Benefits of monitoring the physical condition of the soil with a penetrograph

Brazil is currently the world's largest producer of sugar cane, with a cultivated area of ​​more than eight million hectares. To maintain this standard, increasing production and expanding the cultivated area is necessary. However, the indiscriminate use of agricultural machinery can compact the soil, causing damage to its structure.

Compaction, defined as “the process of reducing soil porosity by increasing its density", is due to the use of heavy machinery that travels in the area and also to agricultural implements that pulverize the crop. As a consequence, the soil has its capacity decreased infiltration and aeration, and there is an increase in surface runoff and resistance to root growth. Thus, both seed germination and the extraction of nutrients and water by plant roots are affected.

This phenomenon is one of the main results of the inappropriate tillage system in the arable layer of the soil. There is no standardized preparation methodology that is applicable to all soils, and producers weigh the potential and limitations of the cultivated area and the needs to meet productive demand. Some studies indicate that subsoil compaction can persist for 30 years after a single pass of combined tractor and trailer heavy machinery. These limitations result in a drop in agricultural income and increase production costs.

SOIL PREPARATION AND RISK OF COMPACTION

Soil preparation is a process that aims to modify its physical state to provide a favorable environment for crop growth, with different preparation operations having different effects on plant growth and yield. In the case of sugar cane, soil management consists of an initial reform preparation, also called post-harvest, which corresponds to subsoiling and incorporation of residues and correctives, and pre-planting preparation for the formation of bed and furrow. This preparation has an important effect on water flow, storage and evaporation through the soil and plant roots. Furthermore, it influences the microbial activity responsible for soil humidity, temperature and aeration, as long as it is treated with appropriate agricultural equipment.

The greatest risk of compacting soils cultivated with sugar cane is found precisely during the harvest, when heavy machinery enters the field. When this happens, the soil receives a load that propagates in depth. The spread of this load and the consequent compaction will depend on the type of tire (one, two wheels or track), the soil moisture, the organic matter present in the soil, as well as the tire inflation pressure and the load per wheel. All these factors together can contribute to increased soil compaction.

Penetrographs are very useful equipment in evaluating compaction, as they simulate root growth in the soil and their data provide an indirect physical measure of the soil's mechanical resistance to penetration - normally expressed in kPa or Mpa, a ratio of force per unit area . Furthermore, they are easy to use and affordable equipment. Monitoring compaction through resistance to penetration serves as a basis for investigating the influence of agricultural machinery traffic on the soil and thus carrying out a survey of the most affected areas.

Adequate penetration resistance monitoring should also include soil moisture monitoring to more accurately determine the most compacted zones within a study area. With the data obtained, the producer can, for example, compare them with productivity and fertility maps. Since compaction is a limiting factor in productivity, it will be possible to compare the production restriction with the levels of compaction found in the field.

In order to provide producers with information on the versatility of the equipment, researchers from the Federal Rural University of Pernambuco have carried out several studies on the use of penetrographs in evaluating soil compaction. Within these studies, researchers carried out a field analysis of the state of soil compaction caused by the traffic of agricultural machinery during the sugarcane harvest in the municipality of Goiana, in Pernambuco, using a SoloTrack PLG5200 penetrograph, configured with a resolution to carry out measurements every 1cm using a type 2 cone with a maximum penetration resistance value of 7.700 kPa.

RESULTS OBTAINED

Of the total of 80 measurements carried out in the area, only 48% managed to reach a depth of 20cm, and 37% of measurements reached a depth of 40cm. In the remainder of the measurements, the equipment performed automatic recoil due to excessive force, as during the measurements the resistance to penetration presented by the soil was greater than the limit value stipulated in the equipment, of 7.700 kPa. The average humidity conditions in the area were slightly below field capacity: 13% for the first 20cm of soil depth and 14% for the depth of 20cm to 40cm. In this case, both soil preparation, humidity and the type of cohesive soil that occurs in the area jointly contributed to the high resistance to penetration.

In specialized literature, the critical limit for root growth is 2500 kPa. Above this value, the root system of most vegetable crops begins to present limitations in its development. In the case of the investigated area, the lowest values ​​observed on the soil surface are already close to the limit of 2500 kPa, while the maximum values ​​observed exceeded more than twice the reference value. This situation shows that there are areas that are more compacted than others within the plot. In this case, it would be interesting to carry out a study of the area in association between the most compacted areas with the water content in them at the time of the penetration resistance test.

TABLE 1: Summary of descriptive statistics for penetration resistance at two depths.

Depth	n	RP	Minimum	Maximum	moisture
		kPa	kPa	kPa	%
0-20 cm	39	4020	2280	6130	13
20-40 cm	30	3220	1540	5780	14

Depth

n

RP

Minimum

Maximum

moisture

kPa

kPa

kPa

%

0-20 cm

39

4020

2280

6130

13

20-40 cm

30

3220

1540

5780

14

Using the penetroLOG software that accompanies the penetrograph, the RP values ​​could be presented in figure form, according to the depth analyzed (Figure 1).

With this tool, it is possible to clearly observe that all values ​​are above the critical value of 2500 kPa (yellow line) and that the layer that presents the greatest resistance to penetration is between 5 and 10 cm, which exceeded 5000 kPa (pink line). This would be the area most affected by agricultural implements as it presents the maximum resistance to penetration in the studied area.

Figure 1. Average Penetration Resistance (kPa) Vs. Depth (mm) for layers of 0-0,20 m (a) and 0,20-0,40 m (b)

MONITORING IS NEEDED

Monitoring the physical condition of the soil using a penetrograph quickly showed the layer most affected by compaction. This demonstrates that the increase in sugarcane production in Brazil may result in the risk of compacting agricultural soils in the long term if their management is not adequate.

From this point on, the producer will be able to consider other ways of preventing or remedying soil degradation due to compaction through measures such as adopting other agricultural practices, reducing the axle load on tractors, increasing the soil-tire contact area , working on soil with adequate moisture, reducing the number of machine passes, controlling agricultural traffic, increasing the introduction of organic matter, removing deep soil compaction and crop rotation.

This article was published in issue 137 of Cultivar Máquinas magazine. Click here to read the edition.