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.� than 7)with a high base exchange(more than 50% ize an equivalent amount of acid and would be a calcium).This is typical for soils forming in an and good choice when potassium fertilization is neces- environment. Groundwater is also predominately sary and the soil is too acidic. .� alkaline as groundwater chemistry reflects the chemistry of surrounding soil and parent rock. Water Infiltration: Low salinity water(less than Severe soil acidification(to pH 5 or lower)is not a 0.5 dS/m and especially below 0.2 dS/m) tends to widespread problem in San Joaquin Valley vine- leach surface soils free of soluble minerals and yards,but it is becoming more and more common, salts, especially calcium, reducing their strong particularly with drip irrigation.Acidic soils(below stabilizing influence on soil aggregates and soil pH 5 to 5.5)can impact vine growth,yield,and fruit structure. Without salts and without calcium, the quality. soil disperses,and the dispersed finer soil particles fill many of the smaller pore spaces, sealing the Vineyards at risk of acidulation are those vineyards surface and greatly reducing the rate at which water on soils with low buffering capacity, irrigated infiltrates the soil surface. Very low salinity water primarily with canal water,and fertilized with acid- (less than Ecw=0.2 dS/m)almost invariably results forming fertilizers. In Tulare County, soil types at in water infiltration problems. greatest risk are sandy soils associated with alluvial fans and with no free lime in the profile(Cajon and Slow water infiltration is a serious problem in some Tujunga series); also, highly weathered, east side vineyards, especially with drip irrigation. noncalcareous hardpan soils developed on old With drip irrigation,irrigation water must infiltrate alluvial fans and terraces (San Joaquin and Exeter a small surface area to satisfy the vines' series). Canal water is low in salt(25 to 50 ppm of evapotranspiration needs. If infiltration is inade- total soluble salts)and calcium,has no neutralizing quate, puddling occurs and the vines will show power, and can dissolve and remove calcium water stress even though more than adequate carbonate from the soil,potentially lowering pH.In amounts of water have been applied. Waterlogging contrast, typical groundwater on the east side of near the surface kills vine roots,and the inadequate Tulare County contains moderate levels of salinity distribution of water in the profile limits the volume (250 to 500 ppm).Much of the pump water actually of the root zone effectively wetted making matters adds calcium carbonate(200 to 600 pounds of lime worse. per acre-foot) which can neutralize an equivalent input of acid. Gypsum and other soluble compounds can double or triple the infiltration rate over that of untreated Acid input to the soil comes from a variety of low-salt water. Gypsum is the amendment most + sources.Most significant is the use of ammoniacal often used, but other calcium sources are also fertilizers which produce H" (acid) when ammo- effective.Calcium nitrate and CAN-17 are effective nium (NH4*) is oxidized to nitrate (NOS). The but can only be used in limited amounts: the nitro- acidification potential of nitrogen fertilizer should gen applied must not exceed the vines'requirement be considered when applied to acid soils. This is for nitrogen fertilizer. shown in the table as the amount of pure lime (pounds calcium carbonate)needed to neutralize the Calcium must be added continuously to the irriga- equivalent of 1 pound of nitrogen for each fertilizer tion water in order to maximize infiltration. When material.Materials with a high acidification poten- application of gypsum to irrigation water is discon- L tial should be avoided on soils with pH of less than tinued, infiltration rates drop to near control levels 6. Instead, use compounds with an alkaline soil within a few weeks. The occasional addition of reaction that raise pH such as calcium nitrate. calcium to the irrigation water is not effective in LPotassium carbonate is very soluble and can easily maintaining infiltration rates. To optimize infiltra- be applied through the drip system. It will neutral- tion rate using canal water, finely ground gypsum LGrape Notes # January 2002 # Page 2