Fertilizer Application Management

by

Dr. Roland D. Meyer Extension Soils Specialist, Emeritus

University of California-Davis email: rdmeyer@ucdavis.edu

Organic matter is the primary storage for nitrogen in most soils

but our Mediterranean climate that has warm temperatures throughout much of the year results in higher decomposition rates than the accumulation from the addition of plant residues.

Thus Inorganic nitrate-nitrogen becomes an important

supply of nitrogen for plants.

A number of research investigations have been conducted on different crops using the nitrogen isotope N15 in fertilizer.

This enables the researcher to assess how much of the nitrogen in the harvested plant comes from the fertilizer versus how much is taken up by the plant from N stored in the soil.

Crop Fertilizer N Soil N in Crop

Plant N from Soil

(lbs/acre) (lbs/acre) (%)

Corn 112-560 55-79 22-47 Corn 90-360 84-148 40-76 Wheat 50-100 64-78 56-79 Rice 50 94-113 79-93 Rice 100 50-57 56-62 Tomato 56-224 196-216 57-90

Table 1. Uptake of soil nitrogen by fertilized annual crops

Based on N15 tracer studies, Broadbent, F.E., 1984.

Note that from 22 % up to 93 % of the nitrogen in several annual crops came from the soil, that released from organic matter plus nitrate-N.

The information in this table also illustrates why we often indicate that the average fertilizer uptake efficiency is around 50%.

Estimating the amount of crop available nitrogen:

Take soil samples and analyze for nitrate in the spring just prior to the initiation of plant growth.

When nitrogen fertilizer is broadcast and water is applied uniformly, combine several cores (two/tree) at the drip-line from ten trees for each sample from a high, medium and low producing area of the field. Sample in 1 foot increments to a depth of three to five feet.

In microirrigated situations, sample two locations on either side of the water outlet equal distance from the emitter approximately 1/3 to ½ the wetted distance away from the emitter under 10 trees for each composite sample. Combine the two cores/tree from ten trees for each 1 foot increment to a depth of 3 to 5 feet from a high, medium and low producing area of the field. Doing this in the spring just prior to the initiation of plant growth should give a fair index of the residual nitrogen that would be available to begin the upcoming production season.

Taking the samples from the same locations and depths at the same time of the year for several years would develop an even more valuable basis for the fertilizer nitrogen necessary to reach optimum yields during the “on” and “off” production pattern of pistachio. Although soils are tested for a number of nutrients, the sodium bicarbonate extractable phosphorus, ammonium acetate extractable potassium and the DTPA extractable zinc are particularly well correlated with annual crop responses.

Soil tests are especially valuable in assessing the presence of excessive salinity and toxic element concentrations of boron (B), sodium (Na) and chloride (Cl).

Soil samples should be taken to a depth of at least 6 feet or more to assess the potential rooting depth and moisture holding capacity as well as adverse environments like salinity or toxic element levels.

Research has demonstrated the large differences in crop nutrient removal by various perennial crops. Note that the nut crops almond, walnut, and pistachio have a much higher nitrogen removal than the fruit crops (See Table 2).

Nutrient Removal Crop Yield Nitrogen Potassium

(tons/acre) (lbs N/acre) (lbs K2O/acre)

Almond 1.5 195 255 Apple 12 72 204 Grape 8 66 104 Orange 10 88 110 Peach 12 76 96 Pear 10 57 63 Pistachio 2.25 188 166

Table 2. Yield and nutrient removal for tree and vine crops.

Western Fertilizer Handbook, Weinbaum, et al., 1995.

Certainly the “on” and “off” yielding characteristic of pistachio adds another dimension to the determination of crop nutrient removal.

Crop yield level dramatically influences the quantity of nutrients removed by crops.

Lesser changes usually result from variation in nutrient concentrations in the crop fractions: nuts, shells and hulls.

The first questions to be addressed when considering fertilization are— how much and when the necessary nutrient quantities should be applied.

Lets address the ‘when’ question first:

Considerable research with a number of crops has demonstrated that if the nutrient can be applied and positioned in the root zone just prior to the greatest need for that nutrient, then the most efficient plant uptake will occur (Please see Figure 1).

Figure 1. Nitrogen uptake pattern by prune.

(Weinbaum, et al., 1978.)

Applications of the nitrogen considerably prior to crop need will result in greater potential for denitrification (loss of nitrogen into the atmosphere) or leaching of nitrate below the crop root zone.

For ease of application into the irrigation systems, usually equal amounts of fertilizer solution are injected 3 to 6 times each year. In general, research has shown that this uptake pattern is similar for many crops for nitrogen and the other nutrients but that certain periods are more critical for various plant functions like reproductive development for the next year’s crop.

Research on almonds conducted over a three year period showed that nitrogen applied early in the season (1/3 of the total N applied on April 1st, May 1st and July 1st) yielded significantly more than when N was applied on a late season schedule (1/3 of the total N applied on June 1st, August 1st and September 1st) (See Table 3).

Timing of N Application Year Early Late

(lbs/acre) (lbs/acre)

1994 2239 a 1832 b 1995 1712 ‘ 1607 ‘ 1996 2063 a 1686 b Three yr ave. 2005 1708

Table 3. Almond yield as influenced by timing of nitrogen application.

