Francois Conradie (Idiom), Johan de Jager (Vinpro) and Dirk Coetzee (L’Avenir).
It often happens that producers jump into the profile pit with me when I am conducting a soil survey and start digging to open the walls. As we discuss the soil, the conversations often evolve and digress to other themes – from how the Boks played over the weekend to our children’s latest antics, and back to the right foliar nutrition or the correct timing for a specific fertilisation. With this article, I will summarise a few regular soil-related questions and answers.
Is one element more important than another?
Although producers sometimes wrongly focus on a specific element, the aim should instead be an optimal balance of all the cations (Ca, Mg, K and Na). When the correct balance of all the cations is reached in the soil, the pH naturally normalises. Other elements that are dependent on optimal pH also then become more accessible.
How should problems with immobile elements like phosphate (P) be handled?
Phosphate is largely immobile in the soil. Both short- and long-term strategies should therefore be followed to address phosphate accessibility and concentrations sufficiently. If I want to correct my soil entirely over the long term, the product should be applied through broadcast spreading to increase the concentration in the soil, and the crop will gradually take up the element via the roots. To address variation within blocks, the additions can be broadcasted with varying spreaders (Photo 1). Over the short term, phosphate should still be included in the fertilisation programme until the roots are able to utilise the long-term solution additions.
PHOTO 1. Example of varying spreader.
How should high sodium (Na) levels in the soil be managed?
In relation to the other basic cations, sodium (Photo 2) should comprise less than 3 – 5% of the cation ratio. This is monitored through routine soil analyses. As soon as the level rises above the critical norm, all manure applications should be discontinued due to the possible high sodium content. Secondly, optimal drainage of the soil should be ensured so excessive sodium can be leached out effectively. Furthermore, calcium levels should comprise at least 68 – 72% of the cation composition. Lastly, organic material should also be built up in the soil. If a sodium problem still occurs after these interventions, it should be washed out with sulphur via a gypsum application.
PHOTO 2. Soil with high sodium levels.
How should a soil with a high pH be handled?
Although it is a time-consuming process, a high pH in soil can be lowered, except when the soil contains free lime from the parent material (Photo 3). If soil drainage is improved and soil samples are taken to monitor the cation ratio, address the deficiencies and improve the balance to rehabilitate the soil, the pH can be lowered. Elementary sulphur can also be used to lower the pH, but good drainage is again essential. The sulphur must convert to sulphate, and the presence of bacteria in the soil is an important requisite for this to happen. In poorly drained soil, bacteria responsible for this conversion will die. Another way to lower the pH is to increase the amount of root growth – more roots result in more organic acids being released, which lowers the pH.
PHOTO 3. Soil with a high pH containing free lime from parent material.
Why is magnesium important for the crop?
Magnesium is a secondary nutrient essential for optimal photosynthesis, and acts as an activator for various enzymes that play a role in carbohydrate synthesis, nitrogen metabolism, etc. Photosynthesis has to be very effective in order to produce energy to ultimately realise the crop and farm profitably. Magnesium is highly mobile in the plant and is readily mobilised from old to younger leaves.
Why are high magnesium (Mg) levels in the soil detrimental?
High levels of magnesium can lead to higher soil pH, poor aeration, delayed water infiltration rate, and so forth (Photo 4). To lower the magnesium in relation to the other cations, the calcium and/or potassium levels in the soil should be increased. If there is already sufficient calcium and potassium in the soil, the following steps can be followed. Attempting to wash magnesium out of the soil will not be successful. Magnesium is a positively charged element and clings to a negatively charged soil particle. Magnesium, therefore, has to be converted to a salt in order to leach. We normally recommend a negatively charged element like sulphur, which will bind with the magnesium. Magnesium sulphate is then formed (Epsom salt), which is mobile and can leach. This will only be successful though, if the soil has good drainage.
PHOTO 4. Example of poor aeration and delayed water infiltration rate.
How does lime work in the soil?
In general, calcium is associated with soil pH, but magnesium and sodium can increase the pH even faster than calcium. Agricultural lime consists of calcium or magnesium carbonate (Photo 5). If the soil pH is low, it indicates a high presence of free hydrogen ions. When calcium or magnesium carbonate (or both) are added to the soil, a chemical reaction occurs with the hydrogen, forming carbon dioxide and water, and the calcium and/or magnesium is then left behind. We use lime to increase pH, but the conversion takes time, and the process can even sometimes take up to four years. The speed of the conversion depends on the fineness of the lime and the amount of water that moved through the profile.
PHOTO 5. Lime is calcium carbonate or magnesium carbonate.
What influences the breakdown or transfer process of herbicides?
Large quantities of manure additions can increase the salt load in the topsoil, which in turn can lead to a decreased prevalence of the bacteria that help to break down herbicides in the soil. Herbicides can break down faster in balanced, healthy soil than in soil with limitations like poor aeration or drainage.
Should I keep the material on the surface or remove it after establishing cover crops?
What is visible above the soil surface is only a part of the remains of cover crops and largely serves as soil cover. The cover crop roots beneath the soil surface are where organic material is built up when the roots break down. If the roots are retained and minimum tillage is applied, they will eventually convert to organic material. In short: make sure you keep the roots.
Does plant material protect the soil surface?
Visualise raindrops and how hard they hit the surface. If you have a protective layer on the surface, the soil will not be stirred up, washed away or blown away when it rains or the wind blows. The disadvantage, however, is that the top layer or hay under the berm will keep the soil cool. If the farm has a history of cold soil, it is not advisable to have a thick layer of organic material on the berms. What also needs to be considered, is that the top layer takes up nitrogen during the breakdown process. This can cause a nitrogen-negative period, during which micro-organisms use the available nitrogen in the soil to break down organic material, leading to nitrogen deficiency for the vineyard. The top layer can also have a water-absorbing sponge effect, especially if the drippers are lifted.
Why is organic material important in the soil?
I can talk about this all day. Organic material in the soil influences everything, from water- and nutrient-retention capacity, to the sponginess of soil and how forgiving your soil is towards environmental factors. Soil with higher levels of organic material will also be less prone to compaction.
How important is copper (Cu) in the plant tissue?
Copper improves disease tolerance and is regarded as the “disease nutrient”. Copper is involved in the synthesis of lignin, which helps to strengthen cell walls and offer resistance against pathogens. If you experience problems with disease pressure, it is a good idea to check your soil and plant’s copper levels. If you can increase the copper levels, the crop will have a natural resistance against diseases. Copper also improves the flexibility of vine shoots. The gap between sufficient copper and a deficiency is very narrow; however, toxic concentrations are not much higher than normal levels.
Is foliar nutrition worthwhile?
Foliar nutrition is really only a temporary solution. The biggest contribution of foliar nutrition is that it stimulates the plant to take up or extract more of a specific element from the soil (if such element is indeed available in the soil). You can nourish the plant with foliar nutrition applications, but if you do not have good soil fertility, the plant will ultimately display the same symptoms again. Early in the season, when the soil temperature is still low and the root activity starts to increase, foliar nutrition applications can help to combat spring fever in the vineyard. Foliar feeds are also mostly micro-element-driven, and these additions are most effective before flowering.
See you in the profile pit!
Johan de Jager and Heinrich Schloms.
For more information, contact Johan de Jager at johan@vinpro.co.za.
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