Efficacy of four phosphate-mobilizing bacteria applied with an animal bone charcoal formulation in controlling Pythium aphanidermatum and Fusarium oxysporum f.sp. radicis lycopersici in tomato
Postma, J. ; Clematis, F. ; Nijhuis, E.H. ; Someus, E. - \ 2013
Biological Control 67 (2013)2. - ISSN 1049-9644 - p. 284 - 291.
pseudomonas-chlororaphis pa23 - rock phosphate - growth promotion - nutrient-uptake - damping-off - soil - phosphorus - biocontrol - diversity - suppressiveness
Four taxonomically different bacteria, with the ability to mobilize phosphate (P) and to colonize animal bone charcoal (ABC), were tested for their capacity to control plant pathogens. Tests were performed in the greenhouse with young tomato plants in (potting) soil and in rockwool. Plants were infested with Pythium aphanidermatum and Fusarium oxysporum f.sp. radicis-lycopersici (FORL) causing respectively damping off and crown and root rot. ABC is a porous, phosphorous containing waste product from the food industry, and was used as carrier to introduce the bacteria into the growing media. Scanning electron microscopy (SEM) pictures showed the intensive colonization of the bacteria in the interior of ABC. Of the four tested strains, Pseudomonas chlororaphis 4.4.1 was most effective in controlling the diseases. It controlled P. aphanidermatum and FORL in tomato in each of the tests. The strain appeared to be a very good root colonizer, since 1–8% of the cultural bacterial population on the tomato roots or in rhizosphere soil consisted of the introduced strain. Population densities of P. chlororaphis 4.4.1 were 0.5–5 × 107 CFU g-1 root or rhizosphere soil. Peanibacillus polymyxa 12.4.1 and Streptomyces pseudovenezuelae 13.4.2 significantly controlled P. aphanidermatum in two tests in potting soil, whereas Bacillus pumilus 4.4.2 was not effective. FORL could be controlled by B. pumilus 4.4.2 and S. pseudovenezuelae 13.4.2 in only part of the tests, whereas P. polymyxa 12.4.1 was not effective. ABC is a novel carrier for delivery of biocontrol bacteria into soil or substrate and combines biocontrol with recycling a phosphorous-rich waste product
Sustainable use of phosphorus : EU tender ENV.B1/ETU/2009/0025
Schroder, J.J. ; Cordell, D. ; Smit, A.L. ; Rosemarin, A. - \ 2010
Wageningen : Plant Research International (Report / Plant Research International 357) - 122
fosfor - kringlopen - duurzaamheid (sustainability) - verarming - verliezen - natuurfosfaat - gebruiksefficiëntie - landbouwbeleid - europese unie - phosphorus - cycling - sustainability - depletion - losses - rock phosphate - use efficiency - agricultural policy - european union
As sustainable phosphorus use will sooner or later become essential for global food security, action is needed. As far as the required actions are concerned, the report has identified short-term and long-term policy options which could improve the current level of phosphorus use efficiency in agriculture. The report emphasizes, however, that policies should not be developed in isolation, let alone for agriculture only, but that all parts of the chain, that is primary production, processing and consumption, should be addressed in an integrative way. The current reliance on imported rock-based phosphorus (‘3 kg P per European citizen per year’) can not be continued in the long run. To become truly sustainable, phosphorus use efficiency must approach a level close to 100% in each chain. Therefore, a full recycling of phosphorus will become a condition sine qua non for global and European food security. The urgency of policies and measures needed for that will be determined by the phosphate rock reserves considered exploitable (including geopolitically and legally accessible), the prevention of accumulation and losses, the size of the global population and its preferences in terms of food, feed, fibers and fuels, and its appreciation of biodiversity. This will require drastic adjustments to the way we manage agriculture, and it may also require adjustments to our society as a whole, including the processing of our ‘wastes’.
