The so-called cold paddy field, also known as cold slurry field in some places, belongs to the paddy field with high soil acidity in the technical terminology of later generations. Generally speaking, the soils in the south are mostly acidic, some of which are less acidic and do not affect cultivation, while others are more acidic, and the crops are more difficult to grow, and the unit yield of the field will be much lower than that of other farmland. From the perspective of geographical conditions, the land that is generally in the shade and lacks sunlight is most prone to strong acidity. Due to the lack of sunshine throughout the year, the soil temperature in these places is lower than that in sunny areas, hence the name of the cold water field.

As early as the Song Dynasty, Chinese folk knew how to improve the cold water fields by applying plant ash, quicklime or animal bone ash to the fields. The ancients explained that the water quality of cold water fields is cold, and the grass ash and quicklime are hot, which can achieve the effect of "warm soil".

As a modern person, Su Hao certainly knows that the reason why quicklime can transform cold water fields is that quicklime is an alkaline substance, which can neutralize and react with the acidic substances in the acidic soil, so as to achieve the purpose of improving the acidic soil.

According to some ancient records, ancient Chinese farmers applied 100 catties of quicklime in an acre of cold-water field, which could increase the yield per mu by 20% to 50%, and the yield increase effect was very obvious. Farmers also understand that the application of quicklime in the field should be reduced year by year, and after the acidity of the soil is basically removed, it is not appropriate to apply quicklime again, otherwise it will lead to soil compaction, which is also not conducive to cultivation.

Su Hao paid attention to the matter of the cold water field because of the inspiration he received when he was digging a well in Caijia Village. At that time, the well location he chose fell on the only two acres of land of the Cheng family's sisters and brothers, and those two acres of land happened to be cold water fields. From the mouth of Cai Youshou, the chief of the li, Su Hao knew that the yield of the cold water field was low, and the land price was also low. Ordinary good land costs more than a dozen taels of silver to buy an acre, while the cold-water fields cultivated by the Cheng family's sisters and brothers are only worth five taels of silver per acre.

After that, Su Hao paid close attention to the situation of the cold water fields everywhere he went, and he found that in the several townships he walked, the cold water fields accounted for about one-third of the cultivated area, and the existence of these cold water fields greatly affected the grain production of the county, and also exacerbated the contradiction of more people and less land.

Su Hao interviewed some old farmers, and from them, Su Hao learned that everyone actually knows the method of transforming cold water fields, but they are only subject to the price of raw materials such as quicklime and plant ash, so they cannot implement this transformation. At that time, the price of a load of quicklime in the north was about 1 penalty, while in the south, the price could reach 2 to 3 yuan. The increased grain yield after the application of quicklime is worth 4 to 5 yuan, and the difference between the two is very small. As a result, farmers are reluctant to do such things that are basically unprofitable.

After Su Hao learned about these situations, he focused on solving the problem on the supply of quicklime. The production process of quicklime is very simple, as long as the limestone is placed in a kiln for high-temperature calcination, it can be obtained. When Su Hao was in the countryside, he found many limestone mines of very good quality, and the rest of the matter was to find enough and cheap enough coal to use as fuel for calcination.

The cheap lime in the north is largely due to the fact that coal is cheap and there is a general shortage of coal in the south, so the price of coal ash is on the high side. Ninety percent of the fuel used in burning lime is briquette, and one percent is firewood, and the price of coal cannot be reduced, and the price of lime is difficult to fall.

Saying that there is a shortage of coal in the south is also relative, Su Hao's mind remembers the distribution of mineral resources in the country, and he clearly remembers that Fengcheng is one of the most important coal-producing areas in the south in later generations.

With these things in mind, Su Hao had a clear roadmap for what he wanted to do: first explore the coal mines, then dig the coal, burn the lime with coal, and then sell the lime to farmers at a low price to transform the cold water fields. He estimated that if 1,000 hectares of cold water fields can be transformed, even if the yield of one acre of land can be increased by one stone of rice, the output of the whole county can be increased by 100,000 stones, which is a very considerable number.

After listening to Su Hao talk about his plan, Wu Zhicheng's face was solemn, and he breathed a sigh of relief for a long time before he said: "Mr. Yang Ming advocates the unity of knowledge and action, how many people can do it in my generation?" The old man is really impressed by the fact that he has not been weak in the year of change, but he is always thinking about the livelihood of the farmer and has been able to put it into practice repeatedly.

