Why Its Important to Measure and to Measure the Right Things

I was talking with a colleague recently about indicators of soil health. He’s a senior manager for a large horticultural enterprise. We agreed on most of them, though he attached more significance to biological activity at the time of sampling, than I do. Suddenly, he raised a challenge:

“Why do we need to measure soil health? I know healthy soil when I see it! You can dig your hands into it. It smells sweet! There are earthworms.”

Its a valid challenge. I am not sure whether I would have given an adequate account if pressed at that moment, so I have given it some thought since then.

The simplest reply is that by quantifying things, we remove the subjective assessment and bias. This is as true when one person plans the soil management/nutrition program over several years as it is when several people have that role in succession. All of them at times and especially as time passes form views that are influenced by their perspectives. Measuring, saving the data and referring to soil attribute data sharpens your perception, or attunes you to odd data. Data shows you changes in the soil that preferences or bias hide.

It should be true, in general, that the intuitive response of a trained and experienced person should be a good guide. Analysis of soil data should not (generally) contradict informed observation. My colleague was however probably pointing in two other directions: soil health on a farm is the product of good operational management, not technical measurement. What farm managers and staff do is what sustains or improves soil health. In addition, he was probably pointing to the need to train your powers of observation and insight. And on that he is right.

The Benefits of Measurement

The primary benefit of measurement is confidence. That confidence leads to action.

It is not uncommon for a manager to want to test a potential solution or concept in the field. Recording data about the trial is not commonly done. With only a visual inspection, and no data to review later, managers often, rightly, conclude that results were uncertain. That uncertainty leads to reluctance to expand the trial. Without confidence, we do not get substantial feedback that improves management practice.

This kind of response probably starts with the observation that an area of soil is performing less well by comparison with the location that is the picture of soil health.

What we measure also matters. There has been a long history of measuring and managing by soil chemical condition, ie the familiar NPK-through-micronutrients like boron and copper, soil analysis. There was little if any measurement of soil texture/physical condition, and soil biology. This led to nutrition/productivity programs that included only bagged fertiliser and at times some pH tests. It can also lead to misunderstandings such as happen in interpreting remote sensing imagery.

The well known NDVI analysis using the red and infra-red bands will detect variation in vegetation. It can assist the farm manager because one can learn the location of variation, its scale, and intensity. In itself however, NDVI won’t tell you the cause of variation. So one cannot assume that if an area is less vigorous, applying more N fertiliser is the solution. There are many possible contributing factors: soil temperature, soil moisture levels/saturation, sandy texture, low pH, low magnesium, low canopy closure/young crop, root damage and more. Most of these are difficult to impossible to determine from light reflected from an area on the earth surface.

Measuring plant vigour in this way does not result in a calculation for the amount of nitrogen to be applied to the soil zone. It identifies to the farm manager or agronomist the locations to visit and assess comprehensively before concluding on what to do, and how likely it is that a business analysis would justify correcting the conditions. In the same way, measuring soil P levels does not provide sufficient information on which to plan to treat low levels of P in the plant.

In short, we can examine the indicators, some of which can tell us what is happening before visible manifestations would be detected. This is very useful. None the less, we need to understand the range of factors that can cause issues, and assess the part that each might play in that location, before taking action.

We now know that soil physical condition, soil chemical status, and soil biological activity all interact with each other and act collectively to influence the fundamental productivity.

The convenience of getting lab tests of soil chemistry has led to over-reliance on it when planning to improve productivity. High levels of nutrients may not make it into the plant. All the nutrients pass through biological intermediates before going into the plant. Mycorrhiza fungi, for example, spread out through the soil mass collecting (among other nutrients) phosphorus and pass it into the roots. This increases the reach of roots significantly. But it only happens when the conditions supporting the fungi prevail in the soil. Soil structure is a key influencing factor in the role of soil biology in plant health and vigour.

Soil structure is a key factor in the functioning of soil, its ability to support plant and animal life, and moderate environmental quality with particular emphasis on soil carbon (C) sequestration and water quality. Aggregate stability is used as an indicator of soil strucuture. Aggregation is mediated by soil organic carbon (SOC), biota, ionic bridging, clay and carbonates.

