In 1993 there were cool summer damages of some crop yields. Especially the damage of rice yield was serious, crop situation indices of rice yield were 74 in the whole country, 28 in Aomori prefecture, 30 in Iwate prefecture, 38 in Miyagi prefecture and 40 in Hokkaido prefecture. Those crop situation indices were the worst since 1945. The difference between in 1993 and in normal year of the mean temperature from the middle of July to the middle of August was the largest among the summer season in 1993, that was -4.0〜—4.3 in Aomori prefecture, Iwate prefecture and Miyagi prefecture. There are many protection methods for cool summer damage of crops. The physical protec-tion method is achieved by changing the volume of heat budget items ; sensible heat, latent heat and soil heat. Wind breaks and deep flood irrigation are the main actual methods. The effects of those protection methods are discussed. Also, the achieving problems of the protection methods are discussed here.
Sugar beet crop was seriously damaged by cold and wet injury at the Tokachi area of Hokkaido in Japan 1993. I tried to estimate relation of sugar beet yield to soil properties using LANDSAT TM data which were acquired on July 8 1993. Geographical distribution of sugar beet yields was mapped from NDVI (normalized difference vegetation index) on the basis of the TM data and the mean temperature in July. The regional variability of the yield was calculated dividing original estimated yield by the broad trend of the yield, and examined with some soil properties. Relatively low regional yields was related to the soil type with poor drainage condition, existence of compact subsoil, and lack of gravel layer. These results suggest that the remotly sensed data may be useful for estimation of the interaction of soil, weather and crop growth.
Summer crop plants grown in rain-fed fields often suffer from drought during a hot and relatively dry summer in Japan, even though there is much precipitation during their growing seasons compared with the semi-arid regions. Crop plants could withstand soil moisture deficiency by means of developing elongated root system under conditions where soil moisture decreases gradually. However, after they have grown under sufficient soil moisture conditions, they could not adapt under conditions of rapid moisture depletion due to shallow and poorly developed root systems and, therefore, would suffer from water deficits even when soil moisture does not deplete considerably. In many regions of Japan, summer field crop plants grow at their best during the vegetative phase in the rainy season called “Baiu” immediately before the hot and dry summer. This might be one of the reasons why they suffer from water deficits in the summer. This assumption has been confirmed by the experiment using the soybean plants ;(1)grown under moderate deficient soil moisture before flowering and (2) grown under sufficient soil moisture. Under severe deficient soil moisture conditions during ripening stage, high dry matter production and high grain yield were attained in the former plants due to (1)a well developed root system and, therefore, high capacity to absorb soil water, (2) maintenance of a high leaf water potential and, therefore, a high photosynthetic rate during daytime and (3) a delay in leaf senescence compared with the latter. The plants with well developed root system also attained higher dry matter production and higher grain yield under well irrigated conditions during ripening stage for the same reasons as in the experiment mentioned above. During the growing seasons of winter crop plants, a rainy season suppressing root system development also exist. Improved cultivation for developing root system, such as drainage in the rainy seasons, would be important in field summer and winter crop plants in Japan.
The soil-crop-atmosphere continuum is a dynamic system in which water is recycling. It is convenient to use models for analyzing dynamic systems like this. There are many models to predict soil water contents profile and crop yields. In this paper, I introduce models which describe water balance and grass production, and discuss some study subjects in order to construct more accurate model. The subjects are summarized as follows :
1) The root zone of grassland has been considered to be shallow. But grass roots in subsoils can absorb nutrients and water as well as roots in shallow layers. We have to know how deep roots can absorb nutrients and water.
2) Unsaturated conductivity is essential to calculate water flux in soil. But it is not so easy to measure unsaturated conductivity, so there are few reports concering it. It is necessary to increase data on unsaturated conductivity.
3) Transpiration rates change with soil water content and weather conditions. There are some empirical equations for estimating transpiration rates. But it is not clear which equation is best for a certain condition.
4) Water requirement is necessary to convert the water use of grass into dry matter production. Examples of water requirement, measured in Hokkaido, have wide ranges of values. More measurements are needed.
Zoosporic fungi such as Pythium, Aphanomyces and Phytophthora depend on high soil moisture for infection, disease development, and rapid spread. The invasion of root tissues by these fungi is made by motile zoospores which are produced in water. This report is dealing with the occurrence, and control (soil solarization) of soil-borne diseases of spinach caused by zoosporic fungi, especially outbreaks mechanisms of root rot caused by Aphanomyces cochlioides Drechsler which is one of the most serious diseases of spinach in growing areas in Hokkaido, Japan. Outbreaks mechanisms of spinach root rot in relation to soil moisture and nitrate nitrogen are
summarized as follows.
1) High soil moisture conditions from rainfall or excess irrigation promote the production of zoospores of A. cochlioides, and such conditions are conducive to the spread of secondary zoospores.
2) Due to leaching and dilution by stagnant water, high soil moisture also causes decrease in mineral salts, especially nitrate nitrogen that inhibits zoospore production from zoosporangia, and causes the encystment and death of secondary zoospores. Consequently, the inoculum potential remains high, and disease outbreaks occur.
3) Severe outbreaks occur in fields of sandy soil where nitrate nitrogen is readily leached, and of heavy clay soil or soils with compact subsoil where nitrate nitrogen concentration is readily diluted by stagnant water.
4) The outbreaks mechanisms of spinach root rot apply to cases of the diseases caused by some zoosporic fungi, A. euteiches and P. aphanidermatum.