Soil management systems in orchard inter-rows

Soil in an orchard is an environment that provides roots with water, oxygen, and nutrients. The chosen inter-row management technology directly determines the physiological state of the plants, and therefore the volume and quality of your harvest. The agronomist’s task is to find a balance: to obtain high productivity of the plantations while maintaining or improving soil fertility.

The basis of orchard management is a proper water regime. Trees are most demanding of moisture in the spring and the first half of the summer, when the processes of intensive shoot growth, leaf formation, and fruit filling take place. Excess water in the second half of summer and autumn is harmful, as it prolongs the growing season and prevents the wood from maturing in time before overwintering. At the same time, an autumn moisture deficit is also dangerous and often leads to winter damage to the root system and the canopy.

How soil management systems affect the organic matter balance

Depending on how the balance of organic matter in the soil develops, there are three main management systems for inter-rows. In a clean fallow system, humus reserves progressively decrease due to constant mineralization. The fallow-sideral system allows for balancing the processes of decomposition and accumulation of organic matter through the ploughing in of green manure crops. The sod-humus system (or intensive grassing) works for the future and ensures a stable accumulation of organic matter in the soil.

The choice of a specific method depends on the design of the orchard itself, the relief of the plot, soil and climatic conditions, the age of the trees, and the availability of irrigation. It is important for an agronomist to protect the soil from water and wind erosion, regulate its temperature regime, and minimize maintenance costs. Each system solves these tasks in its own way, but the clean fallow system remains the most radical in terms of controlling weeds.

Black fallow technology: work regulations, pros and cons

The essence of the black fallow system lies in keeping inter-rows and tree strips constantly loose and free of weeds. Autumn tillage is carried out after harvesting harvesting, taking into account the type of rootstock and the mechanical composition of the soil. In spring and throughout the summer, a series of cultivations and discings are performed, the depth of which is adjusted depending on the species composition of weeds and soil density.

• Ploughing on seed rootstocks — 18–20 cm
• Ploughing on vegetative rootstocks — 16 cm
• Discing of light soils in autumn — 14–16 cm
• Spring loosening of tree strips — 10–12 cm
• Subsequent summer cultivations — 7–8 cm
• Treatment against rhizomatous weeds — 10–12 cm

For autumn tillage in intensive orchards, the PRVN-2,5A cultivator with the PRVN-72000 attachment is used. In the spring period, tree strips are treated with cultivators equipped with retractable sections. If perennial rhizomatous weeds are present on the plot, the depth of summer treatment is increased, which also helps to break the plough pan. On medium and heavy-textured soils, rotary tillers are used for effective crust crushing and weed destruction.

Advantages of the black fallow system:

• effective accumulation and conservation of soil moisture due to the destruction of capillaries;
• improvement of aeration and the air regime of the root zone;
• rapid destruction of weeds, pest hotspots, and pathogens;
• activation of aerobic microorganisms and intensification of nitrification (accumulation of available nitrogen);
• increase in the concentration of mobile phosphorus and exchangeable potassium in the root zone.

Disadvantages of the system:

• destruction of soil structure under the influence of machinery working parts and heavy rainfall;
• increase in the depth of soil freezing in snowless winters;
• reduction in the storability of harvested fruits and deterioration of their market quality;
• delay in technological operations (spraying, harvest transport) due to mud after rains.

It is advisable to maintain black fallow on flat areas in conditions of insufficient moisture or in the absence of irrigation. In the southern horticultural zone, where all agricultural practices are aimed at accumulating and conserving moisture, fallow is an important link. Also, regardless of the zone, it is recommended to maintain the soil using a fallow system in young orchards during the first two to three years.

Fallow-sideral system: a balance of moisture and organic matter

The fallow-siderate system combines black fallow with the sowing and subsequent ploughing under of annual grasses (green manure crops) over the course of a year. Green manure grasses are sown at the same time as spring or winter crops in a specific zone. In regions with a hydrothermal coefficient (HTC) of 1.1–1.2, the most common winter green manures are: wintering peas, grass pea, peas, phacelia, mustard, rye, and a vetch-oat mixture.

The technology for managing green manure includes several sequential stages:

1. Before pre-sowing cultivation, apply nitrogen fertilizer at a rate of 60 kg/ha of active ingredient to improve plant growth.
2. Sow seeds in the middle of row spacings using seed drills such as SZT-3.6, KhST-3.6, or SZS-2.1 in a strip about half the width of the distance between tree rows.
3. During the budding phase of the winter green manures, roll the plants with rollers or shred them with an IKS-3 mower (a converted KIR-1.5 mower).
4. Plough the green mass into the soil as fertilizer.

