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Expertise

The influence of production practices on soil organisms

By Kevin Fourie: Agronomist - Northern Cape Business Unit

Arising from Omnia’s business approach, we tried during the past planting season to not only test Omnia’s intellectual en technological capacity in the Northern Cape, but also to expand and develop it in unique ways in this area to add more value on the farm. This forced us to fall back on the 12 elements which form the basis of Nutriology®, including research and technology, which plays a dominant role. There is a great need in the Northern Cape because of its unique conditions, and all the tools in the technology suitcase had to be unpacked.

The conditions and practices in the Northern Cape differ in many respects from the rest of the country. The area receives much less rainfall, experiences very high temperatures in the summer and suffers from irregular cold spells during winter (Figures 1 and 2). The soils vary from high to low pH, from 35% clay in the top soil to 80% sand, includes rocky and limestone reefs and are uneven to homogenous. These variations can be seen within a radius of 20 km.

The cultivation practices ranges from conventional, minimum and zero tillage. The greatest challenge is the relatively high intensity double cropping system which is followed. This results in two crops per year, which puts a lot of pressure on the cultivation of soils and the time it takes. Because of these time limits, farmers often burn the masses of harvest residue after maize and wheat plantings to prepare the seed bed for the following crop as quickly as possible, which will be planted one or two weeks after harvesting.

Therefore the need arose for a way to determine the general health of the soil and to find out what influence the different practices have on chemical and biological properties of soil, as well as on the growth and development of maize and wheat.

Through the use of Omnia’s technology, the study generated a wealth of data. This article will focus on the reaction of micro-organisms (bacteria and fungi) as well as urease. More specifically we will look at the effect that variations (climatological, soil and cultivation practices) have on soil health as well as the eventual yields.

Two localities were selected: 1. The soil in Jacobsdal (Rietrivier), which uses conventional cultivation practices, had a bulk density of 1 146 kg/m3, sand fraction of 90%, pH of 6.14, ECEC of 4-5 cmolc.kg-1 and organic C of 0.5-0.6%. 2. The soil in Hopetown, which follows a no-till approach,  had a bulk density of 1 050 kg/m3, sand fraction of 80%, pH of 6.38, ECEC of 18-19 cmolc.kg-1 and organic C of 1.3-1.5%.

Various analyses were done at both sites during the year on maize (summer) and wheat (winter) at different growth stages, including soil, OmniSap® and OmniBio™ analyses as well as SPAD meter readings.

From the following graphs, it is obvious that the season had a profound influence on the micro-organisms in the soil. Temperature and moisture are two of the principle determinants of biological activity and general soil health. Following on these are the development of the crop itself (root mass, available sugar, oxidation-reduction reactions, etc.), availability of nutrients (i.e. applied fertilizer), the influence of physical cultivation (wet rip action three to four weeks after plant between rows), and lastly chemical weed and pest control.

The greatest difference between the two cultivation methods is the variation between the micro-organisms during the season with the mentioned variables taking place. The conventional tillage (Figures 3, 4 and 5) showed much more variation and inherent imbalances between micro-organisms than the no-till (Figures 6, 7 and 8) approach, which has a very strong buffered system. This system leads to a more stable micro-organism activity and balance and is important in terms of soil health, development and growth of a crop. The latter leads to a much higher organic matter content, as well as organic carbon. It therefore results in better efficiency of applied fertilizer (Nutrient Use Efficiency), as well as a higher water use efficiency.

The soil in Figure 3 lay fallow after the 2011-2012 maize season. This means after the maize, no wheat was planted in winter and thereafter maize was planted again in summer. The fluctuation in the carbon dynamics due to the fallow system had a great effect on the micro-organisms. The bacterial populations in Figure 3 grew 2.14 times more from the pre-plant of maize to the wheat flag leaf stage. This means that the populations almost doubled. Results by Six et al. (2006) found that a shift in microbial growth can take place because of sub soil C which is influenced by root exudates. This explains the large variation when bacteria in Figure 3 are compared to fungi in Figure 4: the increased root mass contributes to the stimulation of the micro-organisms, for good or for bad. These root exudates therefore contribute to the accumulation of carbon in the soil, which influences the use of carbon by the micro-organisms. This means that the freely available root exudates stimulate the micro-organisms to such a degree that the tempo at which soil organic matter is decomposed is either accelerated or slowed down (Kuzyakov et al. 2001).

The presumption exists that cultivation of soil before planting negatively influenced the bacterial populations, which explains the decreased count of maize at four weeks after emergence. The influence of soil disturbance during tillage has been under-estimated in certain respects.

The fallow system influenced the fungal populations greatly. The long period without disturbance, with sufficient food, caused a rise in fungal counts and can be seen by the high count at maize pre-plant. Again the influence of tillage on fungi can be seen on maize at four weeks after emergence, as well as wheat pre-plant. The possible effect of the cold temperatures on the slow growth of fungi during the wheat season was noticeable and creates concern for the balance between micro-organisms.

Studies by Schimel et al. (2007) showed that fungi are very sensitive to anaerobic conditions. Therefore the conditions in the Northern Cape, where there is a lot of water later in the season almost water logging the soil, can have a negative effect on fungal populations.

Urease activity is influenced by a few factors, of which temperature is key, as well as the amount of available oxygen in the soil (Frankenberger et al., 1980). Both these factors are tested to the limit in the Northern Cape. Although the high temperatures can be beneficial, the extremely warm conditions in the top soil can negatively influence urease activity. Large quantities of water are also applied, sometimes too much, leading to wet, waterlogged soils. This results in anaerobic conditions that suppress urease activity (Frankenberger et al., 1980). Figure 5 shows the effect of large quantities of water later in the season very clearly. Therefore it is assumed that urease activity is under pressure under conventional tillage practices due to large quantities of water and high nitrogen applications later in the season.

Comparing the two localities, the average counts of the different micro-organisms are clearly much higher in Figures 6, 7 and 8 as in Figures 3, 4 and 5. The bacterial populations had a coefficient of variance of only 14% under no-till compared to the 22% coefficient of variation of the conventional tillage. Similar results were found in the fungal populations, where the coefficient of variation was only 29%, compared to the 35% of the conventional tillage. The effect of a healthy soil life on the life cycle of micro-organisms is therefore very clear. It is very interesting that the same trend of a reduction in fungal counts during the wheat season can be seen in Figure 7, which means that more attention should be given to fungal counts in winter - not only on permanent crops but also on annual plants such as wheat.

Comparing the urease activity of the two localities, the average urease activity throughout the season in the conventional system (Figure 5) was 44 units, while it was 57 units in the no-till system (Figure 8).

This can be due to several reasons, but the most logical answer is the prevalence of more stable soil conditions. The average soil temperature is more stable and at the same time the soil did not experience as many wet-dry cycles. There is increased substrate under no-till conditions throughout the year for the urease to function than in conventional tillage practices. The micro-organisms in the soil with higher carbon are less dependent on the inherent carbon formed by root exudates, leading to less fluctuation in populations. Therefore there is a concern over these fluctuations and the influence micro-organsms can have on the efficacy of fertilizer at certain growth stages. Is the urease activity in the soil good enough to convert the applied nitrogen source to a plant available form quickly?

Conclusion

By using available technology, the influence of different practices on soil life and health could be determined throughout the season under Northern Cape conditions. It was clear that more attention had to be given to microbial activity in the soil, as it has an effect on the efficacy of fertilizers, moisture and general growth of the crop. The healthier the soil, the more stable the micro-organisms and the more stable the yields will become. Fungi management in general can also help producers who want to switch over to reduced tillage practices to achieve a more fertile soil faster.