrogen and Phosphorus fertilizer efficiency is still only 40-60%. 

In Slides 30-32 Joe begins to explain the difference of a ecological nutrient managent system. In this system, healthy plants attract biology and the plant creates the environment and nutrients it needs around the rhizosphere. 

Slide 37 explains how having a diversity of roots, including fiburous and taproot. How soil health prinicples support soil function. Maximizing living roots and increasing biodiversity increases soil organic matter, increases nutrient cycling, and enhances plant growth. Minimizes disturbance and maximizing soil cover builds and maintains soil aggregates, minimizing wind erosion and maintianing organic matter. These practices also buffer temperature on soil surface, decreases EVT, increasing soil moisture and soil biology. 

In slide 40, Joe encourges farmers to use ‘strategic tillage’, only disturbing the soil and breaking it up when it serves a purpose that is needed to maintain crop and soil health. He also reminds us that over-tilling can decrease nutrient cycling, reduce habitat for soil biology, and reduce soil aggregate stability. He also reminds us that using excess fertilizer, pesticide or amendents can also cause a disturbance to the biological system. 

In Slide 42, Joe challenges farmers to maintain a ‘stable’ system verses one that is spikes up and down from management practices. 

Slide 43 shows photos of how a farm handled a high rainfall event with various management practices. Joe shows that the practice of keeping the ‘ground cover’ preformed the best in reducing erosion and preventing nutrient runoff. 

Joe closes his presentation by reviewing the ways various soil health practices improve the function of the soil and nutrient cycling. Many of these practices are included in a Soil Health Management Plan.