Optimal application rates have been established for each green manure crop to ensure a dense stand:

• Mustard — 25 kg/ha
• Phacelia — 20 kg/ha
• Wintering peas — 210 kg/ha
• Grass pea — 220 kg/ha
• Rye — 180 kg/ha
• Vetch-oat mixture — 120 kg/ha of vetch + 50–60 kg/ha of oats

The fallow-siderate system is used periodically in the orchard — once every 3 years in each row spacing or every other row spacing, alternating the sowing of grasses by year. In the first half of the growing season, it creates excellent conditions for tree growth, improves soil structure, replenishes organic matter reserves, and prevents late summer and autumn erosion. In the second half of the summer, green manures optimize the trees' preparation for winter: the wood matures better, winter hardiness increases, and fruit coloration improves. Regarding the system's disadvantages, the agronomist should consider the additional costs, the increased demand for irrigation and nutrients, and temporary accessibility difficulties for machinery used in tree maintenance.

Sod-humus system: intensive sodding of row spacings

Under this system, the soil in row spacings is kept under perennial grasses, which are frequently mown, shredded, and left on-site as mulch. Natural grass stands or sown perennial grasses with a shallow root system are used as sod-forming plants. These include Kentucky bluegrass, meadow fescue, ryegrass, timothy-grass, brome grass, orchard grass, and meadow foxtail. Seeds are sown at a rate of 15–20 kg/ha, with two-component mixtures, such as bluegrass and fescue, yielding the best results.

Grasses are mown 4–6 times per season upon reaching a height of 15–20 cm. Meanwhile, tree-row strips are kept under black fallow, cultivated using tillers or herbicides.

The sod-humus system has significant advantages over other methods of soil management:

• humus content increases due to the decomposition of root residues and grass mulch without the application of manure;
• water erosion on slopes is prevented;
• the soil does not compact, as frequent mechanical cultivation of row spacings is excluded, and the risk of damaging roots and trunks with machinery is eliminated;
• soil temperature and humidity are stabilized under the mulch layer, and beneficial fauna develops actively;
• the total length of the fine feeder roots of fruit trees increases in the topsoil layer (0–40 cm);
• fruits acquire better coloration, skin and flesh density increase, and storage life is extended;
• sodded soil facilitates the passage of machinery for spraying and harvesting during rainy weather.

Nevertheless, the sod-humus system has far more advantages than disadvantages. The correct choice of the soil management system in row spacings helps regulate tree life processes and protects them from summer drought and high temperatures — the primary stress factors in the southern part of the country.

The Department of Pomology at KubSAU conducted research to determine the most effective method of soil management in the row spacings of a non-irrigated apple orchard in the southern region, which ensures increased drought and heat resistance of the plants and, consequently, their stable and optimal fruiting in years with varying weather conditions.

To achieve the set goal, research was conducted from 2003–2010 at the "Kuban" educational farm in the zone of leached chernozems (Kuban region) in a non-irrigated apple orchard established in 2002 using a 5×4 m scheme (management system – organic). The study focused on the scab-immune apple cultivar Florina on MM106 rootstock. The following methods of soil management in row spacings were investigated:

1) black fallow (control);

2) alternate-row sodding;

3) inter-row sodding.

To establish the grass stand in the second year after planting the orchard (2003), in early spring, the leveled soil in the inter-rows was compacted using smooth water-filled rollers. Subsequently, the emergence of naturally growing grasses was periodically mown when they reached a height of more than 15–20 cm. The mown mass was left on the soil surface. In the third-year grass stand (2005), a predominance of loose-bunch grasses, which are preferred for orchard inter-row sodding, was noted.

Tree strips 1.0 m wide were kept under black fallow. At the same time, the soil bulk density in the plough layer (0–20 cm) increased to 1.61 g/cm3.

With the beginning of the growing season of the grasses in the inter-rows, plants more resistant to soil compaction emerged. Among them, common purslane and field bindweed predominated (70% of the grass stand). Other weed species appeared stunted and reached a height of up to 10 cm.

