Influence of mulching on the water regime and yield of apple trees

The role of moisture and mulching in horticulture

One of the main limiting factors in the cultivation of fruit crops (especially in the southern zone) is moisture deficit. Meanwhile, all vital processes of plants can proceed normally only when they are sufficiently supplied with water. Water absorbed by the roots transports dissolved nutrients, maintains leaf turgor, is used for the construction of organic compounds, and ensures thermoregulation of the plant organism. In this regard, optimal water supply is the most important condition for the successful growth of fruit plants. Even slight dehydration of plant tissues leads to a suspension of plant growth and a decrease in their productivity.

Irrigation is one of the common methods for regulating the water regime of the soil in plantations. According to many authors, mulching can serve as an alternative method for regulating soil moisture in a young orchard.

Mulching often provides a greater yield increase of fruit crops than other soil management systems in an orchard. This becomes understandable if we recall that in the process of their phylogenesis, the prevailing part of fruit crops evolved in the forest, where the soil under the trees is mulched with forest litter.

“To mulch tree circles,” writes Jill Jesolowsky (2000), “means to follow nature, because in the forest the ground is always covered with fallen leaves, branches, and twigs. In the process of decomposition, this living carpet provides tree roots with the necessary nutrition, helps to preserve moisture in the soil, and gives the forest and the orchard a natural attractiveness. In addition, mulch reduces daily temperature fluctuations, maintains good soil structure, and prevents plants from heaving and their root system from being damaged in winter.”

Experimental study of the effect of mulch on soil

At the Department of Fruit Growing of Kuban State Agrarian University (KubSAU), an experiment was conducted to determine the optimal mulching material for use in the tree row strip of a non-irrigated apple orchard in the south of Russia, which ensures stable fruiting of trees.

The field trial was conducted in 2006–2009 at the "Kuban" training farm (KubSAU) in an orchard planted in 2005 (planting pattern 5×3 m). The scab-immune apple cultivar Florina on M9 rootstock was studied. The soil (leached chernozem) in the orchard rows was maintained under sodding – "vetch–oats" in every other inter-row space. The following types of mulching material were investigated in the tree row strip:

• straw;
• coniferous sawdust, decomposed;
• grass mown in the row;
• "vetch-oats" mown in the inter-row spaces.

The control variant was "black fallow".

According to our data, the stock of productive soil moisture changed during the growing season but was noticeably lower than the optimal values – 180 mm. The use of various mulching materials in the tree row strip contributed to the conservation of productive soil moisture in the meter layer (up to 50 %). At the same time, the mulch layer acted as a kind of soil temperature regulator.

As the experiment showed, in the variant using straw as a mulch layer, with an increase in soil moisture in the root zone by 15–20 % compared to the control, the size of tree leaves increased by 28 %, and the leaf surface area by 1.7 times. mm May June July August September month black fallow (c) straw sawdust mown grass vetch+oats

Figure 23 – Influence of soil mulching of the tree row strip on the stock of productive moisture (mm) in the meter layer, 2007 temperature, C June July August month black fallow (c) straw sawdust grass mown in the row mown vetch-oats

Figure 24 – Influence of soil mulching of the tree row strip on the average monthly soil temperature in the summer period, 2007.

It is noteworthy that against the background of higher water availability for plants in the variants with straw and sawdust, the net photosynthetic productivity of Florina apple trees increased by 25 and 16 % respectively compared to the control. Table 22 – Influence of tree row strip soil mulching on the dynamics of soil temperature change during the day (2007, 0–30 cm layer), °С

8:00 10:00 12:00 14:00 16:00 (control) 26.5 27.0 30.0 31.5 32.0 Mulching: Straw 23.0 23.5 23.5 24.0 24.0 Sawdust 24.5 24.5 25.0 25.5 26.0 grass mown in the row 25.0 25.5 26.0 26.5 26.5 green mass "vetch – oats" mown in the inter-row spaces 24.5 24.5 25.0 25.5 26.0

