|Soil is that part of the earth?s surface in which plant roots grow. In sustaining plant life, its major functions are to supply mineral nutrients and water and serve as a medium for root anchorage. It is difficult to grow grapes without soil (hydroponics) either with nutrient solutions only or with sand or gravel.
The four major components of soil mineral materials, organic matter, water, and air are intimately mixed, but are not independent in nature. If one system is changed, there is a change in all. An average silt loam soil may be composed of about 50 percent pore space (air and water); the solid space is made up of about 45 percent mineral matter and 5 percent organic matter.
|Mineral Constituents of Soil
The mineral or inorganic part of the soil is comprised of small rock fragments and minerals of different sizes, and may be original (e.g., sand derived from quartz) and relatively unchanged form the parent materials, or secondary (e.g, clay) and formed by weathering of less resistant minerals.
Soil texture refers to the size of the individual mineral particles (Buckman and Brady, 1960). Textural designations generally used to describe soils are sand, silt, or clay and the textural triangle shows the relative percentages of these components (Fig. 8-1). For example, a typical loam soil, might have 40 percent sand, 40 percent silt, and 20 percent clay. A ?heavy? soil refers to one that is high in clay and other fine particles, and a ?light? soil is one composed of much sand and coarse particles but little clay. Silts are intermediate in size and have properties midway between sand and clay, the finest of the mineral fractions.
Organic matter has a high water holding capacity that acts like a sponge in the soil. It is also a source of minerals made available during its decomposition. Small particles of organic matter cover mineral particles and keep them from sticking together, thus helping to maintain a good soil structure. A muck soil contains 10 to 40 percent organic matter and a peat soil about 40 to 100 percent. When such soils are properly managed, they can be extremely productive.
If too much water is added to the soil, the air is forced out of it and plant roots may be deprived or oxygen. Flooding of grapes during the dormant season does not appear to be harmful, although prolonged flooding in the growing season can be injurious.
The deeper and more fertile soils usually produce the heaviest crops and are therefore preferred for raisins, common wine grapes, and table grapes such as Thompson Seedless and Tokay. Soils of limited depth are preferable for varieties such as Emperor and Malaga. Wine grapes often produce high quality fruit on infertile or rocky soil; thus good grape crops can often be grown commercially where other crops are unsuccessful.
Characteristics of Good Soil
The best soils for grapes are loams to loamy in texture, well structured, moderately deep to deep and well drained, uniform, and free from harmful salt accumulations, damaging soil pests, and soil pathogens. Relatively flat land is preferred.
A loam soil is a mixture of sand, silt, and clay particles containing about 35-45 percent sand, 35-40 percent silt and 10-25 percent clay. Loams and fine sandy loams are often the best soil textures for agricultural soils. Sands cannot retain much water or supply many nutrients, but good aeration may allow roots to penetrate deeply. Clays have good water holding capacity and nutrient supplying ability, but root penetration may be limited by poor aeration. Silts are similar to clays in many respects.
Loam and fine sandy loams are considered excellent texture, while sand, silt, and clay are often poor (Neja and Wildman, 1973). Coarse-textured sandy soils cannot hold as much water or supply it as rapidly to the vine as can the finer textured loams. As soil depth increases, however, the preference of loams over fine sands decreases. For example, a 6-ft (1.83 m) deep, fine sand can be equal or even superior to a 6-ft (1.83 m) deep loam soil.
Fine textured silts and clayey soils can sometimes limit the depth of rooting of irrigated grapes. Since shallow soils, say 30 inches (76.2 cm) deep, require frequent irrigation, a good irrigation setup is required.
Soil texture is only one of several aspects of the soil environment, and soil structure, depth, uniformity, and compaction must also be considered.
Another type of barrier to root growth is an abrupt change from a fine or moderately fine topsoil to a coarse textured subsoil. Water will not move from the topsoil to the subsoil until several inches of soil above the interface are saturated. This lingering saturated zone remains because particle to particle flow of water is slow from the upper to the lower layer. Good drainage is essential to obtain root depth, and perched water tables block root growth.
Sometimes very dense, unfractured rocklike layers called hardpan occur. Such cemented hardpans are impervious to both water and roots. During the winter, rainfall can accumulate above the hardpan but can not pass through it. Unless the hardpan is mechanically shattered so that drainage is improved, vines may grow poorly (Widlman and Gowans, 1974).