The following year, there was a shift in the species composition of the grasses. The grass stand was dominated by prostrate knotweed and the grass species purple burnet, which are groundcover plants. In the third-year grass stand, a predominance of loose-bunch grasses, preferred for orchard inter-row sodding, was noted. Figure 21 – Stages of natural grass stand formation in an organic apple orchard: a – black fallow (2002); b – introduction of a sod-humus soil management system (2003); c – alternate-row sodding (2005). Table 11 – Changes in the bulk density of leached chernozem during orchard operation using soil compaction, g/cm3 (KubGAU "Kuban" training farm, planted in 2002, planting scheme 5×4 m)

Soil layer, Year after soil compaction cm First (2003) Second (2004) Third (2005)

0–20 1.61+0.03 1.32+0.06 1.16+0.06 20–40 1.28+0.03 1.13+0.03 1.20+0.08 40–60 1.24+0.05 1.13+0.05 1.21+0.04 60–80 1.27+0.05 1.20+0.05 1.22+0.04 80–100 1.29+0.05 1.25+0.06 1.15+0.08 0–100 1.34+0.02 1.23+0.03 1.19+0.03

As a result of using this soil management method, by the end of the fourth growing season, there was an increase (in absolute terms) in the content of labile humus forms by 9.5% compared to the control (fallow tillage), and in total humus by 0.33%. Table 12 – Content of total humus (%) and its labile forms (0.1 N NaOH extract, mg/kg of soil) under different soil management methods in the orchard of the KubGAU "Kuban" training farm

Soil management Total humus, % Labile humus forms system (mg/kg of soil) Ap Ap+AB Ap Ap+AB (0–20) (0–120) (0–20) (0–120) Black fallow 3.48 2.99 980+12 570+45 Grass stand 3.81 3.33 1680+102 1420+115 formation

It is noteworthy that the increase in the studied parameters occurred throughout the entire humus horizon. Based on the presented data, when using soil compaction at the beginning of orchard operation, more severe conditions are created for the development of "weedy" (according to Williams) grass species, while simultaneously fostering the emergence of soil-compaction-resistant species – groundcover grasses. Table 13 – Changes in the species composition of grasses in the inter-rows during orchard operation using soil compaction (KubGAU "Kuban" training farm, planted in 2002, planting scheme 5×4 m)

Year after soil compaction Grass species % in grass stand First (2003) Common purslane 60

Field bindweed 10

Green foxtail 8

Amaranthus blitoides 12

Barnyard grass 10 Second (2004) Prostrate knotweed 40

Purple burnet 30

Green foxtail 15

Field bindweed 13

Meadow foxtail 12 Third (2005) Meadow foxtail 50

Cheatgrass 25

Wall barley 10

Narrow-leaved meadow grass 10

Prostrate knotweed 5

At the same time, the growth and development of the root system of perennial plants, located in the tree strip zone during the first years of life, take place under optimal conditions of nutrition, humidity, and air regime. During the second to third growing seasons, an optimal species composition of grass-based groundcover plants that do not require frequent mowing is formed in the inter-rows. During the same period, the compacted soil undergoes loosening, leading to the optimization of bulk density, humus status, and nutrient regime.

At the same time, due to the growth of grasses, more severe conditions for nutrition and water supply of perennial plants are created, leading to a weakening of their growth and, accordingly, an earlier onset of fruiting.

Based on the data obtained, the long-term use of naturally growing grasses as sod-formers is of great interest for improving the humus status of orchard soils. The nature of the use of naturally growing grasses can be compared with "fallow" (ley farming) previously used in agriculture, which was resorted to after prolonged tillage and soil depletion. As V. R. Williams pointed out, after a fallow period, the harvest of agricultural crops increased to a very significant level.

The soil quality rating of leached chernozem for pome fruits is 100 points. Imperfect orchard soil care technology leads to a decrease in humus content to 3% or lower, an increase in bulk density to critical values (more than 1.50 g/cm3), and the appearance of hydrolytic acidity.

As a result of studies on the use of naturally growing grasses, an improvement in the main soil fertility parameters of leached chernozem was established.

Optimization of the soil structural state was facilitated by the minimization of soil tillage and the phytomeliorative effect of grasses.

Thus, in the variant with grasses, the sum of agronomically valuable structural aggregates (0.25–10.00 mm) was classified as excellent, whereas under clean fallow it was classified as good. The structure coefficient increased from 2.6 (in soil under clean fallow) to 7.8 (in soil under grasses), and the qualitative component of the soil structure in terms of water-stable aggregates also improved. Their total content in soil under grasses increased by 20% and moved from the "satisfactory" category to the "good" category. Table 14 – Aggregate (structural) composition of the arable soil layer (0–20 cm) under different soil management systems* at the "Kuban" training farm, Kuban State Agrarian University

Aggregate size (mm) and their content (%) by weight of air-dry soil Sum of agronomically valuable aggregates (0.25–10 mm) more than 10; less than 0.25; Variant; Ks; Clean fallow (control); 21.6; 23.5; 12.6; 23.6; 12.4; 6.3; 72.1; 2.6; Sodding; 9.4; 41.8; 21.2; 21.0; 4.7; 2.0; 88.7; 7.8

* Dry sieving.