R. D. Meyer, unpublished data.

Addressing the ‘how much or quantity of nitrogen addition question’ is an interesting challenge. The pistachio has yields in the “off” year perhaps ¼ to 1/3 of those in the “on” year. Thus, the amount of nitrogen and other nutrients applied prior to and during the “on” year need to be greater than during the “off” year. Total nitrogen applied should be approximately 200-225 lbs/acre during the “on” year and about one half that amount during the “off” year. The application of N in the “on” year should be 30-75 lbs/acre in April, 50-75 lbs/acre during June and 50-75 lbs/acre during early August. The application of N in the “off” year should be 30-60 lbs/acre in April, 35-50 lbs/acre during June and 35-50 lbs/acre during early August.

Potassium applications will range from very little on heavy high clay content soils to over 200 lbs K20/acre on coarse textured sandy soils with less than 10% clay.

The application of K in the “on” year should be 35-75 lbs/acre in May, 35-75 lbs/acre during June and 35-50 lbs/acre during July to early August.

Potassium application during the “off” year should be 1/3 to1/2 of the amount during the “on” year.

Leaf analysis is an effective tool to monitor the nutritional and potential toxicity of elements in tree crops. Soil sampling and analysis is particularly well suited to evaluating salinity and toxic levels of sodium, chloride and boron.

Proper leaf sampling procedures as well as the identification and correction of nutrient disorders is discussed in great detail in Chapter 16, “Diagnosing and Correcting Nutrient Deficiencies.”

Given in Table 4 are the critical and suggested nutrient concentrations in August leaf samples.

Nutrient Critical Value Suggested Range

Nitrogen (N) 1.8% 2.2-2.5% Potassium (K) 1.6% 1.8-2.0% Chloride (Cl) (?) 0.1-0.3% Boron (B) 90 ppm 150-250 ppm Zinc (Zn) 7 ppm 10-15 ppm Copper (Cu) 4 ppm 6-10 ppm

Table 4. Critical and suggested nutrient concentrations in August leaf samples.

Other considerations: Since many crops are being irrigated with “drip” or “fanjet” systems which apply water to varying amounts of soil volume, the reaction of fertilizers upon soil pH and other characteristics may influence the availability of nutrients. Research in almonds and other tree crops has shown that when urea and other ammonium forms of N are applied, the soil pH may become acidic rather quickly in coarse textured (low clay content, <10%) soils when very limited soil volumes receive all of the water for the tree. When an area of only 2 to 8 square feet of the soil surface is wetted underneath the emitters and utilized to water an individual tree, soil characteristics such as pH may be modified within a few years.

Needless to say this dramatic acidification in the drip zone changes other soil characteristics such as aluminum and manganese solubility. This prompted the initiation of a nitrogen source trial which included several soil pH amelioration treatments (See Table 5).

Treatment Yield Leaf Mn (lbs/acre) (ppm)

Calcium Nitrate 1868 85 c Urea 1963 272 b UN 32 1935 271 b Urea + lime (Ave 3 rates) 1942 301 b Urea + gypsum 1869 377a Urea + NaK nitrate 2123 265 b Calcium Nitrate & Urea 2265 141 c N-phuric 1780 425a

Table 5. Three yr average almond yields and leaf Mn levels.

Meyer and Edstrom, 1998a

Potassium concentrations continued to decline in almond leaf samples in the nitrogen rate by irrigation trial because of the high yields and additions of potassium sulfate resulted in only small increases in yield.

Mitigation of acidic or low pH soil conditions in other crops with phosphate fertilizers suggested the use of mono-potassium phosphate (MKP) as a potassium source rather than potassium sulfate (See Table 6).

Treatment 1995 1996 1997 1998 (lbs/acre) (lbs/acre) (lbs/acre) (lbs/acre)

Control 1541 1581 1576  c 1849 MKP 1718 1916 2115ab ‘ 2016 K sulfate 1573 1872 1777 bc 1867 MKP+K sulfate

1806 2130 2304a ‘ 2239

Table 6. Almond yields as influenced by K fertilizer source.

Meyer and Edstrom, 1998b, MKP = Mono-Potassium Phosphate

The Result:

Potassium sulfate continued to give only small increases (136 lbs/acre over the control) in almond yields (four year average) while MKP alone resulted in a 305 lbs/acre increase over the control and the MKP + potassium sulfate gave an increase of 348 lbs/acre.

However, leaf manganese concentrations remained nearly the same in all four treatments.

Summary

Pistachio production in California occurs on a wide range of soils. Higher clay content soils are predominant in the southern and western San Joaquin Valley while loamy low clay (less than 10%) content soils occur on the East side of the valley and on the West side of the Sacramento Valley production areas. The low clay content soils combined with drip irrigation systems that apply water to limited volumes of the soil greatly increase the acidification of these soils. Under these acidic soil conditions, the micronutrients zinc, iron, manganese and copper are readily available to the tree.

Summary--continued

However, these sandy soils contain limited quantities of nutrients and organic matter to hold nutrients like nitrogen, potassium, zinc and boron which can be rather easily leached. In addition, the irrigation water supplies are generally neutral in pH and low in total salts. The high clay content soils generally have higher pH (7.4-8.4) and free lime (2-8%) which greatly reduces the potential for acidification. In addition, these soils are located in areas where irrigation waters generally contain higher calcium, magnesium and perhaps sodium that neutralize the acidity created by nitrogen fertilizers.

Summary--continued

These conditions of high soil pH, free lime, and perhaps higher salinity cause problems with respect to the availability of micronutrients like zinc, manganese, iron and copper to the pistachio trees. These soils often have higher concentrations of boron.