These academy disciples, if they are willing to do some practical things with the change, I, as a teacher, naturally can't stop them. Even for those who are unwilling to do things, the old man wants to talk to them and persuade them. ”

"It doesn't have to be." Su Hao waved his hand and said, "This kind of thing is voluntary, I would rather have 10 like-minded helpers than 100 bystanders who look like gods." To be honest, I don't really have that much leisure to care about them. ”

"Well, it's up to you to change the matter." Wu Zhicheng said, "If you need to change it and ask the old man to do something, even if he opens his mouth, although the old man is old and dull, he can still do something." ”

"How dare you have Mr. Lao Wu's big driver." Su Hao bowed and said.

After saying that Ma Yu and the others were allowed to ask for leave from the academy to help in the workshop, Su Hao got up to say goodbye to Wu Zhicheng and left the academy. Wu Zhicheng kept sending Su Hao out of the gate, and then he walked back with emotion. Halfway through, Wu Zhicheng suddenly remembered that he was still holding a note in his hand, and Su Hao tricked him out of the study by teaching him the solution to the problem on the note.

"This Su Gaizhi promised good things, but he actually took advantage of the old man's confusion and broke his promise." Wu Zhicheng muttered, he looked at the contents of the note carefully again, and said to himself: "Don't ask him first, I'll figure it out myself, maybe I can come up with a solution." When the time comes, I will use this solution to humiliate him......"

On that slip of paper, there are some equations: 6 = 3 + 3, 8 = 5 + 3, 10 = 5 + 5......

Finally, there is a sentence that reads: Any negative number that is not less than 6 can be recorded as the sum of the two roots.

In ancient China, even numbers were also known as yin numbers, and number roots were prime numbers. Su Hao's question to Wu Zhicheng is translated in modern language:

Any even number not less than 6 can be expressed as the sum of 2 prime numbers.

"What's the matter with the most difficult question, it's a big deal that the old man has spent his whole life trying to solve it." Wu Zhicheng made up his mind with great pride.

Tricking the poor old man Wu to prove Goldbach's conjecture with the knowledge of nine chapters of arithmetic, Su Hao felt no sense of guilt in his heart, who let you close the door and not see the guests, just don't go out for the rest of your life, stay at home and do number problems.

Humming pop songs all the way, Su Hao walked in the direction of his home, and when he passed by the intersection of a small street called Taiping Lane, he suddenly remembered that Lu Xiuer had told him that she sold tea eggs in this alley on weekdays. On a whim, Su Hao decided to go and see how Lu Xiuer was doing business.

Taiping Lane is a commercial street in Fengcheng County, with shops selling all kinds of goods on both sides, as well as tea shops, restaurants, etc., Su Hao inadvertently, and even saw a building that is suspected to be a green building, with red sleeves fluttering on the balcony outside, and the fragrance is fragrant. Su Hao prides himself on being a righteous gentleman, seeing this situation, he hurriedly looked at his nose and nose, recited the multiplication mantra in his heart, and walked quickly past the door of the green building.

A few steps further, a bunch of vendors carrying burdens and stalls appeared in front of them, and Lu Xiuer was among them. I saw her sitting on a small horse, with a small charcoal stove in front of her, and a small pot on the stove, slowly boiling tea eggs, and another pot next to her, presumably containing boiled eggs.

Occasionally, someone walked in front of her and said a few words, and then Lu Xiuer opened the pot, took out a tea egg with chopsticks, held it with bamboo leaves, and handed it to the buyer, and then took a few pennies and carefully put it into a small pocket hanging on her chest, with a smile on her face.

Su Hao stood in the distance, watching his sister enjoy the fun of doing business and making money, and had a warm feeling in his heart. In the middle and late Ming Dynasty, southern China had a stable society and a prosperous life, and there were indeed some peaceful and prosperous times. In an ordinary family, if the family members are more flexible, and the people who can do things in the family are diligent and willing to work, it is not difficult to live and work in peace and contentment in the current society. In fact, the people don't care who is the prince and who is the first assistant, isn't what they pursue is a stable and peaceful life?

Su Hao's mind wandered for a while, and the situation in front of him had been reversed. In front of Lu Xiu'er, I don't know when two little ** with crooked melons and cracked dates appeared, and they threw a tea egg that they had just bought from Lu Xiu'er to the ground, and scolded loudly:

"Where did you come from, countryman, can you boil tea eggs, your mother, Lao Tzu has never eaten such unpalatable tea eggs!"