In-Field Diagnostics

In-Field Diagnostics

A large part of the Plant Management segment of our four-day Certificate in Sustainable Agriculture course is dedicated to demonstrating the use of a range of in-field diagnostic tools. These include a range of pocket tools for immediate feedback and a novel use of conventional leaf analysis. Together, we call this approach Plant Therapy™. These tools can prove invaluable aids but it must be made clear that they should never serve as a substitute for informed observation and hands-on participation in the growing process.

The Farmer's Footsteps

One of the American consultants made the comment that “the very best fertiliser is the farmer’s footsteps in the field” and this is profoundly true. We need to get amongst the crop on a regular basis. Dig up some roots and check root structure. Look for the sticky, fuzzy, brown sleeve that should encase each root. If your acid fertilisers have stripped off this sleeve, then you have created a dependence upon imported inputs because you have compromised the root zone. You will no longer receive a large percentage of your nitrogen free from the atmosphere. You will no longer have access to the massive frozen reserve of phosphate that most soils contain and you will have an increased reliance upon farm chemicals in the absence of disease-suppressive biology.

If you are growing legumes, check for nodulation. If the nodules are few and far between then you may have negatively impacted the roots with unbuffered acid, salt fertilisers or you may have inadvertently redirected the plants messaging system. This is a common phenomenon which is often not anticipated. If a legume is supplied with large amounts of nitrogen, then it is much less likely to message the roots to develop the nodules to house Rhizobium bacteria for nitrogen fixation. Why waste your energy building structures for N-fixers when you have already been supplied with enough nitrogen for your needs?

This phenomenon also applies to non-legumes in relation to nitrogen and phosphorus fertilising. Plants don’t just feed glucose to the microbe army beneath their roots; they lace the sugars with very specific nutrients. In a process not unlike using different bait to catch different fish, they vary their exudates depending upon their requirements during the crop cycle. If nitrogen is needed then the bait is changed and the nitrogen fixers are favoured. If phosphate falls short then a different lure is used to bring in the P solubilisers. Biological farming becomes a question of getting the balance right. The aim is to never apply too much N and P or you risk shutting down your natural supply of these two important minerals.

The other thing to check with legumes is that nitrogen fixation is actually happening in the nodules. The simple trick here is to pinch the nodules and if they are functioning well they should be pink inside. If your nodules are not “in the pink” they are probably short of either molybdenum or boron and application of these two minerals can be very productive.

In the process of leaving your footsteps in the field there are several other things you should check. Observe the thickness of the leaf and stem, the size of the leaves and the presentation of the leaf (as a solar panel) to the sun. Check for mycorrhizal colonisation of the roots and watch out for the blotches, pale colours or stripes that may herald a trace element deficiency.

Success Without the Props

I have a graphic memory of an experience early in my consulting days. I had been speaking to a group of growers about the use of monitoring tools to increase diagnostic precision. A vegetable grower of Italian extraction was fascinated with this new approach. He noted that he had never ever monitored his crop during his growing career. He asked if we could move next door and measure some of his vegetable crops with these tools as he had been growing blind all of this time. I puffed out my chest and agreed to help him with superior science and the whole group moved next door to witness the evidence of his apparent ignorance.

The crops looked perfect but I was sure that I would uncover some hidden yield limiter. I sampled crop after crop, with the expectant crowd watching my every move. I checked brix levels, sap pH, sap conductivity, nitrate levels and potassium in the sap, but could find nothing wrong. In fact, everything was perfect. This grower knew about footsteps in the field. He had learnt to read his crop to perfection without the use of props. Apparently his father had taught him this skill but there is no reason why every grower can’t learn these things.

The use of monitoring tools and leaf analysis can be used to fast-track these skills. If you get a leaf test back that reveals a lack of zinc and manganese then head out to your crop and learn what these deficiencies look like. Take photos with your mobile phone to help as a guideline should the problem recur. If your refractometer shows a clear and distinct line between the two hemispheres, this is usually a calcium deficiency. Observe the crop to familiarise yourself with a calcium shortage. You might note the presence of broadleaf weeds, the tell tale sign of a calcium shortage in the soil or you may detect a tight soil when using a penetrometer. In both cases the flocculating capacity of calcium is required to help the soil breathe and to boost the biology.

Novel Ways to Use Diagnostic Tools

  1. Use your refractometer to monitor weeds in comparison to crop plants. If the brix level of the weed is higher than the crop then you have a mineral balance that is favouring the weed and weed pressure is assured.

  2. Check the lower leaves and the first fully developed leaves with your potassium meter. It is important that the levels are within 5% of each other. The moment that the lower leaves drop below that level, you have detected a potassium deficiency. This is a valuable tip because the standard testing site for potassium at the top of the plant is notoriously ineffective when monitoring this mineral. Potassium determines fruit or grain size so improved monitoring can lead to enhanced profitability.

  3. Monitor the brix levels in your crop in both the morning and the afternoon. Levels should always be lower in the morning as the plant has translocated 30% of its sugars down to the roots overnight. If there is no difference, you may have detected a boron deficiency, as boron opens the trapdoor that allows the transfer of sugars. This simple shortage, that may cost less than $5 to correct, can be costly. When there is no sugar delivery to the army of organisms that deliver minerals and offer protection to the plant there will be expensive consequences.

  4. Use a far infra-red temperature gun (available from electronic stores for less than $50.00) to monitor your crops need for water. Here’s how it’s done. Hold a sheet of white paper 30 cm away from the gun and measure the air temperature between the end of the gun and the paper. This is called ambient temperature. Now point the gun at the foliage, maintaining the same 30 cm distance, and measure the temperature of your crop. There should always be a difference. The crop uses evaporative cooling to always keep its temperature lower than the ambient temperature. If the crop is the same temperature or higher than the surrounding air, then you have detected a water shortage and should act. There are a few rules with this strategy. Test your crop when it is warm enough to detect stress. Keep the sun behind you and the wind must not exceed 15 km per hour.

Call NTS to talk to an Agronomist on +61 7 5472 9900 or email NTS on