August 15 (control) 30.2 32.0 36.5 37.0 42.5 Straw 24.0 24.5 25.0 25.0 25.5 Sawdust 24.5 25.0 25.0 25.5 26.0 grass 24.5 25.0 26.0 26.0 28.5 green mass "vetch – oats" mown in the inter-row spaces 24.5 25.0 25.5 25.5 26.0

Mulching the tree row strip, which allows creating favorable moisture and temperature conditions in the root zone soil layer, has a significant impact on the growth of young trees. The highest growth activity of shoots was recorded in plants where the soil was mulched with straw. In the dry year of 2007, the difference between the variants in shoot growth rates was manifested as early as the beginning of the growing season, and it reached its maximum value by the end of the phenophase. Table 23 – Influence of tree row strip soil mulching on the leaf surface area of apple trees, 2007.

Leaf size, cm² Leaf area, m² per 1 m² on shoots on spurs 1 tree canopy projection (control) 22.6 14.2 6.5 3.4 Straw 29.1 15.4 11.3 4.1 Sawdust 22.6 10.1 7.0 3.5 Grass mowed in the row 20.3 8.49 5.2 2.9 "Vetch–oat" green manure mowed in aisles 21.1 10.3 6.3 3.0

LSD05 0.47 0.71 – –

During the shoot growth phenophase at a soil moisture of 65–72% of field capacity (FC), the straw treatment variant creates a favorable moisture supply regime for the plants. Under these conditions, shoot growth is more uniform and continues until mid-July. By the end of July, terminal buds were formed, and the average shoot length on trees in this variant was 64 cm. A decrease in soil moisture to 50–60% of FC (black fallow, grass mowed in the row) and a rise in temperature to +30 °C or higher led to the cessation of shoot growth and the formation of the terminal bud as early as the beginning of June. Subsequently, this negatively affected the generative activity of the plants.

According to a number of authors, the activation of photosynthesis processes is associated with an increased outflow of plastic substances from the leaves with a simultaneous increase in the fruits. The activation of the influx of assimilates to the generative organs has a positive effect on the tree's productivity as a whole.

It has been established that increasing the soil moisture of the root layer in the straw and sawdust variants contributes to an increase in fruit yield by 23–76% (in total over two years). At the same time, the presence of grass in the row, even with subsequent mowing, leads to a decrease in this indicator by 26%.

It is noteworthy that when studying types of mulching material, differences were noted in the number of weed plants in the tree trunk strip. Thus, per 1 m² with soil mulching using straw and sawdust, there were 67–96% fewer weeds than in the control. Table 24 – Influence of mulching the tree trunk strip soil on the fruit yield of apple trees of the Florina cultivar

(orchard planted in 2005, planting scheme

5×3 m), kg/tree

Years of research Total

Variant for 2008–

1 2 3 4 (control) 3.2 6.8 10.0 Straw 4.9 12.7 17.6 Continuation of Table 24

1 2 3 4 Sawdust 3.8 8.5 12.3 grass 2.3 5.1 7.4 "Vetch–oat" green manure mowed in aisles 2.9 6.4 9.3

LSD05 0.4 0.9 –

Thus, the use of straw as a mulching material in the tree trunk strip of young non-irrigated apple orchards ensures the conservation of soil moisture, a reduction in soil temperature (0–30 cm layer) during hot summer months, and ultimately, a significant increase in fruit yield. At the same time, weed infestation of the plantations decreases sharply.

How does the choice of the soil management system in a young non-irrigated orchard affect the change in its bulk density and humus?

What is the influence of the management system of the soil on yield?

What resistance to summer abiotic stressors is provided by the introduction of inter-row sodding with naturally growing grasses in the orchard?

How do the reserves of productive moisture and soil temperature change during mulching of the tree trunk strip?

Which type of mulching material is the most promising for conserving soil moisture, optimizing soil temperature, and increasing fruit yield in young non-irrigated apple orchards?