Organic anion exudation by lowland rice (Oryza sativa L.) at zinc and phosphorus deficiency
Hoffland, E. ; Wei, C.Z. ; Wissuwa, M. - \ 2006
Plant and Soil 283 (2006)1-2. - ISSN 0032-079X - p. 155 - 162.
lupinus-albus l - root exudation - acid exudation - rock phosphate - zn-efficient - white lupin - genotypes - wheat - soil - iron
The objectives of this paper were to determine (1) if lowland rice (Oryza sativa L.) plants respond similarly to low zinc (Zn) and phosphorus (P) availability by increased root exudation of low-molecular weight organic anions (LMWOAs) and (2) if genotypic variation in tolerance to low soil supply of either Zn or P is related to LMWOA exudation rates. Exudation of LMWOAs can increase bioavailability of both Zn and P to the plant, through partly similar chemical mechanisms. We used seven lowland rice genotypes and showed in two experiments that genotypes that grow relatively well on a soil with low Zn availability also grow well on a sparingly soluble Ca-phosphate (r = 0.80, P = 0.03). We measured exudation rates of LMWOAs on nutrient solution and found that both Zn and P deficiency induced significant increases. Among the LMWOAs detected oxalate was quantitatively the most important, but citrate is considered more effective in mobilizing Zn. Citrate exudation rates correlated with tolerance to low soil levels of Zn (P=0.05) and P (P = 0.07). In a low-Zn-field we found an increased biomass production at higher plant density, which is supportive for a concentration-dependent rhizosphere effect on Zn bioavailability such as LMWOA exudation. We, for the first time, showed that tolerance to low Zn availability is related to the capacity of a plant to exude LMWOAs and confirmed that exudation of LMWOAs must be regarded a multiple stress response.
Exploring diversity in soil fertility management of smallholder farms in western Kenya: II. Within-farm variability in resource allocation, nutrient flows and soil fertility status
Tittonell, P.A. ; Vanlauwe, B. ; Leffelaar, P.A. ; Shepperd, K.D. ; Giller, K.E. - \ 2005
Agriculture, Ecosystems and Environment 110 (2005)3-4. - ISSN 0167-8809 - p. 166 - 184.
african semiarid tropics - guinea savanna zone - representative toposequence - rock phosphate - systems - maize - crops - agroecosystems - nigeria
Strong gradients of decreasing soil fertility are found with increasing distance from the homestead within smallholder African farms, due to differential resource allocation. As nutrient use efficiency varies strongly along these gradients, such heterogeneity must be considered when designing soil management strategies, aimed at an improved overall resource use efficiency at farm scale. Here, we quantify the magnitude and study the origin of farmer-induced, within-farm soil fertility gradients as affected by biophysical and socio-economic conditions, and investigate farmers¿ perceptions of such heterogeneity. Farm transects, participatory resource flow mapping, farmers¿ classification of land qualities, and soil sampling for both chemical and spectral reflectance analyses were performed across 60 farms in three sub-locations (Emuhaia, Shinyalu, Aludeka) representing the variability found in the highlands of western Kenya. Differences between the various field types of a farm were observed for input use (e.g. 0.7¿104 kg N ha¿1), food production (e.g. 0.6¿2.9 t DM ha¿1), partial C (e.g. ¿570 to 1480 kg ha¿1) and N (e.g. ¿92 to 57 kg ha¿1) balances and general soil fertility status, despite strong differences across sub-locations. Concentration of nutrients in the home fields compared with the remote fields were verified for extractable P (e.g. 2.1¿19.8 mg kg¿1) and secondarily for exchangeable K (e.g. 0.14¿0.54 cmol(+) kg¿1), on average, whereas differences for soil C and N were only important when considering each individual farm separately. Farmers managed their fields according to their perceived land quality, varying the timing and intensity of management practices along soil fertility gradients. Fields classified by them as poor were planted later (up to 33.6 days of delay), with sparser crops (ca. 30% less plants m¿2) and had higher weed infestation levels than those classified as fertile, leading to important differences in maize yield (e.g. 0.9 versus 2.4 t ha¿1). The internal heterogeneity in resource allocation varied also between farms of different social classes, according to their objectives and factor constraints. Additionally, the interaction of sub-location-specific socio-economic (population, markets) and biophysical factors (soilscape variability) determined the patterns of resource allocation to different activities. Such interactions need to be considered for the characterisation of farming system to facilitate targeting research and development interventions to address the problem of poor soil fertility
Effectiveness of phosphate rock on ferralsols in Tanzania and the influence of within-field variability
Mowo, J.G. - \ 2000
Agricultural University. Promotor(en): O. Oenema; B.H. Janssen. - S.l. : S.n. - ISBN 9789058082572 - 164
kunstmeststoffen - fosformeststoffen - natuurfosfaat - drievoudig superfosfaat - red soils - ferralsols - mest - peulgewassen - maïs - bodemvariabiliteit - tanzania - fertilizers - phosphorus fertilizers - rock phosphate - triple superphosphate - red soils - ferralsols - manures - legumes - maize - soil variability - tanzania
Crop yields in Tanzania are often limited by P deficiency. Direct application of the locally mined Minjingu phosphate rock (MPR) is considered a possible option in addressing the problem. Being poorly soluble, it is more effective in low pH soils with sizeable P and Ca sinks. In soils rather high in pH, mechanisms are required to promote its dissolution. This research was initiated with the overall objective of increasing our understanding of the multiple interactions affecting the availability of P from MPR in the soil-plant system. This knowledge is required to enable optimal exploitation of the various factors influencing MPR effectiveness for increased crop production. The research program consisted of one laboratory experiment, six greenhouse experiments and four field trials.