The structural state of the soil influences its physical properties – bulk density, porosity, and energy consumption during mechanical tillage. Changes in soil bulk density indicators occurred in the upper root-inhabiting layer. Under clean fallow, a compacted soil layer—a so-called plough pan—formed beneath the arable horizon. This impaired the water and air regimes of the root-inhabiting layer. Under sodding and minimal mechanical impact, the soil was loosened to optimal values. Table 15 – Average soil bulk density over the growing season under different soil management systems ("Kuban" training farm, Kuban State Agrarian University, established in 2002, 2010)

Soil layer, cm

0–20; 20–40; 40–60; 60–80; 80–100; Clean fallow

1.02; 1.41; 1.38; 1.28; 1.26; Sodding; 1.08; 1.23; 1.27; 1.15; 1.18

Enriching the soil with organic grass mass contributes to the stabilization of its humus state. In the soil of the arable horizon of the control variant (clean fallow), the total humus content is 3.63%, which classifies it as a low-humus type. Seven years of sodding promotes the accumulation of organic matter (4.13%), which makes it a moderate-humus type.

Similar changes are observed in the sub-arable horizon. Moreover, the amount of humus in it, in the studied variant, corresponds to the arable horizon of the control. It should be noted that these changes occur against the background of an optimal content of labile forms of humus. Their content increased by 11% compared to clean fallow due to the replenishment of the soil with fresh plant material from grasses. The ratio of total humus to its labile forms, which can be used to assess the degree of soil cultivation or degradation, corresponds to the economically optimal level and was reached in the studied variant by the time the perennial plantations entered the stage of commercial fruiting. Usually, under clean fallow, such indicators are reached only by the end of the orchard's life cycle. Table 16 – Agrochemical indicators of leached chernozem under different soil management systems in the orchard ("Kuban" training farm, Kuban State Agrarian University, established in 2002, 2010)

Variant; Soil layer, cm; Humus content, %; Content of labile forms of humus, mg per 1 kg of soil; Clean fallow; 0–20; 3.63; 980

20–40; 2.71; 560

0–20; 4.13; 1520; Sodding

20–40; 3.68; 1120

As a result of the experiments conducted, in the variant using naturally growing grasses in the aisles of a non-irrigated orchard during the growing season, a noticeable decrease (by 5–18%) in soil moisture was detected compared to the control.

Obviously, the growth of grasses creates more stringent water availability conditions for perennial plants, causing a decrease in the activity of various life processes. The results of special experiments confirm this.

Thus, according to our data, throughout the growing season, the highest apple shoot growth activity was recorded under the "clean fallow" soil management system, while low indicators were noted with sodding of the aisles.

It is noteworthy that naturally growing grasses in the aisles of a non-irrigated apple orchard contribute to an earlier weakening and cessation of shoot growth. Such a result should ensure their better maturation and good preparation of perennial plants (especially young ones) for overwintering. On the other hand, a certain restriction of the orchard's water supply is beneficial for vigorously growing apple trees (for example, the 'Florina' cultivar). Table 17 – Growth dynamics of 'Florina'* apple shoots during the growing season depending on the soil management system in the orchard aisles (averaged over 2009–2010)

Shoot length, cm; May; June; July; August

26.3; 30.5; 38.1; 40.6; Sodding; 23.4; 25.9; 31.5; 32.0

* Rootstock MM106.

In this case, the trees' vegetative growth is weakened and their generative function is enhanced—more fruit-bearing structures are formed, ensuring an increase in fruit yield. Finally, in the presence of naturally growing grasses (sodding) under the influence of a water stressor, apple plants show favorable adaptive adjustments in their organism's functioning. In particular, for the 'Florina' cultivar in the specified variant of the experiment, the water-holding capacity of leaf tissues increases compared to the control (water losses are reduced), which leads to an increase in the efficiency of the production process. Thus, in the case of using sodding, pre-harvest fruit drop in apple plants decreases by 2–8 times, and the initiation and differentiation of flower buds are significantly activated.

However, water deficit is not the only stress factor limiting stable fruit production in the southern regions of Russia. Elevated air temperatures, which occur almost every year in these territories during the summer period, lead to the degradation of protoplasm proteins, the suppression of the vital functions of perennial plants, and a decrease in their productivity.

Under the influence of the stressor, the Florina apple cultivar undergoes adaptive metabolic adjustments.

