Orchard Management (1+1)

EFFECT ON SOIL PROPERTIES AND SOIL ENVIRONMENT

Introduction:

• Soil may be fertile or infertile. Even a fertile soil may not be necessarily productive. Therefore, the aim of a soil management practice is to keep soil in good physical condition, improve its fertility and make it productive.
• Soil management practices greatly influence the soil environment.

Effects of different management practices on soil properties and soil environment have been discussed below.

Soil properties:
Organic matter

Sod culture and cover crops:

• Increased soil organic matter content in orchards appears to be a natural consequence of sod culture (Plate 10.1).
• In the top 5 cm soil, about 2% increase in organic carbon in comparison to cultivation has been noticed.
• Decaying of cover crop roots can increase organic matter, but only to their rooting depth.

Plate 10.1: Sod culture in citrus orchard

• The benefits of increased soil organic matter content can be seen in improved physical and chemical properties, including in nutrient exchange capacity, water absorption, soil structure, and maintenance of a healthy soil flora and fauna.

Mulching

• Addition of organic mulching materials to orchard soil surfaces also conserves and can even increase the organic matter content of the orchard soils.
• The nature of organic mulch can also influence the potential increase in organic matter.
• For example, organic matter content of soil is higher under hay mulch than under seaweed or sawdust, since the later decomposes very slowly.

Cultivation

• It is well known that there is a rapid decline in soil organic matter content under clean cultivation.
• It has been seen that the decline in organic matter content is particularly pronounced for surface layer of the soil.

Herbicides

• In general, the tree rows of orchards, in which herbicides have lower soil organic matter content than grassed alley.
• In some experimental studies, organic matter remained constant for the initial few years after killing of grass with herbicides, but later it tended to decline. It may be because of the fact that the use of herbicides in no-till management may preserve temporarily the original distribution of organic matter in the soil profile, as compared to tillage, but in the later years the organic matter content was found usually lower than that under the sod.
• Soil organic matter is generally higher in soil under continuous tillage, probably because the natural compaction the soil surface provides conditions less favourable for mineralization.
• However, soil organic matter can be expected to decrease with continuous herbicides use due to less plant material being returned to the soil.

Soil pH:

Sod culture and cover crops

• Soil pH is generally higher in orchards under grass than when orchard vegetation are removed.
• However, it is unlikely that the existence of sod can prevent the inevitable pH decline associated with continuous use of acidifying nitrogenous fertilizers.
• A pH decline from slightly acidic to 4.1-4.4 is common in fertilized [(NH₄)₂SO₄] and herbicide-treated plots, in the long term.

Mulching

• Long term use of mulching may result in higher pH due to transfer of calcium and magnesium into mulched area of the orchard.
• However, there is little consistent research data to support this, and more frequently mulches do not affect soil pH.

Cultivation

• Soil pH can decline in cultivated soil in comparison to orchards with permanent sod or cover crops. This has been attributed to increase in leaching of calcium and magnesium from the soil surface of cultivated plots.
• Other ground vegetation treatments may also have the similar effect on pH decline. Incorporation of lime before planting can help to maintain soil pH and prevent the problem of acidity in the orchard.

Herbicides

• Long term use of herbicides in an orchard usually causes decline in soil pH.
• Although, there is a considerable variation in the rate of soil pH change in different locations, which is due to the variable soil properties, fertilizer application, and moisture regimes at different locations, however there is consistent reduction of soil pH with the use of herbicides in all the orchards.
• Moreover, soil in orchards under herbicide treatments has lower pH at all the different tree rooting depths.

