Research was carried out on the possible yield increase of crops in The Netherlands by the use of PSS (partial soil sterilization) on the soil, on the basis of published as well as unpublished data and by experimentation with different disinfectants, soils and plants.
Following review of the literature and the methods employed are chapters dealing with inventory of quantitative data, PSS treatments of random fields without known problems, of soils with known nematode problems, and of fresh uninfested soils, with a final chapter on diagnostic studies on one particular PSS effect in oat.
PSS was applied to 600,000 ha, 1 % of the land, in the U.S.A. and to 40,000 ha, 4 % of the arable land, in The Netherlands in 1969. The literature of the last five years comprises more than 400 scientific articles, but no reliable estimates have apparently been published about the magnitude of the effect of PSS in general, nor has the effect on plant growth been fully explained. The effect of the various PSS treatments, physical as well as chemical, may be direct in connection with the ingredients applied or the soil organisms or the soil itself, and indirect, e.g. in connection with plant growth. Plant growth is the main parameter used to measure PSS effects in this study, although density of pathogens (especially nematodes), and the chemical soilfertility (especially in connection with nitrogen) have sometimes been assessed as well.
The approach and the methods employed in this study were partly statististical and partly experimental; in both cases common techniques were used. The data were usually arranged after calculations and tested by means of the IBM and CDC computerfacilities of the Landbouwhogeschool.
Altogether 2453 individual treatment-soil-plant combinations from trials in The Netherlands during the period 1947-1970 could be collected. They were derived from the author's 545 experiments, from unpublished files of the Plantenziektenkundige Dienst, the Landbouwhogeschool and other institutions or individual researchers, and from literature. For each separate item the growth or yield was calculated as a percentage of untreated, and inclusive of supplementary data inserted into a computer card. After standardization and comprehension of the data special computer programmes furnished a number of tables and diagrams (Tables 2-9, Figs. 1-6). The yield on treated soil showed as general average 141 % of untreated, or 154 % without phytotoxic cases. For PSS treatments with the best known chemicals these figures were 151 %/164 %. If for each crop the average best treatment is selected, 172 %/188 % is reached. The average results of the treatments increased from pentachloronitrobenzene (103 %/118 %), via Vapam, Trapex, dichloropropene and chloropicrin to heat (221 %/240 %). The recommended doses normally used in practice appeared to give optimal average PSS effects. The percentage phytotoxic cases was on an average about 20 %. It, did not increase with higher doses, but was strongly correlated with the boiling point of the fumigants used. The crop averages differed widely. Pea, barley and woody plants seldom reached a good yield unless PSS had been applied. For grass, clover, potato and cabbage an intermediate yield increase was usually reached, whereas the response to PSS of many crops was low on some and very high on other soils. The measure of growth improvement was strongly correlated to the presence of known noxious pests and diseases (Tables 5-7). They were according to this inventory, very low on new polder soil, or clean sand, or fresh peat (99%/114%), and moderate on random soils without known infestation (115%/125%), as compared to the overall average already mentioned above (141%/154 %), The effects varied much with soil type; they decreased from loamy sand, via clay soil, peaty sand, diluvial sand and dune sand to silty clay. The effect for polder sand (mainly fresh soils) was slight and for river sand it was even negligeably small, indicating the absence of stimulatory effects on plant growth by the chemicals themselves. PSS effects also varied with the different localities throughout the country. The relatively high effects for the Winschoten and Wageningen districts are the result of numerous experiments on heavily nematode- infested plots and experiments with imported loads of infested test soils. Therefore they are not representative for these districts. The low effect for Emmeloord district reflects results obtained on new, uninfested polder soils.
The PSS treatments on 11 random fields without known problems (treatments with chloropicrin, dichloropropene and pentachloronitrobenzene, 7 to 14 test crops per field, three replicates) reached an average yield of 113 %, or 122 % excluding the phytotoxic cases, or 156 % for the average of the best treatments of each soil-crop combination. Nearly all crops suffered heavy yield losses on some fields and nearly all fields contained noxious problems for some crops which could be solved by PSS treatment (Fig. 7). The potential effect of PSS applied to all agricultural land including the problem fields must exceed 156 %, but should be less than 188 % found above as the average for all trials in The Netherlands.
Five PSS treatments applied in pottrials to eight soils, with a known, noxious nematode problem and grown with nematode susceptible test plants, illustrated strong growth responses according to expectation, except for two soils where nematodes were probably not the main nor the sole problem, and except for some phytotoxic effects (Tables 11-26, Figs. 9-19). Dichloropropene was as effective as heat and chloropicrin in these soils with respect to nematode control and reached about the same yield increases. The nematicidal effect of Ternik was delayed but sustained; its systemic potencies were confirmed, but sometimes it was phytotoxic. Captan showed poor nematicidal, but not wholly negligeable effect. It was among the best treatments only in one soil with a suspected fungus infestation (Fig. 19) and was strongly phytotoxic in two soils (Figs. 17, 18).
In pot trials with five PSS treatments of soils without or with very few noxious organisms caused only barely measurable growth responses to the crops carrot, rose and ryegrass (Tables 29, 31 and Figs. 20-24). Two unidentified, probably seed-borne problems, i.e. loss of seedlings and branching of roots, encountered in carrot, did not show up in soils previously disinfected with beat, chloropicrin or a very heavy dose of dichloropropene (Fig. 21).
Diagnostic studies on an important PSS effect on oat, in soils infested with a complex of freeliving root-infesting nematodes (Pratylenchus crenatus + Tylenchorhynchus dubius + Rotylenchus robustus)
proved, that inoculation with the nematode could produce the damage for the greater part (Tables 33-37, Figs. 25- 27). The possible contribution of other organisms was however not excluded. The inoculation procedure seemed to cause important losses amongst the nematodes thus inoculated. For some species these were even aggravated by using steamed or heated soil shortly prior to the test.