Graphs allow estimation of available phosphorus in some Hawaii soils representative of different P-fixation types. Levels of P in plant tissues associated with near-maximum yields are given for some common crops

Huang, X.

Hue, N.V.

Ikawa, H.

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Cooperative Extension Service Agronomy and SoilsNov. 1997 AS-2**Replaces “Fact Sheet no. 2 10/06/94”Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Charles W. Laughlin,Director and Dean, Cooperative Extension Service, College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, Honolulu, Hawaii 96822. AnEqual Opportunity / Affirmative Action Institution providing programs and services to the people of Hawaii without regard to race, sex, age, religion, color, nationalorigin, ancestry, disability, marital status, arrest and court record, sexual orientation, or veteran status.Along with nitrogen and potassium, phosphorus (P)is a nutrient that plants need in relatively large quantitiesfor normal growth. In living organisms, P is a structuralcomponent of deoxyribonucleic acid (DNA) and ribo-nucleic acid (RNA), which are essential for reproduction.Plants and animals derive their internal energy from P-containing compounds, mainly adenosine triphosphate(ATP). Inadequate P supply thus results in a decreased syn-thesis of RNA—the protein maker—leading to depressedgrowth. P-deficient plants are stunted, with limited rootsystems and thin stems. Symptoms in cereals include re-duced tillering; corn seedlings look stunted, and olderleaves may be purple because of high levels of anthocya-nin (purple pigment). Fruit trees deficient in P have fewerand shorter new shoots and malformed fruits and seeds.Thus not only low yields but also poor quality result fromP deficiency. Most crops need from 0.2 to 0.5% P in tissuedry matter for normal growth, and P deficiency is likelywhen the P content drops below 0.1% (Table 1).While humans and animals obtain P from various foodsources, plants must get their P from the soil. Unfortu-nately, most soils of Hawaii contain high amounts of ironand aluminum oxides or amorphous aluminosilicate clays,which react strongly with P, making it virtually unavail-able for plant uptake. When this occurs, acceptable cropPredicting Phosphorus Requirements of Some Hawaii SoilsN. V. Hue, H. Ikawa, and X. Huang—Department of Agronomy and Soil Scienceproduction is not possible unless adequate P fertilizers areapplied. The key question for growers is: Does the soilneed P, and if so, how much? An accurate soil test for P,accompanied by a site-specific, research-based recommen-dation on the amount of P fertilizer, is needed to answerthe question. Excessive P fertilization is counter-produc-tive, wastes money and resources, and may pollute theenvironment.CTAHR’s Agricultural Diagnostic Service Center(ADSC) recently changed its P analysis procedure fromthe Rapid Chemical Method to a “research method” knownas the Modified Truog Method. It uses a solution of 0.01M H2SO4 + 0.3% (NH4)2SO4 at a 1:100 soil:solution ratio,which is shaken 30 minutes to extract phosphorus for analy-sis. This change has improved soil test precision for P. How-ever, precise fertilizer recommendations are still difficult,since they depend on site-specific, research-based infor-mation, ideally based on long-term soil fertility trials con-ducted on specific soils. Regretably, for many soils in Ha-waii, data for “calibration” of the P requirement of par-ticular crops, to establish the specific relationship betweenapplied P and the yield and quality of that crop, are oftenunavailable—and expensive to obtain. We do have dataon the “external” P requirements of some crops (Table 1).The soil-solution P concentration is often used to expressTable 1. Levels of P in plant tissue and soil solution associated with 80–95% of maximum yield of some crops grown inHawaii.Crop Plant Px (%) Soil-solution P (mg/liter) Soil seriesyCabbage (Brassica oleracea) 0.4 – 0.8 0.02 – 0.04 KulaCorn (Zea mays) 0.3 – 0.5 0.03 – 0.05 Halii, WahiawaHead lettuce (Lactuca sativa) 0.4 – 1.0 0.20 – 0.40 Kula, LualualeiMacadamia (Macadamia integrifolia)z 0.08 – 0.10 0.02 – 0.04 Kapaa, Puna, WahiawaSorghum (Sorghum bicolor) 0.2 – 0.6 0.02 – 0.06 HonokaaSoybean (Glycine max) 0.25 – 0.50 0.10 – 0.20 Halii, WahiawaSugarcane (Saccharum officinarum) 0.18 – 0.30 0.02 – 0.04 WahiawaTomato (Lycopersicon esculentum) 0.4 – 1.0 0.10 – 0.20 Kula, WaimanaloxPlant P concentration may vary considerably with plant age and part.  ySoils of Hawaii used for establishing critical concentrations of soil-solution P.zMacadamia is an exception to the general rule that tissue levels of P should be in the range 0.2 to 0.5%; iron chlorosis may occur when P is above 0.15%.AS-2 Predicting Phosphorus Requirements CTAHR — Nov. 19972Figure 1. Relationship between sorbed P and soil-solution P in 12 soils of Hawaii.Sorbed P (mg/kg soil)Solution P (mg/liter soil solution)AS-2 Predicting Phosphorus Requirements CTAHR — Nov. 19973Figure 2. Relationship between fertilizer P and modified Truog P for some soils of Hawaii.Modified Truog P (ppm)Fertilizer P (mg/kg soil)AS-2 Predicting Phosphorus Requirements CTAHR — Nov. 19974the external P requirement of crops, because this P require-ment remains nearly constant across soil series, whilechemically extractable P varies from soil to soil. ADSC’sobjective is to establish a clear relationship between soil-solution P (which the plant experiences) and Truog-extract-able P (which we measure in the laboratory) so that wecan reasonably predict (1) whether a soil needs additionalP, and (2) if it does, the amount of P needed for good growthof a given crop.To achieve this objective, we first establish the P sorp-tion curve—the relationship between soil-solution P con-centration and the amount of P sorbed (held in the soil),which varies from soil to soil, as Figure 1 demonstrates.Then we establish the relationship between Truog-extract-able P and fertilizer P applied, as shown in Figure 2. Al-though this latter relationship varies somewhat with  soils,the Modified Truog Method generally extracts (recovers)about half of the fertilizer P (Graph 11, Fig. 2). Finally, wecombine the two relationships to obtain a connection be-tween the soil-solution P and the Truog-extractable P, asshown in Figure 3. Practical use of these graphs is illus-trated in the following examples.(1) Let’s suppose you sent ADSC a soil sample fromyour garden in Lihue, Kauai (the Lihue series) and youwish to grow tomatoes. The test result came back, show-ing 12 ppm P. You then look at Figure 3, which shows that12 ppm Truog P is equivalent to about 0.028 mg/liter P insoil solution. From Table 1, you find that at least 0.10 mgP per liter of soil solution is needed for a good crop oftomato. This translates roughly to 200 ppm Truog P, basedon Figure 3. Then you look at Graph 6 of Figure 2 for theamount of fertilizer P needed in the Lihue soil to maintain200 ppm Truog P, which is 400 mg P per kg soil. Assum-ing that the surface 6 inches (~15 cm) of 1 acre of soilweighs 1 million kg (or 2 million lb), then you should ap-ply about 800 lb/acre of P fertilizer (20 lb P per 1000 ft2).(2) Assume that you want to grow cabbage, and yoursoil test result from ADSC is 100 ppm P. Figure 3 showsthat soils (in general) with this much Truog P have ap-proximately 0.05 mg P per liter of soil solution. Table 1says this level is adequate for good growth of cabbage, soyou may not need P fertilizer.(3) Assume that you’re not certain about the name ororigin of a particular soil, which has a soil-test P of 50ppm, but you know that the intended crop is lettuce. Table1 and Figure 3 predict that lettuce does well only whenthere is at least 0.20 mg P per liter of soil solution, whichis equivalent to 300 ppm Truog P. Using Graph 11 of Fig-ure 2 (or its equation), you can calculate  (2.17 × [300 –50] = 542.5 mg P/kg) that you need to add aproximately1100 lb P per acre, or 25 lb P per 1000 ft2.In summary, with soil-test P data from ADSC, (1) useFigure 3 to determine soil-solution P, (2) examine Table 1 tofind out the critical soil-solution P required for the crop, anddecide whether your soil has sufficient P, then (3) return toFigure 3 to find the Truog P level corresponding to the criti-cal soil-solution P, and finally (4) select the appropriate graphin Figure 2 to determine the amount of fertilizer P to apply.Figure 3. Relationship between soil-solution P and modified Truog P for soils of Hawaii.Modified Truog P (ppm)Soil solution P (mg/liter)

Predicting Phosphorus Requirements of Some Hawaii Soils

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Predicting Phosphorus Requirements of Some Hawaii Soils

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