Thus, by the end of August 2010, which was characterized by abnormally hot weather, the leaf respiration rate respiration increased by 1.2 times, and the glucose content in them decreased by 60% compared to the indicators recorded in the second ten-day period of July. During the same periods, an increase in the concentration of certain organic acids (α-ketoglutaric, fumaric) and amino acids (glutamic, aspartic) was noted in the leaves of this cultivar.

Based on the data presented, it can be assumed that the products of the oxidative conversion of glucose (α-ketoglutaric and fumaric acids) act as a kind of acceptor, binding excess ammonia, which is formed as a result of protein degradation and has a toxic effect on the plant organism. Apparently, this mechanism of ammonia deactivation is characteristic of the heat-tolerant (relatively tolerant) Florina apple cultivar.

At the same time, the manifestation of the cultivar's heat resistance, and consequently its productivity in years with extreme weather conditions, largely depends on the method of soil management in the orchard inter-rows.

As the experiment showed, the potential heat resistance of the Florina apple cultivar (MM106 rootstock) is realized to a greater extent when using naturally growing grasses in the inter-rows (every row or every other row). Even with temperatures rising to 65°C, leaf damage in apple trees did not exceed 47% under these soil management methods.

There is an explanation for this. According to our data, during the noon hours of the hot period of 2010, the soil in the ploughing layer of the control variant (black fallow) heated up to 37°C, while with the use of sodding, it only reached 28°C. At the same time, when heated intensely, the vital functions of plants (including those responsible for the manifestation of resistance) are sharply inhibited. An even greater suppression of the life processes of the plant organism was recorded under the combined action of two stress factors – drought and elevated air temperatures (as was noted, for example, at the end of August – the first half of September 2010).

Table 18 – Heat resistance of the Florina apple cultivar (MM106 rootstock) depending on the method of soil management in the orchard inter-rows

Under such conditions, water loss in leaf tissues by the end of the period under review increased by 3.7–6.4 times in various experimental variants, and their water content decreased by 12–19%. Table 19 – Changes in the water regime parameters of Florina apple leaves under anomalous conditions of the summer-autumn period depending on the method of soil management in the orchard inter-rows (2010)

Variant Water loss, % * Water content, %

23.08 13.09 23.08 13.09 (control) 6.6 42.2 57.2 46.5 every other row 5.9 21.7 58.0 50.9 every row 7.5 28.4 57.9 50.1

* Water loss of leaf tissues after 3 hours of wilting.

However, even during these stages of the growing season, when naturally growing grasses are used in the orchard inter-rows (especially when applied in every other row), apple trees adapt more easily to the complex of unfavorable factors (they show the best water regime indicators in the experiment).

Apparently, under anomalous conditions of the summer period, sodding every other row creates a favorable ratio of soil temperature and moisture regimes, which contributes to the formation of sufficiently high and relatively stable fruit yields even in non-irrigated apple plantations. The average apple yield with this method of soil management is 3.4 times higher than in the control and 2.2 times higher than this indicator in the "every row sodding" variant. Table 20 – Yield of the Florina apple cultivar* depending on the method of soil management in the orchard inter-rows (orchard planted in 2002, planting pattern 5 × 4 m), t/ha

Years of research Average

2008 2009 2010 2011 for 2008–2011 (control) 3.9 5.1 10.2 7.7 6.7 every other row

23.2 18.0 24.2 26.0 22.8 every row

5.5 11.0 7.9 17.6 10.5

LSD05 1.5 3.8 2.2 4.9 –

Moreover, when sodding every other row in the orchard, apples feature the best biochemical composition in the experiment: a higher content of dry matter and sugars. At the same time, the concentration of titratable acids in the fruit is practically the same across the variants. Table 21 – Chemical composition of apple fruits of the cultivar

Florina depending on the method of soil maintenance in the orchard aisles* (orchard planted in 2002, planting scheme 5 × 4 m, 2009)

Mass concentration, %

Variant dry titratable sugars substances acids (control) 14, 2 11,58 0,42 alternate-row 15, 1 13, 18 0,43 between-row 14,9 12,87 040

Thus, in young non-irrigated apple orchards of the Kuban zone (soil leached chernozem), the introduction of alternate-row sodding with naturally growing grasses ensures the enrichment of the soil with organic grass mass, contributes to the optimization of the main parameters of soil fertility, accelerates the onset of fruiting in cultivars (especially vigorous ones), and during the exploitation of plantations, increases plant resistance to abiotic stressors during the summer period, leading to the stabilization of generative activity, as well as an increase in yield and fruit quality.