Predicting wheat yields

I want to get a function that predicts wheat yields on the basis of climate and soil. I have extracted (with Malcolm help at the GIS end) some basic climate data from an old map of Britain. I did a regression of wheat yields against elevation, amounts of rainfall and length of the growing season. The results are below. This is what the results mean: ELEV is negative, meaning the higher up the farm, the less the yield. Makes sense. GS is growing season....strangely the longer the growing season, the lower the yield. The two 'RAINFALL' variables show the effect of rainfall of 1000mm a year and 1250mm a year. The coefficient for the 1250 is greater (at -4.42) than the one for 1000 (at -1.34). Meaning is that the greater the rainfall, a large reduction in yield. See the adjusted R-squared on the right? That gives us the percentage of the variation in wheat yield explained by the model. Here it is 0.3094 or a tad over 30%. This isn't very good, but better than I had expected. Now I need to add soil data and more accurate climate data, such as hours of sunshine.