The laboratory and greenhouse experiments dealt with single factors influencing the effectiveness of PR under controlled conditions. From the laboratory experiment it was established that the high content of CaCO 3 in Minjingu PR was the major factor that determined the amount of HCl extractable P. However, agronomically it did not differ from Khouribga and Mali PRs, which have relatively lower CaCO 3 content. High application rates lowered the effectiveness of PR pointing to a need to balance application rates with proton supply. In a study relating yield and soil pH, increasing dry-matter yield of maize, cowpea and pigeonpea could not noticeably affect the soil pH mainly due to the pH buffer capacity of the soils. However, when intercropped with maize, cowpea or pigeonpea took up more P from MPR and even improved the uptake of MPR-P by maize. This point to a possible proton induced dissolution of MPR by the legumes. Hence, when intercropping involves legume crops capable of fixing N lower rates of PR can be used which is an attractive option for resource poor farmers. A study on the response of maize, cowpea and pigeonpea to MPR and TSP on soils of low and high pH showed that response to MPR was stronger in the low than in the high pH soil. Both legumes gave a higher DM production with MPR than with TSP on the high pH soil and yields were higher on the high than on the low pH soil. This indicates that the legumes prefer a high soil pH to a low soil pH and are able to make use of MPR at a relatively high pH. The later is possible when the legumes can modify rhizosphere soil conditions with respect to pH.
Maize dry-matter yield response to P on soils with different pH and available P was observed where soil available P was low. There was no yield response to P when P-Bray-I was > 7 mg kg -1 (P-Olsen > 10 mg kg -1 ). Utilization of absorbed P was better from MPR than from TSP in low pH soils indicating a liming effect by the MPR.
Field experiments dealt with multiple factors influencing the effectiveness of PR. The influence of spatial soil variability on the effectiveness of PR and crop performance was studied using the Post-mortem Residual Analysis and Nearest Neighbor Means techniques. These techniques were effective in isolating environmental from treatment effects and they were more useful in the large than in the small trials. A study on the method and rate of P application showed that method of application was more important for MPR in high pH soils and that response to MPR will be obtained at modest rates. The residual effects of TSP and MPR were almost the same. Combined application of MPR and the acidifying fertilizer sulfate of ammonia showed that there is little scope for improving the effectiveness of MPR through the acidifying effect of sulfate of ammonia. Combined application of MPR and TSP gave best results in low pH soils when both of them were applied using the same method. Meanwhile, incubating MPR with farmyard manure could not stimulate MPR dissolution given the high pH of the two materials even after 40 days of incubation (pH (H 2 O) = 8.35). It is concluded that the use of MPR could be extended to less acidic P deficient soils when mechanisms are employed that could stimulate its dissolution. They include use of low amounts of MPR (low input strategy), incorporating legumes in the cropping system and using the same method of application of mixtures of MPR and soluble P fertilizers.
Keywords: Minjingu phosphate rock, triple superphosphate, Rhodic Ferralsols, farmyard-manure, protons, dissolution, legumes, maize, management options, spatial soil variability, Tanzania.
Welk fosfaatoverschot nodig op melkveebedrijven?
Boer, D.J. den; Middelkoop, J.C. van; Jagtenberg, C.J. ; André, G. - \ 1994
Praktijkonderzoek / Praktijkonderzoek Rundvee, Schapen en Paarden (PR), Waiboerhoeve 7 (1994)6. - ISSN 0921-8874 - p. 23 - 26.
mineralen - boekhouding - melkvee - melkveehouderij - plantenvoeding - kunstmeststoffen - mest - fosfor - natuurfosfaat - graslanden - zea mays - maïs - minerals - accounting - dairy cattle - dairy farming - plant nutrition - fertilizers - manures - phosphorus - rock phosphate - grasslands - zea mays - maize
Het Nutriënten Management Instituut (NMI) en het PR hebben van een groot aantal melkveebedrijven berekend welk fosfaatoverschot nodig is om de fosfaattoestand van de bovengrond (5 cm zodelaag bij grasland en 20 u 25 cm bouwvoor bij maosland) op peil te houden.