Soil temperature:

Sod culture and cover crops

• Soil temperature affects fruit tree performance.
• Year round temperature fluctuation of top soils (10- 25 cm) under sod is intermediate. It is several degrees Celsius cooler in the summer and warmer in the winter relative to year round cultivation on straw mulch.
• However, day night temperature fluctuation in orchards under sod may be greater in spring, owing to heat losses to radiation.
• Soil temperature may influence fruit tree indirectly by its relationship with air temperature.
• Although summer air temperature in orchards with sod have been observed to be cooler than in orchards with tilled bare soil, differences have been observed mainly in spring during radiation frosts.
• Such frosts occur on clear nights when unchecked soil radiation cools the air nearest the soil below freezing.
• Since, orchards with permanent vegetation have cooler spring soil temperatures than tilled, herbicide used, or plastic-mulched soil, the amount of heat to be radiated is less.
• In addition, grass or other vegetation has a larger surface area for radiation to the air above and looses heat rapidly.
• In the temperate regions, orchards under sod are about 2-3^oC cooler than the bare soil. Therefore, grasses under trees should be cut during blossoming time to reduce spring frost injuries to floral parts.

Mulching

• Organic mulches generally insulate the orchard soil and consequently decrease the fluctuation in daily and annual soil temperature extremes.
• Soil temperatures beneath mulch in summer are frequently lower, especially on day with high incident solar radiation.
• Mean monthly temperatures at 10 cm depth below a 10-20 cm thick straw cover have frequently been 1^o- 2^oC less than those beneath bare soil in the summer months while during the winter similarly measured temperatures could be 1^oC higher under mulch relative to bare soil.
• Mean daily summer season soil temperatures are about 2- 3^oC higher under black plastic mulches than organic mulches.
• Mulch-induced soil temperature changes can be important if they are near critical thresholds. For example, in countries in the Northern hemisphere, where summer temperatures are lower than warmer countries, organic mulching reduces the summer temperature to the level at which it has negative effect on apple fruit quality.
• The reduction in heat transfer from soil under straw mulch to atmosphere during cool spring periods could increase the risk of frost damage at the time of flowering of certain temperate fruits e.g. almond, apricots, apple etc.
• In areas where winter damage to fruit trees can be of concern, extreme soil and air temperature fluctuation under mulching with black plastic also has a risk of accelerating the effect of frost damage to fruit trees.

Cultivation

• Daily summer temperature of soil under cultivation tends to exceed by 1-3^oC than those under mulch or sod.
• In contrast, during winter daily soil temperatures are normally 1^oC cooler.

Herbicides

• Summer soil temperature under herbicide is usually higher than the soil under sod.
• However, it can be lower by 2- 4^oC when compared with soil temperature under black plastic.

Other soil properties/environment:

Sod culture and cover crops:

• Sod and cover crops usually improve other physical properties as reflected by decrease in bulk density and increase in soil porosity.

Mulching:

• Use of organic mulches in orchards is also beneficial in improving soil properties such as increase in porosity and development and stability of a desirable soil structure.
• This useful effect of mulching can be attributed to a regular supply of organic matter, especially in fine textured soils, to the protection of soil surface from raindrop impact and to retardation of soil surface slaking and sealing.

Cultivation

• Compared to other soil management systems, cultivation frequently results in a deterioration of other soil physical properties.
• Increase in bulk density, decrease in stability of soil aggregates, macro-porosity and oxygen diffusion in fine-textured silt soils can occur under clean cultivated plots.
• Other disadvantages of clean cultivation are rapid deterioration of soil structure and possible development of hard pan layer restricting root growth immediately below the zone tillage.
• Over long term cultivation practice in an orchard, soil aggregation deteriorates resulting in increased susceptibility of soil to crusting and impeded drainage.
• However, regular cultivation in orchards with gravely soils is unlikely to cause any serious impact on the soil.
• Decreases in soil infiltration capacity and saturated hydraulic conductivity of cultivated orchard soils can deteriorate physical properties of soil and cause water runoff.
• Thus, this can adversely affect growth of fruit trees, particularly peaches which require well aerated soils.
• For this reason, several alternate soil and water conserving systems of cultivation such as contour or strip cultivation, trashy cultivation and annual cover crops in combination with minimum early spring cultivation can be suggested.

Herbicides

• Bulk density of herbicide-treated soil can be about 10% higher than grassed or cultivated soil.
• Long time herbicide use also affects pore size distribution but the proportion of large pores (> 15 m) may not necessarily reduce.
• A reduction in total pore volume can occur by reducing pores within a limited range of pore sizes or across the entire range, depending on the soil.