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Many agriculturally important soils in the San Joaquin Valley, being compact and of high strength, may pose special problems in water management. The higher than normal frequency of irrigation often required to meet evapotranspiration demand suggests that plant root development has been limited. This study was conducted to determine the influence of high penetrometer soil strength, found naturally occurring in the field, on the root development of cotton (Gossypium hirsutumL.) and corn (Zea maysL.). Additional objectives were to determine if root distribution could be influenced by planting geometry and density, and to relate soil water depletion to observed root density (cm of root/cm3of soil). Cotton and corn plots, having a variable row width and plant density, were established on each of two field soils that differed considerably in strength as measured with a recording soil penetrometer. Root densities of the two crops were related to the independent variables penetrometer strength (measured in situ), and soil depth in a regression model that accounted for 75% of the observed variation. A high‐density plant population of cotton caused higher root densities in the 30 to 122 cm‐depth zone in a low‐strength soil than that with normal plantings. A linear relation between water depletion and root density indicated average water uptake values for fully transpiring corn and cotton to be respectively 2.33 × 10−aand 7.86 × 10−3cm8/cm/day. Field measured soil strength was effective for diagnosing or predicting mechanical root growth restrictions of well drained soils.

Water supplied to a growing crop by capillary rise from a shallow water table can be an important resource. Several thousand hectares in the western San Joaquin Valley in California have a perched water table created by irrigation and a slowly permeable subsurface zone. When irrigations are made according to a schedule that is optimum for soils without a shallow water table, cotton (Gossypium hirsutumL.) frequently shows plantgrowth that is characteristic of excessive irrigation. Field studies were conducted to evaluate the contribution of a perched water table to the evapotranspiration (ET) demand of cotton and to develop an irrigation schedule that uses the resource effectively. Two independent procedures were used to estimate the water table contribution (WT) through capillary rise. A water budget method used a previously developed yield‐ET function to estimate ET from observed production levels. Soil moisture depletion (SMD), stored irrigation water (IW), and deep percolation (PW) were determined from neutron thermalization measurements on volumetric soil water content. WT was determined by rearranging the relation ET = SMD + IW − PW + WT. A second approach, associated the chloride in the water table (17.4 meq/l) with increased chloride concentrations in the soil above the water table. The equivalent water depth necessary to effect this change was calculated. Both procedures estimated WT to be near 36 cm for the growing season. A high standard deviation (18.7 cm) was caused, however, by considerable spatial variability for the chloride tracer method. A revised irrigation schedule for cotton was developed that uses water, of acceptable quality, derived from a water table as a beneficial raource.

Vegetative development of cotton (Gossypium hirsutumL.) in early season affects plant responses through the entire growth period. In the irrigated West, soils are usually irrigated before planting. With soils initially wet, scheduling a first irrigation is important to achieving desired plant growth and development. A 2‐year field study was conducted on contrasting soils to evaluate how first irrigation scheduling affected vegetative growth, maturity rate, and production. Main stem elongation was stimulated by a lower plant water deficit in early irrigations. Although a delayed first irrigation slowed vegetative growth through mid‐June, when irrigations were normalized main stem elongation was accelerated well into the July and August fruiting period. Vegetative growth was frequently increased in the fruiting period by early first irrigations. Either early or late first irrigations delayed maturity. A multiple regression model used to express relative yields (R2= 0.72) had as independent variables the time of first irrigation (days after 30 April) and the water available to the plant in the top 61 cm of soil at planting. This yield equation predicted an optimum time for a first season irrigation from known soil water retention capability.

Reduced growth and fewer plants are common in wheel paths of post harvest equipment in alfalfa (Medicago sativaL.) fields of the San Joaquin Valley. Up to 70% of the soil surface may be covered by equipment wheels in a single harvest. Field and glasshouse studies were conducted to quantify plant and soil responses to wheel traffic in confined paths scheduled to simulate harvest operations. Three field compaction treatments simulated harvest, postharvest, and harvest plus postharvest wheel traffic. A no‐traffic area served as a control treatment. Compaction of a sandy loam soil (Typic Xerorthents) in traffic zones reduced root‐length density (cm of root per cm3of soil) as much as 60% in a 15‐to‐30‐cm depth zone. Postharvest traffic reduced plant population and yield by damaging crowns and regrowth shoots. Field observations on soil‐compaction root‐growth relations were substantiated by a glasshouse study that char. acterized alfalfa root‐length density (Rd, cm of root per cm3of soil) as a function of soil bulk density (Ds, g cm−3) and time (T, days) as Rd= −23.51 +27.88Ds+0.2210T −9.135D,s2−0.0003548T3−0.08360DsT, R2= 0.84. Root growth in a field study was enhanced by delaying the harvest for newly established seedings.

Populations of 100,000 plants per acre produced highest grain yields. Increased row width reduced yields significantly as expressed by the regression Ŷ = 117.11 − 0.72X, where Y is in bushels per acre and ✕ is inches. A significant interaction was found between plant spacing and irrigation water management but not between plant spacing and fertility level. Seasonal moisture use or efficiency was not affected significantly by plant spacing.

During seedling to crown root stages daily dry matter production was 30 to 50 pounds per acre and N uptake about 1 pound; both were increased only slightly by N fertilizer. During crown root through flowering stages increased growth and N uptake rates were further increased by irrigation and N fertilization, ranging from 240 to 340 pounds and from 4 to 6 pounds, respectively. Maximum production and N uptake occurred between soft and hard dough stages.

Grain yields were curvilinearly related to the number of irrigations, depletion of soil moisture, and increments of applied N. Evapotranspiration was related to number of irrigations and to available soil moisture. Plant growth response to applied N increased evapotranspiration slightly. Irrigation and applied N both increased the efficiency of water use. Maximum efficiency occurred at a yield level below maximum production.