Mobilization of rock phosphate by rape (Brassica napus L.)
Hoffland, E. - \ 1991
Agricultural University. Promotor(en): G.R. Findenegg; P.A. Leffelaar. - S.l. : Hoffland - 93
natuurfosfaat - brassica napus var. napobrassica - koolrapen - rock phosphate - brassica napus var. napobrassica - swedes
Rape (Brassica napus) is known as an effective user of sparingly soluble rock phosphates. The research reported in this thesis aimed to establish the cause of this phenomenon.
With the help of an agar plate technique it was established that phosphate-deficient rape plants grown with nitrate as nitrogen source acidify their rhizosphere. The acidification is restricted to a root zone of about 1.5 cm behind the root tip, and is not related to nutrient uptake. Enzymatic analyses revealed that more malic and citric acid is exuded in the acidified part of the rhizosphere than in the alkalinized part and that the concentrations of these acids in the exuding root segments are higher. It is concluded that acidification of the rhizosphere by exudation of organic acids might enable rape to mobilize rock phosphate.
The concentration of citric acid in the shoots of phosphate-deficient rape plants is also higher. The results of experiments in which the shoots of rape plants were exposed to labelled carbon dioxide indicated that the exuded acids originate from the shoot.
To calculate the effect of organic acid exudation on phosphate uptake from rock phosphate a simulation model was used. The first version model describes the uptake of a growth-limiting, dissolved nutrient from soil by a growing root system. The uptake of the nutrient depends on the rate of nutrient supply towards the roots by mass flow and diffusion. Allowance is made for both time-dependent root density and inter-root competition. Each root is assigned a finite cylindrical soil volume delivering nutrients. This soil volume per unit root length declines with increasing root density.
Simulated and experimental results agreed well when uptake of nitrate or (dissolved) phosphate from a quartz sand/nutrient solution mixture was described at growth-limiting supply (a).
An experimentally determined relation was used to describe the effect of decreasing pH on the solubility of Mali rock phosphate (b). Parameters on exudation were measured in rape plants grown without phosphate. The values of these parameters were assumed to describe the potential of rape plants to mobilize rock phosphate. It appeared that simulated phosphate uptake was greater than observed uptake (c). From (a), (b) and (c) it is concluded that measured rates of organic acid exudation are more than sufficient to explain the relatively large uptake of phosphate from rock phosphate by rape.
Rock-phosphate mobilization induced by the alkaline uptake pattern of legumes utilizing symbiotically fixed nitrogen
Aguilar Santelises, A. - \ 1981
Landbouwhogeschool Wageningen. Promotor(en): A. van Diest. - Wageningen : Aguilar Santelises - 111
bodem - stikstof - peulvruchten - fabaceae - kationenwisseling - absorptie - natrium - diffusie - ionen - bodemkunde - natuurfosfaat - stikstofkringloop - bacteriën - soil - nitrogen - grain legumes - fabaceae - cation exchange - absorption - sodium - diffusion - ions - soil science - rock phosphate - nitrogen cycle - bacteria
The ability of plants to exert a certain degree of selection when absorbing nutrients often has as a consequence that on an equivalence basis unequal quantities of cationic and anionic nutrients are absorbed. To maintain electroneutrality inside and outside its tissues, the plant absorbs or extrudes ions not considered as nutritive ones. As a result of such regulatory actions of the plant, the pH in the root environment can be affected which in turn can exert an i nfluence on the solubility and availability of nutrients.In this dissertation, attention is paid to the implications of utilization of symbiotically fixed N 2 by legumes for the usefulness of rock phosphates as phosphatic fertilizers. The chain of thoughts is, as follows: 1. originally present soil phosphate and soil nitrate enable legumes to form nodules in which symbiotic N 2 fixation can take place which for its further growth will make the host plant independent of any form of combined N (NH4+and/or NO3-) ; 2. when the legume has exhausted the initial NO3-supply, but can continue to grow because of the availability of symbiotically fixed N 2 , it will absorb more cationic than anionic nutrients; 3. on account of this cationic uptake pattern, the growth medium in the vicinity of absorbing roots will acidify; 4. as a result of this acidification alkaline rock phosphates, when added as fertilizer, might be partially solubilized; 5. because of this solubilization induced by N 2 fixation, rock phosphates might be more useful P sources for legumes than for other crops such as cereals, and they