Bronick C and Lal, 2005, Soil Structure and Management: a review

key:

• mediates refers to the role of things in causing or influencing eg the role of biota in causing or influencing the joining together of microscopic bits.
  
• biota refers to living things in the soil
  
• ionic bridging refers to the electro-chemical effect of ions that have charges and attach to two or more soil materials that have an opposite charge. They hold the small biological bits together to start building larger biological objects.

Our list of soil attributes to be used in assessing soil health can include not only the soil attributes that are causative, but some that are indicative, because they are sensitive to the effects of management on soil health. Soil biological attributes are the most likely to guide us since they change over appropriate scales of time to provide information on management impacts ie are responsive but not overly responsive.

For physical condition we have aggregate stability and soil texture including the type of clay and the proportions of clay, silt and sand. The type of clay matters because it has a strong physical effect on soil structure. The makeup of smectite clay does not favour aggregation, resulting in dense, unstructured soil that roots find difficult or impossible to grow through. Bulk density is a key measure. Bulk density is the weight of dry soil in a given volume. Sandy soils generally have the highest bulk density.

Measures for soil physical conditions should include:

• aggregate stability
• soil texture and type of clay
• bulk density

Soil chemical condition indicators are more well-known because nutrition plans generally focus on the chemistry and in particular on ensuring sufficient nutrient for optimal plant growth. Since healthy soils¹ must sustain plant and animal productivity, we should collect data on:

• pH, as it is a measure of nutrient availability
• phosphorus (Colwell)
• potassium
• calcium
• magnesium
• electrical conductivity²
• cation exchange capacity (it includes both the soil parent material and the accumulated soil organic matter)

It is generally recognised that more nutrient in the soil does not ensure healthier or more vigorous plants. More is not better. In fact, more might make things worse. There is also some doubt that high levels of nutrient in the soil will reach the plant leading to more productivity.

In some cases, the high residual nutrient levels in the soil will raise the electrical conductivity of the soil, causing reductions in yield. At the point that electrical conductivity (EC) reaches 700 m/S per cm³ it is impacting on the productivity of many crops. Even before that it is probably affecting signalling between the root and the soil biology in the root zone.

Soil biological status is more difficult to measure meaningfully. It is desirable to measure soil organic carbon but the result must be evaluated carefully. If you sample in the same week or weeks each year, you minimise the impact of soil temperature. Small changes in soil organic matter can reflect higher soil temperature and moisture. The same is true of the ratio between bacteria and fungi. Fungi are consumers of resistant carbon; bacteria consume soft carbon. Their balance in the soil changes rapidly in response to changes in the organic matter available to them and its carbon:nitrogen ratio.

The best option is to measure the biological factors that support soil biology:

• labile carbon, the soft pool of carbon that is used by soil microbes for energy
• carbon:nitrogen status of crop litter. It should be about 24:1 C:N. And there should be plenty of it.
• sulfates

In the near future, we will probably have measures of soil microbial guilds that perform certain tasks. We may have measures of the trends in carbon pools in the soil and use these as guides to management. We may use DNA sampling of soil biology to provide early indications of trends in the soil, because soil biology change at about the right rate to serve that purpose. We might use DNA sampling because

…consistent changes in soil microbial communities have been associated with changes in P availability, soil pH, labile organic carbon pools, and soil moisture levels. Likewise, we can often identify particular microbial taxa or functional genes associated with specific soil processes, including nitrification, methane production, denitrification, and cellulose degradation³

For the moment, we are able to foster a healthy and biodiverse soil biology without quantifying the soil biology as a count. Instead we count and monitor the conditions that support it, doing our best to optimise them.

Monitoring and optimising seems to be the heart of agriculture. When we use data to support intuition, its because we have kinds of data that enable us to recognise the trend of change in the soil before it becomes visible to the naked eye. It also refocuses the attention of a farm’s agronomic advisor on the full range of factors that influence productivity.

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