Methods of detecting topsoil depth limitations include a direct study of the soil profile by appearance and feel, a laboratory analysis of texture and bulk density (weight per unit volume of a core of oven dry soil), and a study of the rate of water infiltration at different soil depths (Fig. 8-4).
The uniformity of the soil reservoir is based on evenness of expected root depth and of topsoil across the field. Depth of rooting can be estimated by mapping the differences in soil structure and texture, with depth, over the field. Generally depth uniformity is considered good if deepest soils are no more than 11/2 times the shallowest depth, and is considered unacceptable if deepest soils are greater than twice the depth of the shallowest soils.
A grower makes the best of a nonuniform soil by using several techniques. Vineyard blocks should be laid out so that there is even texture and depth within each, leaving nonuniformity among blocks. Irrigation systems should be set up to allow irrigation of individual blocks. The uniformity of soil depth can be increased by plowing and ripping. One should use grape varieties and/ or rootstocks that are adapted to grow well on the particular type of soil.
The best means of avoiding compaction is to cultivate as little as possible and only when the soil water content is intermediate, not too wet or too dry. Keep all traffic off the freshly loosened soil. The size of water conducting pores can be increased by planting permanent cover crops; deep rooted grasses that do not require tillage are best and can increase water intake rates5 to 10 times over a 5 to 20 year period. In some cases, however, a sod cover in vineyards may have deleterious effects on vine development and fruit production.
Soil compaction result in a loss of good structure, and often shows up as a slight layering or increasing density just below the usual depth of cultivation (Fig. 8-3). Man made compaction often extends to a depth of 2 ft (60.9 cm) (Neja and Wildman, 1974).
Abrupt changes in structure and / or texture can cause a barrier to root growth. For example, if a fine or moderately fine topsoil is under laid with a coarse sand subsoil, roots will not grow across the abrupt boundary. Irrigation water will not pass from the upper layer into the sand until several inches of water above the interface is saturated because of the higher capillarity of upper finer soil compared to that of lower sandy soil.
Temporary saturation above the interface can limit aeration in the area just above the juncture of the two textures and thus restrict root growth. Abrupt boundaries in stratified soils break up the normal downward percolation of water, resulting in poorly aerated zones above the boundaries. A uniformly mixed soil is a much better root medium than a stratified soil.
Deep tillage refers to loosening, breaking up, or mixing restricting subsoil layers located below the depth of normal or ordinary cultivation. Its purpose is to break up man made compact layers in the top 2 feet of soil, break through natural subsoil claypans, hardpans and dense layers, mix stratified soils, and eliminate abrupt boundaries between unlike soil textures (Wildman et al., 1974). Types of equipment usually used in deep tillage include rippers, chisels, subsoilers, slip plows, disc plows and moldboard plows. The rippers and subsoilers, names which are often used interchangeably, operate at a depth of 2 to 7 ft (0.61-2.13 m) or more, where they break up hard layers by cracking and shattering; little mixing or dislocation of layers occurs. They are most effective in moderately dry, brittle soils and least efficient in moist sands and clays. Very dry soils require tremendous power to rip and may produce large clods that are difficult to manage.
Slip plowing is usually done to a depth of 3 to 6 ft. (0.91-1.83 m). A slip plow consists fo a verticle ripping shank with a 12-15 in. (30.5-38.1 cm) wide inclined beam extending to the rear from the ripping point at an angle up to the soil surface. Chunks of subsoil are torn loose by the point, slide up the beam, and are permanently moved from their original position. Surface soils fall into the channel produced, resulting in some permanent mixing and thus improving water and root penetration (Wildman et al., 1974).
Disc plows are valuable for relieving soil compaction and for mixing shallow clays in the upper 2 feet (60.9 cm) of soil. Moldboard plows developed in the 1950s could plow 4 to 6 ft (1.2-1.8 m) deep and effectively loosen and mix soil. Today, only a few 3 and 4 ft(0.91 and 1.21 m) plows are still in operation.
The success of the ripping operation can best be checked by backhoe appraisal. Often slip or plowing ripping along a future vine row may be beneficial, unless you plan to disc plow to 18 to 24 in. (45.7-60.9 cm) after deep tillage. It is important that young vine roots penetrate quickly into loosened and well aerated soil.
Backhoes are probably the most universally available, and easily transportable type of equipment listed for soil modification, and are adaptable mainly to small acreages.
Wheel trenches are usually used to dig trenches for water and drain lines, and have been used to a limited extent to loosen and mix soil layers before planting a vineyard.