might also be more useful for legumes utilizing symbiotically fixed N 2 than for the saw legumes under conditions of an ample supply of NO 3 -N.In a series of experiments, it was investigated whether this chain of thoughts could be verified and, if so, which limiting factors could manifest themselves and how these might be eliminated. These experiments were conducted in greenhouses with the use of two growth media, namely pure quartz sand and a sandy loam low in P which was located at a depth of a few meters in a sand quarry near Lunteren. The experimental crops used were soybean, alfalfa, peanut, and maize. The phosphate fertilizers employed were triple superphosphate and alkaline rock phosphates from Mali and Morocco.In experiment 1, use was made of sand as growth medium, soybean as test crop and superphosphate as P fertilizer. It could be shown that, as long as the crop could make use of NO 3 -N, soybean exerted a pH-raising effect on the sand. Shortly after the stage at which the NO 3 -N was exhausted and the plants transferred to symbiotically fixed N 2 as N source, a decline in pH could be observed. In the first weeks of growth, the pH-values of rhizosphere- and non-rhizosphere sand were found to differ. Probably as a result of intensification of the root system, later on these differences disappeared.In experiment 2, an evaluation was made of the ability of soybean, grown m sand, to make use of P supplied in the form of the two rock phosphates. With an ample supply of NO 3 -N, the plants appeared to be unable to utilize these P sources, but this was different for plants utilizing symbiotically fixed N 2 , on account of the fact that these latter plants succeeded in acidifying their root environment. With the use of superphosphate, the growth of plants utilizing fixed N 2 was very poor, which could be ascribed to P toxicity. To compensate for the absence of NO3-as major anionic nutrient, these plants appeared to have absorbed excessive quantities of phosphate. In the case of NO 3 -fed plants, P uptake was less extreme.Experiment 3 was comparable to experiment 2, except that the sandy loam was used as growth medium. It appeared that in this case the acidifying effect of the plants utilizing fixed N 2 was insufficient to solubilize the rock phosphates used. The short duration of the experiment, the P-fixing capacity and the pH-buffering capacity of the soil, and a shortage of available soil phosphate were factors likely to be responsible for the negative results obtained in this experiment.Since peanut is often grown m light-textured soils having low pH- buffering capacities, this crop was chosen as test crop in experiment 4 to examine its ability to utilize P applied in the 3 different fertilizer form to sand. Even when the superphosphate quantity applied was reduced to me-half of that applied to soybean in a previous experiment, P toxicity was still noticeable. For unknown reasons, symbiosis between host plant and Rhizobium strain was insufficiently effective to bring about an acidification of the sand. As a result, solubilization of rock phosphates did not take place. It was furthermore observed that peanut belongs to the group of plants exhibiting a neutral uptake pattern (equivalent quantities of cationic and anionic nutrients absorbed) when N is absorbed as NO3-.In experiment 5, alfalfa was grown m the sandy loam. With this perennial crop, the acidification induced by plants utilizing fixed N 2 appeared large enough to enable alfalfa to make use of P applied in rock phosphate form. In the case of NO3-nutrition, the soil pH became high enough to render the sparsely soluble Mali rock phosphate largely unavailable to alfalfa. It could be concluded that the influence exerted by perennial legumes m the soil pH can be large enough to overcome a buffering capacity the soil may have.The possibility exists that in extremely poor soils, lack of available P prevents the development of nodules so that symbiotic N 2 fixation cannot take place. In such cases, application of a small quantity of soluble P fertilizer may be sufficient to put into motion the chain of reactions eventually resulting in solubilization of rock phosphate that was also applied. In experiment 6, a small quantity of 32P-labeled KH 2 PO 4 was added to sand or sandy loam which was placed in pots m top of a larger quantity of sand or soil to which either me of the two rock phosphates was added. With the use of sand as growth medium, it was found that with the aid of the small quantity of KH 2 PO 4 applied, soybean could indeed make better use of the rock phosphates. With the soil, such a priming effect of the starter-KH 2 PO 4 was not observed. More so than in other experiments, low light intensity constituted an obstacle to a normal development of the N 2 -fixing mechanism, thus reducing the ability of the root system of soybean to acidify, its