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February 2018

Building soil health: 5 Key Soil Tests to get you started

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First thing to decide is where to do these tests. You might pick a few sample sites in 4 key fields and test them every 6 months. Most of these tests require a decent spade so you can dig 10-20cm depth into the soil profile in order to analyse it.

This tool is made for farmers, we want to enable farmers to understand the ‘pulse’ of the soils on their farm so this is not a precise science — it’s about what is workable on your farm in order to monitor your soil health, better meet your management goals and ultimately have a thriving farm. So the key to all these tests is to be as consistent as possible with what works for you on your farm. We try and keep this as simple as possible to do, so for example, use a spades width and depth to ensure you dig up the same amount of soil each time you do your earthworm count, and use the same spade! Repeat the tests in the same field, at the same time of year (the app tracks this for you). Working with this simple principle you will build up an amazing picture of how your soil is changing, and hopefully even improving!

We have more advice to help you make these decisions here.

1.The VESS test

This is where you get to dig in and really get a feel for your soil. We take a photo as soon as the soil is dug up to see its profile and initial structure (and so we can share it with others later to get their thoughts). Then we look to see if there is an obvious divide between a top soil and the subsoil below. On most farms we have visited the top layer of soil is as thin as 1-10 cm. This is where the aggregation* is happening and there are lots of roots so this is the layer we want to work on building up. Then the next 18cm is relatively uniform in colour and structure. We think it’s helpful to score topsoil and subsoil separately and record the depth of each – one indicator of better soil health is when the topsoil depth begins to increase, as roots reach further and further down and aggregation begins to happen deeper and deeper.

*aggregation: Soil aggregates are clumps of soil particles that are held together by moist clay, organic matter (like roots), gums (from bacteria and fungi) and by fungal hyphae. The aggregates are relatively stable and vary in size. This means that there are spaces of many different sizes in the soil and these spaces are essential for storing air, water, microbes, nutrients and organic matter.

Find full details about doing this test here.

2.Count your earthworms

It has always been explained to me that earthworms are the top of the soil food chain. So if you have lots of earthworms then the soil food web must be doing well! For this test, take the soil sample you’ve dug up and count how many earthworms are present. It’s important to note that this test is quite seasonal: on the farms we visited in the UK in November there were loads of earthworms but when we went out on the farm in Chile last week in the middle of Summer, we saw just one earthworm very deep, across 9 sample sites on 3 fields. When it  is very dry earthworms tend to be hiding somewhere! They also move around depending on heavy rains and other factors, so if you are going to do this test, then it’s best to do it across all the fields you are monitoring in one go. That way you can compare between fields.

Find full details about doing this test here.

3.The Slake test

UK Soil scientist Jenni Dunghait is the expert on this. It’s very easy to do, just put a pea-sized piece of soil in water and leave it. How much the ‘pea of soil’ breaks down indicates how much sticky stuff there is holding your soil together, thanks to the work of all those little microbes. Our three fields had soils that broke down completely differently. In one field where there was the most evidence of aggregation, the ‘pea’ did not break down at all over 24 hours. However, in another field, where the soil was very crumbly, red and easy to dig into but also not much evidence of structure, most of the samples broke down completely within 2 hours and all within 24 hours. As far as I understand this shows that the microbial activity and aggregation activity is very low in this soil. It was deep-ploughed somewhat recently which might explain the lack of compaction but it’s lack of aggregate structure means it’s not particularly alive!

Find full details about doing the test here.

4.How much ground cover and bare soil is there? What is the percentage cover of weeds (undesirables), herbs, grasses?

For many of us a key reason healthy soils is important is because we want healthy plants above ground, and importantly healthy plants that support the bottom line. For PFLA members that often means increased forage, more grass species, less buttercups. On our farm in Chile that means healthy vines and olives, and fostering warm season grasses and perennials for fire retardant ground cover in Summer.

This measure is a great way to understand the link between healthy plants and healthy soils. The 1st soil health principle is a living root, so lots of bare soil is not a good sign. What’s on the surface of our soils can tell us a lot about what is happening below, so for this test record % cover of undesirable species, herbs, grasses and bare soil. Our farm is not pasture-based but this is still helpful as a measure – one of our main tools for managing damage from fires is to shift our ‘undesirables’ to plants that remain green all Summer long.

To do this test you need to make yourself a quadrat. Full details on doing these tests here.

5.Measure the sugar content and health of your groundcover with a Brix reading

Brix is a measure of photosynthetic activity. The building block for production and plant immunity/health. Brix measures how much photosynthesis is occuring in the plant by showing the amount of sugar and dissolved solids in the sap. Higher values indicate the plant is photosynthesizing more rapidly. Thus growing faster or having a better immune response and a higher nutrient profile. Brix is already used by many fruit producers as an indicator of when their fruit is ready to harvest. Research has shown that Brix readings show the actual sugar content in pasture, as well as other plants.

I first heard about it from Australian farming advisor Graeme Saite as he explained if you take a Brix reading in the morning and then another in the afternoon, there should be a big difference in sugar content because late afternoon the plant moves all it’s sugar from its leaves (solar cells) to its roots to be in conversation with the world below. If this isn’t the case then the system connecting your plant to the soil isn’t working.

We mainly use Brix as an indicator of plant health. We compare Brix readings across fields at the same time to see which plants have more sugars. It’s then interesting to do see how the Brix value evolves over time. An increase in Brix value could be a good indicator of improvements in soil health and healthier plants. Brix is very dependent on the time of day you do it (as explained above) as well as the season, so if you want to compare across fields you need to get round and do the Brix readings all in one go and then try to do them again, on more or less the same day and time a year later. Also Brix doesn’t work in wet conditions as the rainwater dilutes the reading.

To perform a brix test you will need a refractometer and garlic crusher. Find full details on how to do this test here.

So, those are the first soil tests to get you started! Keen to learn a few more? Head over to the free soil testing guide on our website to check out the full list and learn about our Sectormentor for soils app here.