Importance of the ideal subsoiler for each soil condition

The use of the direct planting management system (SPD), which emerged in Brazil in the 1970s, currently predominates in Brazilian territory. According to the Brazilian Federation of Direct Planting in Straw, in 2006 Brazil reached 25,5 million hectares, currently, it must have already exceeded 30 million hectares. Through sowing, without soil preparation and on the straw of the previous crop, SPD is recognized as the biggest revolution in agriculture in tropical regions and emerges as one of the greatest technologies in the scientific world in the area of ​​agricultural science, as it reduced operations and costs with agricultural machinery, and is also a great benefit to erosion problems caused by soil removal.

The benefits are numerous, as when straw from previous crops is maintained there is greater moisture retention. As there is no direct incidence of sunlight, the soil temperature remains lower, consequently enabling greater crop resistance in periods of drought, reduced erosion and fertilizer losses, increased organic matter and improvements in physical and chemical structures, contributing to greater crop productivity.

However, after years that this system has been implemented, the soil has been presenting some compaction problems, which are made worse by the lack of soil disturbance and its intensive use. The increase in the size of tractors, seeders and fertilizer spreaders, resulting from the demand for high operational capacity over time, has increased the load on the ground, and this, combined with inadequate tire inflation and operations in high soil moisture conditions , exacerbates the problem.

The soil, when compacted, has a reduction in macroporosity, which is the available amount of air that the soil provides to the plants, and below 10% the plant's development begins to be compromised due to the lack of it. Water infiltration decreases, causing greater water runoff over the soil, generating erosion and, mainly, a reduction in water storage, making drought problems increasingly aggravated and noticeable over time.

In operations, from the machine's point of view, compaction generates: increased tractor power requirements to pull an implement; increased rate of slipping and wear on its mechanisms, significantly increasing fuel consumption and maintenance costs; decrease in depth and irregular cutting of straw during planting.

Faced with this, there are two alternatives to solve the problem of compaction: the first is the use of plants known as “decompactors", as these plants have an aggressive root system that can break through some layers, improving the physical structure. But the use of this technique requires time and good management, sometimes it may be unfeasible, as some crops are sown exclusively for this purpose and do not bring direct economic return, only indirect. The second alternative is scarification or subsoiling, which solves the problem immediately, however, there are doubts about when to do it and how long its effects will last on the soil.

Using scarifiers on soil under direct seeding is called reduced cultivation or minimum cultivation, a term commonly used by researchers in the field of agricultural sciences. This process consists of mobilizations up to 30cm deep. It is worth highlighting the difference between a scarifier and a subsoiler. The subsoiler has the same mechanisms as the scarifier, however, they are more robust, since this implement operates at depths of up to 60cm.

The conventional subsoiler has some operational problems such as changes in the surface roughness of the soil, incorporation of straw and bushing. Who has never subsoiled an area with good straw and had to stop the operation to clear it? Faced with these problems, a survey carried out at IFRS - Campus Sertão, sought to compare the efficiency between these subsoilers, in the percentage of straw on the surface, amount of straw incorporated and soil resistance (to see how the work on the soil was). In a crop with rye straw, approximately 4,9t/ha, the conventional subsoiler kept only 35,75% of the straw on the surface, compared to 75,13% for the subsoiler with a cutting disc, as shown in Table 1, or In other words, the cutting disc left 2/3 of the straw above the ground, while the other was almost the opposite, placing almost 2/3 below.

Tabela 1 - Quantity in tons and incorporation of straw in relation to the type of subsoiler used compared to direct planting

Treatment¹	IP	Straw	Productivity
	%	t/ha	kg.ha-1
no-till	-	4,88 to	4196 to
Conventional subsoiler	75,13 a*	2,57 c	4030 to
Subsoiler with cutting disc	35,75 b	3,50 b	4052 to
Variation Coefficient (%)	15,87	14,67	20,37

Treatment¹

IP

Straw

Productivity

%

t/ha

kg.ha^-1

no-till

-

4,88 to

4196 to

Conventional subsoiler

75,13 a*

2,57 c

4030 to

Subsoiler with cutting disc

35,75 b

3,50 b

4052 to

Variation Coefficient (%)

15,87

14,67

20,37

In practice, operationally the conventional subsoiler generated several bushings, as can be seen in Figure 2A, which was never found in the subsoiler with a cutting disc. Another relevant point was the change in the surface roughness of the soil, as seen in Figure 2B, where in the right part the change in roughness is visible, the variation in straw on the surface, while in the left area it appears that it was not disturbed. or it looks like it was sown.

Looking at the coverage data, straw on the surface, the conventional subsoiler left 2,57t/ha compared to 3,5t/ha for the other, this only proves the incorporation that was carried out by the first.

Although the subsoiler with cutting disc presented these benefits in straw maintenance and soil surface roughness as well as operational benefits, they did not reflect on corn production, as their productions were close. This is attributed to the physical characteristics of the soil, as both left similar soil resistance, that is, the work carried out by the rods below is similar, the differences are above.

This observation made above can be seen in Figure 3, in which there is a graph that demonstrates a kind of X-ray of the soil on the resistance in the area around the roots. The planting line would be right in the middle of each figure, it is visible that in the soil under direct planting the colors become lighter at 8cm, that is, greater resistance, not in the subsoil areas. Taking 2.000kPa as a basis as a restriction for the plant, it is visible that in subsoils it occurs between 15cm and 30cm, whereas in direct planting it is 8cm. Comparing the resistance between the subsoilers there is no visible difference, proving that the work of the rods is similar.

The work in decompacting the soil by the subsoilers is expected to be similar, as what happens between them is that the soil turrets that rise due to the action of the subsoiler tip will encounter the leveling/declogging roller, which will break them down and, with The action of the disc in front of the stem, in addition to cutting the straw, will also create a “tear” in the soil, that is, a slight break that will facilitate the action of the stem.

Care must be taken when using a subsoiler, because, although what is sought in practice is to increase production, it is necessary to think about protecting the soil, as in the face of rainfall, areas with greater surface roughness will suffer more intense erosive processes, as well as greater straw coverage helps maintain soil temperature, an important parameter in seed germination and maintenance of soil processes.

This article was published in issue 135 of Cultivar Máquinas magazine. Click here to read the edition.