root environment.It is known that vesicular- arbuscular mycorrhiza (VAM) can make a contribution to the P nutrition of plants growing m P-depleted soil or m soil to which sparsely soluble rock phosphate was applied. In experiment 7, it was examined whether the joint actions of Rhizobium and VAM in mobilizing soil P and P added as rock phosphate would be more effective than the single actions of each of the micro-organisms . The experimental crop was again alfalfa grown m the sandy loam. It was found that for the control treatment (no fertilizerP added) and for the Mali rock phosphate treatment the quantities of P absorbed by the plants were indeed largest when both Rhizobium and VAM were active. In the superphosphate- and Morocco rock phosphate treatments, VAM did not make a contribution to the P nutrition of the plants, which can be seen as an indication that the availability of these fertilizers was high enough to meet the requirements of the alfalfa plants without any aid rendered by VAM.In experiment 8, it was shown that maize, as an example of a crop that in the case of NO3-nutrition raises the pH of its root environment, lacks the ability to utilize P applied in rock phosphate form. In this respect, the behavior of maize is similar to that of leguminous crops, like soybean and alfalfa, when these crops are amply supplied with NO 3-N and do not utilize symbiotically fixed N 2 .The conclusion to be drawn from the results obtained is that alkaline rock phosphates are relatively useful as P sources to c-raps which are capable of acidifying their root environment. Legumes possess this ability, provided that they utilize symbiotically fixed N 2 . In such cases, they withdraw from the soil more cationic than anionic nutrients, thereby exerting an acidifying effect m their root environment.Such an ability to solubilize rock phosphates is, therefore, dependent m the extent to which N 2 fixation can take place. Such a fixation can be hampered by low light intensities, but also by lack of a small quantity of readily available phosphate needed by the young plants to build up an N 2 -fixation mechanism. Once such a mechanism is operative, it can indirectly make a contribution to the mobilization of alkaline rock phosphates when these have been applied to the soil. From the results obtained it appeared that the initiation of a chain of reactions eventually leading to the solubilization of rock phosphate can be brought about by adding a small quantity of superphosphate or by establishing a symbiosis of legume and VA mycorrhiza.The results obtained provide opportunities for those countries which possess alkaline rock phosphates that do not lend themselves to being used as basic materials for the manufacturing of superphosphates. Application of these rock phosphates creates a possibility to grow legumes with only a slight investment in the form of rock phosphate needed to supply the plants with two highly important and expensive nutrients, namely nitrogen and phosphate.
Proefnemingen met hyperfosfaat en rhenaniafosfaat
Prummel, J. - \ 1972
Haren : [s.n.] (Rapport / Instituut voor bodemvruchtbaarheid 1972, no. 10) - 19
efficiëntie - kunstmeststoffen - metafosfaten - rhenaniafosfaat - natuurfosfaat - efficiency - fertilizers - metaphosphates - rhenania phosphate - rock phosphate
Bemestingswaarde van het natuurlijke fosfaat "Hyperfosfaat Reno"
Paauw, F. van der; Prummel, J. - \ 1949
's-Gravenhage : Staatsdrukkerij Uitgeverijbedrijf (Verslagen van landbouwkundige onderzoekingen 55.01) - 27
natuurfosfaat - rock phosphate
Fijnheid en verdeeling van een als meststof toegediend natuurlijk fosfaat
Leijenaar, G.H.A. - \ 1932
Wageningen University. Promotor(en): J. Hudig. - Wageningen : Veenman - 113
natuurfosfaat - fosfor - bodem - kunstmeststoffen - samenstelling - onderzoek - proeven - rock phosphate - phosphorus - soil - fertilizers - composition - research - trials
This study was to examine whether grinding of an insoluble fertilizer to different degrees of fineness produced a limit below which further grinding did not accelerate solution of the particles. Such a limit was indeed found. Below 30 mmu* the distribution of the particles became very uneven because individual particles tended increasingly to stick together. Mixing fine particles with coarser ones could improve the situation, but the percentage of coarse particles should not be too high.
Grinding natural phosphates to colloidal fineness was useless, since the colloidal state was lost during drying of the product. The smaller particles tended to aggregate thus preventing uniform distribution throughout the soil.
The availability of P in natural phosphates improved with the physiologically acid reaction of N fertilizers such as NH 4 HCO 3 and NH 4 NO 3 . A first requirement for rapid absorption of P from natural phosphates was a low concentration of Ca 2+ .